Patent Application
20090252754
The present invention relates to vaccines comprising Tat, biologically active derivatives thereof or precursors therefor, including nucleic acids encoding such, as well as to methods for vaccination comprising the use of such vaccines.
The Tat protein of HIV-1 is produced very early upon virus entry and is required for virus replication and infectivity. Recently, we have shown that biologically active Tat is very efficiently taken up by dendritic cells and activates them, increasing Th-1 type responses against heterologous antigens. In addition, Tat-based vaccines in monkeys have been shown to be safe, and to induce protective immunity which correlates with the generation of Th-1 type immune responses.
Tat is a regulatory protein of HIV-1 and is produced very early after infection. It is essential for HIV-1 gene expression, replication, and infectivity. During acute infection of T cells by HIV-1, Tat is released in the extracellular milieu in a biologically active form in the absence of cell death or permeability changes1,2. Extracellular Tat is taken-up by neighbour cells where it modulates cellular functions, depending on the concentration, oxidation state, and cell type.
In EP-A-1279404, we show that biologically active monomeric Tat protein is very efficiently taken-up by monocyte-derived dendritic cells (DC) and that, after internalisation, it induces DC maturation and augments allogeneic and antigen-specific presentation by DC, increasing Th-1 responses against recall antigens3. Fanales-Belasio et al. (Journal of Immunology 2002, vol 168 (1), pp. 197-206) also discloses the ability of Tat to augment presentation.
In addition, studies in mice and monkeys have shown that Tat-based vaccines are safe and induce protective immunity against pathogenic virus challenge that correlates with Th-1 type immune responses and cytotoxic T cells4,5.
Cytotoxic lymphocytes (CTLs) play an essential role in the control of intracellular pathogens, including HIV, suggesting that vaccines eliciting optimal CTL responses have applications for the prevention and/or for the control of virus-associated diseases and tumours.
CTLs recognise peptide epitopes expressed at the surface of target cells in association with MHC class I molecules6. The epitope is generated in the cytosol by degradation of the antigen, from where it is transported into the endoplasmic reticulum, where it associates with newly synthesised class I molecules. Often, CTL responses are directed to a single immunodominant peptide out of a larger number of potential epitopes within the same antigen. This phenomenon, known as immunodominance, is still poorly understood. However, the generation and presentation of peptides, the availability of responsive T cells, and little understood immunoregulatory effects can all influence the activation of an efficient immune response to a particular epitope.
The major enzymatic activity responsible for the generation of class I-associated peptides is the proteasome, a large multicatalytic protease that is essential for the degradation of intracellular proteins and the maintenance of cell viability7,8.
Proteasomes consist of a 20S catalytic core arranged as four heptameric rings. The two outer rings contain structural α-subunits (α1-α7), while the inner rings contain β subunits (β1-β7), three of which (β1, β2, β5) exert catalytic activity through a nucleophilic attack on the peptide bond by the N-terminal threonine9. Biochemical studies on the specificities of the proteasome reveal three distinct proteolytic components, which are involved in chymotryptic, tryptic and post-acidic (also called caspase-like) hydrolysing activities. Analysis of the contribution of the individual β-subunits has demonstrated a clear correlation between the individual subunits and the cleavage after preferred amino acids10. When cells are exposed to IFN-γ, the three catalytic β-subunits are substituted by LMP2, LMP7 and MECL1 (also referred as LMP10). These subunits are also expressed in a constitutive manner in specific cell types such as dendritic cells and B cells11,12, and their incorporation in the proteasome alters its activity and enhances the production of certain peptides13.
Proteasomes equipped with LMP2, LMP7 and MECL1 have been called immunoproteasomes, as distinct from the constitutively expressed standard proteasomes. The catalytic activity of immunoproteasomes is characterised by a reduced cleavage after acidic amino acids and an increased cleavage after hydrophobic and basic residues, the most frequent residues found at the COOH terminus of MHC class I binding peptides14. It has been demonstrated that proteasomes generate the exact COOH terminus of MHC class I binding peptides, whereas the NH2-terminal cleavage is not always as precise and that aminopeptidases located in the endoplasmic reticulum may cut the NH2 extensions to generate the correct peptide epitope15-18.
For full and regulated proteasome function, the 20S proteasome core must assemble with other proteasome components, such as the 19S cap complex, to form the 26S proteasome which is able to degrade ubiquitin-conjugated proteins or/and the PA28 proteasome regulator to form the PA28-proteasome complex. The association of PA28 with the 20S proteasome seems to favour the generation of immunogenic peptides19. The generation of immunogenic peptides is a critical step in the activation of epitope-specific CTL responses. Indeed, there is evidence demonstrating that proteasome-mediated proteolysis contributes to the hierarchy of epitopes presented by MHC class I molecules. Subdominant T cell epitopes, in contrast to the immunodominant epitopes, are generated with less efficiency or are destroyed at cleavage sites located within the epitope20,21.
Cafaro et al (Nature Medicine (1999), Vol 5, pp. 643-650), shows that the use of biologically active Tat in an HIV-1 vaccine for monkeys is safe and elicits a broad (both cellular and humoral) but specific immune response and reduces infection with SIV.
WO 00/43037 discloses that Tat and Nef are chemotactic agents for CD4+ cells and that vaccine efficacy may be boosted by the recruitment of CD4+ cells to the site of vaccine injection, when said vaccine is supplemented with Tat and Nef.
WO 02/019968 discloses a co-expression DNA vaccine (CED) that displays immunogenic properties. In particular, a vaccine encoding both an antigen and Tat is disclosed, the antigen benefiting from Tat-mediated immune deviation or immunomodulation/immunoregulation.
We have now, surprisingly, found that the Tat protein induces modifications of the subunit composition of immunoproteasomes in cells either expressing Tat or exposed to exogenous, biologically active Tat protein. In particular, Tat up-regulates the expression of the IFN-γ inducible catalytic subunits LMP7 and MECL1, but down-modulates LMP2. These changes correlate with an increase of all three of the major proteolytic activities of the proteasome. Proteasomes play a key role in the production of MHC class I binding peptides, and we found that Tat decreases the generation and presentation of immunodominant epitopes, while increasing the generation and presentation of subdominant T cell epitopes.
We have also found that modulation of proteosome subunit composition may be achieved by not only wild type Tat, but also by mutated Tat and Tat-derived peptides.
Thus, in a first aspect, the present invention provides the use of Tat, a biologically active equivalent, or a precursor therefor, in the preparation of a vaccine suitable to elicit an immune response against an antigenic substance having a plurality of epitopes, the epitopes including both immunodominant and sub-dominant epitopes, the vaccine comprising at least a part of the antigenic substance encoding or comprising a sub-dominant epitope thereof.
Thus, Tat, when used in a vaccine, causes MHC-I to expose subdominant epitopes of a variable antigen, thereby enabling a persistent immune response to be generated within an individual against variants of the antigen, such as might be encountered with HIV or influenza viruses.
In a preferred embodiment, there is provided the use of Tat, a biologically active equivalent thereof or a precursor therefor, in the preparation of a vaccine suitable to elicit an immune response against a plurality of strains of an infectious organism, the vaccine comprising antigenic material from at least one strain of the organism, said material encoding or comprising a subdominant epitope.
It will be understood that a “precursor” includes any suitable material leading to the presence of Tat in the patient in a manner suitable to act as an adjuvant. This may include peptide precursors, such as fusion proteins, including fusions with signal peptides, which may be cleaved to yield active Tat, or which may be active without cleavage, or may include nucleic acid sequences in a form suitable to be expressed in situ.
Preferably, the Tat used is the wild type Tat shown in SEQ ID NO 284 or is a mutant and/or fragment thereof.
In one aspect, Tat is mutated. Any number of mutations, whether by substitution, deletion or insertion is envisaged, provided that the mutant is capable of increasing the number of subdominant epitopes presented, preferably by modulation of the proteosome subunits, as described above.
Preferably, the mutant has 90% homology to wild type Tat, according to SEQ ID NO 284, preferably 95% and more preferably 99% homology or sequence identity, as measured by known methods, such as the BLAST program.
In a particularly preferred embodiment, Tat is mutated at position 22. Preferably, the cysteine residue present in the wild type Tat at this position is substituted, preferably by glycine. Other suitable amino acids may also be used, such as alanine or any other non-polar amino acid.
In one embodiment, it is preferred that a fragment of Tat is used in the present invention. Any length peptide may be employed, provided that the above effect is seen. It is particularly preferred, however, that the fragment comprises or encodes at least amino acid numbers 47-86 of SEQ ID NO 284, which are given separately as SEQ ID NO 285. Preferably, the precursor is a polynucleotide, preferably DNA or RNA, encoding at least the above amino acids. It is also preferred that the fragment is a polypeptide comprising these amino acids. More preferably, the polypeptide consists of amino acids 47-86 of SEQ ID NO 284.
Preferably, however, the fragment comprises or encodes for up to: 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150 or 200 or more amino acids.
Preferably, Tat may be expressed in situ, preferably in a target cell. The cell may be targeted, preferably, in vitro or more preferably, in vivo. Alternatively a benign, transformed organism may be introduced into the patient, the organism preferably expressing both Tat and the antigen against which it is desired to raise an immune response, but at least expressing Tat. The organism is suitably a virus or bacterium, and may be an attenuated form of an organism against which it is desired to stimulate an immune response.
Preferably, Tat is expressed in situ under the control of an inducible promoter, such that expression of Tat in the target cell can be induced by the user at or near the same time as administration or expression of the antigen. Suitable inducible promoters are well known but include those activated by physical means such as the heat shock promoter, although this is not generally preferred, or those activated by chemical such as IPTG or Tetracycline (Tet). The Tet promoter system is particularly preferred as it allows both on/off control of expression and control of the level of expression.
For Tat to be expressed in the target cell, it is preferable that the vaccine comprises an expression sequence. This sequence, element or vector is capable of expressing Tat in said cell and may be a viral vector, preferably attenuated, preferably an adenoviral vector capsid that can induce expression of Tat in the target cell. Other methods of inducing gene expression in target cells are known in the art and are also mentioned below.
It is, therefore, also preferred that the antigen is administered with a factor that controls or induces expression of Tat from an inducible promoter.
Alternatively, it is preferred that the antigen is expressed by administering a polynucleotide sequence encoding the antigen and that either a further polynucleotide sequence encoding Tat or a polynucleotide encoding a factor capable of inducing expression of Tat, is also provided, preferably substantially contemporaneously.
Expression in situ can be achieved by known methods of gene expression, such as the use of vectors, preferably viral vectors, that lead to expression of foreign DNA or RNA in a host. Preferably, polynucleotides encoding Tat are delivered and expressed by adenoviral or attenuated HIV systems. Alternatively, the polynucleotides can be delivered and expressed by methods such as the use of so-called “gene-guns.” Thus, it is preferred that Tat is endogenously expressed by the patient or vaccinee.
It will also be understood that where reference to Tat is made in the present application, it is intended to include mutants and fragments thereof, as also discussed herein, unless otherwise apparent to the skilled person. For instance, Tat may be wild type Tat or a shortened fragment of the Tat polypeptide sequence, or may be a tat mutant.
It is also preferred that Tat is exogenously produced and provided as a peptide. Preferably, Tat is administered as a precursor that may be cleaved in vivo to provide active Tat.
The patient or vaccinee is preferably a mammal, preferably an ape or monkey and mostly preferably a human. In an alternative embodiment, however, it is also preferred that the method, use or vaccine is not applied or administered to humans.
In a further aspect, the present invention also provides a vaccine or method for modulating proteosome subunit composition, preferably down-regulating particular subunit or subunits, preferably the LMP2 subunit.
In a still further aspect, the present invention also provides vaccines for eliciting an immune response against an antigenic substance having a plurality of epitopes, or for eliciting an immune response against a plurality of strains of an infectious organism, as discussed above. In one embodiment, the vaccine preferably comprises Tat, a biologically active equivalent, or a precursor therefor, the epitopes including both immunodominant and sub-dominant epitopes, and at least a part of the antigenic substance encoding or comprising a sub-dominant epitope thereof.
In a further embodiment, the vaccine preferably comprises Tat, a biologically active equivalent, thereof or a precursor therefore, and antigenic material from at least one strain of the organism, said material encoding or comprising a subdominant epitope.
As discussed above, Tat is preferably that shown in SEQ ID NO. 284. Furthermore, it is preferred that Tat and the antigen are provided as proteins or peptides.
The present invention also provides for the use of these vaccines, preferably to stimulate cross-strain immunity preferably in the treatment of disease, preferably HIV or influenza.
Preferably, the vaccine comprises a suitable vehicle for delivery of Tat and the antigen. Such vehicles are well known in the art.
While a major application of the present invention is in the fight against viruses well-known to generate escape mutants, it is equally important for use against cancer and immunomediated diseases, where the ability to target sub-dominant epitopes is a significant advantage.
In addition, it will be understood that Tat may be used with a vaccine of the present invention to enable the identification of subdominant epitopes of any antigens, especially from disease forms. Identification may effectively take the form of subtractive analysis. An example of this might be to take identical animals, immunise one against a tumour using standard vaccine, and another with a similar vaccine containing Tat, and then identifying what CTL epitopes the second animal had extra, by comparison with the first. Epitopes identified in this manner could be identified and isolated and used in vaccination programs, or in screening.
Subdominant epitopes are commonly found, not only in infectious organisms, but also in tumour and immunomediated disease antigens. Without being bound by theory, the presence of Tat appears to expose a greater number of regions of such antigens, thereby generating a more potent immune response against subdominant epitopes.
What is especially surprising is that it has been found that the use of Tat actually results in a reduction of CTL responses to immunodominant epitopes, despite the fact that these epitopes are still present and efficient. Indeed, the responses to these epitopes are reduced, while the response to subdominant epitopes becomes highly significant. It is particularly advantageous that the reduction of CTL responses against immunodominant epitopes is useful to avoid or reduce the formation of escape mutants.
In general, the effect of Tat appears to be to “equilibrate” CTL responses to the epitopes of an antigen, favouring a broader immunodominant/subdominant epitope-specific set of CTL responses to any given antigen. In addition, as stated above, the decrease of immunogenicity of immunodominant CTL epitopes may avoid escape mutants.
Previous vaccine studies in animals with Tat have shown the ability of this protein to stimulate an immune response, with the Tat protein acting both as an antigen and as an adjuvant. However, none of the studies, with Tat employed as an adjuvant or co-antigen, have shown or suggested the unexpected, unpredictable and striking ability of TAT to stimulate an immune response against the subdominant epitopes as demonstrated by the present invention. Essentially, no one would expect that Tat or, indeed, any other protein, to so significantly alter epitope generation and presentation.
Instead, we have now found that Tat can be used with a broad range of antigenic materials and that strong immune responses can be stimulated and observed against subdominant epitopes which rarely generate a response, or even which generate no response in most individuals.
Subdominant epitopes may either be observed on antigenic materials also comprising dominant epitopes, or may be comprised in molecules not associated with dominant epitopes. The location of the subdominant epitope is not important to the present invention, although it is generally preferred that it be available for recognition by CTLs during the life cycle of this organism, in order that the immune response generated be able to affect the course of the infection. Preferably, the infection will be controlled or eliminated by the immune response generated.
Escape mutations of dominant epitopes are very common, and have been observed in most disease organisms. For example, influenza and HIV are both associated with a number of strains where the dominant epitopes have mutated. Such mutation is widely believed to be a defence mechanism and, while the mutation has little or no effect on the virus, it is sufficient that an individual immune to one strain of the organism has no effective immunity against the organism carrying the new mutation.
By way of contrast, there has been no evolutionary pressure on subdominant epitopes to be able to mutate, so that there has also been no pressure on these epitopes to become dissociated from active functions in the organism. Accordingly, subdominant epitopes are substantially conserved and unable to mutate without crippling the organism.
Thus, by employing Tat in the vaccines of the present invention, it is possible to generate immunity against subdominant epitopes which are otherwise obscured by the dominant epitopes, and thereby to generate immunity against the CTL epitopes, both subdominant and immunodominant, within a given antigen, permitting an immune response to be generated against not only the strain of pathogen immunised against, but also a majority of, if not all, future and existing variants of the pathogen.
As used herein, the term “variant” includes all forms of the antigen that may be presented by the disease form in question, provided that the antigen in question is still presented by the disease form. It is not readily possible to define the term more closely, but it will be appreciated that escape mutations often mutate the immunodominant epitopes substantially, so that one form of variant might include those where only the immunodominant epitopes vary, but the remainder of the antigen remains 99% and preferably 100% unchanged.
Antigens of the present invention may be derived from a number of sources, including plants, parasites and fingi. However, it is preferred that the antigen or antigens are derived from bacteria, preferably Mycobacteria, preferably Mycobacterium tuberculosis, M. bovis, or M. africanum. The antigen may also, preferably, be derived from staphylococcal or bacilli bacteria.
It will be understood that the term derived from includes antigenic peptide fragments from an organism or virus or even the organism or virus itself, provided that an epitope is provided.
It is particularly preferred that the antigen is derived from viral sources, preferably herpes viruses or from the family of pox viridae, preferably from respiratory-disease causing viruses, especially Adenoviruses Picornaviruses, Rhinoviruses, Echoviruses and Coxsackieviruses, preferably those that are responsible for influenza.
Indeed about 30 to 50% of all colds are caused by one of the >100 serotypes of rhinoviruses. At any one time only a few viruses are prevalent. Often a single virus is responsible during outbreaks in relatively closed populations, such as in a school or barracks. However, new disease-causing strains rapidly evolve which immunised or tolerant individuals are not capable of reacting to rapidly. The present invention helps to overcome this by increasing the number of sub-dominant epitopes, which can often be more highly conserved.
Preferably, the antigen is derived from Acute Respiratory Syndrome viruses, such as those leading to SARS.
Even more preferred is that the Antigen is derived from, is a fragment of or comprises an Immunodeficiency virus, preferably SIV, but most preferably HIV. Various HIV antigens are known, including Gag, Pol, Rev and Env. Preferably, the antigen is derived from Gag or Env. Indeed, we have shown in the accompanying Examples that Tat is particularly useful against both Gag and Env as the number of epitopes Gag or Env recognised by a host immune systems is greater in the presence of Tat.
Preferably, the antigen is derived from a cancer or tumour, preferably, a bowel, stomach, lung, colon, or pancreatic tumour, or a melanoma.
It is also preferred that tat is useful in the treatment of immunomediated diseases, preferably allergies, asthma, bronchitis, autoimmune diseases, arthritis, gout and allied conditions, infections, gastroenteritis, dysentery, constipation, neoplasia or conditions associated with immunosuppression.
It is also preferred that Tat can be used a as co-antigen, that is, that Tat can be administered or expressed together with an antigen, Tat having the beneficial effect of increasing the number of epitopes, particularly sub-dominant epitopes of the antigen.
A further advantage of administering or expressing Tat, preferably with a further antigen, is that an immune response will also be raised against Tat itself and it is envisaged that this could lead to an immune response to both the antigen and Tat.
Preferably, the antigen is administered as a protein or peptide. As discussed above, the protein or peptide may be modified such that it is protected from digestion or breakdown, for instance by use of glycosylation, provided that the protection can later be removed at the appropriate site, for instance the bloodstream, for instance by blood-borne glycosylases or glycosylases administered to the blood.
Preferred routes of administration are discussed below, include oral, intravenous, intramuscular, or subcutaneous. Preferably, the antigen is provided in a form adapted for such delivery, and may be in the form of a tablet, pill, suppository or liquid suitable for injection, or it may be contained with polysaccharide spheres or particles or nanoparticles.
The vaccines of the present invention comprise Tat, a biologically active equivalent mutant or fragment thereof, thereof or a precursor therefor. As shown in the accompanying Examples, oxidised Tat has little or no effect, so that it is important to retain the biological activity of Tat. Within this requirement, it is possible to alter the Tat molecule, provided that the enhanced proteolytic activities of the immuno-proteasomes is conserved. This level of activity should be at least 30% of that shown in the accompanying Examples for each proteolytic activity. Preferably, the proteolytic activity should be at least 50% of the activity shown in the Examples, and preferably 80% and more preferably at least 90% of the activity shown in the Examples. For the avoidance of doubt, where more than one level of increased proteolytic activity is demonstrated in the accompanying Examples, then the above definition applies to the least of the listed activities, but may apply to any of the others, and preferably applies to the greatest activity.
Tat contains four domains. The acidic domain (amino acid residues 1 to 21) is important for interaction with cellular proteins. The cysteine rich region (amino acid residues 22 to 37) corresponds to the transactivation domain and is highly conserved among primary isolates. For example, replacing cysteine 22 with a glycine residue, leading to a so-called Tat22 mutant, abolishes the ability of Tat to transactivate the HIV-LTR. Likewise, the core domain (amino acid residues 38 to 48) is highly conserved, and simple substitution of lysine 41 with a threonine also incapacitates the transactivating ability of Tat on HIV-LTR. The fourth domain is the basic domain (amino acid residues 49 to 57), which is rich in arginine and lysine, and is responsible for the nuclear localisation of Tat, binding specifically to target RNA. This fourth domain is also responsible for binding extracellular Tat to heparin and heparansulphate proteoglycans. The carboxy terminal region is not necessary for LTR transactivation, but contains an arginine-glycine-aspartic acid sequence (RGD), common to extracellular matrix proteins; responsible for the interaction and binding of Tat to the integrin receptors α5β1 and αvβ3.
Mutation of any of the domains or the carboxyl terminal is encompassed within the present invention, provided that the resultant biologically active Tat is still sufficient to stimulate the proteolytic activity of the immunoproteasomes as defined above.
In place of Tat, or a biologically active equivalent thereof, it is possible to use a nucleic acid sequence encoding either Tat or a biologically active equivalent thereof. In particular, the Tat, if not administered as part of the vaccine, may be expressed in situ, either by microbial systems in the vaccine, or as a result of administration of suitable expression sequences to the patient.
Likewise, the antigen comprised in the vaccine may also be presented in the form of a nucleic acid sequence encoding the antigen, or in the form of the original or a partially digested version of the original antigen, or a peptide. Although the subdominant epitope may be incorporated per se within the vaccine, this is not generally necessary when Tat is used, as Tat is capable of causing the presentation of subdominant epitopes by MHC-I.
It is convenient simply to incorporate antigenic material from the desired organism into the vaccine, as the unique activity of Tat is sufficient to decrease the immune response to the dominant epitope while substantially increasing the immune response to the subdominant epitope or epitopes. Although it is not essential to completely inactivate the infectious organism for the purposes of the vaccine, it is highly preferred, and this may be achieved by heat treatment or attenuation, for example. Further purification may be effected, if desired, such as by HPLC, ultrafiltration or centrifugation. Immunosorbent columns may also be used to separate ingredients.
The vaccines of the present invention may be used both for priming and boosting an immune response, and it is generally preferred that the composition of both the primary vaccine and booster is the same, although this is not necessary, provided that both the primary vaccine and the booster are to the same species of infectious organism, as the subdominant epitopes are conserved within the species.
Subsequent boosters may be applied as recommended by the skilled physician, and it is an advantage that it is not necessary to use the current virulent strain of an infectious organism to provide an effective vaccine.
The present invention further provides a method for providing an immune response against a plurality of strains of an infectious organism, comprising administering a vaccine comprising:
antigenic material from at least one strain of the organism, said material encoding or comprising a subdominant epitope; and
Tat, a biologically active equivalent thereof or a precursor therefor.
Preferably, Tat is as disclosed in SEQ ID NO. 284 or is a mutant and/or fragment thereof, as discussed elsewhere herein, and references to Tat and associated terms should be construed accordingly, in the absence of any indication to the contrary.
It is preferred that the infectious organism be a disease organism, and it is particularly preferred that the organism be a virus, although this is not necessary. Suitable sources of antigens are well known and are further discussed above.
Vaccines for use in the present invention may be provided in any suitable form and may be for administration by any suitable route. For example, vaccines of the invention may be provided intravenously, intramuscularly, intraperitoneally, subcutaneously, transdermally or in the form of eyedrops, or even as pessaries or suppositories.
Vaccines of the present invention may comprise any suitable ingredients in addition to the Tat and antigen ingredients, including, for example, stabilisers, buffers, saline, and isotonicity agents for injections, and any suitable ingredients, such as emulsifying agents and solid vehicles for applications such as pessaries and suppositories. Antibacterial and sterilising agents may also be employed, if desired.
In accompanying Example 1, we show that native HIV-1 Tat protein, an early product of HIV-infected cells, modifies the subunit composition and the activity of proteasomes. In particular, proteasomes in cells of B and T cell origin, either expressing endogenous Tat or exposed to a biologically active Tat protein, show up-regulation of LMP7 and MECL1 subunits and down-modulation of the LMP2 subunit. Strong down-regulation of the LMP2 subunit was shown to occur in splenocytes isolated from mice after treatment with native Tat but not with oxidised Tat protein, and selective down-regulation of LMP2 by viral gene products has been reported35,36. It is known that the substitution of standard β-subunits with IFN-γ-inducible subunits alters the hydrolytic activity of proteasomes towards tri- and tetra-peptides, and the quality of the peptide products derived from polypeptides10,23-25. We demonstrate here that changes in proteasome subunit composition induced by Tat result in the increase of all three major proteasome proteolytic activities10,23-25.
Perturbation of the proteasome system by viral infection, cell transformation or pharmacological treatments is often a key event in the modulation of the immune response to pathogens37-44, since proteasomes play a pivotal role in the generation of the majority of antigenic peptides presented by MHC class I molecules8. In particular, immunoproteasomes are very efficient for the generation of specific CTL epitopes, and it has been shown that substitution of standard β-subunits with LMP2, LMP7 and MECL1 subunits improves the production of peptide antigens with the correct C termini for binding to MHC class I45-48. By way of contrast, there is evidence, both in humans and mice, that the presence of LMP2 may inhibit the presentation of specific peptide antigens12,49,50.
We now show that the variations in proteolytic activity of proteasomes in Tat-expressing cells or in cells exposed to Tat protein correlate with a different presentation of EBV-derived epitopes, for example. In the accompanying Example, we show that Tat decreases the presentation of two immunodominant CTL epitopes (IVT and AVF) presented by HLA-A11 molecules, and increases the presentation of two subdominant epitopes (YLQ and CLG) presented by HLA-A2. HLA-A2-associated peptides present a valine at the C terminus, and it has been demonstrated that the β1 subunit, replaced by LMP2 in the immunoproteasomes, is responsible for cleavage beyond acidic residues and beyond residues with branched chains, such as valine10,51.
It is, therefore, preferred that the present invention stimulates the down-regulation or replacement of LMP2 subunits, and preferably an up-regulation of β1 subunits in proteasomes. It is also preferred that the present invention stimulates an increase in the number of peptides cleaved at Valine. Also preferred is a vaccine or method for increasing the number of epitopes recognises, particularly sub-dominant epitopes.
Preferably, Tat, its equivalent mutant or fragment, or precursor, is capable of down-regulating levels of LMP2 in the intended recipient of the vaccine.
Proteasomes in Tat-expressing/treated cells present low levels of LMP2 and higher post-acidic activity, compared with untreated cells or with cells that do not express Tat, which may account for the greater enhancement in the generation and presentation of YLQ and CLG CTL epitopes that present a C-terminal valine10,51. We also show that proteasomes from Tat-expressing cells are very efficient in the degradation of a CLG peptide precursor and can generate immunogenic peptide fragments therefrom, in contrast to proteasomes isolated from control cells.
A similar phenomenon was observed for an HLA-A2 presented epitope expressed in melanoma cells12, suggesting that the presence of LMP2 may particularly affect the range of peptides presented by some HLA class I alleles, such as HLA-A2. This suggests that the presence of LMP2 is critical for the generation of CTL epitopes. Indeed, it has been demonstrated that influenza-specific CTL responses to the two most dominant determinants decrease in LMP2 knock-out mice, whereas responses to two subdominant epitopes are greatly enhanced50. Similarly, we demonstrated that the Tat-dependent LMP2 down-modulation induces changes in the hierarchy of CTL responses directed to Ova-derived CTL epitopes.
What we have demonstrated, for the first time, is that Tat increases CTL responses directed to subdominant epitopes and decreases these directed to the immunodominant SII peptide.
This is achieved by modifying the catalytic subunit composition and activity of immunoproteasomes in B and T cells which either express Tat, or have been treated with biologically active exogenous Tat protein. This results in modulation of the in vitro CTL epitope hierarchy. In particular, both intracellularly expressed and exogenous native Tat protein increase the major proteolytic activities of the proteasome by up-regulating LMP7 and MECL1 subunits and by down-modulating the LMP2 subunit. This results in a more efficient generation and presentation of subdominant CTL epitopes
Decreasing the presentation of immunodominant epitopes, accompanied with an increase in the presentation of subdominant epitopes, is particularly beneficial for the elimination of virally infected cells, given that it is well established that immunodominant epitopes are very prone to mutation and to viral-escape, while subdominant epitopes are more stable while being capable of inducing protection52.
Thus, Tat protein is useful to drive the induction of MHC-I restricted immune responses, broadening the spectrum of the epitopes recognised and increasing the chances to prevent the appearance of viral escape.
As mentioned above, we have shown that the presence of Tat results in a more efficient generation and presentation of subdominant CTL epitopes. Since the amount of MHC-I/epitope complexes is crucial in determining the presence and the strength of epitope-specific CTL responses and to verify the biological relevance of these findings for vaccination strategies, we went on to evaluate epitope-specific CTL responses against ovalbumin in mice vaccinated with both Tat and ovalbumin.
Surprisingly, we also found that Tat decreases CTL responses directed to the immunodominant epitope while inducing those directed to subdominant and cryptic T-cell epitopes that were not present in mice vaccinated with ovalbumin alone.
This finding suggests that Tat favours the generation of CTL responses directed to “weak” CTL epitopes and could therefore be used as a tool to increase CTL responses to heterologous antigens.
In addition, we found that a mutated form of the HIV-1 Tat protein, carrying glycine instead of cysteine 22 (Tatcys22) (SEQ ID NO 286), like wild-type Tat, modifies the subunit composition of proteasomes. The Tatcys22 mutant, in contrast to wild-type Tat, has no effect on the transactivation of the HIV-1 LTR, and does not induce reactivation of latent infection. This is particularly advantageous as administration or expression of biologically inactive tat may be appropriate in some circumstances.
Thus, we have also shown that the Cys residue at position 22, although key to the function of wild type Tat, is not required for Tat's effect on proteosome subunit composition.
It is a particular advantage of the TatCys22 mutant (SEQ ID NO 286) that it shows an improved effect compared to wild type Tat. That is, the TatCys22 mutant has actually been shown to increase the number of subdominant epitopes processed and, thereby, presented.
Indeed, our results in Experiment 3 show that T cell responses induced by vaccination with Gag+Tatcys22 are directed to 11 different T cell epitopes, 7 more than mice immunised with Gag alone, and 4 more than mice immunized with Gag and wild-type Tat.
We also demonstrate that peptide 47-86, derived from the wild-type Tat protein, is sufficient to down-modulate the LMP2 subunit.
Therefore, mutated forms of Tat, or Tat-derived peptides, represent an important alternative to the use of wild-type Tat in vaccination strategies aimed at increasing epitope-specific T cell responses directed to heterologous antigens.
We exploited the effect of Tat (both wild-type Tat protein and mutant Tatcys22 protein) on T cell responses against structural HIV gene products in vivo. We showed that, surprisingly, Tat increases the number of CTL epitopes within HIV Gag and Env antigens.
Balb/C mice were immunised with HIV-1 Gag or Env protein antigens, either alone or in combination with wild-type Tat or mutant Tatcys22. We found that both wild-type Tat and mutated Tatcys22 increase the number of epitope-specific T cell responses against Gag and Env antigens. In particular, we demonstrated that mice vaccinated with Gag, in combination with wild-type Tat or with the mutant Tatcys22, responded to 7 or 11 T cell Gag-derived epitopes respectively, in contrast to mice vaccinated with Gag alone, which responded to 4 T cell Gag-derived epitopes. Similarly, mice vaccinated with Env, in combination with wild-type Tat or with the mutant Tatcys22 responded to 12 Env-derived pools of epitopes, in contrast to mice vaccinated with Env alone, which responded to 8 T cell Env-derived peptide pools.
Our results show that Tat is not only an antigen but also an adjuvant capable of increasing T cell responses against heterologous antigens. Therefore, the Tat protein, as well as mutant Tatcys22, represents an important tool in HIV-1 vaccine strategies aimed at broadening the spectrum of the epitopes recognized by T cells.
Thus, Tat is a useful tool for inducing epitope-specific CTL responses against HIV antigens and can be used as co-antigen for the development of new vaccination strategies against AIDS.
In the following Examples reference is made to the accompanying Figures, in which:
Left panel: Equal amount of total proteins from cell lysates from MIN and MON LCL's transduced with pBabeP (MIN-0 and MON-0) or with pBabeP-Tat (MIN-Tat and MON-Tat) were fractionated by SDS-PAGE, transferred onto nitrocellulose filters, and probed with mAbs or polyclonal anti-sera specific for the α-2 subunit, PA28α, LMP2, LMP7 and MECL1. Right panel: The intensity of specific bands was measured by densitometry. Data are expressed as % increase in optical densities of Tat expressing cells relative to control cells. One representative experiment out of four performed is shown.
Purified proteasomes from cell lysates of MIN and MON LCL's transduced with pBabeP (MIN-0 and MON-0) or with pBabeP-Tat (MIN-Tat and MON-Tat) were tested for chymotryptic-like, tryptic-like and post-acidic activities using Suc-LLVY-AMC, Boc-LRR-AMC and Ac-YVAD-AMC as substrates, respectively. Peptide substrates (100 μM) were incubated with 5 μg of purified proteasomes at 37° C. for 30 min. Data are expressed as arbitrary fluorescence units. One representative experiment out of three performed is shown.
Left panel: Equal amount of total proteins from cell lysates from MIN and MON LCL's treated with the indicated concentrations of the native Tat protein were fractionated by SDS-PAGE, transferred onto nitrocellulose filters, and probed with mAbs or polyclonal anti-sera specific for the α-2 subunit, LMP2, LMP7 and MECL1. Right panel: The intensity of specific bands was measured by densitometry. Data are expressed as % increase in optical densities of Tat expressing cells relative to control cells. One representative experiment out of three performed is shown.
Mice were treated with native Tat protein (panel a) or with oxidized Tat (panel b) and after 3 i.m. treatments, splenocytes were isolated and lysed. Equal amount of total proteins from cell lysates were fractionated by SDS-PAGE, transferred onto nitrocellulose filters, and probed with an antibody specific for LMP2. The intensity of LMP2 bands was evaluated by densitometry and normalised to the correspondent expression of proteasomes evaluated with a polyclonal sera specific for α-subunits. Data are expressed as % increase in optical densities compared to the mean of LMP2 expression in control splenocytes from 6 untreated mice.
The HLA-A2, -A11 positive MN and MON LCL's transduced with pBabeP (MIN-0 and MON-0) or with pBabeP-Tat (MIN-Tat and MON-Tat) were used as target in cytotoxic assays of CTLs specific for the HLA-A11 presented, EBNA4-derived IVT and AVF epitopes, the HLA-A2-presented Lmp1-derived YLQ epitope, and the HLA-A2-presented Lmp2-derived CLG epitope, respectively. Results are expressed as % specific lysis. One representative experiment out of three performed is shown.
The HLA-A2, -A11 positive MIN LCL's, treated or not with Tat, were used as target in cytotoxic assays of CTLs specific for the HLA-A11 presented, EBNA4-derived IVT and AVF epitopes, the HLA-A2-presented Lmp1-derived YLQ epitope, and the HLA-A2-presented Lmp2-derived CLG epitope. Results are expressed as % specific lysis. One representative experiment out of three performed is shown.
Panel A: the CLG+5 peptide was incubated with proteasomes purified from MIN-Tat or from MIN-0 LCLs. The precursor degradation was followed at different time points and the degradation of CLG+5 was evaluated by HPLC analysis. Data are expressed as % degradation. The mean of the results from three independent experiments is shown.
Panel B: the digestion products obtained after 120 min of degradation were purified by HLPC, the indicated fractions were collected and tested by IFN-γ Elispot for their capacity to activate CLG-specific CTLs. Data are expressed as spot-forming cells (SFC) per 106 cells. The mean of the results from three independent experiments, performed in triplicates, is shown.
Mice were vaccinated with Ova alone or with Ova and Tat protein. After 2 immunisations, fresh splenocytes were pooled and tested in cytotoxicity against EL4 cells pulsed with SII, KVV, or CFD peptides. Data are expressed as % specific lysis calculated by subtracting lysis of untreated EL4 cells (always below 10%). Mean of two independent experiments performed in triplicate.
Jurkat cells were treated for 12 (
Proteasomes (2.5 μg) purified from cell lysates of the indicated cell lines were incubated for 30 min at 37° C. with Suc-LLVY-AMC, Boc-LRR-AMC and Ac-YVAD-AMC to evaluate chymotryptic-like, tryptic-like and post-acidic activities, respectively. Data are expressed as arbitrary fluorescence units.
Jurkat cells were treated for 24 with 0.1 μg/ml with Tat or with peptides 1-38, 21-58 and 47-86 covering the wild-type sequence of Tat. Equal amounts of proteins from total cell lysates were fractionated by SDS-PAGE, transferred onto nitrocellulose filters, and probed with α-2 subunit- and LMP2 subunit-specific mAbs. One representative experiment out of the three performed is shown.
Mice were immunized with 25 μg of ovalbumin alone or in combination with 5 and 10 μg Tat protein. After 2 immunizations, fresh splenocytes were pooled and tested in cytotoxicity against EL4 cells pulsed with SII, KVV, or CFD peptides. Data are expressed as % specific lysis calculated by subtracting lysis of untreated EL4 cells (always below 10%). The mean of the results from three independent experiments, performed in triplicates, is shown.
Mice (n=5) were immunized subcutaneously with Tat, TatCys22 and Env proteins alone or in combination, as described in materials and methods. Splenocytes (pools of spleens) of immunized mice were stimulated with pools of Env peptides, and tested for IFNγ production in the presence of each pool, medium alone (negative control) or Concanavaline A (positive control). Results are expressed as the number of spot forming units (SFU)/106 cells subtracted from the SFU/106 cells of the negative controls, as described in Example 3. Responses ≧50 SFU/106 cells are considered positive. Filled boxes mark reactive pools.
Mice (n=5) were immunized with 5 μg of Gag alone or in combination with 5 μg Tat protein. After 3 immunizations, fresh splenocytes were pooled, stimulated with the indicated pools of Gag peptides and tested for IFNγ release by Elispot assay. Results are expressed as SFU/106 cells subtracted from the SFU/106 cells of the negative controls, as described in Example 3. Responses ≧50 SFU/106 cells are considered positive.
Mice (n=75) were immunized with 5 μg of Gag alone or in combination with 5 μg Tat protein. After 3 immunizations, fresh splenocytes were pooled, stimulated with the indicated peptides of Gag peptides and tested for IFNγ release by Elispot assay. Results are expressed as SFU/106 cells subtracted from the SFU/106 cells of the negative controls, as described in Example 3. Responses ≧50 SFU/106 cells are considered positive.
Mice (n=5) were immunized with 5 μg of Gag alone or in combination with 5 μg Tatcys22 protein. After 3 immunizations, fresh splenocytes were pooled, stimulated with the indicated pools of Gag peptides and tested for IFNγ release by Elispot assay. Results are expressed as SFU/106 cells subtracted from the SFU/106 cells of the negative controls, as described in Example 3. Responses ≧50 SFU/106 cells are considered positive.
Mice (n=5) were immunized with 5 μg of Gag alone or in combination with 5 μg Tatcys22 protein. After 3 immunizations, fresh splenocytes were pooled, stimulated with the indicated peptides of Gag peptides and tested for IFNγ release by Elispot assay. Results are expressed as SFU/106 cells subtracted from the SFU/106 cells of the negative controls, as described in Example 3. Responses ≧50 SFU/106 cells are considered positive.
Pools 1-7 and pools 12-30 were designed so that 2 independent pools contain one peptide in common.
Pool 1, contains Env 1-Env 19+Env 8771, 8772, 8773
Pool 2, contains Env 20-Env 38+Env 8789, 8790, 8791
Pool 3, contains Env 39-Env 57+Env 8805, 8806
Pool 4 contains Env 58-Env76+Env 8822
Shows matrix used for use with the Gag peptides in Example 3.
The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. Indeed, while the invention will now be illustrated in connection in connection with the following Examples, it will be understood that it is not intended to limit the invention to these particular embodiments. On the contrary, it is intended to cover all alternatives modifications and equivalents, as may be included within the scope of the invention as defined by the appended claims.
The following methods were used in this and the following Examples.
PG13 murine amphotropic packaging cell line54 was cultured in DMEM supplemented with 10% FCS. Jurkat T cell transfectants (pRPneo-c and pRPneo-c-Tat)22 were cultured in RPMI 1640 medium, supplemented with 10% FCS and 800 μg/ml neomycin (Sigma). Lymphoblastoid cell lines (LCL) were established by in vitro infection of normal B-lymphocytes from healthy donors with the B95.8 strain of EBV. LCL's were cultured in RPMI 1640 medium supplemented with 10% FCS.
HIV-1 Tat cDNA sequence was amplified by PCR from pGEM-3-Tat plasmid22 using primers Tat A: 5′-GGGGAATTCATGGAGCCAGTAGAT-3′ (forward) (SEQ ID NO 271) and Tat B: 5′-CAAGAATTCCTATTCCTTCGGGCC-3′ (reverse) (SEQ ID NO 272) (annealing temperature 57° C.). The purified PCR product was sequenced and cloned into the EcoRI site of pBabePuro vector to generate pBabePuro-Tat55.
The pBabePuro and pBabePuro-Tat vectors were transfected into PG13 packaging cell line by the calcium phosphate method56. Transfected cells were cultured in selective medium containing 3 μg/ml of puromycin (Sigma). Production of recombinant retroviruses from selected cultures was tested by semiquantitative RT-PCR on cell-free DNase treated supernatants, using of primers PuroA/PuroB (PuroA: 5′-CGAGCTGCAAGAACTCTTCC-3′ (forward) (SEQ ID NO 273), PuroB: 5′-AGGCCTTCCATCTGTTGCTG-3′ (reverse) (SEQ ID NO 274); annealing temperature 57° C.) and TatA/TatB respectively.
MIN and MON LCL's were transduced with pBabePuro-Tat (MIN-Tat and MON-Tat) or pBabePuro (MIN-0 and MON-0) recombinant retroviruses by co-cultivation with packaging cell lines using transwell-clear tissue culture membranes. Subconfluent PG13 pBabePuro and PG13 pBabePuro-Tat cells, grown in the lower chamber, were co-cultivated in the presence of 8 μg/ml polybrene (Sigma) with MIN or MON LCL's (3×106/well) added to the upper chamber in 2.5 ml of RPMI 1640 medium supplemented with 10% FCS. After 48 hrs of co-cultivation, cells were harvested from the membranes and grown in culture medium containing puromycin (0.3 μg/ml) for 6 weeks. All cell lines were characterised by DNA-PCR, RT-PCR and Northern blot analysis.
Total DNA was extracted from 5×106 cells with the NucleoSpin Blood kit (Macherey-Nagel), as specified by the manufacturer. For amplification of Puromycin and Tat genes, primers PuroA/PuroB were used, under the conditions described above, and Tat 1: 5′-gAAgCATCCAggAAgTCAgCC-3′ (SEQ ID NO 275) Tat 2: 5′-ACCTTCTTCTTCTATTCCggg-3′ (SEQ ID NO 276) (annealing temperature 55° C.).
RNA was extracted from cell-free supernatants of packaging cells, MIN and MON LCL's, MIN-0 and MON-0, MIN-Tat and MON-Tat cells (5×106) with NucleoSpin RNA II (Macherey-Nagel), as specified by the manufacturer. Total RNA (1 μg) was incubated with 20 mM MgCl2 and 500 IU/ml pancreatic DNase I (Boehringer Mannheim) at 37° C. for 1 hr and purified by phenol-chloroform. DNase digestion was repeated three times upon the addition of fresh DNase I.
RNA was reverse transcribed by using the random hexamer method with RT-PCR Systems (Promega) according to manufacturer's instructions. cDNA was tested by PCR using actin-specific primers:
annealing temperature 63° C.). PCR's for Puromycin and Tat genes were performed as described.
Equal amounts of total RNA's (40 μg) were electrophoresed onto formaldehyde-agarose gel (1.5%) for 12 hours, transferred onto nylon membranes (Hybond N; Amersham) and hybridised with DNA probe. Probes were randomly labelled with [32P] dCTP, using the Prime-It II kit (Stratagene).
Equal amounts of proteins were loaded on a 12% SDS-PAGE gel and electroblotted onto Protran nitrocellulose membranes (Schleicher & Schuell, Keene, Hampshire, USA). The blots were probed with antibodies specific for α2, LMP2, LMP7, MECL1, and PA28α subunits (Affinity, Exeter, UK) and developed by enhanced chemiluminescence (ECL, Amersham Pharmacia Biotech, Uppsala, SW).
HIV-1 Tat from the human T lymphotropic virus type IIIB isolate (subtype B) was expressed in E. Coli and purified by heparin-affinity chromatography and HPLC as a Good Laboratory Practice (GLP) manufactured product as described previously. The Tat protein was stored lyophilised at −80° C. to prevent oxidation and reconstituted in degassed buffer before use, as described2. Different GLP lots of Tat were used with reproducible results and in all cases endotoxin concentration was below 0.05 EU/μg.
Cells (5×107) were washed in cold PBS and resuspended in buffer containing 50 mM Tris-HCl pH 7.5, 5 mM MgCl2, 1 mM dithiothreitol (DTT, Sigma), 2 mM ATP, 250 mM sucrose. Glass beads equivalent to the volume of the cellular suspension were added and cells were vortexed for 1 min at 4° C. Beads and cell debris were removed by 5 minutes centrifugation at 1000×g, followed by 20 minutes centrifugation at 10,000×g. Supernatants were ultracentrifuged for 1 hour at 100,000×g44. Supernatants were loaded into an affinity column containing an agarose matrix derivatised with the MCP21 mAb specific for the α2 subunit of the proteasome (Affinity, Exeter, UK). The column was washed, eluted with 25 mM Tris-HCl pH 7.5 containing 2 M NaCl, and 0.5 ml fractions were collected. Fractions containing proteasomes were combined and dialysed against 25 mM Tris-HCl pH 7.5. Protein concentration was determined using BCA protocol (Pierce Chemical).
The fluorogenic substrates Suc-LLVY-AMC, Boc-LRR-AMC and Ac-YVAD-AMC were used to measure chymotrypsin-like, trypsin-like and post-acidic proteasome activities, respectively. Peptide substrates (100 μM) were incubated at 37° C. for 30 min with purified proteasomes in 75 μl of buffer containing 50 mM Tris-HCl pH 7.4, 5 mM MgCl2, 500 μM EDTA pH 8.0, 1 mM dithiothreitol, and 2 mM ATP. Fluorescence was determined by a fluorimeter (Spectrafluor plus, Tecan, Salzburg, Austria) using an excitation of 360 nm and emission of 465 nm. Proteasome activity is expressed in arbitrary fluorescence units11.
All peptides were synthesised by solid phase methods57. Crude deprotected peptides were purified by HPLC to >98% purity. Structure verification was performed by elemental and amino acid analysis and mass spectrometry. Peptide stocks were dissolved in DMSO at a concentration of 10−2 M, kept at −20° C., and diluted in PBS before use.
The synthetic peptide CLGGLLTMVAGAVW (CLG+5) (SEQ ID NO 279) was dissolved in DMSO at a concentration of 20 μg/μl. 500 μg of synthetic peptide were incubated with 127 μg of purified proteasomes in 300 μl of buffer (25 mM Tris HCL pH 7.4, 5 mM MgCl2, 500 μM EDTA pH 8.0, 1 mM DTT, 2 mM ATP) at 37° C. At the indicated time points, 60 μl of sample were collected and the reaction was stopped by adding 2 volumes of ethanol at 0° C.
Digestion mixtures were centrifuged at 5000 rpm for 5 minutes. 80 μl of supernatant were collected and peptide digests were separated by reverse-phase HPLC at the flow rate of 0.7 ml/min, as follows: linear gradient of 0-100% of solution B (acetonitrile 100% with 0.1% TFA) for 25 min, followed by linear gradient of 0-100% for solution A (water 100% with 0.1% TFA) for 5 min. The fractionation was simultaneously monitored at 210 and 280 nm. Fractions were collected every 30 seconds and stored at +4° C. and tested in ELISPOT.
HLA A11-restricted EBV-specific CTL cultures reacting against the EBNA4-derived IVTDFSVIK (SEQ ID NO 280) (IVT) and AVFSRKSDAK (SEQ ID NO 281) (AVF) epitopes, corresponding to amino acid 416-424 and 399-40827, were obtained by stimulation of monocyte-depleted PBL's from the HLA-A11-positive EBV-seropositive donor MC with the autologous B95.8 virus-transformed LCL. HLA A2-restricted EBV-specific CTL cultures reacting against the Lmp2-derived CLGGLLTMV (SEQ ID NO 282) (CLG) epitope, corresponding to amino acid 426-43428, and the Lmp1-derived YLQQNWWTL (SEQ ID NO 283) (YLQ) epitope corresponding to amino acid 159-16729, were obtained by stimulation of monocyte-depleted PBL's from the HLA-A2-positive EBV-seropositive donor RG with peptide-pulsed T2 cells, as previously described32. The first stimulation was performed in RPMI 1640 medium containing 10% FCS. A second and a third stimulation were performed in the same conditions on day 7 and day 14. Starting from day 8 the medium was supplemented with 10 U/ml rIL-2 (Chiron, Milan, Italy).
Target cells were labelled with Na251CrO4 for 90 min at 37° C. Cytotoxicity tests were routinely run at different effector: target ratios in triplicate. Percent specific lysis was calculated as 100×(cpm sample−cpm medium)/(cpm Triton X-100−cpm medium)27. Spontaneous release was always less than 20%.
CTLs (4×104 cells) were plated in triplicate on microplate 96-wells unifilter (Whatman) previously coated with 100 μl of an anti IFN-γ mAb (Endogen, Woburn, Mass.) overnight at 4° C. CTLs were incubated with medium alone as a negative control, with phytohaemagglutinin (PHA) as a positive control, or with 20 μl of each HPLC fractions derived from the in vitro digestion by proteasomes of epitope precursors.
Plates were incubated for 24 h at 37° C., 5% CO2 and then washed three times with PBS and three times with washing buffer (PBS 0.05% Tween 20) before 100 μl of biotinylated anti-IFN-γ MAb (1 μg/ml; Endogen, Woburn, Mass.) were added, and incubated at 37° C. for 60 min. After the plates were washed again, HRP-conjugated streptavidin (Endogen, Woburn, Mass.) was added and the plates were incubated at room temperature for 45 min. Wells were washed, and individual IFN-γ producing cells were detected using ABC chromogen kit (Sigma, Saint Louise, Mo.). IFN-γ-secreting T cells were counted by direct visualisation. The number of specific IFN-γ-secreting T cells, expressed as spot-forming cells (SFC) per 106 cells, was calculated by subtracting the negative control value. Negative control values were always <500 SFC per 106 input cells.
Animal use was according to national and institutional guidelines. Seven-to-eight week old female Balb/c mice (Nossan, Milan, Italy) were injected with native monomeric biologically active Tat protein (1 μg) resuspended in degassed sterile PBS. Control mice were injected with oxidised Tat (1 μg) or with PBS alone. Samples (100 μl) were given by intramuscular (i.m.) injections in the quadriceps muscles of the posterior legs. Each experimental group consisted of three mice, and the experiment was repeated twice. Mice were boosted at days 11 and 20 after the first injection. Seven days after the last injection, animals were anaesthetised intraperitoneally (i.p.) with 100 μl of isotonic solution containing 1 mg of Inoketan (Virbac, Milan, Italy), and 200 μg Rompun (Bayer, Milan, Italy) and sacrificed to collect spleens. Mononuclear cells from individual spleens were purified using cells strainers, resuspended in PBS containing 20 mM EDTA, and treated with a red blood cells lysis buffer for 4 minutes at room temperature. Cells were washed twice in PBS, lysed and used for western blot analysis as described above.
Seven- to eight-week old female C57BL/6 mice (H-2b) (Nossan) were injected with 25 μg ovalbumin (Sigma, St. Louis, Mo.) alone or in combination with native monomeric biologically active Tat protein (5 and 10 μg) and resuspended in degassed sterile PBS in Freund's adjuvant (CFA for the first injection, and IFA for subsequent injections). Control mice were injected with PBS alone in Freund's adjuvant. Samples (100 μl) were given by subcutaneous (s.c.) injection in one site in the back. Each experimental group consisted of five mice, and the experiment was repeated twice. Mice were boosted at day 24. Two weeks after the last injection, animals were anaesthetised i.p., as described above, and sacrificed to collect spleens. Mononuclear cells from individual spleens were purified as described above, pooled and tested in cytotoxic assay against peptide-pulsed EL4 cells.
Endogenously expressed Tat modulates proteasome composition and activity To evaluate proteasome expression in the presence of endogenous Tat, lymphoblastoid cell lines (LCL) expressing Tat (MIN-Tat and MON-Tat) were prepared by retroviral transduction and assayed (
The level of expression of proteasomes was then analysed by Western blot analysis in both Tat-expressing cells as compared to control cells. No difference in proteasome expression was detected in these cells by the use of a monoclonal antibody specific for the α2-subunit (
To investigate whether the differences in subunit composition detected by Western blot analysis correlated with differences in enzymatic activity, we analysed the cleavage specificity of equal amount of proteasomes isolated from MIN-Tat and MON-Tat cells or control cells. We tested chymotryptic-like, tryptic-like, and post-acidic activities using Suc-LLVY-AMC, Boc-LRR-AMC and Ac-YVAD-AMC as substrates, respectively. All three enzymatic activities were higher using proteasomes purified from cells expressing Tat as compared to activities of proteasomes purified from control cells (
To test the effect on proteasomes in cells after the up-take of exogenous Tat protein, MIN and MON LCL's were cultured in the absence or presence of increasing concentrations of biologically active Tat protein for 24 hours at 37° C. After treatment, the expression of the different subunits was analysed and compared to that of untreated cells. No difference in the expression of the α2-subunit was detected by Western blot analysis, suggesting that exogenous Tat does not alter the expression of proteasomes (
To evaluate the in vivo effect of Tat, we treated Balb/c mice with biologically active Tat or oxidised Tat. After 3 consecutive treatments, splenocytes were isolated and total cell lysates tested for the expression of IFN-γ inducible catalytic subunits.
Splenocytes isolated from all mice treated with native Tat demonstrated the down-regulation of LMP2, while no effect was observed in splenocytes isolated from mice treated with oxidised Tat (
Tat Modifies the Generation of CTL Peptide Epitopes Derived from EBV Latent Antigens
Since proteasomes play a key role in the generation of CTL epitopes, we investigated the effect of proteasome variation induced by Tat on the generation and presentation of CTL epitopes in LCL's expressing endogenous Tat or exposed to Tat protein.
LCL's express the total set of EBV latent antigens, including nuclear antigens (EBNA) 1, 2, 3, 4, 5, 6 and latent membrane protein (LMP) 1 and 2. These antigens, except for nuclear antigen 1, are targets of cytotoxic T lymphocytes and a large number of CTL epitopes have been identified26. In this set of experiments we evaluated: the immunodominant IVT and AVF HLA-A11-presented epitopes, that derive from the EBNA4 antigen27, and of the subdominant YLQ and CLG epitopes, two HLA-A2-presented epitopes derived from the latent membrane protein 1 (Lmp1) and 2 (Lmp2), respectively28,29. It has been recently shown that the generation of these epitopes depends on proteasome activity30,31. To this purpose, the HLA-2 and -A11 positive MIN LCL and the HLA-A2 positive MON LCL, transduced or not with Tat, were tested as target in cytotoxic assays using CTL cultures specific for the IVT, AVF, CLG and YLQ epitopes (
The expression of endogenous Tat caused a decrease of IVT- and AVF-specific CTL killing and an increase of YLQ- and CLG-specific killing as compared to control cells transduced with the empty vector (
In a second set of experiments we evaluated CTL sensitivity of MIN LCL untreated or treated with 0.1 μg/ml of the Tat protein for 24 hours. In agreement with the results of the previous experiments, LCL's treated with Tat were less sensitive to IVT- and AVF-specific CTL killing but were lysed at higher efficiency by YLQ- and CLG-specific CTLs (
These findings suggest that the effect of Tat on proteasome composition and activity results in changes of epitope presentation at the surface of virally infected cells.
Efficient In Vitro Generation of the CLG Epitope by Proteasomes Purified from Tat-Expressing Cells
To test whether proteasomes from Tat-expressing cells generate the CLG epitope with more efficiency, we analysed the in vitro degradation of a CLG peptide precursor that contains 5 amino acids at the C-terminus (CLG+5) corresponding to the wild-type sequence of the LMP2 antigen. The in vitro assay was performed using proteasomes purified from MIN-Tat and proteasomes purified from MIN-0. The precursor degradation was followed at different time points and evaluated by HPLC analysis. We found that proteasomes isolated from Tat-expressing cells degraded the CLG+5 peptide precursor more efficiently than proteasomes purified from cells expressing the empty vector (
To characterise the digestion products, we separated the digests obtained at different time points by HPLC. All fractions were collected and used in ELISPOT to activate CLG-specific CTLs. Fractions purified from digests obtained after 30, 60, and 90 min of incubation did not activate CTL responses (not shown). Only HPLC fractions 4 and 8 obtained after 120 min of degradation with proteasomes purified from MIN-Tat did stimulate CLG-specific CTL responses. This demonstrates that these fractions contain the CLG epitope or a longer but immunogenic epitope. A weak CLG-specific CTL response was also observed in HPLC fraction 8 obtained after 120 min of degradation using proteasomes isolated from MIN-0. These results demonstrate again that proteasomes purified from Tat-expressing cells exhibit a different proteolytic activity and that generate with more efficiency the immunogenic CLG peptide.
As shown above, Tat alters, both in vitro and in vivo, the subunit composition of proteasomes which, in turn, modulates the presentation of EBV-derived CTL epitopes at the cell surface of EBV-infected B cells. Down-modulation of two immunodominant CTL epitopes and up-regulation of two subdominant CTL epitopes (
The presence of specific CTL responses directed to the three Ova-derived epitopes was evaluated on fresh splenocytes using EL4 target cells pulsed or not with the relevant CTL epitopes (
Jurkat T cells expressing Tat or mutated Tat (referred to as Tat22, wherein a Cys at position 22 is mutated) have previously been described (1). Cells were cultured in a medium supplemented with 800 μg/ml neomycin (Sigma, St. Louise, Mich.).
HIV-1 Tat from the human T lymphotropic virus type IIIB isolate (BH10 clone) was expressed in E. Coli and purified by heparin-affinity chromatography and HPLC as previously described (2). The lyophilised Tat protein was stored at −80° C. to prevent oxidation, reconstituted in degassed buffer before use, and handled as described (3). Different lots of Tat were used with reproducible results, and, in all cases, endotoxin concentration was undetectable (detection threshold: 0.05 EU/μg).
Cells were lysed with glass beads as previously described (4). Supernatants were ultracentrifuged for 1 h at 100,000 g and loaded into an affinity column containing a matrix derivatised with the MCP21 mAb specific for the α2 subunit of the proteasome (Affinity, Exeter, UK). Proteasomes were eluted with 25 mM Tris-HCl pH 7.5 containing 2 M NaCl, and 0.5 ml fractions were collected. Homogeneity of the eluted material was confirmed by analysis of an aliquot by SDS 12% PAGE and Coomassie blue staining of the gel. Fractions containing proteasomes were combined and dialysed against 25 mM Tris-HCl pH 7.5. Protein concentration was determined using the BCA method (Pierce Chemical, Rockford, Ill.).
Equal amounts of proteins, or equal amounts of purified proteasomes, were loaded on a 12% SDS-PAGE and electro-blotted onto Protran nitrocellulose membranes (Schleicher & Schuell, Keene, Hampshire, USA). Blots were probed with Abs specific for α2, LMP2 LMP7, MECL1, and PA28α subunits (Affinity), and developed by enhanced chemi-luminescence (ECL, Amersham Pharmacia Biotech, Uppsala, SW).
The chymotrypsin-like, trypsin-like and post-acidic activities of purified proteasomes were tested using the fluorogenic substrates Suc-LLVY-AMC, Boc-LRR-AMC and Ac-YVAD-AMC, respectively, as previously described (4). Fluorescence was determined by a fluorimeter (Spectrafluor plus, Tecan, Salzburg, Austria). Proteasome activity is expressed as arbitrary fluorescence units.
Peptides were synthesized by the solid phase method and purified by HPLC to >98% purity, as previously described (5). Structure verification was performed by elemental and amino acid analysis and mass spectrometry. Peptides were dissolved in DMSO at 10−2 M, kept at −20° C., and diluted in PBS before use.
We have shown in Example 1 that the HIV-1 Tat modifies the catalytic subunit composition and activity of immunoproteasomes in lymphoblastoid cell lines which either express Tat or have been treated with exogenous biological active Tat protein. Similarly, the endogenous expression of Tat in Jurkat cells induces down-regulation of LMP2 and up-regulation of LMP7 and MECL1 (see Example 1 and
To assay whether the exogenous Tat protein modulates the expression of the catalytic subunits of proteasomes in Jurkat cells, we evaluated the expression of proteasomes from Jurkat cells cultured for 12 (
These results demonstrate that both endogenously expressed Tat and exogenous native Tat protein modify the subunit composition of immunoproteasomes in Jurkat cells.
To investigate whether the differences in subunit composition correlated with differences in enzymatic activity, we analysed the cleavage specificity of equal amounts of proteasomes purified from Jurkat cells treated for 24 h with 1 μg/ml Tat protein or from control cells. Chymotryptic-like, tryptic-like, and post-acidic activities were all augmented in proteasomes purified from Jurkat cells treated with Tat, as compared to control cells (
Tat does not require Cysteine 22 to modulate proteasome composition and activity
In the next set of experiments we evaluated the effect of Tat mutants stably expressed in Jurkat cells. Cysteines at position 22 and/or 37 were substituted with glycine and serine, respectively, to obtain three mutant Tat molecules (Tat22, Tat37 and Tat22/37). Tat22 and Tat22/37 mutants, in contrast to wild-type Tat, have no effect on the transactivation of the HIV-1 LTR, and do not induce reactivation of latent infection.
The level of expression of proteasomes was then analysed in Tat-expressing cells and compared to cells expressing Tat mutants. No difference in proteasome expression was detected in these cells by the use of a monoclonal antibody specific for the α2-subunit (
The Tat-derived 47-86 peptide is sufficient to down-modulate the LMP2 subunit To identify the region of Tat responsible for the modulation of the catalytic subunits of immuno-proteasomes, we tested the effect of peptides 1-38, 21-58 and 47-86 covering the wild-type sequence of Tat. Jurkat cells were treated for 24 h with 0.1 μg/ml of Tat-derived peptides and, after treatment, total cell lysates were assayed for proteasome expression by western-blot. As shown in
We have shown here that a mutated form of the HIV-1 Tat protein, carrying a glycine instead of cysteine 22 (Tat22), like wild-type Tat, modifies the subunit composition of proteasomes. In addition, we demonstrated that peptide 47-86, derived from the wild-type Tat, protein is sufficient to down-modulate LMP2 subunit. We have recently shown that LMP2 down-regulation by wild-type Tat results in a different generation of CTL epitopes in virally infected cells (6). Furthermore, we have produced evidence suggesting that Tat modifies in vivo CTL responses against heterologous antigens favouring the generation of subdominant CTL epitopes (unpublished results). Therefore, mutated forms of Tat or Tat-derived peptide may represent an important alternative to the use of wild-type Tat in vaccination strategies aimed at increasing epitope-specific T cell responses directed to heterologous antigens.
We shown above that the HIV-1 Tat protein modulates in vitro CTL epitope hierarchy by modifying the catalytic subunit composition of immunoproteasome. In particular, by up-regulating LMP7 and MECL1 subunits and by down-modulating the LMP2 subunit, both intracellularly expressed or exogenous native Tat protein increase the major proteolytic activities of the proteasome resulting in a more efficient generation and presentation of subdominant CTL epitopes. Since the amount of MHC-I/epitope complexes is crucial in determining the presence and the strength of epitope-specific CTL responses and to verify the biological relevance of these findings for vaccination strategies, we evaluated epitope-specific CTL responses against ovalbumin in mice vaccinated with both Tat and ovalbumin.
We found that Tat slightly decreases CTL responses directed to the immunodominant epitope while induces those directed to subdominant and cryptic T-cell epitopes that were not present in mice vaccinated with ovalbumin alone. Due to these effects we exploited the effect of Tat on T cell responses against structural HIV gene products. We found that Tat increases the number of CTL epitopes within Gag and Env antigens. Thus, Tat may represent a new tool to induce new epitope-specific CTL responses against HIV antigens and could be used as co-antigen for the development of new vaccination strategies against AIDS.
HIV-1 Tat and the mutant Tatcys22 (C→G) from the human T lymphotropic virus type IIIB isolate (BH10 clone) was expressed in E. Coli and purified by heparin-affinity chromatography and HPLC as described previously (2). The Tat proteins were stored lyophilised at −80° C. to prevent oxidation, reconstituted in degassed buffer before use, and handled as described (4). Different lots of Tat were used with reproducible results, and in all cases endotoxin concentration was undetectable (detection threshold: 0.05 EU/μg). HIV-1 GagSF2 and HIV-1 EnvSF2 proteins were obtained from Chiron and NIH AIDS Reagent Program respectively (HIV-1 gp120 SF162; #7363). The Gag sequence (HIVSF2 p55) is given in SEQ ID NO 266. The Env sequence (HIV-1 SF162 gp120) is given in SEQ ID NOS 267 (without linker) and 288 (with linker). The entire HIV-1 Env gp160 SF162 sequence is given in SEQ ID NO. 287.
Peptides were synthesized by solid phase method and purified by HPLC to >98% purity, as previously described (5). Structure verification was performed by elemental and amino acid analysis and mass spectrometry.
Gag and Env peptides, 15 amino acid long and overlapping by 10 to 11 amino acids, spanning the entire Gag (HIV-1 consensus subtype B Gag complete set, #8117) and Env sequences (SHIV SF162P3 env set; #7619 and HIV-1 consensus subtype B Env complete set, #9840), were provided by NIH AIDS Reagent Program. Peptides were dissolved in DMSO at 10−3 M, kept at −20° C., and diluted in PBS before use. The Gag peptides are listed in Table 1 and the Env peptides are listed in Table 2. The amino acid number relative to the full sequences are given, together with the appropriate SEQ ID NO from the Sequence Listing.
C57BL/6 mice (H-2b) (Harlan Nossan, Udine, I) were immunized subcutaneously, in a single site in the back, with 25 μg of ovalbumin (Sigma) alone or in combination with native monomeric biologically active Tat protein (5 and 10 μg, respectively) in Freund's adjuvant (CFA for the first injection, and IFA for subsequent injections). BALB/c mice (H-2d) (Harlan, Udine, Italy) were immunized subcutaneously, in a single site in the back, with 5 μg of HIV-1 Gag or Env proteins alone or in combination with 5 μg of native monomeric biologically active Tat protein or with the mutant Tatcys22 protein in Freund's adjuvant or in Alum. Each group was composed of 5 animals. Immunogens were given subcutaneously in 100 μl, at days 1, 14 and 28. Mice were sacrificed 10 days after the last boost (day 38). During the course of the experiments, animals were controlled twice a week at the site of injection and for their general conditions (such as liveliness, food intake, vitality, weight, motility, sheen of hair). No signs of local nor systemic adverse reactions were ever observed in mice receiving the immunogens as compared to control or untreated mice. Animal use was according to European and institutional guidelines.
Splenocytes were purified from spleens squeezed on filters (Cell Strainer, 70 μm, Nylon, Becton Dickinson). Spleens of each experimental group were pooled. Following red blood cell lysis with of 154 mM NH4Cl, 10 mM KHCO3 and 0.1 mM EDTA (5 ml/spleen) for 4 minutes at room temperature, cells were diluted with RPMI 1640 containing 3% FBS (Hyclone), spun for 10 minutes at 1200 rpm, resuspended in RPMI 1640 containing 10% FBS and used immediately for the analysis of antigen-specific cellular immune responses (fresh). Cellular responses were also measured after in vitro re-stimulation. Therefore, cells (3-5×106/ml) were stimulated in 14 ml with the Env 10-mer (3 μg/ml), or with pools of Env peptides (15-mers) (10−6 M) for 5 days before Elispot analysis.
Target cells were labelled with Na251CrO4 for 90 min at 37° C. Cytotoxicity test were routinely run at different effector:target ratios in triplicate. Percent specific lysis was calculated as 100×(cpm sample−cpm medium)/(cpm Triton X-100−cpm medium) (6). Spontaneous release was always less than 20%.
Elispot (IFN-γ) was carried out using a commercially available kit provided by Becton Dickinson (murine IFNgamma ELISPOT Set; #551083), according to manufacturer's instructions. Briefly, nitrocellulose 96-well plates were coated with 5 μg/ml of anti-IFN-γ overnight at 4° C. The following day, plates were washed 4 times with PBS and blocked with RPMI 1640 supplemented with 10% foetal bovine serum for 2 hours at 37° C. Splenocytes (2.5 and 5×105/200 μl for assays on fresh cells, and 5×104/200 μl for assays on cells in vitro re-stimulated) were added to the wells (duplicate wells) and incubated with peptides (10−6 M) for 16 hours at 37° C. Controls were represented by cells incubated with Concanavaline A (Sigma; 5 μg/ml) (positive control) or with medium alone (negative control). The spots were read using an Elispot reader (Elivis, Germany). The results are expressed as neat number of spot forming units (SFU)/106 cells: [mean number SFU of peptide treated wells minus mean number SFU of the negative control].
We demonstrated in Example 1 that, by altering the antigen processing machinery, Tat influences the number of MHC class I-epitope complexes at the cell surface of antigen presenting cells, thereby modulating CTL responses directed against immunodominant and subdominant epitopes within heterologous antigens. To determine the relevance of these in vitro findings, the effect of Tat on the induction of epitope-specific CTL responses was investigated in vivo.
To this end, Kb-restricted CTL responses to ovalbumin (Ova) that are directed to the immunodominant SIINFEKL (SII) epitope (SEQ ID NO 268) and potentially to the subdominant KVVRFDKL (KVV) (SEQ ID NO 269) and the cryptic CFDVFKEL (CFD) (SEQ ID NO 270) epitopes (7, 8) were used as model systems. In fact, it has been shown that KVV- and CFD-specific CTLs are not found upon immunization of C57BL/6 mice with Ova and that the lack of these responses is due to a poor generation of KVV and CFD epitopes by proteasomes (9, 10).
To address the generation of CTL responses to the Kb-restricted Ova-derived epitopes, C57BL/6 mice were vaccinated with Ova alone or in combination with the Tat protein. We then evaluated the presence of epitope-specific CTL responses in fresh splenocytes that were tested against EL4 target cells pulsed with the relevant peptides (
To address the effect of Tat on epitope specific T cell responses directed to Gag and Env antigens, BALB/c mice were vaccinated with the HIV-1 Env or HIV-1 Gag proteins alone, or in combination with the Tat protein. The presence of peptide-specific T cell responses was evaluated by IFN-γ Elispot assays using fresh splenocytes stimulated with pools of peptides spanning the entire sequence of Env and Gag proteins.
Fresh splenocytes isolated from mice immunized with Env, either alone or in combination with Tat, did not respond to stimulation with any of the Env-derived peptide pools (data not shown). Subsequently, splenocytes from Env-vaccinated mice were stimulated with the immunodominant Kd-restricted RGP CTL epitope (amino acid 311-320), or with pools of peptides covering the entire Env sequence. After 5 days, all cultures were tested for specificity by IFN-γ Elispot following stimulation with pools of peptides using a peptide-based matrix approach. Matrices consisted of pools of peptides in which each peptide was present in two separate pools (see
As shown in
These results indicate that Tat and TatCys22 generally broaden the immune response to Env.
Fresh splenocytes isolated from mice immunized, with Gag alone or with Gag and Tat, were stimulated with pools of peptides using a peptide-based matrix approach and assayed by IFN-γ Elispot. Matrices consisted of pools of peptides in which each peptide was present in two separate pools (see
Responses were regarded as positive if they had at least three times the mean number of SFU in the control wells, and had to be ≧50 SFU/106 cells.
As shown in
Interestingly, splenocytes from mice vaccinated with Tat and Gag, as compared to mice vaccinated with Gag alone, did not respond to pool 17 (containing the TTS peptide), whereas they recognized pools 13 and 19. We then assayed 36 individual peptides identified as potential targets by the matrix approach. As shown in
In the next set of experiments we evaluated the effect a Tat mutant carrying a glycine at position 22 instead of cysteine (referred to as Tatcys22). Tatcys22, in contrast to wild-type Tat, has no effect on the transactivation of the HIV-1 LTR, and does not induce reactivation of latent infection.
To address the effect of Tatcys22 on epitope specific T cell responses directed to Gag, BALB/c mice were also vaccinated with HIV-1 Gag protein alone, or in combination with the Tatcys22 protein, and assayed as previously described.
As shown in
We have shown that native HIV-1 Tat protein and the mutant Tatcys22 protein modulate in vivo epitope specific T cell responses to the HIV-1 Gag and Env antigens. In particular, we have demonstrated that mice vaccinated with Gag, in combination with wild-type Tat or with the mutant Tatcys22, responded to 7 or 11 T cell Gag-derived epitopes respectively, in contrast to mice vaccinated with Gag alone, which responded to 4 T cell Gag-derived epitopes. Similarly, mice vaccinated with Env, in combination with wild-type Tat or with the mutant Tatcys22 responded to 12 Env-derived pools of peptides epitopes in contrast to mice vaccinated with Env alone which responded to 8 T cell Env-derived peptide pools.
These observations, together with our previous findings (2, 3, 11, 12), suggest that Tat is not only an antigen but also a novel adjuvant capable of increasing T cell responses against heterologous antigens. Therefore, the Tat protein, as well as mutant Tatcys22, may represent an important tool in HIV-1 vaccine strategies aimed at broadening the spectrum of the epitopes recognized by T cells.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0322840.9 | Oct 2003 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP04/11950 | 10/11/2004 | WO | 00 | 2/2/2007 |
