1. Technical Field
The present invention relates to compositions and methods for prophylaxis and treatment of malaria infection.
2. Background Art
Malaria is a devastating infectious disease. There are over 300 million cases per year worldwide and it is responsible for over one million deaths per year. Malaria is caused by protozoan parasites of the genus Plasmodium. There are four species that infect humans and they are all transmitted by the bite of an infected Anopheline mosquito. Plasmodium falciparum is responsible for most of the death due to malaria; however, Plasmodium vivax is the most prevalent species worldwide and causes a significant amount of morbidity. Plasmodium falciparum, the cause of the most virulent form of malaria, has developed resistance to currently used drugs. This in turn has led to an increase in the incidence of malaria and to fewer drugs for both treatment and prophylaxis of the disease.
Malaria infection is initiated when an infected Anopheline mosquito injects sporozoites into a subject during the mosquito's blood meal. After injection, the parasite enters the bloodstream and undergoes a series of changes as part of its life-cycle. The sporozoite travels to the liver where it invades hepatocytes. One sporozoite can generate over 10,000 hepatic merozoites, which will then rupture from the hepatocyte and invade erythrocytes. All of the symptoms of malaria are associated with the erythrocytic stage of the disease and treatment of malaria infection requires targeting this stage. The anti-malarial drugs currently on the market target the erythrocytic stage of the parasite. Unlike Plasmodium falciparum, in most parts of the world, Plasmodium vivax is still sensitive to chloroquine. However, in Plasmodium vivax malaria, treatment of the erythrocytic stages is not adequate for eradicating the infection because this parasite has dormant liver stages that can cause relapses months to years after the blood infection has been cleared. Plasmodium vivax-infected individuals must also take primaquine, the only drug that is effective against liver stages of the disease. Primaquine is contraindicated in people with glucose-6 phosphate dehydrogenase deficiency and in pregnant women. Thus, at least two drugs must be taken to prevent Plasmodium vivax infection.
Cell invasion by Plasmodium is an ordered process in which the parasite forms a close association with the host cell plasma membrane and then actively enters the cell (Sinnis, et al. (1997)). This process is aided by the sequential and regulated secretion of proteins from apical organelles (Carruthers, et al. (1999)). One of these secreted proteins is the circumsporozoite protein (hereinafter, “CSP”). CSP forms a dense coat on the parasite's surface. Studies have shown that CSP mediates sporozoite adhesion to target cells (Sinnis, et al. (2002)) and that it is required for sporozoite development in the mosquito (Menard, et al. (1997)). CSP forms a dense coat on the sporozoite and is constitutively secreted onto the parasite's surface. However, secretion of the protease that cleaves CSP appears to be regulated since there is a dramatic increase in the kinetics of CSP cleavage when sporozoites are added to cells. In the absence of cells, it takes two hours for newly synthesized CSP to be cleaved. In contrast, within minutes of contacting target cells, the majority of sporozoites no longer have full-length CSP on their surface.
A similar phenomenon occurs in the merozoite stage of Plasmodium where a low level of MSP-1 cleavage is observed in the absence of cells.
During invasion, however, processing goes to completion within minutes (Blackman, et al. (1990) and Blackman, et al. (1993)). It is likely that low-level cleavage in the absence of cells is due to leaky secretion from apical organelles whereas exocytosis of larger amounts of protease is mediated by specific signals that are transduced upon cell contact.
The Plasmodium proteins that are proteolytically processed during cell invasion can be divided into 2 groups: those that are secreted onto the parasite surface during invasion (e.g., TRAP and AMA-1) and those that are already on the surface (e.g., CSP and MSP-1), which are the major surface proteins of sporozoites and merozoites respectively. In neither case is the precise function of cleavage in the invasion process known. However, it is noteworthy that in the case of the surface proteins, CSP and MSP-1, the C-terminal fragment remaining with the parasite contains a known cell adhesive motif. Proteolytic cleavage can control exposure of these cell-adhesive motifs.
Previous work has demonstrated that proteolytic cleavage of Plasmodium proteins during invasion is accomplished primarily by serine proteases. CSP, however, is cleaved by a cysteine protease. In addition, a recent study found that the cysteine protease falcipain-1 is required for merozoite invasion of erythrocytes (Greenbaum, et al. (2002)). Thus, in addition to serine proteases, cysteine proteases are important components of Plasmodium's invasion machinery.
More specifically referring to the CSPs, they all contain a central repeat region whose amino acid sequence is species-specific. Immediately before the repeat region is a highly conserved five amino acid sequence called region I and in the C-terminal portion of CSP is a known cell adhesive sequence with similarity to the type I thrombospondin repeats (hereinafter, “TSR”)(Goundis, et al. (1988)). CSP has a canonical glycosylphosphatidyl inositol (hereinafter, “GPI”) anchor addition sequence in its C-terminus (Moran, et al. (1994)) and the CSP is GPI-anchored to the sporozoite plasma membrane. CSP immunoprecipitated from metabolically-labeled sporozoites consists of 1 to 2 high MW bands (that differ by ˜1 kDa) and a low MW band that is 8 to 10 kDa smaller (Yoshida, et al. (1981); Cochrane, et al. (1982); Krettli, et al. (1988); and Boulanger, et al. (1988)). Biosynthetic studies show that an initial label is incorporated into the top band(s) and the lower MW band appears only later as a processed product (Yoshida, et al. (1981) and Cochrane, et al. (1982)).
CSP is proteolytically cleaved by a papain-family cysteine protease. Typically, the entire N-terminal third of the CSP is removed. This is consistent with previous structural studies of CSP. In one study, CSP from several Plasmodium species were analyzed by two-dimensional gel electrophoresis and in all cases the low MW form had a significantly lower isoelectric point than the high MW form (Santoro, et al. (1983)). Since the N-terminal portion of CSP is basic compared to the rest of the protein, its removal explains the lower pl of the low MW CSP form. Other studies showed that the difference in mobility by SDS-PAGE between the high and low MW CSP forms corresponds to a size difference of approximately 8 to 10 kDa (Yoshida, et al. (1981); Cochrane, et al. (1982); Krettli, et al. (1988); Nardin, et al. (1982); Gonzalez-Ceron, et al. (1998); Bruana-Romero, et al. (2001); and Boulanger, et al. (1995)). The N-terminal portion of CSP, beginning after the signal sequence and ending just before the repeat region, is approximately this size.
Currently, there are no previously described drugs that target the sporozoite stage of the parasite. The advantages of targeting this stage of the parasite include, but are not limited to, preventing malaria infection in travelers or military personnel going into endemic areas, no longer requiring treatment of P. vivax infections with primaquine, and slowing the development of drug resistance because it targets a stage of the parasite that does not multiply and that uses very low numbers to establish infection.
The incidence of malaria is increasing owing to several factors including resistance of the parasite to currently available anti-malarial drugs. In addition, efforts to develop an effective malaria vaccine have not been successful. Therefore, there is an urgent need to identify new parasite drug targets both for prophylaxis and therapy. Potential new targets include Plasmodium proteases due to their critical roles in the parasite life cycle and the feasibility of developing specific inhibitors.
Current research in both Plasmodium and other Apicomplexan parasites such as Toxoplasma demonstrates that proteolytic cleavage of parasite surface and secreted proteins is necessary for successful invasion of host cells (Blackman, M. J., Howell, S. A., et al., and Kim, K.). It has been recently shown that the major surface protein of sporozoites, the circumsporozoite protein (CSP), is proteolytically processed by a parasite-derived cysteine protease. This cleavage event is temporally associated with sporozoite invasion of hepatocytes.
One particular cysteine protease inhibitor is allicin. Allicin is one of the active compounds of freshly crushed garlic that has been shown to possess a number of antimicrobial activities (Ankri, S. and Harris, L. C.). Allicin has a broad spectrum of antibacterial effects, demonstrating activity against Gram-positive, Gram-negative, and even acid-fact bacteria (Uchida, Y.). Antifungal properties of allicin have been observed not only in vitro, but also recently in vivo (Shadkchan, Y.).
Less work has been done to elucidate the effect of allicin on parasitic protozoa. Allicin inhibits the growth of various parasitic protozoa and extracts of allicin have been effective against a host of infections, including Giardia, Leishmania, and Trichomonas (Reute, H. D.). Because of its sulfydryl modifying activity (Willis, E.), the effects of allicin are thought to involve the inhibition of thiol-containing enzymes in microorganisms (Rabinkov, A.). In fact, allicin has been shown to irreversibly inhibit papain (Rabinkov, A.). Allicin rapidly penetrates cell membranes (Miron, T.), allowing it to quickly exert its biological effects. In the parasitic protozoan Entamoeba histolytica, allicin inhibits cysteine proteases of the parasite, inhibiting parasite growth (Mirelman, D.) and preventing its cytopathic effects (Ankri, S.).
Accordingly, there is a need for a composition and related methods for the prophylaxis and treatment of malaria infection whereby proteolytic cleavage of sporozoites' CSP is prevented, which results in inhibiting cell entry of the sporozoites. Thus, malaria infection is prevented or aborted.
The present invention provides a composition for preventing malaria infection including a protease inhibitor. Further, the present invention provides a pharmaceutical composition for preventing malaria infection including an effective amount of a protease inhibitor and a pharmaceutical carrier. The present invention also provides a method of malaria infection prophylaxis including the step of administering an effective amount of the composition of the present invention. Additionally, the present invention provides a method of malaria prophylaxis by inhibiting circumsporozoite protein processing. Furthermore, the present invention provides a method of malaria prophylaxis by inhibiting a protease of a sporozoite. Finally, the present invention provides various methods of preventing sporozoite cell invasion or preventing circumsporozoite processing.
Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
Generally, the present invention provides a composition and related methods for preventing or treating malaria infection. Specifically, the present invention is based on affecting and/or targeting the sporozoite stage of the parasite. More specifically, the present invention provides protease inhibitors that can inhibit sporozoite infection and thereby completely prevent malaria infection.
As used herein, the phrase, “malaria infection” means an infectious febrile disease caused by protozoa of the genus Plasmodium, which is transmitted by the bites of infected mosquitoes of the genus Anopheles. Malaria infection can be caused by any Plasmodium protozoa, including, but not limited to, Plasmodium vivax and Plasmodium falciparum.
The term “effective amount” as used herein, means, but is not limited to, the amount determined by such consideration as are known in the art of preventing or affecting malaria infection. The effective amount must be sufficient to provide an effect on malaria infection such as the elimination of infection or reduction thereof, which results in the elimination, reduction, or prevention of malaria symptoms or other measurements as appropriate and known to those of skill in the medical arts.
The term “protease inhibitor” as used herein includes, but is not limited to, peptide epoxides, of which L-trans-epoxysuccinyl-leucylamide-[4-guanido]-butane (E-64) is the prototype, Phenylmethylsulphonylfluoride (PMSF), Leupeptin, fluoromethyl ketones, acyloxymethyl ketones, chloromethyl ketones, peptide diazomethanes, allicin, combinations thereof, and any other similar protease inhibitor known to those of skill in the art. Preferably, the present invention utilizes cysteine protease inhibitors.
The term “subject” as used herein includes, but is not limited to, humans, and any other similar subject capable of contracting and developing a malaria infection.
The present invention is based on the discovery that the high molecular weight circumsporozoite protein (hereinafter, “CSP”) form is proteolytically cleaved by a protease, specifically a papain-family cysteine protease, which gives rise to the low molecular weight form. The protease that cleaves CSP is of parasite origin and cleavage occurs on the sporozoite's surface. Cleavage necessarily occurs during target cell invasion and is temporally associated with cell contact. Inhibitors of CSP processing inhibit cell invasion in vitro and in vivo.
The present invention has numerous embodiments. In one embodiment, the present invention provides a composition for the prophylaxis of malaria infection. More specifically, the composition includes a protease inhibitor including, but not limited to, peptide epoxides, of which L-trans-epoxysuccinyl-leucylamide-[4-guanido]-butane (E-64) is the prototype, PMSF, Leupeptin, fluoromethyl ketones, acyloxymethyl ketones, chloromethyl ketones, peptide diazomethanes, allicin, combinations thereof, and any other similar protease inhibitor known to those of skill in the art. The protease inhibitor prevents proteolytic cleavage of sporozoites' CSP by a papain-family cysteine protease. The cleavage occurs on the sporozoite's cell surface. Cleavage of the CSP necessarily occurs during target/host cell invasion and is temporally associated with cell contact. Therefore, inhibitors of CSP processing inhibit cell invasion of the sporozoite in vitro and in vivo.
In another embodiment of the present invention, there is provided a pharmaceutical composition for preventing malaria infection including a protease inhibitor and a pharmaceutical carrier. The protease inhibitor can include, but is not limited to, peptide epoxides, of which L-trans-epoxysuccinyl-leucylamide-[4-guanido]-butane (E-64) is the prototype, PMSF, Leupeptin, fluoromethyl ketones, acyloxymethyl ketones, chloromethyl ketones, peptide diazomethanes, allicin, combinations thereof, and any other similar protease inhibitors known to those of skill in the art.
Either the composition or the pharmaceutical composition of the present invention is administered and dosed in accordance with good medical practice, taking into account the clinical condition of the individual subject, the site and method of administration, scheduling of administration, subject age, sex, body weight and other factors known to medical practitioners. The pharmaceutically “effective amount” for purposes herein is thus determined by such considerations as are known in the art. The amount must be effective to achieve improvement including, but not limited to, improved survival rate or more rapid recovery, or improvement, prevention, or elimination of symptoms and other indicators as are selected as appropriate measures by those skilled in the medical arts.
The composition or the pharmaceutical composition of the present invention can be administered in various ways. It should be noted that it can be administered as the compound or as a pharmaceutically acceptable salt and can be administered alone or as an active ingredient in combination with pharmaceutically acceptable carriers, diluents, adjuvants and vehicles. The compounds can be administered orally, subcutaneously or parenterally including intravenous, intraarterial, intramuscular, intraperitoneally, and intranasal administration as well as intrathecal and infusion techniques. Implants of the compounds are also useful. Generally, the subject being treated is a warm-blooded animal and, in particular, mammals including man. The pharmaceutically acceptable carriers, diluents, adjuvants and vehicles as well as implant carriers generally refer to inert, non-toxic solid or liquid fillers, diluents, or encapsulating material not reacting with the active ingredients of the invention.
It is noted that humans are treated generally longer than mice or other experimental animals exemplified herein, which treatment has a length proportional to the length of the disease process, subject species being treated, and compound or drug effectiveness. The doses can be single doses or multiple doses over a period of several days, but single doses are preferred.
When administering the composition of the present invention parenterally, it can generally be formulated in a unit dosage injectable form (solution, suspension, or emulsion). The pharmaceutical formulations suitable for injection include sterile aqueous solutions or dispersions and sterile powders for reconstitution into sterile injectable solutions or dispersions. The carrier can be a solvent or dispersing medium containing, for example, water, ethanol, polyol (for example, glycerol, propylene glycol, liquid polyethylene glycol, and the like), suitable mixtures thereof, and vegetable oils.
Proper fluidity can be maintained, for example, by the use of a coating such as lecithin, by the maintenance of the required particle size in the case of dispersion and by the use of surfactants. Nonaqueous vehicles such a cottonseed oil, sesame oil, olive oil, soybean oil, corn oil, sunflower oil, or peanut oil and esters, such as isopropyl myristate, can also be used as solvent systems for compound compositions. Additionally, various additives that enhance the stability, sterility, and isotonicity of the compositions, including antimicrobial preservatives, antioxidants, chelating agents, and buffers, can be added. Prevention of the action of microorganisms can be ensured by various antibacterial and antifungal agents. Examples of these agents include, but are not limited to, parabens, chlorobutanol, phenol, sorbic acid, and the like. In many cases, it can be desirable to include isotonic agents, for example, sugars, sodium chloride, and the like. Prolonged absorption of the injectable pharmaceutical form can be brought about by the use of agents delaying absorption, for example, aluminum monostearate and gelatin. According to the present invention, however, any vehicle, diluent, or additive used would have to be compatible with the compounds.
Sterile injectable solutions can be prepared by incorporating the compounds utilized in practicing the present invention in the required amount of the appropriate solvent with various of the other ingredients, as desired.
A pharmacological formulation of the present invention can be administered to the patient in an injectable formulation containing any compatible carrier, such as various vehicles, adjuvants, additives, and diluents. The compounds utilized in the present invention also can be administered parenterally to the patient in the form of slow-release subcutaneous implants or targeted delivery systems such as monoclonal antibodies, vectored delivery, iontophoretic, polymer matrices, liposomes, and microspheres. Examples of delivery systems useful in the present invention include: U.S. Pat. Nos. 5,225,182; 5,169,383; 5,167,616; 4,959,217; 4,925,678; 4,487,603; 4,486,194; 4,447,233; 4,447,224; 4,439,196; and 4,475,196. Many other implants, delivery systems, and modules are well known to those skilled in the art.
A pharmacological formulation of the compound utilized in the present invention can be administered orally to the patient. Conventional methods such as administering the compounds in tablets, suspensions, solutions, emulsions, capsules, powders, syrups and the like are usable. Known techniques, which deliver it orally or intravenously and retain the biological activity, are preferred.
In one embodiment, the compound of the present invention can be administered initially by intravenous injection to bring blood levels to a suitable level. The patient's levels are then maintained by an oral dosage form, although other forms of administration, dependent upon the patient's condition and as indicated above, can be used. The quantity to be administered can vary for the patient being treated and can vary from about 100 ng/kg of body weight to 100 mg/kg of body weight per day and preferably can be from 1 mg/kg to 10 mg/kg per day.
In another embodiment of the present invention, there is provided a method of malaria prophylaxis including the step of administering an effective amount of the composition of the present invention. The composition, which includes a protease inhibitor, prevents cleavage of CSP.
Another embodiment of the present invention provides a method of malaria prophylaxis by inhibiting circumsporozoite protein processing. Circumsporozoite processing is necessary during target cell invasion and is associated with host cell contact. Therefore, by preventing circumsporozoite processing, cell entry into the host cell is prevented or inhibited.
Additionally, the present invention provides a method of malaria prophylaxis by inhibiting a cysteine protease of the sporozoite. Again, as set forth above and supported in the examples, inhibition of cysteine proteases of the sporozoite results in the prevention of host cell entry by the sporozoite.
Further embodiments of the present invention are directed towards compositions for preventing malaria infection that include allicin. Additionally, the present invention also provides a pharmaceutical composition for preventing malaria infection including allicin and a pharmaceutical carrier. As supported herein, administration of allicin, a protease inhibitor, prevents circumsporozoite protein processing or prevents proteolytic cleavage of sporozoites' circumsporozoite protein. Since cleavage of the circumsporozoite protein is necessary for host cell invasion, prevention of the cleavage results in prevention of infection of the host cell by the sporozoite.
The invention is further described in detail by reference to the following experimental examples. These examples are provided for the purpose of illustration only, and are not intended to be limiting unless otherwise specified. Thus, the present invention should in no way be construed as being limited to the following examples, but rather, be construed to encompass any and all variations which become evident as a result of the teaching provided herein.
Materials and Methods:
Chemicals and Reagents:
All chemicals were obtained from Sigma-Aldrich (St. Louis, Mo.) except for aprotinin, antipain, AEBSF, leupeptin, pepstatin, 3,4-dichloroisocoumarin (3,4-DCI), chymostatin and L-trans-epoxysuccinyl-leucylamide-[4-guanido]-butane (E-64), which were obtained from Roche Applied Science (Indianapolis, Ind.). Western blot reagents were purchased from Amersham Pharmacia Biotech (Piscataway, N.J.) and other secondary antibodies were from Sigma-Aldrich.
Parasites:
Plasmodium berghei and Plasmodium yoelii sporozoites were grown in Anopheles stephensi mosquitoes as previously described (Sultan, A. A., et al.) and were obtained from infected salivary glands on the day of the experiment.
Antibodies:
mAb 3D11, directed against the repeat region of P. berghei CSP (Santoro, F., et al.), was conjugated to sepharose as previously described (Boulanger, N., et al.) and biotinylated using D-biotinoyl-ε-aminocaproic acid-N-hydroxysuccinimide ester as outlined in the manufacturer's protocol (Roche Applied Science).
Allicin Preparation:
Allicin was prepared by passing the synthetic substrate alliin [(+)S2-propenyl L-cysteine Soxide] through an immobilized alliinase column (Pinzon-Ortiz, C., et al. and Gonzalez-Ceron, L., et al.). The concentration of allicin was confirmed by HPLC (Krettli, A. U., et al.), and it was stored in a dark tightly closed tube at 4° C. for less than 3 months.
Antibodies and Peptides:
mAb 3D11 is directed against the repeat region of P. berghei CSP (Yoshida, et al. (1980)); mAbs 2F6 (P. Sinnis, F. Zavala, M. Tsuji, unpublished data) and NYS1 (Charoenvit, et al. (1987)) are directed against the repeat region of P. yoelii CSP; and mAb 2A10 is directed against the repeat region of P. falciparum CSP (Nardin, et al. (1982)). For immunoprecipitations, mAbs 3D11 and 2A10 were conjugated to sepharose using the protocol outlined in (Harlow, et al. (1988)). For gliding motility assays, mAbs 3D11 and NYS1 were biotinylated using D-Biotinoyl-e-aminocaproic acid-N-hydroxysuccinimide ester (Roche) as outlined in (Harlow, et al. (1988)). Antisera to the N- and C-terminal thirds of P. berghei CSP were generated using peptides from Institute of Biochemistry, at the University of Lausanne. The sequence of the N- and C-terminal peptides are GYGQNKSIQAQRNLNELCYNEGNDNKLYHVLNSKNGKIYIRNTVNRLLADAP EGKKNEKKNKIERNNKLK (SEQ ID NO. 1) and NDDSYIPSAEKILEFVKQIRDSITEEWSQCNVTCGSGIRVRKRKGSNKKAED LTLEDI DTEICKMDKCS (SEQ ID NO. 2), respectively.
Rabbits were injected three times, at one month intervals. The first injection (200 mg of peptide in complete Freund's adjuvant) was intramuscular and subsequent injections (50 mg peptide in incomplete Freund's adjuvant) were subcutaneous. Overlapping peptides and the repeat peptides were synthesized by Midwest Bio-Tech (Indianapolis, Ind.) and purified by reverse-phase HPLC. The sequence was confirmed by mass spectrometry.
Sporozoites.
P. berghei and P. yoelii sporozoites were grown in Anopheles stephensi mosquitoes. P. falciparum infected mosquitoes were obtained from the NMRC Malaria Program (United States Navy).
Sporozoites were dissected from mosquito glands on the day of the experiment and where indicated, were purified by passage through two 3 mm pore polycarbonate membranes (Whatman, Clifton, N.J.).
ELISAs:
Peptides were coated onto wells of Immunlon 2HB microtiter plates (model 3455, ThermoLabsystems, Frankline, Mass.) overnight at 4° C., blocked and antisera was added at the indicated dilutions. Binding of antisera was revealed with either anti-mouse or anti-rabbit immunoglobulin (Ig) conjugated to alkaline phosphatase followed by the fluorescent substrate, 4-methylumbelliferyl phosphate. Fluorescence was read in a Fluoroskan II plate reader.
Metabolic Labeling Studies:
P. berghei sporozoites were incubated in Dulbecco's Modified Eagle Medium (DMEM; Invitrogen, Carlsbad, Calif.) without cysteine (Cys) and methionine (Met), containing 1% BSA and 400 mCi/ml L-[35 S]-Cys/Met (Pro-Mix, Amersham Pharmacia) in a total volume of 200 μl for one hour at 28° C. They were then washed and resuspended in DMEM/BSA containing unlabeled Cys/Met at 28° C. for the indicated time in the presence or absence of the indicated protease inhibitor. Sporozoites were then lysed and CSP was immunoprecipitated and analyzed by autoradiography as outlined below.
When sporozoites were incubated with specific inhibitors, their concentrations were as follows: 10 mM E-64; 1 mM phenylmethylsulfonyl fluoride (PMSF); 0.3 mM aprotinin; 100 mM 3,4 DCI; 75 mM leupeptin; 100 mM TLCK; 1 mM pepstatin; 1 mM 1,10 phenanthroline; 5 mM EDTA; 0.5% sodium azide. To check for toxicity by propidium iodide (PI) staining, P. berghei sporozoites were incubated in the presence or absence of the various protease inhibitors for two hours at 28° C., PI was then added to a final concentration of 1 mg/ml for five minutes at 28° C. The sporozoites were washed and viewed under a fluorescent microscope.
For the pronase experiment, sporozoites were labeled in medium without BSA for 45 minutes at 28° C., washed, and resuspended in DMEM with unlabeled Cys/Met and cycloheximide (100 mg/ml) for 10 minutes at 28° C. Then, they were kept on ice or chased at 28° C. for one hour. Sporozoites were then resuspended in pronase (100 mg/ml) with or without pronase inhibitor cocktail [500 mg/ml antipain, 30 mg/ml aprotinin, 600 mg/ml chymostatin, 5 mg/ml EDTA, 5 mg/ml leupeptin, 10 mg/ml AEBSF, 7 mg/ml pepstatin, 2 mM PMSF; (Wieckowski, et al. (1998))] for one hour at 4° C., washed with pronase inhibitor cocktail, lysed in buffer supplemented with pronase inhibitor cocktail and 1% BSA, and CSP was immunoprecipitated and analyzed as outlined below.
Immunoprecipitation and SDS-PAGE Analysis:
Metabolically-labeled sporozoites were lysed in lysis buffer (1% Triton X-100, 50 mM Tris-HCl pH 8.0) with 150 mM NaCl containing a protease inhibitor cocktail (Complete Mini-Tablets, Roche) for one hour at 4° C. The lysates were incubated with mAb 3D11 conjugated to agarose overnight at 4° C. with agitation, and the beads were then washed sequentially with lysis buffer containing 150 mM NaCl, high salt buffer (500 mM NaCl in lysis buffer), lysis buffer without added NaCl, and pre-elution buffer (0.5% Triton X-100, 10 mM Tris-HCl pH 6.8). CSP was eluted with 1% SDS in 0.1 M glycine pH 1.8, neutralized with 1.5 M Tris-HCl pH 8.8, and run on a 7.5% SDS-polyacrylamide gel under nonreducing conditions.
For experiments with P. falciparum, a 10% SDS-polyacrylamide gel was used. Gels were fixed in 25% methanol/12% acetic acid, enhanced with Amplify (Amersham Pharmacia) for 30 minutes, dried, and exposed to film.
Immunoblot of Sporozoite Lysates:
Lysates of 5×104 P. berghei sporozoite equivalents were loaded onto each lane of a 7.5% SDS-polyacrylamide gel under non-reducing conditions.
Proteins were transferred to PVDF membrane and incubated with either mAb 3D11 (4 mg/ml), N-terminal antiserum (1:3000) or C-terminal antiserum (1:3000), followed by anti-mouse or anti-rabbit Ig conjugated to horseradish peroxidase (HRP; 1:100,000). Bound antibodies were visualized using the enhanced chemiluminescence detection system (ECL).
Biotinylation of Sporozoites:
P. berghei sporozoites expressing green fluorescent protein [GFP; (Natarajan, et al. (2001))] were biotinylated using sulfo-succinimidyl-6′-(biotinamido)hexanoate according to the manufacturer's instructions (Pierce, Rockford, Ill.). Lysates of biotinylated sporozoites were immunoprecipitated with either mAb 3D11 or polyclonal antibodies to GFP (1:200; Molecular Probes, Eugene, Oreg.) followed by Protein A coupled to agarose beads. Beads were washed and bound proteins were eluted according to the protocol outlined above. 5×104 sporozoite equivalents were loaded onto each lane of a 4-12% Tris-Glycine gel (Invitrogen) under nonreducing conditions, transferred to PVDF, and incubated with either mAb 3D11 followed by anti-mouse Ig conjugated to HRP, anti-GFP Ig (1:500), followed by anti-rabbit Ig conjugated to HRP, or streptavidin conjugated to HRP (1:100,000). Bound antibodies were visualized using ECL.
Immunofluorescence Assay:
Live P. berghei sporozoites were incubated with N-terminal antiserum (1:500 in DMEM/BSA) at 4° C. for two hours, washed three times at 4° C., and allowed to air dry on slides at 4° C. They were then incubated with anti-rabbit Ig-FITC, washed, and mounted in Citifluor (Ted Pella Inc., Redding, Calif.).
Sporozoite Invasion Assay:
Invasion assays were performed as previously described (Pinzon-Ortiz, et al. (2001) and Renia, et al. (1988)), with some modifications. For assays with P. berghei and P. yoelii, Hepa 1-6 cells (ATCC CRL-1830, American Type Culture Collection, Manassas, Va.), a mouse hepatoma cell line permissive for P. yoelii sporozoite development (Mota, et al. (2000)) was seeded (8×104 cells/well) in Lab-Tek permanox chamber slides (Nalgene Nunc Corp., Naperville, Ill.) and grown until confluent.
For assays with P. falciparum, HepG2 cells (ATCC HB8065, American Type Culture Collection) were used. On the day of the experiment, sporozoites were pre-incubated with DMEM/BSA alone or with the indicated protease inhibitor for two hours at 28° C. and plated on cells in the continued presence of the inhibitor for one hour at 37° C.
In a control, Hepa 1-6 cells were incubated with E-64 for two hours at 37° C., the medium was removed and then untreated P. berghei sporozoites were added. The inhibitors used were: 10 mM E-64; 1 mM PMSF; 75 mM leupeptin; 0.3 mM aprotinin; 100 mM 3,4 DCI; 1 mM pepstatin. After incubation with sporozoites, cells were washed, fixed with 4% paraformaldehyde and sporozoites were stained with mAb 3D11 (P. berghei), mAb 2F6 (P. yoelii) or mAb 2A10 (P. falciparum) followed by anti-mouse Ig conjugated to rhodamine. Cells were then permeabilized with cold methanol and stained again with mAbs 3D11, 2F6 or 2A10 followed by anti-mouse Ig conjugated to FITC. All sporozoites were FITC-positive whereas only extracellular sporozoites stained with rhodamine. The percentage of sporozoites that invaded the cells is calculated using the following equation:
Cell Contact Assay:
Hepa 1-6 cells were seeded on glass coverslips at a density of 2×105 cells per coverslip and grown until confluent. P. berghei sporozoites were incubated in DMEM ±10 mM E-64 at 4° C. for two hours. Thirty minutes before sporozoites were added to coverslips, Cytochalasin D (CD) was added to all samples (final concentration, 1 mM). Sporozoites were centrifuged onto coverslips (1250×g) for five minutes at 4° C. Coverslips were then brought to 37° C. for two minutes, fixed with 4% paraformaldehyde and stained with either mAb 3D11 followed by anti-mouse Ig FITC or the N-terminal antiserum (1:100) followed by anti-rabbit Ig FITC. When P. berghei sporozoites transgenic for GFP were used, the cells were only stained with the N-terminal antiserum followed by anti-rabbit Ig FITC. As a control, sporozoites were spun onto coverslips without cells using the protocol outlined above.
Sporozoite Motility Assay:
Lab-Tek glass slides (model 177402, Nalgene Nunc Corp., Naperville, Ill.) were coated with 10 mg/ml of mAb 3D11 (P. berghei) or mAbs 2F6 and NYS1 (P. yoelii) and sporozoites pre-incubated with DMEM/BSA alone or supplemented with 10 mM E-64 or CD for two hours at 28° C. were added in the continued presence of the inhibitor for one hour at 37° C. in a humidified chamber with 5% CO2. The slides were then fixed with 4% paraformaldehyde, incubated with 1:100 dilution of either biotinylated mAb 3D11 (P. berghei) or biotinylated mAb NYS1 (P. yoelii) followed by streptavidin-FITC.
Sporozoite Migration Assay:
Migration assays were performed as previously described (Mota, M., et al.). Sporozoites were pre-incubated ±10 mM E-64 for two hours at 28° C. and added to monolayers of Hepa 1-6 cells in the continued presence of inhibitor with 1 mg/ml rhodamine-dextran, 10,000 MW (Molecular Probes). After one hour at 37° C., the cells were washed, fixed and the number of rhodamine-positive cells in each field was counted.
Staining of Sporozoites in Dextran-Positive Cells:
P. berghei sporozoites were added to Hepa 1-6 cells in the presence of 1 mg/ml rhodamine-dextran for 45 minutes at 37° C., washed, and fixed with 4% paraformaldehyde. Extracellular sporozoites were stained with mAb 3D11 followed by anti-mouse Ig conjugated to 10 nm gold (1:50, Amersham Pharmacia). Hepa 1-6 cells were then permeabilized with 0.1% saponin, which does not allow the escape of intracellular dextran, and intracellular sporozoites were stained with the N-terminal antiserum (1:100) followed by anti-rabbit Ig FITC (1:500, Molecular Probes). Coverslips were washed and developed using the Intense M Silver Enhancement kit (Amersham Pharmacia) for fifteen minutes at room temperature.
Assay for Sporozoite Infectivity In Vivo:
Swiss/Webster mice were given three intraperitoneal injections of DMEM with or without E-64 (50 mg/kg/injection) at 16 hours, 2.5 hours, and 1 hour prior to intravenous injection of 15,000 P. yoelii sporozoites. Forty hours later, livers were harvested and total RNA was isolated using Tri-Reagent (Molecular Research Center, Cincinnati, Ohio). Malaria infection was quantified using reverse-transcription (RT) followed by real time PCR as outlined in (Bruna-Romero, et al. (2001)). RTs were performed with 0.5 mg of total RNA and random hexamers (PE Applied Biosystems, Foster City, Calif.). Real time PCR was performed using primers that recognize P. yoelii-specific sequences within the 18S rRNA (Bruna-Romero, et al. (2001)) and the SYBR Green Core PCR kit (PE Applied Biosystems). Ten-fold dilutions of a plasmid construct containing the P. yoelii 18S gene were used to create a standard curve.
Metabolic Labeling, Immunoprecipitation and SDS-PAGE Analysis:
P. berghei sporozoites were metabolically labeled as previously described (Bruna-Romero, O., et al.). Briefly, sporozoites were labeled in Dulbecco's Modified Eagle Medium containing 1% BSA (DMEM/BSA) without Cys/Met and with 400 μCi/ml L-[35S]Cys/Met for one hour at 28° C. and chased in DMEM/BSA with Cys/Met at 28° C. in the presence of 10 μM E-64 or the indicated concentrations of allicin. Labeled sporozoites were lysed in 1% Triton X-100/150 mM NaCl/50 mM Tris-HCl pH 8.0 with protease inhibitors, and lysates were incubated with 3D11-sepharose overnight at 4° C. CSP was eluted with 1% SDS in 0.1 M glycine pH 1.8, neutralized with Tris-HCl pH 8.8, and run on a 7.5% SDS-polyacrylamide gel under nonreducing conditions. The gel was fixed, enhanced with Amplify (Amersham Pharmacia), dried and exposed to film.
Allicin Toxicity Assay:
P. berghei sporozoites were incubated with the indicated concentrations of allicin for ten or sixty minutes at 28° C., washed with DMEM, and then incubated with 1 μg/ml propidium iodide for five minutes at 25° C. The number of fluorescent sporozoites in each sample was counted using a Nikon Eclipse E600 microscope. Control samples consisted of sporozoites that were incubated for sixty minutes at 28° C. in DMEM without allicin and sporozoites that were heat killed at 65° C. for ten minutes.
Gliding Motility Assay:
Glass 8-chambered Lab-tek wells (Nalgene #) were coated with 10 μg/ml 3D11 in PBS overnight at 25° C. and then washed three times with PBS. 2×104 P. berghei sporozoites were incubated with 50 μM allicin in DMEM without -Cys/-Met for ten minutes at 28° C., the medium removed and replaced with DMEM/3% BSA containing 50 μM or 4.2 μM allicin before sporozoites were added to the coated Lab-Tek wells. The sporozoites were incubated for one hour at 37° C., the medium was removed, and the wells were fixed with 4% paraformaldehyde, washed, blocked with PBS/1% BSA, and incubated with biotinylated 3D11 followed by Streptavidin-FITC (1:100 dilution; Amersham Pharmacia). All incubations were performed at 37° C. for one hour. Controls included untreated sporozoites and sporozoites added to wells in the presence of 1 μM cytochalasin D. For each group, gliding motility was assessed by determining the percentage of sporozoites associated with trails, and for those sporozoites with trails, counting the number of circles in each trail.
Sporozoite Invasion Assays:
Invasion assays were performed as previously described (Pinzon-Ortiz, C., et al.) with some modifications. P. berghei sporozoites were preincubated with the indicated concentrations of allicin for ten minutes at 28° C., diluted 12-fold with DMEM/BSA and added to Hepa 1-6 cells (CRL-1830: American Type Culture Collection) for one hour at 37° C. Cells were then washed, fixed, and sporozoites were stained with a double staining assay that distinguishes between intracellular and extracellular sporozoites (Renia, L., et al.).
Assay for Sporozoite Infectivity In Vivo:
Swiss/Webster mice were given either 5 or 8 mg/kg of allicin (in DMEM without Cys/Met) intravenously (i.v.) at 60 minutes, 30 minutes, or immediately before i.v. injection of 104 P. yoelii sporozoites. Forty hours later, livers were harvested, total RNA was isolated, and malaria infection was quantified using reverse transcription (RT) followed by real time PCR using primers that recognize P. yoelii-specific sequences within the 18S rRNA as previously described (Bruna-Romero, O., et al.). Ten-fold dilutions of a plasmid construct containing the P. yoelii 18S rRNA gene were used to create a standard curve. For allicin preincubation experiments, P. yoelii sporozoites were preincubated with or without 50 μM allicin (in DMEM without Cys/Met) for ten minutes at 28° C. and diluted 12-fold with medium before i.v. injection into mice. All in vivo data were analyzed using the Student t-test for unpaired samples.
To study the structure of the high and low molecular weight CSP forms, polyclonal antisera to peptides representing the entire N-terminal and C-terminal thirds of CSP from Plasmodium berghei, a rodent malaria parasite, were made. As shown in
The antisera recognized the appropriate full-length peptides and did not recognize peptides representing the central repeat domain (
Western blot analysis of a P. berghei sporozoite lysate using the N- and C-terminal specific antisera (
The present data provides evidence that the conserved region I, found at the end of the N-terminal portion of CSP, contains the cleavage site. In order to determine what class of protease is responsible for cleavage, pulse-chase metabolic labeling experiments were performed in the presence of protease inhibitors. Previous studies showed that after one hour of labeling, radioactivity is found in the high molecular weight CSP form and at 28° C., the half-life of this species is between sixty to ninety minutes (Yoshida, et al. (1981) and Cochrane, et al. (1982)).
As supported by
As set forth above, sporozoites were labeled with [35S]-Cys/Met for one hour and chased with medium containing unlabeled Cys/Met for two hours in the presence or absence of protease inhibitors (
To further examine the role of serine proteases, two other serine protease inhibitors, aprotinin and 3,4 DCI, were assayed. Aprotinin inhibits most classes of serine proteases and would be predicted to inhibit the serine proteases of Plasmodium, which are subtilisin-like serine proteases (Wu, et al. (2003)). 3,4 DCI is a serine protease inhibitor that has some activity against cysteine proteases, but does not react with papain-like cysteine proteases (Harper, et al. (1985)). Neither compound had an effect on CSP processing. Taken together, the data proves that the processing enzyme is a cysteine protease.
In addition to the above, pulse-chase metabolic labeling experiments were performed with sporozoites of the human malaria parasite, Plasmodium falciparum. As shown in
Immunofluorescence experiments with live sporozoites showed that the sporozoites were recognized by the N-terminal antiserum, suggesting that full-length CSP was on the surface (
Sporozoites isolated from salivary glands of infected mosquitoes are invariably contaminated with mosquito debris. For this reason, it could not be determined whether the protease that cleaves CSP is of parasite or mosquito origin. To address this question, the kinetics of CSP processing in purified and unpurified sporozoites was compared and no difference was found between these groups. However, even purified sporozoites are associated with a small amount of mosquito debris. Therefore, sporozoites were dissected and purified in the presence of E-64, an irreversible inhibitor of cysteine proteases. After their isolation, sporozoites were washed and metabolically labeled in medium without E-64. Cysteine proteases of mosquito origin would be extracellular and therefore irreversibly inhibited by the E-64 present during sporozoite isolation. Sporozoites, however, continue to synthesize and/or secrete protease after the removal of E-64, allowing newly labeled CSP to be processed. As shown in
Proteolytic cleavage of cell surface and secreted proteins occurs during invasion of erythrocytes by the merozoite stage of Plasmodium (Blackman, et al. (2000). To determine whether CSP cleavage was required for sporozoite entry into cells, a variety of protease inhibitors were tested for their ability to inhibit sporozoite invasion of a hepatocyte cell line.
As set forth in
The results, as set forth in
In the presence of E-64, the number of extracellular sporozoites was always enhanced, showing that there was an accumulation of attached sporozoites that were prevented from entering cells (
a P. berghei sporozoites (wildtype in experiment 1; GFP in experiment 2) were preincubated +/−E−64 and before addition to cover slips, CD was added to the indicated samples. Sporozoites were then spun onto cover slips, with or without cells as indicated, brought to 37° C. for two minutes, fixed, and stained with the indicated antisera.
b Each point was plated in duplicate, 50 fields per cover slip were counted and shown are the means with standard deviations. When sporozoites were plated without cells, 100 to 200 sporozoites per cover slip were counted and shown is the percentage staining with the N-terminal antiserum.
As set forth in
Since sporozoite motility is required for cell invasion (Sultan, et al. (1997)), it was tested whether CSP processing is required for motility. E-64 had no effect on the percentage of P. yoelii or P. berghei sporozoites that exhibited gliding motility (
Migrating sporozoites would retain full-length CSP on their surface. In order to test this, sporozoites with cells in the presence of dextran conjugated to rhodamine were incubated and then stained intracellular sporozoites with the N-terminal antiserum. Very few intracellular sporozoites reacting with the N-terminal antiserum were observed. However, the sporozoites that were recognized by this antiserum were always in dextran-positive cells, suggesting that they were migrating through these cells (
As shown in
As set forth in
It has been previously shown that the cysteine protease inhibitor E-64 prevents proteolytic cleavage of the major surface protein of sporozoites, the circumsporozoite protein (CSP) (Coppi, A.). Further, allicin has been shown to react with free sulfhydryl groups (Rabinkov, A.), and in this way can reversibly inhibit cysteine proteases (Ankri, S.). Pulse chase metabolic labeling experiments in the presence of allicin indicate that CSP cleavage is inhibited by 10, 25, and 50 μM allicin (
As set forth in
In order to determine if the effect of allicin on CSP cleavage was due to a toxic effect on the sporozoites, parasites were incubated for ten minutes or one hour with different concentrations of allicin and then propidium iodide was added. Propidium iodide is a dye that is excluded by viable cells but penetrates the cell membranes of dying or dead cells. When sporozoites were incubated with either 1 or 10 μM allicin for up to one hour, the percentage of sporozoites that took up the dye was no different from the untreated control (
As set forth in
Since uptake of propidium iodide is a terminal event, it was determined whether sporozoites incubated with allicin were still motile. Plasmodium sporozoites exhibit a unique form of substrate-dependent locomotion, termed gliding motility, which is required for cell invasion (Sultan, A., et al.). If sporozoites were motile in the presence of allicin, then the compound was not affecting the overall metabolic activity of the parasites. In motility assays, allicin was tested and found that preincubation with 50 μM allicin for ten minutes followed by dilution to 4.2 μM had no effect on gliding motility (
According to
Since allicin inhibited CSP cleavage and previous studies showed that cleavage is associated with cell invasion, it was tested whether allicin would inhibit invasion of host cells. For these experiments and as set forth above, P. berghei sporozoites were pretreated with 10, 25, and 50 μM allicin for 10 minutes, diluted 12-fold and added to host cells. As shown in
As set forth in
Allicin was tested to determine its ability to inhibit sporozoite infectivity in vivo using the rodent malaria parasite P. yoelii. Mice were injected with allicin or buffer alone at different times before injection of sporozoites. Forty hours after sporozoite injection, the parasite burden in the liver was determined by RT followed by real time PCR. As shown in
The decrease in efficacy of allicin over time is a consequence of its rapid decomposition in vivo (Brodnitz, M. H., et al.). In order to test the inhibitory activity of allicin in vivo, before its catabolism in the blood, a second set of experiments were performed in which mice were injected with P. yoelii sporozoites preincubated with 50 μM allicin or buffer alone. As shown in
Throughout this application, various publications, including U.S. patents, are referenced by author and year and patents by number. Full citations for the publications are listed below. The disclosures of these publications and patents in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The invention has been described in an illustrative manner, and it is to be understood that the terminology, which has been used, is intended to be in the nature of words of description rather than of limitation.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the described invention, the invention can be practiced otherwise than as specifically described.
This application claims the benefit of priority under 35 U.S.C. Section 119(e) of U.S. Provisional Patent Application No. 60/600,547, filed Aug. 11, 2004, which is incorporated herein by reference in its entirety.
Research in this application was supported in part by a contract from National Institute of Health (R01 AI044470). The government has certain rights in the invention.
Number | Date | Country | |
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60600547 | Aug 2004 | US |