Patent Application
20100216661
The present invention relates to mimotopes of blood group antigens, and uses thereof.
Blood group antigens exist on membrane proteins on the surface of erythrocytes. The genes encoding all but one of the 29 blood group systems (Rhesus, Kell, Duffy, Kidd, ABO, MNS, P, Lutheran, Lewis, Diego, Yt, Xg, Scianna, Dombrock, Colton, Landsteiner-Wiener, Chido-Rodgers, Hh, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, JMH, I, Globoside and GIL) have been cloned, sequenced, the amino acid sequences deduced, and the membrane structure predicted.
Of the Rhesus, Kell, Duffy and Kidd blood systems etc, the Rhesus antigens are the most complex being the products of two highly homologous genes, RHD and RHCE. The antigens are predicted to have six extracellular loops that are available for antibody binding.
The RhD polypeptide, the most commonly expressed of the Rhesus antigens, differs by 36 amino acids from its nearest equivalent, the RhCcEe polypeptide. Both are co-expressed with the Rhesus-associated glycoprotein (RAG) polypeptide.
The RHCE gene product exists in four allelic forms and each allele determines the expression of two antigens in combination i.e. Ce, ce, cE or CE. The RhC antigen differs from Rhc by one amino acid in loop 2 (Ser103Pro), and for E/e, a single exofacial amino acid difference is predicted in loop 4 (Pro226Ala).
The Kell protein is a single pass membrane structure, and the K/k alleles are the result of a single nucleotide polymorphism (SNP) resulting in a single amino acid change (Met193Thr) in the extracellular region.
The Duffy protein is predicted to have 3 extracellular loops with the N-terminus being extracellular, and the Fya/Fyb alleles are the result of a SNP giving rise to an amino acid substitution (Asp42Gly) in the N-terminal region.
The Kidd glycoprotein is predicted to have 10 membrane-spanning domains with intracellular N- and C-termini. The Jka/Jkb polymorphism results from a SNP giving rise to a single amino acid substitution (Asp280Asn) on the fourth extracellular loop.
The immunological determinants on an antigen that bind antibodies are defined as B-cell epitopes since they are recognised by the surface-membrane immunoglobulin receptors of B-lymphocytes. These epitopes may be linear i.e. a continuous stretch of amino acids from the protein sequence, or conformational and depend on the spatial juxtaposition of amino acids which are not contiguous. Study of the B-cell epitope structure of blood group antigens has largely been restricted to the RhD antigen, and to date over 30 epitopes have been identified serologically.
A single amino acid substitution can affect more than one epitope, even though the predicted loops involved are some distance apart in the linear sequence, and it is concluded that the RhD B-cell epitopes are conformational.
The B-cell epitopes on C/c and E/e are predicted to be fewer, but are likely to be conformational since full antigenicity requires co-expression of the RAG polypeptide.
The K/k polymorphism is predicted to disrupt a glycosylation site, which may account for the antigenicity of the K allele, and it is anticipated that the epitopic determinants will also be conformational.
Blood group antigens may induce the production of alloantibodies that can cause destruction of transfused red blood cells. Reactions to transfused blood may, therefore, occur when a recipient of transfused erythrocytes has antibodies to those erythrocytes, resulting in their destruction. An immediate reaction including fever, hypotension, nausea and vomiting, and pain in the back and chest can occur, with the severity of the reaction depending on the class and amount of antibodies involved.
Alloantibodies to blood group antigens may also cross the placenta of a pregnant woman and give rise to haemolytic disease of the newborn (HDNB). Haemolytic disease of the newborn (HDNB) can occur when the mother has been sensitised to antigens on the infant's erythrocytes and makes IgG antibodies to these antigens. These antibodies can cross the placenta and react with the foetal erythrocytes causing their destruction. RhD is the most commonly involved antigen.
Blood group antigens may also induce the production of autoantibodies involved in autoimmune diseases such as autoimmune haemolytic anaemia. The trigger for this autoimmune disease is unknown and therefore it may occur at anytime and results in the body producing autoantibodies of broad Rh group specificity which attack the body's own red blood cells.
The most important blood groups associated with alloantibody generation are Rhesus, Kell, Duffy, and Kidd.
Current methods for detecting alloantibodies that may destroy transfused erythrocytes or cross the placenta to cause HDNB, or for detecting autoantibodies, rely on blood donor red cell reagents. These have a short shelf-life and vary considerably in the antigen expression between donors. Use of human blood products also introduces safety implications, for example, involving the potential transfer of viral infections. The rigorous testing and screening required when using human reagents, together with inconsistencies between samples slows the availability of reagents for detecting alloantibodies, as well as increasing the cost of obtaining suitable reagents.
The present invention, accordingly, seeks to overcome the problems associated with the prior art, in particular, the problems associated with existing methods for detecting alloantibodies and autoantibodies.
According to an aspect of the present invention there is provided a mimotope of a blood group antigen.
The term “mimotope” refers to a peptide which is capable of mimicking an epitope of a native peptide. Usually such peptides are synthetic peptides. In this respect, “synthetic peptide” includes any peptides which are synthesised rather than expressed natively in a biological system. Synthesis may be by recombinant technology such as phage display, F-moc chemistry or any other method for expressing or synthesising foreign peptides in biological or non-biological systems or environments.
The present invention thus provides novel peptides which are capable of binding to antibodies which have been raised to blood group antigens. These peptides are not necessarily from natural sources such as human blood and, therefore, do not carry the health risks, storage problems and variability associated therewith.
Conveniently, the blood group antigen is selected from Rhesus, Kell, Duffy, Kidd, ABO, MNS, P, Lutheran, Lewis, Diego, Yt, Xg, Scianna, Dombrock, Colton, Landsteiner-Wiener, Chido-Rodgers, Hh, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, JMH, I, Globoside or GIL.
The deduced amino acid structure of a mimotope may not necessarily correspond to the amino acid structure of the antigen which it mimics. Indeed, in the present study it was discovered that the majority of the blood group mimotopes identified have amino acid sequences which differ widely from the amino acid sequence of the native antigen that they mimic.
As the epitopes of many blood group antigens are thought to be complex in conformation, it is surprising that the synthetic peptides, identified in the present invention by means of immunopanning, are capable of binding antibodies raised to native antigens in a specific manner. This finding suggests that the synthetic blood group mimotopes assume and maintain a three dimensional conformation which allows them to specifically bind the antibodies raised to their corresponding antigen.
Conveniently, the mimotope is synthesised. This removes the problems associated with using human blood reagents, for example, the potential risk of contamination and infection, difficulties regarding storage, donor identification, variability and screening. These problems have associated costs which will be reduced through use of synthetic mimotopes. Use of synthetic mimotopes also provides a greater degree of control over the reagents that are available for use, ensuring that shortages never occur and unnecessary waiting for reagents can be avoided.
Through the present study, numerous mimotopes of blood group antigens were identified. Some of the peptides conveniently identified as blood group mimotopes are identified in Tables 2-5 and include peptide fragments, immunoreactive analogues or derivatives thereof.
According to a further aspect of the present invention there is provided a method for identifying a mimotope of a blood group antigen, the method comprising contacting a random phage display library with an antibody to a blood group antigen and detecting binding of the antibody to peptides in the phage display library, wherein binding of an antibody to a peptide is indicative of the peptide being a mimotope of a blood group antigen.
Preferably, the blood group antigen is Rhesus, Kell, Duffy, Kidd, ABO, MNS, P, Lutheran, Lewis, Diego, Yt, Xg, Scianna, Dombrock, Colton, Landsteiner-Wiener, Chido-Rodgers, Hh, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, JMH, I, Globoside or GIL. These are the blood group antigens associated with the generation of alloantibodies and autoantibodies.
The antibody to which the mimotope binds may be a monoclonal antibody or a preparation of polyclonal antibodies. Methods for generating monoclonal antibodies and polyclonal antibodies are well known in the art.
Random phage display libraries comprising 7-mer, 12-mer and 15-mer peptides were used to identify peptides which could bind antibodies raised to native blood group antigens. Identification and sequencing of peptides which bound to the antibodies enabled identification of potential blood group antigen mimotopes. Subsequent analysis of these peptides by ELISA followed by synthesis of these peptides by SPOTs and analysis of the membrane-bound SPOTs confirmed the strength and specificity of the peptide-antibody binding. This confirmed that synthetic peptides structurally different from the native antigens which they mimicked were successfully able to specifically bind antibodies raised to native blood group antigens, thus representing effective mimotopes of the blood group antigens in question. Immobilisation of the mimotopes onto, for example, microsphere beads or superparamagnetic beads did not affect the ability of the mimotopes to specifically bind antibodies raised to native blood group antigens. The method described therefore represents an effective and efficient method for identifying synthetic mimotopes of blood group antigens which can be used in a clinical setting, for example, in the diagnosis of diseases associated with the production of alloantibodies or autoantibodies.
Binding of antibodies to the phage-peptides may be detected by any means suitable, such means being known to a person skilled in the art. Examples of the type of binding which can be used is described in the Examples.
According to a further aspect of the present invention there is provided a method for detecting an antibody to a blood group antigen in a sample. The method comprising reacting a mimotope of said blood group antigen with the sample to be tested, detecting any reaction between the sample and the mimotope, wherein a reaction between the sample and the mimotope is indicative of the presence of an antibody to the blood group antigen.
Conveniently before carrying out the above identified method the mimotope is immobilised on a solid support. Immobilisation of the mimotopes onto a solid support such as, microsphere beads or superparamagnetic beads did not affect the ability of the mimotopes to specifically bind antibodies raised to native blood group antigens. In this connection, the method comprises immobilising a mimotope of a blood group antigen on a solid support, contacting the immobilised mimotope with the sample to be tested and detecting binding of the sample to the mimotope, wherein binding of the sample to the mimotope is indicative of the presence of an antibody to the blood group antigen mimicked by the mimotope.
Conveniently, the blood group antigen is Rhesus, Kell, Duffy, Kidd, ABO, MNS, P, Lutheran, Lewis, Diego, Yt, Xg, Scianna, Dombrock, Colton, Landsteiner-Wiener, Chido-Rodgers, Hh, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, JMH, I, Globoside or GIL.
In this respect, given that many epitopes of native blood group antigens are thought to be conformational it is surprising that the synthetic mimotopes identified herein are capable of presenting a three dimensional conformation which specifically bind antibodies raised to native blood group antigens, even when bound to a solid support. The solid support may comprise any support suitable for the purpose but conveniently comprises a membrane, glass slide or bead (glass, polystyrene etc.). Attachment to the support can be by any suitable means, for example using Ni-NTA groups attached to the surface of polystyrene microsphere beads or using superparamagnetic polystyrene beads. Polyhistidine tags may be incorporated into the C-terminal or N-terminal of mimotopes to aid attachment to the support. Membranes having a lower background and which can be successfully stripped and re-probed, for example INTAVIS AG membranes, are preferable.
The present invention, therefore, provides a method which effectively identifies antibodies in biological samples that have been raised to blood group antigens. Use of synthetic mimotopes removes the problems identified with regard to use of human reagents. This will increase consistency in antibody detection and enable detection of rarer antibodies for little or no extra cost, compared with existing techniques for screening antibodies raised to blood group antigens. Avoiding use of human blood products also increases safety, avoids the rigorous testing required when using human blood products and increases the consistency of the reagents available for use. This reduces the cost of sourcing and clearing reagents for use. The method described is therefore more reliable, safe and convenient than existing detection methods, making detection of antibodies to blood group antigens in biological samples quicker, cheaper and easier to conduct.
The antibodies detected may be alloantibodies or autoantibodies to blood group antigens. The method described, therefore, provides improved methods of detecting and diagnosing diseases or a reaction to blood product's involving the generation of alloantibodies or autoantibodies. Such diseases include haemolytic reactions to red cell transfusions (HTR), haemolytic disease of the newborn and autoimmune haemolytic anaemia.
Conveniently, the antibody is an alloantibody, allowing one to reliably and safely diagnose the risk of HTR or haemolytic disease of the newborn.
According to another aspect of the present invention there is provided a device for detecting the presence of an antibody in a sample to a blood group antigen. The device comprising a solid support (membrane, glass slide, microarray chip or bead(s) etc.) having an array of mimotopes of a blood group antigen immobilised thereon or having one mimotope on each solid support and having provided a plurality of solid supports.
An array may be constructed comprising only Rhesus, Kell, Duffy, Kidd, ABO, MNS, P, Lutheran, Lewis, Diego, Yt, Xg, Scianna, Dombrock, Colton, Landsteiner-Wiener, Chido-Rodgers, Hh, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, JMH, I, Globoside or GIL antigen mimotopes. Alternatively, an array may be developed comprising any combination of blood group mimotopes, with mimotopes from multiple blood groups being arranged on the same diagnostic array. The composition and design of the array may depend on the intended purpose of the device and bespoke arrays may be developed as required. Arrays required for specific purposes can be constructed quickly, reliably and cost-effectively, without having to consider the availability of reagents.
With regard to the construction of arrays, different types and length of spacer for attaching peptides to a solid membrane support can be used, for example an amino-PEG spacer or a limited number of amino acid residues could be used to link the mimotope peptide sequence to the solid support.
Conveniently, the antibody is an alloantibody. The device can therefore be used for detecting alloantibodies in a sample of blood obtained from a patient suspected of generating alloantibodies to blood group antigens, without requiring the use of human blood group reagents. Removal of the need for blood group reagents will conveniently increase the availability of reagents for detection of antibodies in blood group samples.
Antigen-capture systems as described herein can also increase the sensitivity and accuracy of antibody detection, thus providing an improvement over existing methods for the detection of blood group.
By showing that phage-peptides, selected by biopanning, specifically bind the antibodies originally used to identify them, the experiments described herein confirm the potential value of the selected phage-peptides as diagnostic reagents.
The mimotopes identified in the present study, when coupled to a suitable solid phase, can be used as screening reagents to detect antibodies to blood group antigens.
In this respect, the mimotopes may be used to diagnose diseases associated with blood group antigens, such as diseases associated with the production of alloantibodies or autoantibodies.
Further potential uses of the mimotopes identified and described herein, together with uses of the arrays formed therewith, will be evident to a person skilled in the art.
The present invention will now be described by way of illustration only with reference to the accompanying drawings:
Identification of mimotopes of blood group antigens commenced with biopanning of four phage-peptide libraries with a series of blood group antigen-specific monoclonal antibodies (3× anti-RhD, 3×RhE, 2×Rhe, 1×Fya, 1×Fyb) and an RhD polyclonal preparation. In total, 490 phage were selected and initially characterised by DNA sequencing and ELISA. DNA sequence analysis identified 295 unique phage-peptide sequences, which were synthesised on membranes by SPOTS technology. The specificity of the peptide/antibody reaction was tested by ELISA and SPOTS assays and identified 84 peptides capable of detecting specific RhD, RhE, Rhe, Fya & Fyb antibodies.
Biopanning
Three different phage libraries from New England Biolabs. (Ph.D. 7 and Ph.D. 12 displaying linear 7- and 12-mer peptides, respectively and Ph.D. C7C displaying 7-mer peptides constrained by a disulphide bridge at their termini) and a linear 15mer library (kindly provided by Prof. George P. Smith, University of Missouri) were used in biopanning experiments with 10 different monoclonal antibodies and a polyclonal preparation (Table 1).
All monoclonal antibodies listed were purified from culture supernatants. Polyclonal anti-RhD preparation was obtained from the Scottish National Blood Transfusion Service.
For each target antibody/library combination, a separate well of a 96-well Immulon 4 flat-bottomed ELISA plate (Thermo Labsystems) was coated with 5 μg of antibody in 150 μl ELISA coating buffer (15 mM Na2CO3, 35 mM NaHCO3, pH9.6). Plates were sealed and incubated at 4° C. overnight. Unbound antibody was discarded and 350 μl of blocking buffer (0.1M NaHCO3/3% skimmed milk powder) was added to each well. Plates were sealed and incubated for 1 hour at 4° C. The blocking buffer was removed and the wells were washed six times with 300 μl TBS/0.1% Tween 20. Approximately 2×1011 plaque forming units (pfu) in 100 μl TBS/0.1% Tween 20 were added to each well and the plates were incubated at room temperature for 1 hour. Wells were washed ten times with 300 μl TBS/0.1% Tween 20. Bound phage were eluted by incubation with 100 μl of 0.2M glycine-HCl/BSA, pH2.2 (1 mg/ml) for 10 minutes, then neutralised with 15 μl of 1M Tris-HCL (pH9.1). An aliquot of the Round 1 eluate was removed and titred. The remainder of the eluted phage were amplified by infection of E. coli ER2538 cells for 4.5 hours at 37° C. The cultures were centrifuged for 10 minutes at 10,000 rpm at 4° C. The supernatants were recovered, ⅙ volume PEG/NaCl (20% polyethylene glycol 8000; 2.5M NaCl) added, then incubated overnight at 4° C. The PEG-precipitated phage were centrifuged at 10,000 rpm for 15 minutes at 4° C. The supernatant was removed and the phage pellet resuspended in 200 μl TBS. Two further rounds of panning were undertaken and approximately 2×1011 pfu of the Round 2 or Round 3 amplified eluates were used as input phage. Round 3 eluates were titrated but not amplified. Individual plaques were amplified and subjected to PCR amplification, prior to DNA sequence analysis. The specificity of peptide-antibody binding was subsequently determined using both phage ELISA and SPOTs techniques.
Phage Culture
Individual phage clones were randomly selected from the Round 3 titration plates and amplified in 1 ml volumes of 1:100 dilution of an overnight culture of E. coli ER2538, grown in L-Broth. Cultures were incubated at 37° C. for 4.5 hours, then centrifuged at 13000 rpm for 1 minute. The phage supernatants were recovered for further analysis by PCR amplification, DNA sequencing and phage ELISA.
PCR Amplification
To determine the peptide sequences displayed in each of the selected phage clones, the DNA insert encoding the peptide sequence was obtained by PCR. PCR amplification was performed in a final volume of 25 μl containing 1× buffer (Bioline), 400 μM dNTPs, 1.5 mM MgCl2, 1 unit of Taq DNA polymerase (Bioline), 1 μl of phage culture supernatant and 0.5 μM each of the appropriate forward and reverse primers. The primers 12MER1For: 5-CGCAATTCCTTTAGTGGTAC-3 and 12MER2Rev: 5-CCCTCATAGTTAGCGTAACG-3 were used for amplification of the C7C, linear 7-mer and 12-mer phage/peptides. The primers Fuse5For1: 5′-ACCGATACAATTAAAGGCTC-3′ and Fuse5Rev1: 5′-TGAATTTTCTGTATGAGG-3′ were used for the amplification of the 15-mer phage/peptides. PCR was carried out in a thermal cycler (Techgene, model FTGene2D; Techne, Cambridge, UK) under the following conditions: denaturation at 95° C. for 1 minute; annealing at 55° C. for 1 minute; and extension at 72° C. for 3 minutes. After 30 cycles, extension was continued at 72° C. for an additional-9 minutes. PCR products were recovered using the QIAquick PCR Purification Kit (Qiagen) for sequence analysis.
Sequence Analysis
The nucleotide sequences of the phage peptides were obtained using the BigDye Terminator Cycle Sequencing Ready Reaction Kit and an ABI Prism 377 DNA Sequencer (Applied Biosystems). Primer 12MER2Rev was used to determine the random peptide sequences of the C7C, linear 7-mer and 12-mer PCR products. Primer Fuse5Rev1 was used for the 15-mer PCR products. Amino acid sequences were deduced from the nucleotide sequences using Lasergene software (DNASTAR Inc., Wisconsin, USA).
Phage ELISA
The ability of the recovered phage-peptides to react with the antibodies used in their selection was confirmed by capture ELISA. A 10 μg/ml stock of appropriate monoclonal antibody was prepared in ELISA coating buffer and 150 μl aliquots were added to wells of an Immulon 4 ELISA plate, and then incubated overnight at 4° C. Unbound antibody was discarded and the wells were blocked with 350 μl of blocking buffer (0.1M NaHCO3/3% skimmed milk powder) for 1 hour at 4° C. The wells were washed three times with 300 μl TBS/0.5% Tween 20 and 100 μl of selected phage culture supernatants were added to appropriate wells for 1 hour at room temperature. Unbound phage were removed and the wells washed six times with 300 μl TBS/0.5% Tween 20. A 1:5000 dilution of horseradish peroxidase (HRP) conjugated anti-M13, secondary antibody was prepared in blocking buffer and 100 μl aliquots added to each well. Following incubation for 1 hour at room temperature the wells were washed six times with 300 μl TBS/0.5% Tween 20. To develop the reactions, a 200 μl aliquot of o-phenylenediamine dihydrochloride (OPD) substrate (Sigma) was added to each well. The absorbance at 450 nm was measured, following incubation at room temperature for 1 hour, by a microplate ELISA reader. An OD450 signal greater than 0.1 was considered positive.
SPOT Synthesis
The peptide mimotopes were synthesised by the SPOT method (Frank; 1992) using F-moc chemistry. The peptides were anchored to an amino-PEG derivatised cellulose membrane (Intavis AG) and synthesised as discrete spots. Synthesis was by stepwise elongation of the peptide chain from C-terminus to the N-terminus, by manual pipetting of the respective amino acids, according to the Sigma Genosys protocol. Incubation times were modified when using Intavis AG membranes. After addition of the final amino acid, the terminal residues and side chains of the peptides were deprotected and the membrane was washed with methanol; air dried and then stored at −20° C. prior to use.
Screening of SPOT Membranes
A membrane was placed in a 50 ml Falcon tube and blocked overnight at 4° C. with gentle agitation in TBS/0.25% Tween 20/5% skimmed milk powder. A 2 μg/ml dilution of the primary (1°) antibody (monoclonal or polyclonal) was prepared in TBS/0.25% Tween20/5% skimmed milk powder before incubation with the membrane for 1 hour at room temperature on a spiral rotor. Following 4 washes, each for 5 minutes with 20 ml of TBS/0.25% Tween20, the membrane was incubated for 1 hour at room temperature with a 1/5000 dilution of the secondary (2°) antibody, prepared in TBS/0.25% Tween20/5% skimmed milk powder. Peroxidase labelled rabbit anti-mouse IgG (Sigma) was used for the detection of the murine anti-Fya (LM487) and anti-Fyb (LM447) 1° antibodies and peroxidase labelled goat anti-human IgG (Pharmacia) for the detection of the human 1° antibodies. Following 4 washes, each for 5 minutes with 20 ml of TBS/0.25% Tween20, the membrane was blotted to remove excess fluid. Bound antibody was detected by chemiluminescence. ECL detection reagents (Amersham Biosciences) were mixed and overlaid on the membrane for 2 minutes. The chemiluminescent substrate was removed and the membrane blotted to remove excess fluid. Positive spots were visualised following exposure to Hyperfilm ECL (Amersham Biosciences) for a series of short incubation times (5 seconds to 10 minutes).
The anti-Fya (LM487) and anti-Fyb (LM447)-specific monoclonal antibodies were developed using immunogenic peptides derived from epitopes on the Duffy glycoprotein (Colligan et al, 1998). These antibodies were used to pan phage libraries for peptides that mimicked the native Duffy antigens. Clones from the final third round of each panning experiment were chosen at random for amplification and DNA sequencing. The amino acid sequences encoded by the phage-peptides were deduced from the DNA sequence data.
Biopanning of each of the 4 phage-peptide libraries was undertaken with the anti-Fyb monoclonal antibody (LM447) and resulted in the identification of multiple copies of particular sequences from each of the libraries. In total, 100 unique Fyb peptide mimotopes were identified (see Table 2/peptides Fyb1-71 & Fyb88-116). None of the sequences identified resembled the 14-mer immunisation peptide (PDGDYDANLEAAAP; CPep75) used in the development of the LM447 antibody. However, analysis of the amino acid sequences of these “Fyb” peptide mimotopes identified 2 consensus sequences:
M(F/Y)QPD(N/P) (P/L) (T/P)T(K/L) (N/Q) (P/V/A/S)
D(H/M/V)HYT(S/N)NTDPL(H/N/R) (A/P/V)P
Certain positions within each consensus sequence had a limited number of amino acids (shown in brackets). These occurred with equal frequency amongst the total population of Fyb peptide mimotopes selected.
Biopanning with the anti-Fya monoclonal, LM487, identified a total of 37 unique peptide sequences (see Table 3/Fya peptides Fya1-37). A small number of the “Fya” phage-peptides, contained sequence identical to the Fya polymorphism, present within the original 14-mer immunisation peptide (PDGDYGANLEAAAP; CPep118), thus validating the use of phage display as an approach to identifying blood group antigen mimics. Sequence analysis of the amino acid sequences of these “Fya” peptide mimotopes also identified 2 putative consensus sequences:
YNYQSYPNPFPV
GI(A/S) (E/D) (D/G)DYGALSW
However, in contrast to the “Fyb” results, many of these “Fya” peptide mimotope sequences did not conform to either consensus sequence identified above.
The data shows that phage-peptides were identified that mimicked the Duffy antigenic epitopes. The anti-Fya antibody bound phage-peptides (Peptides Fya15 & Fya20) that contained sequences identical to the Fya polymorphism and the anti-Fyb antibody selected peptide mimotopes, confirming the use of phage display as a valid means of identifying blood group antigen mimics.
Phage-peptides that mimic RhD antigenic epitopes were identified with 3 monoclonal anti-Ds (T10, T22 & T27). Biopanning of the 4 phage-peptide libraries with this limited number of monoclonal antibodies selected 80 RhD peptide mimotopes (see Table 4/Peptides RhD 1-80). Additionally, biopanning with a polyclonal anti-RhD preparation identified a further 25 RhD peptide mimotopes (see Table 4/peptides RhD81-105). Peptide RhD52 was selected with both T10 and the polyclonal anti-RhD preparation.
Identification of Phage-Peptides which Mimic RhEe Antigenic Epitopes
Biopanning was undertaken with 3 anti-RhE monoclonal antibodies (1×IgG, 2×IgM; see Table 1) and resulted in the identification of multiple copies of many sequences from each of the 4 phage-peptide libraries. Sequence similarities were also evident between the selected phage-peptides from the different libraries. Peptides RhE18 and RhE24 were selected with antibodies E0002 and E0003 and Peptide RhE19 was selected with antibodies E0001 and E0002 These results indicate that, regardless of class, antibodies raised against the same epitope will identify some phage with the identical peptide sequence. In total, 36 unique RhE peptide mimotopes were identified (see Table 5; Peptides RhE1-36). Biopanning with 2 anti-Rhe monoclonal antibodies (2×IgM; see Table 1) identified 17 unique Rhe peptide mimotopes (see Table 5; Peptides Rhe1-17).
The relevance of the phage-peptides selected by biopanning was tested in immunological studies. Initially, 20 “Fyb” phage (Peptides Fyb1-13, Fyb 20-21, Fyb 24-28; see Table 2 for sequence details) were selected for investigation by both phage ELISA and SPOTs analysis, to confirm that synthetic peptides could bind specifically the original monoclonal antibodies.
The phage ELISA results confirmed the binding of the anti-Fyb monoclonal, LM447. A 10 μg/ml stock of this monoclonal antibody was prepared in ELISA coating buffer and 150 μl aliquots were added to wells of an Immulon 4 ELISA plate, and then incubated overnight at 4° C. Unbound antibody was discarded and the wells were blocked with 350 μl of blocking buffer for 1 hour at 4° C. The wells were washed three times with 300 μl TBS/0.5% Tween 20 and 100 μl of selected phage culture supernatants were added to appropriate wells for 1 hour at room temperature. Unbound phage were removed and the wells washed six times with 300 μl TBS/0.5% Tween 20. A 1:5000 dilution of horseradish peroxidase (HRP) conjugated anti-M13, secondary antibody was prepared in blocking buffer and 100 μl aliquots added to each well. Following incubation for 1 hour at room temperature the wells were washed six times with 300 μl TBS/0.5% Tween 20. To develop the reactions, 200 μl aliquots of o-phenylenediamine dihydrochloride (OPD) substrate (Sigma) was added to each well. The absorbance at 450 nm was measured following incubation at room temperature for 1 hour. The absorbance values at 450 nm, confirmed the binding of 14 (Peptides Fyb1, Fyb2, Fyb4-5, Fyb7-13, Fyb25, 27 & 28) of the 20 phage assayed (see
The 20 Fyb peptide mimotopes were synthesised in duplicate, by SPOTs. Binding experiments using the anti-Fyb antibody (LM447) showed that the antibody successfully bound 2 of the 20 peptide sequences (Peptides Fyb12 & Fyb13) tested (
Phage ELISA analysis of 19 “Fya” phage-peptide clones (Peptides Fya1-18 & Fya25; see Table 3 for sequence details) confirmed the binding specificity of the LM487 antibody for the Fya phage-peptides.
These results mirrored the observations obtained for the Fyb phage-peptides when tested in parallel by both phage ELISA and SPOTs. A positive phage ELISA signal did not automatically guarantee a positive signal when the corresponding peptide mimotope was synthesised and tested by SPOTs hybridisation. Moreover, sequence analysis identified no obvious sequence similarities amongst either positive or negative peptides.
The results from both the Fya and Fyb phage ELISA and SPOTs assays suggested that the mode of presentation of the peptide is important in antibody binding. Peptides that form part of a fusion protein in the phage coat may adopt a different conformation when synthesised on a membrane by SPOTs. Without the surrounding protein, antibody recognition may be subsequently affected. Therefore, rather than simply using ELISA to select phage-peptide clones for analysis by SPOTs, all phage-peptides (Fyb, Fya, RhD and RhEe) selected by biopanning were characterised by DNA sequence analysis, prior to SPOTs analysis with the monoclonal antibody originally, used in their selection, in order to ensure no potentially diagnostic peptide was overlooked.
Reciprocal Hybridisation to Identify Cross-Reactive Peptides
A series of 44 peptides was synthesised in duplicate (
Generation of Consensus Sequence-Derived Synthetic Fyb Peptide Mimotopes by Amino Acid Substitution
Sequence analysis of the 100 unique “Fyb” peptide mimotopes (Fyb1-71 & Fyb88-116) had identified 2 consensus sequences. Consensus 1: M(F/Y)QPD(N/P)(P/L)(T/P)T(K/L)(N/Q)(P/V/A/S) contains a series of positions (amino acid 2, 6, 7, 8, 10, 11 & 12) which have a limited number of residues occurring with equal frequency. A series of 16 peptide mimotopes (Peptides Fyb72-87; see Table 2) were synthesised by SPOTs, with amino acid substitutions at positions 2, 6, 8 and 12. On testing, 2 of these consensus sequence-derived peptide mimotopes (Peptide Fyb84; Spot 13 and Peptide Fyb85; Spot 14) proved capable of binding the anti-Fyb antibody LM447 (see
The RhD, RhE and Rhe peptide mimotopes (see Tables 4 & 5) were synthesised by SPOTs and hybridisation of the membranes was undertaken with the appropriate antibodies (Table 1).
A further 28 RhD peptide mimotopes were synthesised by SPOTs. Peptide mimotopes RhD81-105 (Spots 1-25) were originally selected with the polyclonal anti-RhD preparation and Peptides RhD68, RhD71 and RhD79 (Spots 26-28) were identified through biopanning with the anti-RhD antibody T10. The peptides CPep75 (Spot 29) and CPep76 (Spot 30) were included in the assay as negative controls.
Overall, the results achieved with the Rh peptide mimotopes were comparable to those obtained with the Fya and Fyb peptides and Fya and Fyb monoclonal antibodies. SPOTs analysis identified Rh peptides that reacted specifically with their cognitive antibody.
Further confirmation that the peptide mimotopes can specifically bind antibodies raised to native blood group antigens was achieved following the immobilisation of peptide mimotopes onto polystyrene LiquiChip Ni-NTA microsphere beads (Qiagen).
Peptide mimotopes (e.g. Fyb16; RhD12) were synthesised with a C-terminal tag consisting of 6 Histidine amino acids. Stock solutions (4 μg/ml) of the 6×His tagged peptide mimotopes were prepared in PBS/0.1% BSA. The peptides were immobilised onto the beads using the Ni-NTA groups attached to the surface of the beads, as follows. A 50 μl volume of a 4 μg/ml peptide stock (Fyb16 or RhD12) was mixed with an equal volume of LiquiChip Ni-NTA beads (Bead 50 or Bead 52, respectively). Following incubation with mixing at 4° C. overnight, the Bead/peptide mixture was diluted with 900 μl of PBS/0.1% BSA. The Bead/peptide mixture was vortexed for 2 minutes and then a 10 μl volume was aliquoted into the well of a 96-well, round-bottomed. ELISA plate (Nunc). A 10 μl volume of the test monoclonal antibody (LM447: anti-Fyb or T27: anti-RhD) was added to the well of the ELISA plate and the reaction volume was made up to 60 μl with PBS/1% BSA. The plate was incubated at room temperature for 90 minutes, in the dark and then a 10 μl volume (200 ng) of phycoerythrin conjugated sheep anti-mouse IgG (added to wells containing the LM447: anti-Fyb monoclonal antibody) or phycoerythrin conjugated goat anti-human IgM (added to wells containing the T27: anti-RhD monoclonal antibody) was added into the well. The plate was incubated at room temperature for a further 90 minutes, in the dark. The plate was briefly vortexed; then loaded into a Luminex 100 Analyser (Luminex). Confirmation of bound monoclonal antibody was determined by the detection of a phycoerythrin fluorochrome signal. The intensity of the signal equates to the quantity of monoclonal antibody bound to the immobilised peptide mimotope.
The results are set out in
The ability of peptide mimotopes to specifically bind antibodies was further tested using superparamagnetic polystyrene beads (TALON Dynabeads; Invitrogen) in a modified gel agglutination assay.
A 50 μl volume of the magnetic beads was added to 700 μl of TBS (50 mM Tris-HCl pH7.5; 150 mM NaCl). Following mixing for 5 minutes at room temperature, the magnetic beads were isolated using a Magnetic Particle Concentrator (Invitrogen). The supernatant was discarded and the beads were washed for 5 minutes in 700 μl of TBS. The beads were again separated from the buffer using the MPC magnet, and then resuspended in 700 μl of TBS. Peptide mimotopes (e.g. Fyb16; RhD12) were synthesised with a C-terminal tag consisting of 6 Histidine amino acids. Stock solutions (100 μg/ml) of the 6×His tagged peptide mimotopes were prepared in PBS.
A 50 μl volume of diluted TALON beads was placed in the reaction chamber of an NaCl gel card (ID Microtyping System; Diamed), together with a 50 μl volume of diluted His-tagged peptide mimotope (e.g. RhD12) and 100 μl of test human monoclonal antibody (e.g. T27: anti-RhD). The gel card was incubated at 37° C. for 1 hour in an Incubator-ID (DiaMed). Following incubation, the card was centrifuged for 10 minutes in an ID Centrifuge 24S (DiaMed). Antibody-positive reactions are visualised by a layer of beads upon the surface of the gel matrix. In negative reactions, non-agglutinated beads pass through the gel matrix and form a pellet.
To test the binding of a murine monoclonal antibody (e.g. LM447: anti-Fyb monoclonal antibody), a 1 μl volume of sheep anti-mouse IgG is first added to the reaction chamber of an NaCl gel card and allowed to equilibrate with the gel matrix for 10 minutes at room temperature, prior to the addition of a 50 μl volume of diluted TALON beads, a 50 μl volume of diluted His-tagged peptide mimotope (e.g. Fyb16) and 100 μl of the test murine monoclonal antibody (e.g. LM447: anti-Fyb). The gel card was incubated at 37° C. for 1 hour in an ID Incubator (DiaMed). Following incubation, the card was centrifuged for 10 minutes in an ID centrifuge (DiaMed). Again the reaction was read visually.
These examples demonstrate that the synthetic peptide mimotopes are capable of specifically binding antibodies raised to native blood group antigens even when bound to a solid support. In these examples the solid support used is polystyrene Ni-NTA microsphere beads or supermagnetic polystyrene beads but it is envisaged that the synthetic peptide mimotopes will be capable of specifically binding antibodies raised to native blood group antigens when immobilised on any suitable support.
The results are identified in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0706558.4 | Apr 2007 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB08/01200 | 4/3/2008 | WO | 00 | 2/25/2010 |
