Failure to find holes in the T-cell repertoire

A journey to your self: The vague definition of immune self and its practical implications

The identification of immunogenic peptides has become essential in an increasing number of fields in immunology, ranging from tumor immunotherapy to vaccine development. The nature of the adaptive immune response is shaped by the similarity between foreign and self-protein sequences, a concept extensively applied in numerous studies. Can we precisely define the degree of similarity to self? Furthermore, do we accurately define immune self? In the current work, we aim to unravel the conceptual and mechanistic vagueness hindering the assessment of self-similarity. Accordingly, we demonstrate the remarkably low consistency among commonly employed measures and highlight potential avenues for future research.

CXCR3 Identifies Human Naive CD8+ T Cells with Enhanced Effector Differentiation Potential

In mice, the ability of naive T (TN) cells to mount an effector response correlates with TCR sensitivity for self-derived Ags, which can be quantified indirectly by measuring surface expression levels of CD5. Equivalent findings have not been reported previously in humans. We identified two discrete subsets of human CD8+ TN cells, defined by the absence or presence of the chemokine receptor CXCR3. The more abundant CXCR3+ TN cell subset displayed an effector-like transcriptional profile and expressed TCRs with physicochemical characteristics indicative of enhanced interactions with peptide-HLA class I Ags. Moreover, CXCR3+ TN cells frequently produced IL-2 and TNF in response to nonspecific activation directly ex vivo and differentiated readily into Ag-specific effector cells in vitro. Comparative analyses further revealed that human CXCR3+ TN cells were transcriptionally equivalent to murine CXCR3+ TN cells, which expressed high levels of CD5. These findings provide support for the notion that effector differentiation is shaped by heterogeneity in the preimmune repertoire of human CD8+ T cells.

Selection of immunodominant epitopes during antigen processing is hierarchical

MHC II proteins present processed antigens to CD4 + T cells through a complex set of events and players that include chaperons and accessory molecules. Antigen processing machinery is optimized for the selection of the best fitting peptides, called 'immunodominant epitopes', in the MHC II groove to which, specific CD4 + T cells respond and differentiate into memory T cells. However, due to the complexity of antigen processing, understanding the parameters that lead to immunodominance has proved difficult. Moreover, immunodominance of epitopes vary, depending on multiple factors that include; simultaneous processing of multiple proteins, involvement of multiple alleles of MHC II that can bind to the same antigen, or competition among several suitable epitopes on a single protein antigen. The current dogma assumes that once an antigenic determinant is selected under a specific condition, it would emerge immunodominant wherever it is placed. Here we will discuss some established parameters that contribute to immunodominance as well as some new findings, which demonstrate that slight changes to antigen structure can cause a complete shift in epitope selection during antigen processing and distort the natural immunodominant epitope.

Distorted Immunodominance by Linker Sequences or other Epitopes from a Second Protein Antigen During Antigen-Processing

The immune system focuses on and responds to very few representative immunodominant epitopes from pathogenic insults. However, due to the complexity of the antigen processing, understanding the parameters that lead to immunodominance has proved difficult. In an attempt to uncover the determinants of immunodominance among several dominant epitopes, we utilized a cell free antigen processing system and allowed the system to identify the hierarchies among potential determinants. We then tested the results in vivo; in mice and in human. We report here, that immunodominance of known sequences in a given protein can change if two or more proteins are being processed and presented simultaneously. Surprisingly, we find that new spacer/tag sequences commonly added to proteins for purification purposes can distort the capture of the physiological immunodominant epitopes. We warn against adding tags and spacers to candidate vaccines, or recommend cleaving it off before using for vaccination.

Junctional diversity prevents negative selection of an antigen-specific T cell repertoire

Endogenous mouse mammary tumor proviruses (MMTV; Mtv loci) deletes Vbeta6 expressing T cells in the thymus of Mtv-7(+) DBA/2 (H2(d)) mice through negative selection. We found that in Mtv-7(-) BALB/c (H2(d)) mice, Vbeta6 is a dominant V gene used in T cell responses to an 18 amino acid long peptide antigen: EYKEYAEYAEYAEYAEYA [abbreviated as K5 or EYK(EYA)(5)]. It was therefore surprising to find that despite the deletion of Vbeta6+ T cells, vigorous K5 specific T cell responses that use Vbeta6 can be raised in DBA/2 mice. Sequence analysis of Vbeta6 junctional diversity in K5 specific T cell lines revealed that the DBA/2 K5 repertoire compensates for the loss of most Vbeta6 T cells by overusing and amplifying Vbeta6+ T cells escaping central deletion and peripheral tolerization. In order to address the inability of some Vbeta6 T cells to recognize Mtv-7(+) we analyzed a panel of BALB/c Vbeta6 expressing T cell hybridomas. This data supported the argument that certain Vbeta6 junctional sequences preclude Mtv recognition and allows their escape from central deletion in DBA/2 mice. These cells are not anergic and can be activated with cognate peptide antigen in periphery. We suggest that junctional diversity at the V region of some of the T cell receptors does not allow these cells to recognize self-superantigens with high enough affinity and thus they escape negative selection in the thymus. These results for the first time provide a molecular explanation of how the immune system compensates for "hole in the repertoire" caused by deletion of the majority of T cells carrying certain V region segments.

Negative selection imparts peptide specificity to the mature T cell repertoire

The T cell alphabeta receptor (TCR) recognizes foreign peptide antigens bound to proteins encoded in the MHC. The MHC portion of this complex contributes much to the footprint of the TCR on the ligand, yet T cells are usually very specific for individual foreign peptides. Here, we show that the development of peptide-specific T cells is not intrinsic to thymocytes that undergo thymic-positive selection but is an outcome of eliminating, through negative selection, thymocytes bearing TCRs with extensive peptide cross-reactivity. Hence, thymic-negative selection imposes peptide specificity on the mature T cell repertoire.

Alternate interactions define the binding of peptides to the MHC molecule IA(b)

We have solved the crystal structure of the MHCII molecule, IA(b), containing an antigenic variant of the major IA(b)-binding peptide derived from the MHCII IEalpha chain. The four MHC pockets at p1, p4, p6, and p9 that usually bind peptide side chains are largely empty because of alanines in the peptide at these positions. The complex is nevertheless very stable, apparently because of unique alternate interactions between the IA(b) and peptide. In particular, there are multiple additional hydrogen bonds between the N-terminal end of the peptide and the IA(b) alpha chain and an extensive hydrogen bond network involving an asparagine at p7 position of the peptide and the IA(b) beta chain. By using knowledge of the shape and size of the traditional side chain binding pockets and the additional possible interactions, an IA(b) peptide-binding motif can be deduced that agrees well with the sequences of known IA(b)-binding peptides.

MHC polymorphism can enrich the T cell repertoire of the species by shifts in intrathymic selection

The murine class I molecule H-2Kb and its natural gene conversion variant, H-2Kbm8, which differs from H-2Kb solely at 4 aa at the bottom of the peptide-binding B pocket, are expressed in coisogenic mouse strains C57BL/6 (B6) and B6.C-H-2bm8 (bm8). These two strains provide an excellent opportunity to study the effects of Mhc class I polymorphism on the T cell repertoire. We recently discovered a gain in the antiviral CTL repertoire in bm8 mice as a consequence of the emergence of the Mhc class I allele H-2Kbm8. In this report we sought to determine the mechanism behind the generation of this increased CTL diversity. Our results demonstrate that repertoire diversification occurred by a gain in intrathymic positive selection. As previously shown, the emergence of the same Mhc allele also caused a loss in positive selection of T cell repertoire specific for another Ag, OVA-8. This indicates that a reciprocal loss-and-gain pattern of intrathymic selection exists between H-2Kb and H-2Kbm8. Therefore, in the thymus of an individual, a new Mhc allele can select new T cell specificities, while abandoning some T cell specificities selected by the wild-type allele. A byproduct of this repertoire shift is a net gain of T cell repertoire of the species, which is likely to improve its survival fitness.

Assessment of the Role of Determinant Selection in Genetic Control of the Immune Response to Insulin in H-2b Mice

The immune response to insulin is regulated by MHC class II genes. Immune response (Ir) gene-linked low responsiveness to protein Ags can be mediated by the low affinity of potential antigenic determinants for MHC molecules (determinant selection) or by the influence of MHC on the functional T cell repertoire. Strong evidence exists that determinant selection plays a key role in epitope immunodominance and Ir gene-linked unresponsiveness. However, the actual measurement of relative MHC-binding affinities of all potential peptides derived from well-characterized model Ags under Ir gene regulation has been very limited. We chose to take advantage of the simplicity of the structure of insulin to study the mechanism of Ir gene control in H-2b mice, which respond to beef insulin (BINS) but not pork insulin (PINS). Peptides from these proteins, including the immunodominant A(1–14) determinant, were observed to have similar affinities for purified IAb in binding experiments. Functional and biochemical experiments suggested that PINS and BINS are processed with similar efficiency. The T cell response to synthetic pork A(1–14) was considerably weaker than the response to the BINS peptide. We conclude that the poor immunogenicity of PINS in H-2b mice is a consequence of the T cell repertoire rather than differences in processing and presentation.

Identification of Streptococcus mutans PAc peptide motif binding with human MHC class II molecules (DRB1*0802, *1101, *1401 and *1405)

A surface protein antigen (PAc) of Streptococcus mutans, in particular the A-region of this PAc molecule, has been noted as a possible target in research for an effective dental caries vaccine. To identify the antigenic peptide binding to major histocompatibility complex (MHC) class II (HLA-DR) molecules in the A-region, we prepared a panel of overlapping synthetic peptides in the second unit of the A-region, and established that a simple enzyme-linked immunosorbent assay (ELISA) binding assay could be achieved by incubating the DR-crude. Binding to DR molecules of these peptides from nine donors was investigated by using the ELISA binding assay. It was revealed that the PAc(316-334) peptide bound more strongly to the HLA-DR molecule in seven out of nine subjects. In particular, DR8 (DRB1*0802), DR5 (DRB1*1101) and DR6 (DRB1*1402 and *1405), which bound strongly to PAc(316-334) peptide, were identified. Moreover, we synthesized glycine-substituted peptide analogues of the peptide and examined the binding motif of the binding region. As a result, the multiple binding motif in DR8, DR5 and DR6 was found in L-RV-K-A. It is suggested that a peptide vaccine for dental caries that is more effective for humans, with fewer adverse side-effects, could be designed by combining the multiple binding motif with the B-cell epitope to produce only the inhibiting antibody against dental caries. The peptide could therefore be useful for peptide vaccine development in the general human population.

Interaction of pigeon cytochrome c-(43-58) peptide analogs with either T cell antigen receptor or I-Ab molecule

We determined that a pigeon cytochrome c-derived peptide, p43-58, possesses two anchor residues, 46 and 54, for binding with the I-Ab molecule that are compatible to the position 1 (P1) and position 9 (P9) of the core region in the major histocompatibility complex (MHC) class II binding peptides, respectively. In the present study to analyze each binding site between P1 and P9 of p43-58 to either I-Ab or T cell antigen receptor (TCR), we investigated T cell responses to a series of peptides (P2K, P3K, P4K, P5K, P6K, P7K, and P8E) that sequentially substituted charged amino acid residues for the residues at P2 to P8 of p43-58. T cells from C57BL/10 (I-Ab) mice immunized with P4K or P6K did not mount appreciable proliferative responses to the immunogens, but those primed with other peptides (P2K, P3K, P5K, P7K, and P8E) showed substantial responses in an immunogen-specific manner. It was demonstrated by binding studies that P1 and P9 functioned as main anchors and P4 and P6 functioned as secondary anchors to I-Ab. Analyses of Vbeta usage of T cell lines specific for these analogs suggested that P8 interacts with the complementarity-determining region 1 (CDR1)/CDR2 of the TCR beta chain. Furthermore, sequencing of the TCR on T cell hybridomas specific for these analogs indicated that P5 interacts with the CDR3 of the TCR beta chain. The present findings are consistent with the three-dimensional structure of the trimolecular complex that has been reported for TCR/peptide/MHC class I molecules.

T cells can be activated by peptides that are unrelated in sequence to their selecting peptide

We tested the ability of CD4+ T cells, selected in the thymus by reaction with class II protein bound to a single peptide, to react with the same class II protein bound to other peptides. The T cells reacted with all peptides tested, including one that was quite unlike the selecting peptide in T cell receptor binding residues. The receptors on class II/peptide-reactive T cells from class II/single peptide mice were similar but not identical to some of those from normal animals. Thus, class II bound to a single peptide selects a subset of T cells that is related to that selected by class II bound to many peptides.

Determination of the allele-specific antigen-binding site on I-Ak and I-Ab molecules

Residues 46 and 54 on a pigeon cytochrome c 43-58 analog, 50E, function as major histocompatibility complex class II contact sites. A peptide, 46F50E54A, with phenylalanine (F) at position 46 and alanine (A) at 54 on 50E bound to Ab and a peptide, 46D50E54A, with aspartic acid (D) at 46 and alanine at 54, bound to Ak. To determine the allele-specific peptide contact sites on I-A molecules corresponding to the I-A contact sites of the peptides, we analyzed responses of Ak- and/or Ab-restricted T cell hybridomas to 46F50E54A or 46D50E54A using L cell transfectants expressing recombinant I-A molecules between Ak and Ab or point mutants of Ak as antigen presenting cells. It was shown that the N-terminal half of the alpha helix of the A alpha chain determined the allele-specific T cell responses. Furthermore, with arginine (k type amino acid) or alanine (b type amino acid) at position 56 of the Ak alpha chain, these T cell hybridomas were stimulated predominantly by 46D50E54A (Ak binding peptide) or 46F50E54A (Ab binding peptide), respectively. Thus, the amino acid at position 56 of the A alpha chain determines allele-specific antigen presentation. This postulate was confirmed by direct binding analysis of 50E analogs of various I-A molecules. A single amino acid change (arginine to alanine) at position 56 of the Ak alpha chain altered the peptide binding specificity (46D50E54A to 46F50E54A).

A promiscuous T cell hybridoma restricted to various I-A molecules

In a previous study, we identified T cell receptor and major histocompatibility complex (MHC) contact sites on the pigeon cytochrome c p43-58 peptide. Positions 46 and 54 of p43-58 were shown to be the MHC-binding sites. Specific amino acids were identified on the MHC-binding sites which bound to the relevant I-A molecule. In the present study, using NOD (I-Ag7) mice, we established a T cell hybridoma specific for a p43-58 analog 46R50E54A with arginine (R) and alanine (A) at positions 46 and 54, respectively. Interestingly, NOE 33-1-2 recognized 46R50E54A in the presence of not only I-Ag7, but also I-Ad, s, u and v. In contrast to previous reports that promiscuous T cells were able to recognize peptide antigens with various HLA-DR or I-E molecules consist of monomorphic alpha and polymorphic beta chains, the promiscuous T cell clone NOE33-1-2 recognized peptides with various I-A molecules lacking the monomorphic chain.

T cell receptor repertoire for a viral epitope in humans is diversified by tolerance to a background major histocompatibility complex antigen

Two unusual characteristics of the memory response to the immunodominant Epstein-Barr virus (EBV) epitope FLRGRAYGL, which associates with HLA B8, have provided an unique opportunity to investigate self tolerance and T cell receptor (TCR) plasticity in humans. First, the response is exceptionally restricted, dominated by cytotoxic T lymphocytes (CTL) with identical TCR protein sequences (Argaet, V. P., C. W. Schmidt, S. R. Burrows, S. L. Silins, M. G. Kurilla, D. L. Doolan, A. Suhrbier, D. J. Moss, E. Kieff, T. B. Sculley, and I. S. Misko. 1994. J. Exp. Med. 180:2335-2340). Second, CTL expressing this receptor are cross-reactive with the alloantigen HLA B* 4402 on uninfected cells (Burrows, S. R., R. Khanna, J. M. Burrows, and D. J. Moss. 1994. J. Exp. Med. 179:1155-1161). No CTL using this conserved public TCR could be reactivated from the peripheral blood of EBV exposed individuals expressing both HLA B8 and B*4402, demonstrating the clonal inactivation of potentially self-reactive T cells in humans. A significant FLRGRAYGL-specific response was still apparent, however, and TCR sequence analysis of multiple CTL clones revealed an oligoclonal TCR repertoire for this determinant within these individuals, using diverse V and J gene segments and CDR3 regions. In addition, a significant public TCR component was identified in which several distinct alpha/beta rearrangements are shared by CTL clones from a number of unrelated HLA B8+, B*4402+ donors. The striking dominance of public TCR in the response to this EBV epitope suggests a strong genetic bias in TCR gene recombination. Fine specificity analysis using peptide analogues showed that, of six different antigen receptors for FLRGRAYGL/HLA B8, none associate closely with the peptide's full array of potential TCR contact residues. Whereas the HLA B*4402-cross-reactive receptor binds amino acids toward the COOH terminus of the peptide, others preferentially favor an NH2-terminal determinant, presumably evading an area that mimics a structure presented on HLA B*4402. Thus, tolerance to a background major histocompatibility antigen can effectively diversify the TCR repertoire for a foreign epitope by deflecting the response away from an immunodominant combination of TCR-binding residues.

Fine structure of a virus-encoded helper T-cell epitope expressed on FBL-3 tumor cells

Antigen peptide fn20 representing Friend murine leukemia virus env122-141 (DEPLTSLTPRCNTAWNRLKL) is recognized by two independent Friend virus-induced, FBL-3 tumor-specific helper T-cell (Th) clones. We isolated more Th clones from mice immunized with fn20 peptide. We examined the fine structure of the peptide required to activate a large group of fn20-specific Th clones. A systematic analysis of peptides of decreasing lengths eliciting Th proliferation defined the minimum core length as 13 amino acids (LTSLTPRCNTAWN). Functional proliferation and competition assays with variant peptides with alanine substitutions permitted the assignment of five peptide residues in two major histocompatibility complex-interacting and three T-cell-receptor-interacting sites. Th clones were different in their reactivities toward peptides of various lengths and the variant peptides.

Determination of the agretopic residues of a peptide co-restricted to different class II isotypes, I-Au and I-Eu, and its application for preparation of a synthetic peptide vaccine against influenza virus A/Aichi/2/68

We have defined that residues 46 and 54 on a synthetic peptide composed of residues 43-58 of pigeon cytochrome c (p43-58) work as agretopes (sites bound to an MHC molecule) in I-Ab mice. Substitution of amino acid residues on these positions altered the peptide to bind with the other MHC molecules. Furthermore, by substituting the agretopic residues with a variety of amino acids, we could determine the class II binding motif for each MHC molecule. In the present study, immunogenicity of a peptide, 46R50V54A, carrying valine (V) at epitopic (site bound to TCR) position 50, arginine (R) and alanine (A) at agretopic positions 46 and 54 of the p43-58, respectively has been analyzed in B10.PL (H-2u) mice. We found that this peptide bound to two different class II isotypes, I-Au and I-Eu. Arginine at position 46 or alanine at position 54 of the 46R50V54A was shown to be critical for binding to I-Au or I-Eu, respectively. Further, on the basis of this class II binding motif we could prepare potent peptide vaccines against influenza A/Aichi/2/68 virus in B10.PL mice.

A potential peptide vaccine against two different strains of influenza virus isolated at intervals of about 10 years

We have developed a strategy for making synthetic peptide vaccines, in which a peptide, HA127-133, derived from the hemagglutinin (HA) of A/Aichi/2/68(H3N2) influenza virus (Aichi/68) is introduced into the Ab binding component consisting of 43-46 and 54-58 residues of a pigeon cytochrome c analogue peptide, 46F50V54A. Indeed, this hybrid peptide, 46F/HA127-133/54A, induced impressive T-cell responses and antibody production neutralizing infectivity of Aichi/68 in vitro. In a subsequent study we found that 46F/HA127-133/54A(18mer) peptide antigen, which had been prepared by substitution at the central five residues of 46F50V54A with HA127-133, generated T-cell responses and neutralizing antibody responses as well. On the basis of these prior findings, in the present study we analyzed immunopotency of 46F/HA127-133/54A(18mer) in vivo administered in several ways to I-Ab mice. We show herein that this peptide vaccine loaded in multilamellar liposomes without adjuvant protects the mice against infection with Aichi/68 within 2 weeks after final immunization. Further, this peptide vaccine was shown to be effective in preventing infection with a naturally occurring antigenic variant, A/Texas/1/77(H3N2), carrying the same sequence at 127-133 of the HA as Aichi/68 virus. Since this part of the HA is relatively conserved among H3 subtype influenza viruses, our peptide vaccine may become the basis for a new strategy to prepare effective vaccines that will overcome the ineffectiveness of classical vaccines attributable to antigenic drift of influenza viruses.

Fine specificity of CD4+ T cell responses to the dominant encephalitogenic PLP 139-151 peptide in SJL/J mice

PLP 139-151(S) is the major encephalitogenic epitope of PLP in the SJL/J mouse. CD4+ T cells specific for PLP 139-151(S) induce a relapsing-remitting form of EAE which is similar to the human demyelinating disease MS in both clinical course and histopathology. We are interested in events involved in activation of autoreactive T cells and how to specifically regulate these immune response to both prevent and treat ongoing demyelinating disease. In the current study, we examined the effect of both amino acid substitutions and deletions in the native PLP 139-151(S) peptide to identify which residues are critical for immunogenicity and encephalitogenicity. Conservative and nonconservative substitutions at position 145 diminished or completely destroyed the encephalitogenic potential of the peptide without affecting the ability to recall a proliferative response in lymph node T cells primed with the native PLP 139-151(S) peptide indicating an interesting dichotomy between ability to induce T cell proliferation and ability to induce active clinical disease. In addition, tryptophan at position 144 was identified as a critical TCR contact site as a peptide containing an alanine for tryptophan at this position (A144) primed a unique population of T cells which did not cross react with the native PLP 139-151(S). In addition, A144 was unable to stimulate PLP 139-151(S)-specific T cells in vitro or to induce active relapsing EAE in vivo. The significance of these results to the potential development of new strategies for preventing and treating T cell-mediated autoimmune diseases is discussed.

Analysis of epitopic residues introduced into the hybrid peptide vaccines prepared according to the cassette theory

In our previous study, we prepared a synthetic peptide vaccine (46F/HA127-133/54A) against influenza strain A/Aichi/2/68 (H3N2) virus by introducing haemagglutinin (HA) 127-133 to an I-Ab,b binding component that consisted of residues 43-46 and 54-58 of an I-Ab,d binding peptide, 46F50V54A. This hybrid peptide vaccine induced considerable immunological responses against A/Aichi/2/68 as well as against the peptide vaccine in I-Ab mice. In the present study, we have attempted to increase the immunogenicity of the peptide vaccine by introducing HA peptides of various lengths into the I-Ab,d binding components consisting of residues 43-46 and 54-58 or 43-47 and 53-58 of 46F50V54A. We demonstrate here that, among the peptide vaccines prepared, 46F/HA127-133/54A (18 mer) consisting of HA127-133 and the I-Ab,d binding component constructed from 43-47 and 53-58 of 46F50V54A induces the most vigorous T-cell responses and neutralizing antibodies against A/Aichi/2/68 in both I-Ab and I-Ad mice.

Determination of amino acids on agretopes of pigeon cytochrome c-related peptides specifically bound to I-A allelic products

In our prior study it was demonstrated that residues 46 and 54 on a synthetic peptide, AEGFSYTVANKNKGIT (50V), work as an agretope (site contacts with major histocompatibility complex molecules) and residues 50 and 52 function as an epitope (site contacts with T cell receptor), when tri-molecular complexes are formed among 50V,I-Ab and the T cell receptor. 50V was composed of residues 43 to 58 of pigeon cytochrome c (p43-58) except that the aspartic acid (D) at residue 50 was substituted by valine (V). Substitution of agretopic residues on 50V changed this I-Ab-binding peptide to an I-Ak-binding peptide, suggesting that positions 46 and 54 work as an agretope in I-Ak-restricted T cell responses. In the present study we examined whether residues 46 and 54 of 50V worked as agretopes in T cell responses restricted to other I-A haplotypes. The 50V-related peptides with phenylalanine (F) at position 46 and alanine (A) at position 54 bound tightly to I-Ab, I-Ad, I-Aq and I-As molecules and stimulated T cells most potently in mice bearing these I-A haplotypes. In contrast, 50V-related peptides carrying D at position 46 and A at position 54 bound most potently to I-Ak molecules, and the peptides with arginine (R) at position 46 and A at position 54 bound most efficiently to I-Av molecules. The present findings, thus, demonstrate that the agretopic positions on the p43-58 related peptides are preserved in T cell responses restricted to each I-A haplotype studied, and that the specific amino acids on the agretopic positions exist a priori for each I-A allele-specific structure.

A strategy for making synthetic peptide vaccines

We have determined the H-2 class II allele-specific amino acid motif of the agretope (the site of contact between the peptide antigen and the major histocompatibility complex) for a synthetic peptide composed of residues 43-58 of pigeon cytochrome c (p43-58). Residues 46 and 54 functioned as the agretope, and residues 50 and 52 functioned as the epitope (the site for contact between the peptide antigen and the T-cell antigen receptor). In general, agretopes and epitopes function independently. Thus, substitution of amino acids in the epitope does not significantly affect binding of the peptide antigen to a class II molecule. On the basis of these findings, synthetic peptide vaccines against influenza Aichi (H3N2) virus were prepared by introducing seven residues of the influenza virus hemagglutinin into the frame component residues 43-46 and 54-58 of p43-58 analogues including the agretopes for Ak or Ab previously determined on the p43-58 segment. These peptide vaccines induced both helper T-cell responses and production of antibodies that were specific for influenza Aichi hemagglutinin but not for the major histocompatibility complex binding frame in mice bearing Ak or Ab. The antibodies produced neutralize the infectivity of influenza Aichi in vitro. The present findings should provide a basis for preparing potent peptide vaccines that function without producing side effects.

Nonresponsiveness to an immunodominant Epstein-Barr virus-encoded cytotoxic T-lymphocyte epitope in nuclear antigen 3A: implications for vaccine strategies

An immunodominant Epstein-Barr virus (EBV)-encoded cytotoxic T lymphocyte (CTL) epitope has been mapped to the EBV nuclear antigen 3A. The epitope, represented by the peptide sequence AWNAGFLRGRAYGLD (hereafter termed AWNA), is restricted through the HLA-B8 allele and is expressed by type A but not type B-infected transformants. Herein, we show that EBV-specific memory CTLs from an HLA-B8+ healthy virus carrier, JS, did not respond in vitro to AWNA, even though that individual's endogenously infected transformants processed and presented the natural equivalent of this peptide to AWNA-specific CTLs from another B8+ individual. Instead, an epitope, represented by the peptide sequence QLSDTPLIPLTIFVGENTGV, was the dominant EBV-specific CTL epitope in donor JS. This epitope mapped to EBV nuclear antigen 2A, was restricted by an HLA-A2 subtype, and specifically associated with type A strains of EBV. No AWNA-specific CTL precursors were detected by limiting dilution analysis of peripheral blood mononuclear cells from donor JS whereas the precursor frequency of AWNA-specific CTLs from a responder donor, LC, was estimated at 1:4500. The presentation in vivo of an immunogenic epitope-HLA antigen complex is clearly insufficient to guarantee an effective memory CTL response to that foreign epitope. Thus, vaccination strategies based on peptides inducing CTL responses may need to take into account not only the polymorphism of HLA antigens but also possible allelic variation in the repertoires of T-cell receptors.

Ia restriction specificity of KLH-specific T cells from allogeneic bone marrow chimeras is influenced by histocompatibility at the H-2 and minor histocompatibility loci

Ia restriction specificity involved in T cell proliferative responses to keyhole limpet hemocyanin (KLH) has been analyzed using a variety of allogeneic bone marrow chimeras. The chimeric mice were prepared by reconstituting irradiated AKR, SJL, B10.BR and B10.A(4R) mice with bone marrow cells from B10 mice. When such chimeric mice had first been primed with KLH in complete Freund's adjuvant (CFA), T cells from H-2 incompatible fully allogeneic chimeras showed significantly higher responses to KLH in the presence of antigen-presenting cells (APC) of donor strain (B10) than APC of recipient strain. However, in H-2 subregion compatible chimeras, [B10----B10.A(4R)], which were matched at the H-2D locus and at minor histocompatible loci, the T cells could mount vigorous responses to KLH with antigen-presenting cells (APC) of either donor or recipient type. The same results were obtained as well with chimeras that had been thymectomized after full reconstitution of lymphoid tissues by donor-derived cells. A considerable proportion of KLH-specific T cell hybridomas established from [B10----B10.A(4R)] chimeras exhibited both I-Ab and I-Ak restriction specificities. The present findings indicate that the bias to donor Ia type of antigen specific T cells is determined by donor-derived APC present in the extrathymic environment but that cross-reactivity to the recipient Ia is influenced to some degree by histocompatibility between donor and recipient mice, even though the histocompatible H-2D locus and minor histocompatibility loci seem not to be directly involved in the I-A restricted responses studied herein.

Allelic variations of human TCR V gene products

A central problem confronting the immune system is how to discriminate among vast numbers of antigens. Novel genetic ploys that aid the discriminative process, including complex gene rearrangements (in antibody and T-cell receptor (TCR) genes) and extensive allelic polymorphism (in major histocompatibility complex (MHC) genes), have been described. Recent evidence has suggested a further level of diversity; TCR V gene allelic variation. In this article David Posnett summarizes evidence in favour of this possibility and speculates on the possible functional consequences of TCR allelism.

Sequential analysis of the thymocyte differentiation in fully allogeneic bone marrow chimera in mice. I. Relationship between functions and surface characteristics of thymocytes

Differentiation of thymocytes according to surface phenotype, functional status and cell size was investigated using fully allogeneic bone marrow chimeras. Most of the donor-derived thymocytes obtained from chimeras 9 days after hematopoietic reconstitution were CD4-8- and IL2R+. At day 14, CD4+8+ cells became prominent in the thymus. Eighty-six per cent of thymocytes were CD4+8+ and 9% were CD4-8- at this stage. After day 21, the proportion of CD4+8- or CD4-8+ single positive cells transiently increased and then declined to normal level at day 42. Further, the mean size of CD4+ or CD8+ single positive cells in chimeric thymuses at day 21 after reconstitution was markedly larger than that at day 35. When proliferative responses to various stimuli (PMA + rIL2, anti-CD3 mAb (2C11) and anti-V beta 8 mAb (F23.1] were evaluated, significant responses were generated by thymocytes for the first time at around day 28 and the responses reached their peaks at day 35. These findings demonstrated that the process of thymocyte differentiation in the fully allogeneic chimeras was similar to ontogenic development as observed in fetal mice. However, the tempo at which the differentiation of surface phenotypes and development of functions proceeded was quite different from that seen in normal mice. The relationship among surface phenotypes, cell size and functions of developing thymocytes of bone marrow chimeras is discussed.

The primary lesion theory of autoimmunity: a speculative hypothesis

Autoaggression and autoimmune attack are established vernacular in the literature of autoimmune disease, and reflect the popular view that autoimmunity is a disease of immune dysregulation in which the immune system inappropriately attacks healthy tissues. The aim of this article is to focus attention on an alternative, indeed opposite, view--that autoimmunity represents the response to a primary lesion in the target tissue, rather than its cause, and that like alloimmunity, autoimmunity is physiological appropriate and protective. The cell death and tissue damage which results is characteristic of an immune response programmed to eliminate immunogen, remove detritus and isolate the lesion.

Structural requirements for class I MHC molecule-mediated antigen presentation and cytotoxic T cell recognition of an immunodominant determinant of the human immunodeficiency virus envelope protein

In H-2d mice, the immunodominant determinant of the HIV-1-IIIB gp160 envelope glycoprotein recognized by CD8+ CTL is represented by a 15-residue synthetic peptide (315-329: RIQRGPGRAFVTIGK). This peptide is seen in association with the Dd class I MHC molecule expressed on H-2k L cell fibroblast targets. We explored the structural requirements for CTL recognition of this peptide at the levels of both the peptide molecule and the class I MHC molecule. Using several transfectants expressing recombinant Dd/Ld molecules, we found that presentation of this epitope required both the alpha 1 and alpha 2 domains of the Dd molecule, in contrast to certain instances of allorecognition for which alpha 1 of Dd was sufficient in association with alpha 2 of Ld. Because this peptide derives from a hypervariable segment of the HIV envelope, substituted peptides could be used to define not only the structures affecting interaction of peptide with class I MHC molecule and with the TCR, but also the structural basis for the effect of naturally occurring viral variation on CTL recognition. The CTL-LINE specific for this HIV-1-IIIB-derived sequence could not recognize the HIV-1-RF variant-derived sequence from exactly the same site (315-329:--HIGPGRVIYATGQ). Peptides with single amino acid substitutions from the HIV-1-IIIB sequence toward the HIV-1-RF sequence were made to test the effect of each residue significantly affected recognition, and only one, 324(F), was obligatory. Moreover, both 322(R) and 324(F) substituted peptides failed to inhibit the binding of the wild type peptide to the MHC molecule. Therefore, the amino-acids 322(R) and 324(F) seem to be involved in regulating peptide interaction with the Dd class I MHC molecule. In contrast, 325(V) appeared to affect interaction with the TCR. We suggest that sequence variations among known HIV-1 isolates that affect peptide binding to MHC such as those described here, if occurring during the course of infection of an individual, could result in failure of the MHC molecules of that individual to present the peptide. If the number of dominant HIV CTL epitopes is indeed very limited, such a blind spot could allow the virus to escape immune control, proliferate rapidly, and cause AIDS.

A single amino acid interchange yields reciprocal CTL specificities for HIV-1 gp160

For the IIIB isolate of human immunodeficiency virus type-1 (HIV-1), the immunodominant determinant of the envelope protein gp160 for cytotoxic T lymphocytes (CTLs) of H-2d mice is in a region of high sequence variability among HIV-1 isolates. The general requirements for CTL recognition of peptide antigens and the relation of recognition requirements to the natural variation in sequence of the HIV were investigated. For this purpose, a CTL line specific for the homologous segment of the envelope from the MN isolate of HIV-1 and restricted by the same class I major histocompatibility (MHC) molecule (Dd) as the IIIB-specific CTLs was raised from mice immunized with MN-env-recombinant vaccinia virus. The IIIB-specific and MN-specific CTLs were completely non-cross-reactive. Reciprocal exchange of a single amino acid between the two peptide sequences, which differed in 6 of 15 residues, led to a complete reversal of the specificity of the peptides in sensitizing targets, such that the IIIB-specific CTLs lysed targets exposed to the singly substituted MN peptide and vice versa. These data indicate the importance of single residues in defining peptide epitopic specificity and have implications for both the effect of immune pressure on selection of viral mutants and the design of effective vaccines.

Specific immune responses restored by alteration in carbohydrate chains of surface molecules on antigen-presenting cells

Two class I major histocompatibility (MHC) mutant mouse strains, H-2bm14 and H-2bm6, differ from the strain of origin C57BL/6 (B6, H-2b) in one and two amino acids of the H-2Db and H-2Kb molecule, respectively. The bm14 Db mutation results in specific failure of female bm14 mice to generate a cytotoxic T lymphocyte (Tc) response to the male-specific antigen H-Y. The allospecific Tc response of CD8+ B6T cells against bm6 Kb mutant spleen cells, in contrast to that against other Kb mutants, is absolutely CD4+ T helper cell dependent. Purified CD8+ T cells completely fail to respond. We now report that the inability to mount these specific immune responses is restored by the use of dendritic cells (DC) as antigen-presenting cells (APC). Comparison of MHC expression on various types of APC by cytofluorimetry and quantitative immunoprecipitation showed very high expression of class I and class II MHC molecules on DC. Strikingly, examination of class I and class II molecules by isoelectric focusing revealed qualitative differences as well. We show that the surface MHC class I molecules of DC are present in greater quantity and carry on average fewer sialic acids than the same molecules isolated from other APC types such as spleen cells, lipopolysaccharide blasts or concanavalin A blasts. That sialic acids on cell surface molecules, including MHC, may play a role in antigen presentation is suggested by our finding that removal of sialic acids, by neuraminidase, can restore specific responses to nonresponder APC as well.

Vaccine T-cell epitope selection by a peptide competition assay

The binding of several peptides derived from the Plasmodium falciparum circumsporozoite protein (CS protein) to the human major histocompatibility complex class II proteins HLA-DR5 and -DRw6 was examined in a competition assay. Fixed antigen-presenting cells (APCs) were incubated with various concentrations of each peptide and suboptimal concentrations of stimulator peptides. The binding of the CS peptides to DR5 or DRw6 proteins was then determined in a proliferation assay using two established DR5 or DRw6-restricted T-cell clones with specificity for the stimulator peptides as responder cells. One of five CS peptides, comprising together about 50% of the CS protein sequence, was found to compete with the binding of the stimulator peptides to DR5 and DRw6. The CS peptide CS-(378-398), binding to DR5 and DRw6, was then shown to be able to induce primary in vitro responses of T cells from donors with DR5 and DRw6 haplotypes. CS-(378-398)-induced T-cell clones responded not only to the homologous peptide but also to the native CS protein in the presence of appropriate APCs. The strategy we have applied is of considerable general interest for the engineering of vaccines against any pathogen, since it greatly facilitates the selection of appropriate T-cell epitopes to be incorporated in the vaccine.

Self-tolerance alters T-cell receptor expression in an antigen-specific MHC restricted immune response

The influence of major histocompatibility complex (MHC) gene products on the T-lymphocyte alpha beta receptor (TCR) repertoire is well documented, but how specificity is also generated for a diverse array of foreign peptide antigens is unknown. One proposed mechanism is that the TCR repertoire is selected by the recognition of processed self-antigens bound to MHC molecules. Here, we examine the influence of non-MHC-encoded self-antigens on the TCR repertoire expressed in an antigen-specific immune response. Most pigeon cytochrome c-specific, Ek alpha Ek beta (Ek) Ia-restricted T cells from B10.A mice express a product of the V alpha 11 gene family in association with a V beta 3 gene-encoded protein. We therefore examined V alpha 11 and V beta 3 gene expression in cytochrome c-specific T-cell lines derived from various mouse strains with different non-MHC genetic backgrounds. T cells from several strains failed to express any V beta 3 due to tolerance induced by Mlsc-encoded self-antigens. Variable levels of V alpha 11 messenger RNA (mRNA) were expressed by antigen-specific T cells from all the strains. In one strain V beta 3 was expressed in the relative absence of V alpha 11. These results directly demonstrate that self-tolerance alters TCR gene usage in the immune response to a foreign antigen, and indicate that TCR V alpha and V beta proteins may, in part, be independently selected.

Inducibility of Ia molecules on keratinocytes reflects genetic control on astrocytes in the brain

We have recently observed strain differences in interferon-gamma (IFN-gamma) induction of Ia on astrocytes in rats and mice that are susceptible or resistant to experimental allergic encephalomyelitis (EAE) (Massa et al., 1987). We now found that keratinocytes cultured from rat skin show the same strain differences as astrocytes. Keratinocytes of EAE-susceptible Lewis rats are induced to higher levels of Ia expression compared to relatively resistant Brown-Norway (BN) rats, independent of the dose of IFN-gamma applied. This difference in regulation is specific for class II histocompatibility molecules because no such differences are seen with class I antigen induction by IFN-gamma. That keratinocytes reflect similar strain differences as astrocytes may be explained by a common ectodermal origin of these epithelial-like cells. This observation allows the determination of the genetic predisposition of an autoimmune response in the central nervous system by skin biopsy. This approach may be of importance in studies of immunopathological phenomena of brain tissue in man such as multiple sclerosis.

Self-tolerance eliminates T cells specific for Mls-modified products of the major histocompatibility complex

In mice the product of the Mlsa locus is an unusual antigen capable of interaction with certain products of the major histocompatibility locus (MHC) to form a ligand for a large portion of the T-cell alpha/beta receptor repertoire, including nearly all receptors that use V beta 8.1. The presence of Mlsa/MHC during T-cell development results in the deletion of T cells that express V beta 8.1, documenting the importance of clonal deletion in establishing tolerance to self antigens.

Limiting dilution comparison of the repertoires of high and low responder MHC-restricted T cells

To approach the mechanism that determines Ir gene-controlled high or low responsiveness to whole proteins, such as sperm whale myoglobin (SWMb), we compared the repertoires of high and low responder haplotype-restricted T cells for different myoglobin epitopes by limiting dilution frequency analysis. Poisson analysis was performed using long-term limiting dilution cell lines of (B10.BR [low] X B10.D2[high])F1 T cells maintained on high or low responder APCs. The cell lines were tested with SWMb peptides and fragments for T cell repertoire fine specificities and Ia restrictions. The frequency of SWMb-specific F1 T cells responsive on B10.BR (H-2k) APCs was 2.5-3.6-fold lower than on B10.D2 (H-2d) APCs. Strikingly, all of the H-2k-restricted T cells used I-Ek as a restriction element, whereas both I-Ad- and I-Ed-restricted T cells were found among the H-2d-restricted lines. The I-Ad-restricted T cells were dominant, and the majority was specific for the synthetic peptide 102-118. T cells specific for peptide 132-146, dominant in association with I-Ed, were less frequent. However, no detectable H-2k-restricted T cells were specific for either of these peptides, but instead they were specific for fragment 1-55 or peptide 59-80. Fragment 1-55 also stimulated a similar number of H-2d-restricted T cells. Therefore, the low response of F1 T cells on H-2k-presenting cells may be due to the failure to see myoglobin plus I-Ak, in particular the immunodominant site around Glu 109, in contrast to the dominant response of high responder mice (both H-2d and H-2s) focused on the I-A molecule and the site around residue Glu 109. The I-E- low responder B10 strain also failed to respond to peptide 102-118, supporting the idea that the low responder status results from a limited repertoire lacking response to 102-118 plus I-A. In those strains that respond to the immunodominant site 102-118, the frequency of T cells in the repertoire specific for this site was always considerably greater than that for other sites. These results suggest that there is an important difference between immunodominant epitopes and minor epitopes and that Ir gene-controlled low responsiveness to a natural whole protein may be due primarily to the failure to respond to a single immunodominant site, even though a number of other epitopes can be recognized.