Working principles in the immune system implied by the "peptidic self" model

Immunoglobulin-specific T-B cell interaction. II. T cell clones recognize the processed form of B cells' own surface immunoglobulin in the context of the major histocompatibility complex class II molecule

In the preceding report (Eur. J. Immunol. 1989. 19: 1677) we have demonstrated that normal B cells, including small B cells, are capable of presenting Ig kappa-1b allotypic determinants of their endogeneously synthesized Ig+ to Ig kappa-1b-immune major histocompatibility complex (MHC) class II-restricted T cells. A panel of Ig kappa-1b allotype-specific T cell clones from August rats has been developed to study further the presentation of self surface Ig by B cells from Ig kappa-1-congeneic August.1b rats. All the clones studied were of the T helper/inducer phenotype (W3/25+,OX8-) and restricted by the RT-1Bc molecule. These clones responded both to the serum IgG(Ig kappa-1b) in the presence of irradiated spleen cells (SC) from August rats and to the Ig kappa-1b-bearing irradiated B cells from August.1b rats. SC presentation of secreted IgG was much less effective than B cell presentation of membrane Ig. Using CNBr cleavage of isolated C kappa (Ig kappa-1b) domain followed by high-performance liquid chromatography fractionation of the derived antigenic peptides, the kappa chain sequence between amino acids 176 and 214 has been identified as the T cell epitope recognized by all T cell clones in association with RT-1Bc. The fragment 176-214 of the Ig kappa-1b allotype differs from that of Ig kappa-1a allotype by three amino acid substitutions at positions 184, 185, 188. T cell recognition of pL kappa-1b(176-214) required no additional processing by the antigen-presenting cell: the efficient presentation of the peptide but not of intact IgG(Ig kappa-1b) by the paraformaldehyde-fixed SC was observed. These data provide clear-cut evidence for an absolute requirement of the processing of Ig molecules for T cell recognition to occur in our experimental system. Although the fixation of B cells from August.1b rats diminished their Ig kappa-1b-presenting ability, fixed Ig kappa-1b+ B cells were still able to induce Ig kappa-1b-specific T cell clone responses. Our results suggest that B cells can express the processed form of self-synthesized surface Ig in addition to intact surface Ig molecules. The former can be recognized by MHC-restricted T cell.

I-A alpha polymorphic residues that determine alloreactive T cell recognition

An individual's T lymphocytes are highly reactive to allogeneic MHC molecules. As a step in deciphering the mechanism of allorecognition by T lymphocytes, we have attempted to identify the TCR's target on MHC class II molecules, in particular the polymorphic residues that determine the specificity of recognition. We have generated a panel of Ak-reactive, Ab-nonreactive T cell hybridomas, and sets of L cell transfectants displaying A alpha A beta molecules with wild-type, chimeric or single site-mutated A alpha chains, with reciprocal interchanges between Ak and Ab. We then measured the stimulation of the T hybridomas in response to the transfectants. The results indicate that the hybridomas recognize diverse and complex determinants, with contributions from both A alpha and A beta chains, and from several regions or amino acids of the A alpha chain. The data are most consistent with a model in which alloreactivity results from the presentation of peptides to the T cell by an allogeneic MHC molecule, peptides that cannot be presented by the responder's own MHC complexes. The specificity of allorecognition seems to be imparted mainly by peptide/MHC molecule rather than TCR/MHC molecule contacts.

The tumor-rejection antigens of the 1591 ultraviolet fibrosarcoma. Potential origin and evolutionary implications

Previously, we cloned and sequenced the three novel MHC class I genes expressed by the C3H UV fibrosarcoma, 1591. We have extended the analysis of the polymorphic nature of these genes relative to the C3H strain. Scattered nucleotide differences among the tumor genes as compared with the C3H H-2 and Qa sequences make it highly unlikely that the novel tumor genes were generated by recombination between endogenous C3H sequences. Given that two of the tumor clones, A149 and A166, are remarkably similar in amino acid and DNA sequence to H-2Lq and H-2Dq, respectively, we also examined the 1591 RP2 and GUS loci for evidence of polymorphism. Compared with C3H and B10.AKM, 1591 appears to be heterozygous at each of these loci, consistent with an H-2q origin for the two novel 1591 class I genes. Interestingly, the third tumor gene, designated A216, shares certain characteristics with the H-2Ks antigen, reminiscent of the naturally occurring combination of H-2Ks, H-2Dq, and H-2Lq antigens found in some Swiss mouse strains. As a result, we propose that the non-C3H/HeN characteristics displayed by the 1591 tumor point to a non-C3H origin for the novel tumor class I genes of 1591.

T cells sensitized to synthetic HLA-DR3 peptide give evidence of continuous presentation of denatured HLA-DR3 molecules by HLA-DP

T cell clones raised against a synthetic peptide, identical to the third hypervariable region of the DR3 BI chain, were tested for secondary proliferative responses against a panel of PBLs. All seven DR3 DPw3+ stimulators could induce proliferation. DR3- DPw3+ PBLs were recognized when the synthetic peptide was added to the cultures. Inhibition studies with mAbs showed that in both cases the HLA-DP molecule is involved in the recognition of both types of stimulators. We conclude that the clones recognize the DR3 peptide presented by HLA-DPw3. This stimulus can be obtained in two different ways: (a) by addition of synthetic peptide to DPw3+ PBLs or (b) by using DR3 DPw3+ stimulator cells where DR3 peptides are present in the culture as a product of denaturation of the DR3 molecule. Because all DR3 DPw3+ PBLs tested could stimulate the line and clones, we assume that the presentation of the DR3 peptide by DP is a naturally and continuously occurring phenomenon.

Creation of H-2 class I epitopes using synthetic peptides: recognition by alloreactive cytotoxic T lymphocytes

A major role that cytotoxic T lymphocytes (CTLs) play in the immune response is the specific destruction of viral-infected cells and tissue from foreign grafts. Class I molecules encoded within the major histocompatibility complex are the target structures for these CTLs. Recently, viral-restricted CTLs have been shown to recognize viral peptides in association with class I molecules, while several studies with cloned alloreactive CTLs have indicated that these T cells can recognize class I-derived peptides in association with class I molecules. Together, these observations suggest that peptide binding is an important function of class I molecules. In this paper, we show that the recognition of a particular class I molecule by a bulk population of alloreactive CTLs can be altered by incubating with it a peptide derived from another class I molecule. Specifically, we used the hybrid D/Ldm1 molecule as a target structure together with the peptide Ld61-85, and we have shown that their associative recognition by Ld-specific CTLs depends on sequence and configuration of the peptide and is specific for Ld using a cold-target inhibition assay. Our results are discussed in light of three possible models for the target structure(s) that can be recognized by alloreactive CTLs and in terms of the role peptides may play during allorecognition.

Recognition of xeno-(HLA, SLA) major histocompatibility complex antigens by mouse cytotoxic T cells is not H-2 restricted: a study with transgenic mice

Cytotoxic T lymphocytes (CTLs) recognize antigens in the context of major histocompatibility complex (MHC) class I gene products. The T-cell receptor (TCR) that mediates this MHC-restricted antigen recognition recognizes short peptide fragments rather than the intact antigen. Presentation of peptides to the TCR may thus be a major function of the MHC. An intriguing question emerging from this model is whether peptide presentation also applies to foreign MHC antigens and which of the available MHC molecules can present preferentially the peptides of the foreign MHC molecule. Allo- and xenoreactive CTLs might either recognize native MHC class I molecules or peptides presented by self MHC or by the foreign class I MHC itself. The finding that synthetic peptides corresponding to MHC class I regions are recognized by allo- and xenoreactive CTLs suggests that recognition of foreign MHC by CTLs might involve degraded fragments presented by syngeneic class I molecules. We used MHC transgenic mice as a tool to study these questions. The CTL responses against human (HLA) antigen B27 were analyzed by using HLA-B27 transgenic mice with various H-2 haplotypes. We report here that mouse xeno-MHC-specific (anti-B27) CTLs are perfectly able to kill human and mouse cells expressing the appropriate xenoantigen and that in primary and secondary responses to xeno-MHC, the mouse T-cell repertoire does not use self-H-2 as a restriction element. Absence of H-2 restriction was confirmed by the lack (less than 1/10(6] of H-2-restricted HLA-specific CTL precursors. Therefore, H-2-restricted recognition of xeno-MHC antigens cannot be generalized as part of a classical MHC class I-specific response. These results indicate that xenoreactive CTLs usually recognize intact MHC molecules or MHC peptides preferentially presented by their native MHC molecule. We suggest the latter possibility.

B-lymphoma cells process and present their endogenous immunoglobulin to major histocompatibility complex-restricted T cells

Antigen-presenting B-lymphoma cells were transfected with the gene encoding the immunoglobulin lambda 2 light chain of MOPC315 cells (lambda 2(315). The lambda 2 chain is expressed on the cell surface of the transfectants together with the endogenous heavy chain. The transfectants present an idiotope of the lambda 2(315) light chain to class II-restricted T-cell clones. Recognition by the T cells requires processing of the lambda 2(315) light chain. From these data we conclude that B-lymphoma cells constitutively process and present their immunoglobulins. Secretion and reuptake of the light chain was not necessary for the presentation. Thus, B cells bear two types of idiotypes on their membrane, a native form as surface immunoglobulin and a processed form in the context of products of the major histocompatibility complex.

Molecular characterization of the C3HfB/HeN H-2Kkm2 mutation. Implications for the molecular basis of alloreactivity

The C3HfB/HeN (C3Hf) mouse strain expresses an H-2Kk molecule, previously denoted H-2Kkv1, that is structurally and functionally distinct from H-2Kk of the parental C3H strain. By molecular genetic analysis, we demonstrate that the C3Hf H-2K gene carries a homozygous coding region mutation relative to the C3H allele, revealing that C3Hf meets the requirements for assignment of a mutant haplotype, H-2km2. C3Hf H-2Kkm2 bears a single clustered substitution of four nucleotides within 14 contiguous nucleotides in exon 3. Since this sequence also is present intact at the homologous position in H-2Dk of both C3H and C3Hf, the origin of the H-2Kkm2 mutation is consistent with a nonreciprocal sequence transfer from the H-2Dk donor gene, analogous to the mechanism proposed for generation of the H-2Kb mutations. The H-2Kkm2 mutation encodes three clustered amino acid substitutions, at positions 95, 98, and 99, that map to one of the large beta strands at the bottom of the peptide antigen binding cleft of the H-2Kkm2 molecule. The nature and location of these amino acid substitutions are unique relative to any other known H-2 mutant or HLA variant, and underscore the importance of the beta-pleated sheet in influencing CTL recognition. These results indicate that H-2Kkm2 alloantigenicity may derive largely from altered presentation of self cellular peptides.

T-immunogenic peptides are constituted of rare sequence patterns. Use in the identification of T epitopes in the human immunodeficiency virus gag protein

The sequences of a set of 63 peptides of demonstrated T immunogenicity have been analyzed and compared with two different randomly generated sets of sequences. This study indicates a statistically significant tendency of T immunogenic peptides to be constituted of clusters of rare tetrapeptides, as evaluated from the available sequence data banks. This result has been used to locate potential T epitopes in the human immunodeficiency virus (HIV) gag protein. Four peptides corresponding to the best candidate T epitopes (chosen in regions of conserved sequence among different virus isolates) have been synthesized and found to be recognized by a HIV-1-specific, HLA-A2-restricted human cytotoxic T cell line.

Direct demonstration of critical amino acid residues required for cytotoxic T-lymphocyte allorecognition of H-2 class I antigens

To identify critical amino acid residues recognized by alloreactive cytolytic T lymphocytes (CTL) generated between H-2Kb and H-2Kbm1, we have derived a series of cloned L-cell lines expressing the following mutant H-2Kb class I genes. Cell line L-KbTyr-Tyr expresses a mutant gene in which positions 155-156 of the Kb molecule have been changed from Arg-Leu to Tyr-Tyr, leaving position 152 unchanged. Cell line L-KbAla expresses the reciprocal mutant gene that has position 152 of the Kb molecule mutated from glutamic acid to alanine, leaving positions 155-156 unchanged. Electrophoretic mobilities of the mutant Kb molecules reflect only those changes predicted by the mutations. Mutant-specific (anti-Kbm1) and native-specific (anti-Kb) CTL lyse L-KbTyr-Tyr and L-KbAla target cells equally well. Unlabeled target inhibition of lysis revealed a pattern of recognition and inhibition that suggests that the amino acid differences between Kbm1 and Kb create at least two discrete determinants that can be recognized by different populations of CTL. The results suggest that these determinants consist, at least in part, of a linear amino acid sequence from which critical amino acid residues can be identified.

Allospecific cytotoxic T lymphocytes recognize an H-2 peptide in the context of a murine major histocompatibility complex class I molecule

We have isolated cytotoxic T lymphocytes (CTL) preferentially reactive with the alpha 1 external domain of the H-2Ld antigen by selecting for T cells capable of recognizing a variant major histocompatibility complex (MHC) class I antigen sharing alpha 1 sequences with H-2Ld. Using these CTL, we demonstrate that a synthetic alpha 1 peptide corresponding to one of the helices derived from the H-2Ld molecule can be presented by a class I restriction element to reconstitute a CTL determinant borne by intact H-2Ld. Moreover, several other H-2L-reactive CTL generated independently were also able to recognize H-2Ld either as an intact alloantigen or as a peptide in conjunction with appropriate class I restriction elements. These data demonstrate that an H-2 peptide can reconstitute a CTL target structure and suggest that some alloreactive T cells in fact might be directed against allogeneic class I peptides in the context of a class I framework.

Functional polymorphism of each of the two HLA-DR beta chain loci demonstrated with antigen-specific DR3- and DRw52-restricted T cell clones

HLA-DR3- and HLA-DRw52-associated functional polymorphism was investigated with selected tetanus toxoid (TT)-specific T cell clones. We have shown earlier that HLA-DR antigens are encoded by two distinct loci, DR beta I and DR beta III. The alloantigenic determinant(s) defined by the serological HLA-DR3 specificity map to the former, while the supratypic HLA-DRw52 determinants map to DR beta III. Furthermore, we have recently recognized by DNA sequencing three alleles of HLA-DRw52 at locus DR beta III, referred to as 52 a, b, and c. Our objective was to correlate the pattern of T cell restriction with the gene products of individual DR beta chain loci and with the three newly described alleles of locus DR beta III. Among the selected T cell clones, 5 reacted exclusively when TT was presented by HLA-DR3+ APCs (TT-DR3-APC). In contrast, two T cell clones were stimulated by TT-DRw52-APC. More specifically, these two T cell clones (Clones 10 and 16) were stimulated by different subsets of TT-DRw52-APC. Clone 16 responded to some DR3 and TT-DRw6-APC, while clone 10 was stimulated by other TT-DR3 and TT-DRw6, and all TT-DR5-APC. This same pattern of DRw52 restriction was found in panel, as well as in family studies. Because this suggested a correlation with the pattern of DRw52 polymorphism observed earlier by DNA sequencing and oligonucleotide hybridization, the APC used in these experiments were typed for the 52 a, b, and c alleles of locus DR beta III by allele-specific oligonucleotide probes. This distribution overlapped exactly with the stimulation pattern defined by the T cell clones. Clone 16 responded to TT-52a-APC, clone 10 to TT-52b-APC, and both clones to a TT-52c-APC. The response of the T cell clones was inhibited differentially by mAbs to DR. Raising TT concentration, or increasing HLA-class II expression with INF-gamma both affected the magnitude of response of the TT-specific clones but did not modify their specificities. These results demonstrate that a restriction specificity can be attributed to the DR beta III locus and illustrate the functional relevance of the polymorphism observed at this locus. This is of special interest in view of the striking difference in the pattern of structural diversity among alleles of DR beta I and DR beta III.

Rare sequence motifs are common constituents of hypervariable antibody regions

An analysis of the amino acid sequences of variable regions of human and mouse antibody molecules was performed. It involved comparison of their constituent tetrapeptides with those found in a reference set (the somatic self) built with non-immunological proteins found in a protein data base. It appeared that hypervariable regions, particularly CDR1 and CDR3, are often made up of rare tetrapeptides not present in the reference set. As assessed by simple statistical tests, this bias was significant. We discuss its possible connection with the problem of antibody immunogenicity. This result provides indirect support for the existence of idiopeptides predicted by the "peptidic self model".