Structural comparison of I-A antigens produced by a cloned murine T suppressor cell line with B-Cell-Derived I-A

A cloned, antigen-specific T suppressor cell line derived from a CBA mouse expresses large amounts of I-A and I-E antigens. Comparative two-dimensional polyacrylamid gel electrophoresis of biosynthetically labeled I-A antigens immunoprecipitated with a variety of monoclonal I-Ak-specific antibodies suggested that alpha, beta and Ii polypeptide chains are identical with B-cell-derived I-A. Dimeric complexes formed by I-A chains derived from B or T suppressor cells were also similar with two major exceptions. Pulse-labeled T-cell-derived Ia antigen was complexed with two additional unknown components of about 31K. These components were not visible in pulse-chased (processed) materials. In addition, T suppressor-cell-derived I-A antigens did not contain S-S linked dimers consisting of processed alpha and beta chains, which are usually formed during solubilization of B cells. We consider the possibility that in T cells these chains are associated with other structures, thus preventing S-S linkage between alpha and beta chains.     

Influence of asparagine-linked oligosaccharides on tumor cell recognition in the mixed lymphocyte reaction

The ability of an Ia+ B cell lymphoma, AKTB-1b, to stimulate thymocytes in the allogeneic mixed lymphocyte reaction is dependent on its prior treatment with either swainsonine or deoxynojirimycin, two inhibitors of the processing of asparagine-linked oligosaccharides. In the absence of drug treatment, the tumor cells fail to stimulate thymocytes, whereas pretreatment of the tumor cells with either drug results in a five- to 10-fold increase in their ability to induce thymocyte proliferation. Drug-treated AKTB-1b stimulates thymocytes at levels comparable to those obtained with allogeneic splenocytes. In contrast, the untreated lymphoma does stimulate splenic lymphocytes, and pretreatment with either inhibitor only marginally increases the response. Genetic studies demonstrate that the thymocyte response is still H-2 locus restricted and can be blocked by monoclonal antibodies against two tumor cell major histocompatibility antigens, H-2K and I-A. Drug treatment does not change cell surface I-A expression, and H-2K levels are apparently decreased one-third by deoxynojirimycin but are not affected by swainsonine. To verify that the drug protocol used was capable of altering the glycoconjugates of membrane-associated proteins, the endo-beta-N-acetylglucosaminidase H (endo H)-sensitivity of immunopurified H-2K and I-A was analyzed by SDS-PAGE. These studies demonstrated that swainsonine treatment does result in cell surface expression of glycoconjugates with altered oligosaccharide moieties. Likewise, deoxynojirimycin treatment results in the cell surface expression of an I-A alpha polypeptide with altered oligosaccharide chains while only marginally affecting H-2K and not affecting the I-A beta chain. An intracellular form of the I-A beta chain sensitive to endo H digestion in the presence of deoxynojirimycin is not detectable at the cell surface. Neuraminidase-digested AKTB-1b are also capable of stimulating allogeneic thymocytes. These studies demonstrate that changes in the glycosylation state of the tumor cell can markedly influence its recognition by allogeneic lymphocytes, and further, that different T cell populations differ in their response to such changes.     

The I-A beta b region (65-85) is a binding site for the superantigen, staphylococcal enterotoxin A

Ia antigen is a receptor for the superantigen staphylococcal enterotoxin A (SEA). Peptides I-A beta b(30-60), I-A beta b(50-70), I-A beta b(65-85), and I-A beta b(80-100) of the MHC class II antigen beta chain on mouse (H-2b) accessory cells were synthesized. Only I-A beta b(65-85) inhibited SEA binding to the mouse B-cell lymphoma line, A20 (H-2d) and the human Burkitt's lymphoma line, Raji (HLA-DR). The I-A beta b(65-85) sequence is a predicted alpha-helix along the hypothetical antigen binding cleft of the Ia molecule. I-A beta b(65-85) also directly and specifically bound both the intact SEA molecule and its Ia binding site, represented by the peptide SEA(1-45). The results suggest that I-A beta b region (65-85) is a necessary site for Ia molecular interaction with the superantigen SEA. Further, the data suggest that the same helical region of other Ia antigens binds SEA irrespective of haplotype and species.     

A single monoclonal anti-Ia antibody inhibits antigen-specific T cell proliferation controlled by distinct Ir genes mapping in different H-2 I subregions

A xenogeneic rat anti-mouse Ia monoclonal antibody, M5/114 (gamma 2b, kappa), was studied for its effects in vitro on T cell proliferative responses. Strain distribution studies revealed that M5/114 could inhibit I-A subregion-restricted T cell responses of the H-2b,d,q,u but not the H-2f,k,s haplotypes, indicating that this xenoantibody recognizes a polymorphic determinant on mouse Ia molecules. This same monoclonal antibody was found to inhibit BALB/c (H-2d) T cell proliferation to both G60A30T10 and G58L38 phi 4. The Ir genes regulating responses to these antigens map to either the I-A subregion (GAT), or the I-A and I-E subregions (GL phi), raising the possibility that M5/114 recognizes both I-A and I-E subregion-encoded Ia glycoproteins. It could be shown, using appropriate F1 responding cells, that M5/114 does in fact affect GAT and GL phi responses by interaction with both the I-A and the I-E subregion products, and not by any nonspecific effect resulting from binding to the I-A subregion product alone. These results are consistent with genetic and biochemical studies directly demonstrating that M5/114 recognizes A alpha A beta and E alpha E beta molecular complexes. The existence of a shared epitope on I-A and I-E subregion products suggests the possibility that these molecules arose by gene duplication. Finally, the precise correlation between the Ia molecules recognized by M5/114 and the ability of this antibody to block T cell responses under Ir gene control strengthens the hypothesis that Ia antigens are Ir gene products.     

DNA-mediated transfer of major histocompatibility class II I-Ab and I-Abm12 genes into B lymphoma cells: molecular and functional analysis of introduced antigens

A20.2J B lymphoma cells have been co-transfected with the A alpha b, A beta b or with the A alpha b, A beta bm12 and neomycin resistance genes. The transfected cell lines constitutively express the I-Ab or I-Abm12 class II molecules at a level comparable with that of the endogenous I-Ad antigen. The I-Ab antigens expressed on three independently transfected B cell clones (A20.Ab.1, A20.Ab.2, and A20.Ab.3) are serologically and functionally indistinguishable from the I-Ab molecules expressed by control H-2bxd B hybridoma cells (LB cells). These transfected cell lines were potent I region-restricted antigen-presenting cells to a large panel of antigen-specific, autoreactive and alloreactive T cell hybridomas, as well as normal T cell clones. There were not significant differences in the efficiency of antigen presentation by the Ia molecules encoded by the transfected, as compared with the endogenous, I-A genes. The expression of a functional I-Ab antigen on the surface of cells transfected with A beta bm12 and A alpha b genes is consistent with previous work that implicated the A beta-chain alone in the bm 12 mutation. Furthermore, because the transfected A20.Ab and A20.Abm12 cells display the serologic and functional properties of normal spleen cells from the wild-type and mutant mouse strains, respectively, it is clear that class II genes do not undergo unexpected and unpredictable alterations after transfection in this system. This system permits us to investigate the structural requirements for interactions between class II major histocompatibility complex antigens, a foreign antigen, and the T cell receptor by in vitro site-directed mutagenesis coupled with DNA-mediated gene transfer.     

Structural studies of the protein portion of the H-2-linked Ia glycoprotein antigens of the mouse: tryptic peptide comparison of products from the I-A and I-C subregions of B10-HTT.

Ia antigens from the I-A8 and I-Ck subregions of the B10.HTT (H-2t3) strain of mice were isolated by indirect immunoprecipitation of arginine-labeled, nonionic detergent-solubilized materials. After biochemical purification the electrophoretically homogeneous 28,000 dalton glycoprotein beta chains from the Ia precipitates were digested with trypsin and the resultant radiolabeled tryptic peptides were compared by analytical ion exchange chromatography. These comparisons reveal that the beta chains of Ia antigens from the A (I-A8) and C (I-Ck) subregions of B10.HTT share only two out of 12 to 14 of their arginine tryptic peptides. Thus these noncross-reactive Ia antigens are structurally quite diverse, and would possess sufficient structural variability to account for their lack of antigenic cross-reactivity.     

Use of molecular techniques for analysis of Ia structure-function relationships

An overview of a strategy for the molecular analysis of class II major histocompatibility complex (Ia) gene product structure-function relationships is presented, and results obtained to date by using this approach are summarized. The A beta, A alpha, E alpha, and E beta genes have been cloned and sequenced to yield information on gene organization and primary protein sequence. Comparison of sequences from allelic forms of these genes show the NH2-terminal domain to be the locus of most intraspecies polymorphism. Transfection of I-A alpha and A beta genes into B lymphoma cells or L cells has generated cells expressing the transfected gene products on their membrane. Such Ia+ transfectants present antigen to various T cells, which use the expressed I-A as a restriction element. Exon shuffling has shown the beta 1 domain of A beta to play a predominant role in such restricted antigen recognition. Preliminary data refining this analysis to sites within beta 1, as well as data on control of alpha: beta chain association, are reviewed, and future prospects for use of this approach in resolving questions of immunological interest are discussed.     

Ia specificities on parental and hybrid cells of an I-A mutant mouse strain

Splenic B cells and B cell blasts from the I-A mutant mouse strain B6.C-H-2bm12 were tested by serology with a series of new monoclonal anti-Iab antibodies. Four out of 5 of those monoclonal antibody-defined specificities that are determined by wild-type I-Ab antigens were undetectable on B6.C-H-2bm12 cells. Specificities both present and absent on mutant cells appear to be determinants on the same wild-type molecule, as indicated by sequential precipitation experiments with soluble H-2b antigens. The lack of expression of certain Ia specificities on mutant cells was found not to be the result of disparate control by the Xid gene, which was previously shown to control the expression of Ia.W39, another specificity absent in B6.C-H-2bm12 mice. Serologic testing of Ia specificities on cells and blasts from F1-hybrid mice suggested that the Iabm12 antigens are codominantly expressed, indicating a failure to detect trans regulation or complementation of the mutant phenotype. Another monoclonal antibody-defined Ia specificity dependent on the expression of the E beta polypeptide was normally expressed in B6.C-H-2bm12 mice. These data thus suggest that the lesion of these mutant mice occurred in the A alpha and/or A beta structural gene, resulting in the loss of several Ia specificities.     

Three classes of signalling molecules on B-cell membranes

The question of whether surface immunoglobulin and Ia molecules have a signalling function in helper T cell-dependent activation of B cells has been evaluated. Two sources of B cells have been used, one a purified population of hapten-binding B cells, the other a B-cell lymphoma, CH12, with known antigen specificity. Evidence is presented that both immunoglobulin and Ia molecules are receptors actively involved in the initial activation of resting B cells. Nevertheless, the requirements for ligand binding to either receptor can be bypassed under appropriate conditions, and the implications of this result for the function of these molecules is discussed. With respect to B-cell Ia, the authors present data that demonstrate two distinct functions of this molecule, one as a restricting element for T-cell activation, the second as a signalling receptor for B-cell excitation. On the CH12 surface, the I-A molecule fulfills the former function, but T-cell interactions with I-A fail to result in B-cell stimulation, suggesting that B-cell Ia may limit helper T cell-B cell interactions. We suggest that the binding of antigen surface immunoglobulin and binding of helper T-cell receptors to the appropriate Ia molecule(s) results in the activation of genes that encode for a third class of membrane B-cell receptors, those that bind B-cell stimulating factors.     

Co-dominant restriction by a mixed-haplotype I-A molecule (alpha k beta b) for the lysozyme peptide 52-61 in H-2k x H-2b F1 mice

Helper (CD4+) T lymphocytes recognize protein Ag as peptides associated to MHC class II molecules. The polymorphism of class II alpha- and beta-chains has a major influence on the nature of the peptides presented to CD4+ T lymphocytes. For instance, T cell responses in H-2k and H-2b mice are directed at different epitopes of the hen egg lysozyme (HEL) molecule. The current studies were undertaken with the aim of defining the role of mixed haplotype I-A (alpha k beta b and alpha b beta k) molecules in T cell responses to HEL in (H-2k x H-2b)F1 mice, as well as the nature of the immunogenic peptides of HEL recognized in the context of I-A alpha k beta b and I-A alpha b beta k. A series of HEL-reactive T cell lines and hybridomas derived from MHC class II heterozygous (C57BL/6 x C3H F1) mice were established. Their responsiveness to HEL and synthetic HEL peptides was analyzed with the use of L cells transfected with either I-A alpha k beta b or I-A alpha b beta k as APC. Out of 28 clonal T cell hybridomas tested, 13 (46%) only responded to HEL presented by I-A alpha k beta b, 11 (40%) by I-A alpha b beta k (and to a minor extent I-A alpha k beta k), only 4 (14%) were primarily restricted by I-Ak, and none by I-Ab. All the I-A alpha k beta b-restricted T cell hybridomas responded to the HEL peptide 46-61 and to its shorter fragment 52-61, even at concentrations as low as 0.3 nM. As this determinant has been previously defined as immunodominant for I-Ak but not for I-Ab mice, these results suggest a role for the I-A alpha k chain in the selection and immunodominance of HEL 52-61 in H-2k mice. The fine specificity of I-A alpha k beta b-restricted T cell hybridomas for a series of different HEL peptides around the sequence 52 to 61 suggests that peptide 52-61 binds to I-A alpha k beta b with higher affinity than to I-A alpha k beta k. The peptides recognized in the context of I-A alpha b beta k and I-A alpha k beta k were not identified.     

The expression of a tissue-specific self-peptide is required for allo-recognition

We have compared the functional properties of I-Ad expressed on different cell types. Specifically, we have transfected I-A alpha d and I-A beta d cDNA into a panel of T cell thymomas of various phenotypes. Excellent class II surface expression was achieved in all T cell tumors, equivalent in level to that found on the B cell lymphoma A20. Interestingly, however, two allo-I-Ad-specific Thy differed in their recognition of the transfected tumor cells: whereas the 42H11 T cell hybridoma (THy) was stimulated very efficiently by all transfectants, the RK38.2.2 Thy did not react to any of them. Both THy responded equally well to I-Ad on A20 B lymphoma cells. Purified macrophages isolated from various sources were also differentially recognized by the two THy, although there was only a quantitative difference in stimulation. Taken together, these results are best interpreted to show that the TCR of the RK38.2.2 THy is specific for I-Ad in the context of a B cell-specific determinant, possibly a self-peptide that is naturally associated with Ia. A cross-reactive molecule could be expressed by macrophages and COS-1 cells, but not by T cells.     

Qualitative and quantitative studies of antigen-presenting cell function by using I-A-expressing L cells

I-A-expressing transfected murine L cells were analyzed as model antigen-presenting cells. Four features of accessory cell function were explored: antigen processing, interaction with accessory molecules (LFA-1, L3T4), influence of Ia density, and ability to stimulate resting, unprimed T lymphocytes. I-A+ L cells could present complex protein antigens to a variety of T cell hybridomas and clones. Paraformaldehyde fixation before but not subsequent to antigen exposure rendered I-A+ L cells unable to present intact antigen. These results are consistent with earlier studies that made use of these methods to inhibit "processing" by conventional antigen-presenting cells. The ability of anti-L3T4 antibody to inhibit T cell activation was the same for either B lymphoma or L cell antigen-presenting cells. In striking contrast, anti-LFA-1 antibody, which totally blocked B lymphoma-induced responses, had no effect on L cell antigen presentation, measured as interleukin 2 (IL 2) release by T hybridomas, proliferation, IL 2 release, or IL 2 receptor upregulation by a T cell clone. I-A+ L cell transfectants were found to have a stable level of membrane I-A and I-A mRNA, even after exposure to interferon-gamma-containing T cell supernatants. In agreement with earlier reports, a proportional relationship between the (Ia) X (Ag) product and T cell response was found for medium or bright I-A+ cells. However, dull I-A+ cells had a disproportionately low stimulatory capacity, suggesting that there may be a threshold density of Ia per antigen-presenting cell necessary for effective T cell stimulation. Finally, I-A-bearing L cells were shown to trigger low, but reproducible primary allogeneic mixed lymphocyte responses with the use of purified responder T cells, indicating that they are capable of triggering even resting T cells. These studies confirm the importance of antigen processing and I-A density in antigen-presenting cell function, but raise questions about the postulated role of the LFA-1 accessory molecule in T cell-antigen-presenting cell interaction. They also illustrate the utility of the L cell transfection model for analysis and dissection of antigen-presenting cell function.     

Characterization of the molecules on SJL/J lymphomas which stimulate syngeneic T cells

I-A antigens isolated from SJL reticulum cell sarcoma (RCS) cells show greater heterogeneity with respect to charge and size of the A alpha chains than do I-A antigens isolated from normal SJL spleen cells. The relevance of these structural changes in RCS I-A to the previously shown syngeneic T cell stimulatory properties of RCS was investigated. It was shown that RCS cells expressed acidic forms of the A alpha chain on their cell surface which were not present on SJL spleen cells, peritoneal cells, or dendritic cells. The only normal cells which showed A alpha chain heterogeneity approaching that of RCS cells were LPS-induced B cell blasts. Treatment with tunicamycin completely abolished the RCS A alpha chain heterogeneity, whereas exposure to neuraminidase removed the charge heterogeneity, but not the size heterogeneity. Parallel studies of the syngeneic T cell proliferative response to RCS showed that tunicamycin abolished, while neuraminidase enhanced, the ability of RCS cells to stimulate syngeneic T cells. It is suggested that polysaccharide chains on RCS I-A molecules are particularly important for the biologic properties of these lymphoma cells. The possibility that highly glycosylated I-A antigens on normal B cell blasts are also involved in the stimulation of syngeneic T cells is discussed.     

Acquisition of syngeneic I-A determinants by T cells proliferating in response to poly (Glu60Ala30Tyr10)

The T cell proliferative response in mice to the synthetic polymer GAT is under Ir gene control, mapping to the I-A subregion of the H-2 major histocompatibility complex (MHC). Antigen-dependent proliferation in vitro of in vivo GAT-primed lymph node cells can be inhibited by a monoclonal antibody to Ia-17, an I-A public determinant. Using this antibody for direct immunofluorescent analysis, T cells in GAT-stimulated proliferative culture are identified that express syngeneic I-A during culture. This expression is strictly antigen dependent, requires restimulation in vitro, and requires the presence of I-A-positive adherent antigen-presenting cells. T cells bearing I-A can be enriched by a simple affinity procedure, and I-A-positive cells separated on a FACS are shown to retain antigen-specific reactivity. The acquisition of I-A determinants by T cells under these culture conditions is not nonspecific. The Ia determinants borne by T cell blasts appear to be dictated by the I subregion to which the relevant Ir gene maps, and which codes for the Ia molecule involved in presentation of the antigen. Thus, (B6A)F1 (H-2b X H-2a)F1 LNC express I-Ak antigens when proliferating to GAT but not when stimulated by GLPhe, the response to which is under I-E subregion control. The relation of Ir gene function to Ia-restricted antigen presentation and self-Ia recognition is discussed.     

Alteration of a non-polymorphic residue in a class II E beta gene eliminates an antibody-defined epitope without affecting T cell recognition

The interaction between the clonally selected T cell receptor, antigen, and Ia molecule is poorly understood at the molecular level. A cell line bearing an altered E beta k molecule has been examined to provide more information about the relationship between Ia structure and function. The cell line, 2B1, was derived from the TA3 B cell hybridoma through a series of negative and positive immunoselection steps. The 2B1 mutant lacked the binding site recognized by the 17.3.3 monoclonal antibody (mAb) but presented antigen normally to all I-Ek-restricted T cell hybridomas and clones examined. Sequence analysis of the mutant E beta k gene showed a single base transition (G----A) that resulted in an arginine to a histidine substitution at amino acid 49 of the beta 1 domain. This mutation demonstrates that residue 49 is not involved in antigen presentation to T cells but can be involved in B cell recognition (mAb binding).     

Identification of distinct predominant epitopes recognized by myoglobin-specific T cells under the control of different Ir genes and characterization of representative T cell clones

We characterized the fine specificity of antigen recognition of myoglobin-immune T cells from B10.D2, B10.GD, and B10.A(5R) mice. Polyclonal H-2d T cells show a predominant response to an epitope centering on Glu 109 and His 116. These residues localize a dominant epitope to one segment of the myoglobin surface, but probably are not the only amino acid residues involved. This response pattern maps genetically to I-Ad (or Kd) based on the B10.GD recombinant strain. A different epitope specificity was detected in B10.A(5R) T cells mapping to I-E beta b E alpha k or I-Cd, but no difference was detected between strains differing at the Igh locus. Thus, epitope specificity varied with Ia haplotype but not Igh allotype. Myoglobin-specific B10.D2 T cell lines were established, and five clones specific for the Glu 109, His 116 epitope were isolated; these were all restricted to I-Ad antigen-presenting cells. These clones represent the dominant specificity in the polyclonal T cell response to myoglobin and will be useful in characterizing the structure and function of T cell receptors for antigen and Ia. The differences in number and nature of T cell epitopes compared to B cell epitopes of myoglobin are discussed.     

Biochemical characterization of Ia antigens. I. Characterization of the 31K polypeptide associated with I-A subregion Ia antigens

Procedures are presented for the preparative isolation of murine Ia antigens directly from splenocyte detergent extracts with monoclonal immunoadsorbents. Utilizing these procedures, three Ia (I-A subregion) polypeptides (alpha, 31K, beta) were isolated and their m.w. and pI values characterized. Evidence is presented that indicates that: 1) the 31K polypeptide probably does not associate with the Ia alpha and beta chain complex during the Ia isolation procedure; 2) the 31K polypeptide is not tightly bound to the alpha/beta Ia complex and can be selectively removed by freezing and thawing and by washing the Ia-immunoadsorbent with buffers containing pyrrolidinone (a polar solvent); and (3) unlike the alpha and beta chains, the 31K polypeptide is not intrinsically radiolabeled with 3H fucose and 3H glucosamine, indicating that the 31K polypeptide either contains a carbohydrate structure that is different from that of the alpha and beta chains or it is not a glycopeptide. These data suggest that although Ia antigens are probably comprised of three polypeptides in the intact cell, only two (alpha and beta) are required to maintain alloantigenic determinants.     

Lead influences translational or posttranslational regulation of Ia expression and increases invariant chain expression in mouse B cells.

The molecular mechanisms governing the increased cell surface expression of major histocompatibility complex (MHC) class II molecules (Ia) on lead-treated mouse B cells was investigated. Lead has been shown to directly cause a selective, two-fold increase in the B cell's surface density of both products of the I region of the mouse MHC, I-A and I-E. In the present study, Western blot analysis showed that Pb increases the total cellular pool of I-A beta-chain by twofold. The increase in cellular I-A was not found to be due to increased messenger RNA (mRNA) for either the alpha- or the beta-chain of I-A. Biosynthetic labeling studies showed that Pb increases the translation or the stability of the Ia-associated invariant chain (Ii or gamma) and possibly the beta-chain of Ia. Collectively these results suggest that Pb increases the B cell's surface Ia by influencing translational or posttranslational regulation of Ia and/or Ia-associated chains.     

Lead influences translational or posttranslational regulation of ia expression and increases invariant chain expression in mouse B cells

The molecular mechanisms governing the increased cell surface expression of major histocompatibility complex (MHC) class II molecules (Ia) on lead-treated mouse B cells was investigated. Lead has been shown to directly cause a selective, two-fold increase in the B cell's surface density of both products of the I region of the mouse MHC, I-A and I-E. In the present study, Western blot analysis showed that Pb increases the total cellular pool of I-A β-chain by twofold. The increase in cellular I-A was not found to be due to increased messenger RNA (mRNA) for either the α- or the β-chain of I-A. Biosynthetic labeling studies showed that Pb increases the translation or the stability of the Ia-associated invariant chain (Ii or γ) and possibly the β-chain of Ia. Collectively these results suggest that Pb increases the B cell's surface Ia by influencing translational or posttranslational regulation of Ia and/or Ia-associated chains.     

Separation of the immune response genes for LDH-B and MOPC-173. II. Description of an immune response defect in B10.ASR7

By using the intra-I region recombinant mouse strain B10.ASR7 (H-2as3), the immune response (Ir) genes for LDH-B and MOPC-173 were genetically and serologically separated, as assayed by T cell proliferation. Previous work demonstrated that H-2s and H-2b strains respond to LDH-B and MOPC-173 whereas H-2a and H-2k strains failed to respond due to haplotype-specific suppression of I-Ak molecule-activated T helper cells by I-Ek molecule-activated T suppressor cells. In the experiments reported here, B10.ASR7 mice, which lack I-Ek expression, mounted a significant T cell proliferative response to LDH-B but not to MOPC-173. Separation of the Ia determinants used in restricting these two antigen responses was further confirmed when pretreatment of B10.S(9R) (A beta sA alpha sE beta sE alpha k) macrophages with A.TL anti-B10.HTT serum (anti-As beta Es beta Js) adsorbed with B10.ASR7 spleen cells blocked the MOPC-173 response but not the LDH-B response. Unadsorbed serum blocked both antigen responses. The B10.ASR7 E beta allele was determined to be s due to the ability of (A.TL X B10.ASR7)F1 hybrids to mount a T cell proliferative response to the terpolymer GLPhe. Monoclonal antibody blocking of the B10.ASR7 T cell proliferative response to LDH-B demonstrated that the Ia.2 and Ia.17, and not the Ia.15 epitopes are spatially related to the Ia epitopes involved in the restriction of the B10.ASR7 LDH-B T cell proliferative response. In addition, B10.ASR7 helper T cells generated in response to LDH-B were suppressed in a haplotype-specific manner by I-Ek molecule-restricted suppressor T cells in that (A.TL X B10.ASR7)F1 hybrids failed to respond to LDH-B. This nonresponsiveness was eliminated by treatment with monoclonal antibodies directed against the I-Ek molecule. These results suggest the possibility that the immune response defect in B10.ASR7 could be related to the site of recombination.