Genetic control of T-cell proliferative responses to poly(glu40ala60) and poly(glu51lys34tyr15): Subregion-specific inhibition of the responses with monoclonal Ia antibodies

The relationship betweenIr genes and Ia antigens was studied in the T-cell proliferative responses to two synthetic polypeptides poly(glu⁴⁰ala⁶⁰) (GA) and poly(glu⁵¹lys³⁴tyr¹⁵) (GLT¹⁵). The response to GA was found to be controlled by anIr gene in theI-A subregion, whereas the anti-GLT¹⁵ response was shown to be under dual control, oneIr gene mapping probably in theI-A subregion, and the other in theI-E subregion. We obtained two different lines of evidence suggesting identity ofIr and Ia genes. First, the presence of certain serologically identified allelic forms of the I-A-encoded A molecule correlated with the responder status to GA both in inbred strains and in B10.W lines, the latter carrying wild-derivedH-2 haplotypes. Thus the Ir and Ia phenotypes were not separable in strains of independent origin. Second, the anti-GA response was completely inhibited by monoclonal antibodies against determinants on the A molecule (Ia.8, 15, and 19), but not by a monoclonal antibody against a determinant on the E molecule (Ia.7). In contrast, the anti-GLT¹⁵ response was only inhibited by a monoclonal antibody against the E molecule, but not by antibodies against the A molecule. Our data support the hypothesis that Ia antigens, as restriction elements for T-cell recognition, may in fact be the phenotypic manifestation ofIr genes.     

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.     

Analysis of Ir gene function using monoclonal antibodies: independent regulation of GAT and GLPhe T cell responses by I-A and I-E subregion products on a single accessory cell population

T cell proliferative responses to the synthetic polypeptides GAT and GLPhe are under Ir gene control. GAT responses are regulated by gene(s) in the I-A subregion, and GLPhe responses are controlled by a pair of complementing genes mapping to the I-A and I-E subregions. We demonstrate that monoclonal antibody to the I-A gene product inhibits GAT proliferation but not the GLPhe response, whereas a monoclonal antibody to the I-E associated Ia-7 determinant inhibits GLPhe but not GAT proliferation, which indicates independent involvement of each Ia determinant in antigen presentation for the T cell response to these antigens. Use of the same subregion-specific monoclonal antibodies in complement-dependent lysis demonstrates that the antigen-presenting cells for GAT and GLPhe express both I-A and I-E products. The possibility that an Ia subregion-specific "self-receptor" functions on the reactive T cells as a regulatory element is discussed.     

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.     

Immune responses to complex protein antigens I. MHC control of immune responses to bovine albumin

Murine T cell proliferative and antibody responses to the multi-determinant protein bovine serum albumin (BSA) are controlled by Ir genes mapping within the H-2 gene complex. Strains possessing the H-2k, H-2a, and H-2d haplotypes are classified as high responders to BSA. In contrast, H-2b strains are low responders to BSA. Genetic mapping experiments employing strains with recombinant H-2 haplotypes indicate that both T cell proliferative and antibody responses are at least in part regulated by genes within the I-A subregion. Studies on the inhibition of T cell proliferation by monoclonal anti-Ia antibodies are consistent with the assignment of an Ir gene for BSA to the I-A subregion and strongly suggest a role for genes within the I-E/C subregions as well. The MHC-mediated control of antibody responses did not affect the affinity or the isotype of the antibody produced. The relative quantities of antibody specific for each of the three domains of BSA appears to be regulated by H-2-linked BSA Ir genes, and domain III antigenic determinants were found to be dominant in the responses of low-responder mice and in the early response of high-responder mice. This domain III epitope dominance essentially disappears by the tertiary response of high-responder mice.     

Regulation of immune responses via genetically restricted cellular interactions. II. I-region-restricted cooperation between idiotypic and anti-idiotypic B lymphocytes

Immunization of BALB/c mice with MOPC-104E myeloma protein induced idiotype-specific enhancing cells which acted on anti-dextran antibody-producing cells. The enhancing cells have surface phenotypes of B cells. Using BALB/c H-2 congenic strains, it was found that the cooperation between anti-idiotypic-enhancing B lymphocytes and dextran-primed B lymphocytes was controlled by major histocompatibility gene complex. Here we have described the loci which restrict the successful cooperation between B lymphocytes, wherein it was revealed that the interaction was restricted to the I-A and I-E subregions in H-2k haplotype and the I-A subregion in H-2b haplotype. Utilizing several monoclonal antibodies specific for Ia antigens, it was revealed that the enhancing B lymphocyte activity was completely inhibited by the pretreatment of antibody-producing B cells with anti-Ia.7 in H-2d haplotype as well as H-2k, and with anti-I-A antibody in H-2b haplotype. The results suggest that the anti-idiotypic B-lymphocyte response to the self idiotype is under control of H-linked immune response (Ir) gene.     

T lymphocyte-enriched murine peritoneal exudate cells. III. Inhibition of antigen-induced T lymphocyte Proliferation with anti-Ia antisera.

The recent development of a reliable murine T lymphocyte proliferation assay has facilitated the study of T lymphocyte function in vitro. In this paper, the effect of anti-histocompatibility antisera on the proliferative response was investigated. The continuous presence of anti-Ia antisera in the cultures was found to inhibit the responses to the antigens poly (Glu58 Lys38 Tyr4) [GLT], poly (Tyr, Glu) ploy D,L Ala-poly Lys [(T,G)-A--L], poly (Phe, Glu)-poly D,L Ala-poly Lys [(phi, G)-A--L], lactate dehydrogenase H4, staphylococcal nuclease, and the IgA myeloma protein, TEPC 15. The T lymphocyte proliferative responses to all of these antigens have previously been shown to be under the genetic control of major histocompatibility-linked immune response genes. The anti-Ia antisera were also capable of inhibiting proliferative responses to antigens such as PPD, to which all strains respond. In contrast, antisera directed solely against H-2K or H-2D antigens did not give significant inhibition. Anti-Ia antisera capable of reacting with antigens coded for by genetically defined subregions of the I locus were capable of completely inhibiting the proliferative response. In the two cases studied, GLT and (T,G)-A--L, an Ir gene controlling the T lymphocyte proliferative response to the antigen had been previously mapped to the same subregion as that which coded for the Ia antigens recognized by the blocking antisera. Finally, in F1 hybrids between responder and nonresponder strains, the anti-Ia antisera showed haplotype-specific inhibition. That is, anti-Ia antisera directed against the responder haplotype could completely block the antigen response controlled by Ir genes of that haplotype; anti-Ia antisera directed against Ia antigens of the nonresponder haplotype gave only partial or no inhibition. Since this selective inhibition was reciprocal depending on which antigen was used, it suggested that the mechanism of anti-Ia antisera inhibition was not cell killing or a nonspecific turning off of the cell but rather a blockade of antigen stimulation at the cell surface. Furthermore, the selective inhibition demonstrates a phenotypic linkage between Ir gene products and Ia antigens at the cell surface. These results, coupled with the known genetic linkage of Ir genes and the genes coding for Ia antigens, suggest that Ia antigens are determinants on Ir gene products.     

Ir genes of different high responder haplotypes for staphylococcal nuclease are not allelic

The Ir gene controlling high responsiveness to staphylococcal nuclease in the H-2d haplotype has been mapped to the I-A subregion, in contrast to that in the H-2k and H-2a haplotypes, which maps in the I-B subregion. The nonallelic high responder genes also confer differences in fine specificity on the antibodies produced. This nonallelism of Ir genes for the same antigen in different haplotypes is consistent with Ir gene mechanisms involving associative recognition of antigen plus Ia by T lymphocytes. It further suggests that products of different subregions can perform the same function.     

Molecular associations of class II MHC antigens in mitogen-stimulated B cells

Previously, we showed that murine B cell membrane proteins undergo rearrangements in the plasma membrane to form new molecular associations in response to mitogenic stimulation. These complexes were covalently stabilized by photoreactive cross-linking agents and were analyzed by SDS PAGE. We have now identified certain complexes that involve class II MHC products, the Ia antigens. Upon stimulation of B cells with LPS, Ia surface molecules (as identified by radioimmunoprecipitation with polyclonal anti-Ia antiserum) enter into a molecular complex with a 95-kd membrane-associated protein (p95) to form a 200-kd complex that may be stabilized by the cross-linking agent dithiobisphenylazide (DTPA). This molecular association is not observed upon stimulation with mitogenic anti-Ig reagents, nor with the polyclonal B cell activator 8-bromoguanosine. p95 is not a disulfide-linked molecule itself, and by separate immunoprecipitation experiments we have established that it is not a component of surface Ig, transferrin receptor, the B cell Fc receptor, or CR1, the receptor for complement component C3b. Further analysis of the association of Ia antigens with surface proteins, with the use of monoclonal antibodies directed against I-A or I-E, has demonstrated that each subregion gene product forms a unique molecular association. Precipitation of radiolabeled lysates from LPS-activated B cells with anti-I-A reveals the aforementioned association with p95. In contrast, the I-E antigen apparently forms complexes with a multimer of a 15-kd protein to give complexes of 45, 60, 75, and 90 kd. When analyzed by two-dimensional diagonal gels (nonreducing/reducing), only the I-E bands are revealed by autoradiography, indicating that the putative p15 that associates with I-E may not be accessible to surface labeling. The disparate molecular associations for I-A and I-E suggest that the formation of these distinct protein complexes may be functionally related to a different role in the process of cellular activation for each of these Ia subregion gene products.     

Identity of molecules bearing murine Ia alloantigenic specificities Ia.7 and Ia.22: evidence for a single type of I-E molecule in homozygotes

In homozygous mice bearing I regions derived from haplotype k, only a single type of Ia molecule bearing the alloantigenic specificities Ia.7 and Ia.22 was found using techniques of sequential immune precipitation and tryptic peptide analysis. As suggested at the fourth Ir Gene Workshop (Sachs 1978), Ia.7 is considered here to be an antigenic determinant associated with I-E-subregion-encoded molecules, i.e., it is excluded from the I-C subregion. The I-C subregion is currently defined mainly by functional traits. It is now known that the I-E molecules are composed of an alpha chain encoded in the I-E subregion, and a beta chain encoded in the I-A subregion. Since the I-C subregion is not involved with the determination of these Ia molecules, and since in homozygotes there is apparently only a single type of molecule bearing both specificities Ia.7 and Ia.22, the term "I-E/C" molecule should probably be dropped in favor of the simpler designation I-E.     

Role of macrophages in the augementation of mouse natural killer cell activity by poly I:C and interferon.

The genetic control of T lymphocyte proliferative response to the five synthetic antigenic sites of myoglobin, two synthetic nonantigenic control peptides, and one "nonsense" peptide was determined in independent and recombinant strains of mice. In all the strains examined, the nonantigenic control peptides and the "nonsense" peptide did not invoke a response in myoglobin-primed mice. Further, when mice were not primed with whole myoglobin, no response was obtained with any of the antigenic sites. Haplotypes H-2d, H-2f, and H-2s are higher responders to sites 1 and 2, whereas haplotypes H-2d and H-2s are high responders to site 5. Response to site 3 may be controlled by a non-H-2-linked gene. Site 4 can stimulate H-2b and H-2k haplotypes that are nonresponders to the whole myoglobin. Studies with the recombinant strains suggested that Ir genes to sites 1 and 2 map in the I-A subregion and I-C subregion and were designated Ir-Mb-1,2(A) and Ir-Mb-1,2(C). Ir genes to sites 4 and 5 mapped only in the I-A subregion and were designated Ir-Mb-4(A) and Ir-Mb-5(A). These studies suggest that individual antigenic sites in a molecule are controlled by unique Ir genes.     

Genetic control of immune responses of inbred mice: Responses against terpolymers poly(glu57lys38ala5) and poly(glu54lys36ala10)

The immune response patterns of inbred and congenic strains of mice against terpolymers poly(glu⁵⁷lys³⁸ala⁵) and poly(glu⁵⁴lys³⁶ala¹⁰) have been studied. Initial recognition of the polymers is ascribed to ‘GA’ receptors (Ir-GA gene product) on T cells of mice ofH-2 haplotypes,a,b,f,k ands, and ‘GL’ receptors (Ir-GL gene product) of mice ofH-2 ^p,H-2 ^q andH-2 ^j haplotypes, and to GA and/or GL receptors of mice ofH- 2^d andH- 2^r haplotypes. The specificity of the antibody is directed predominantly against GL. The inability to elicit antibody with GA specificity has been ascribed to the lack of significant concentrations of GA sequences in the polymers to interact with appropriate receptors on B cells. The weakest responders were mice of H-2^b haplotype. F¹ hybrids (responders×nonresponders) were all responders demonstrating the dominant character of responsiveness. Wide variations in antibody levels produced among strains of mice of theH-2 ^k andH-2 ^b haplotypes are ascribed to genes not linked toH-2.     

I-Region genetic restrictions imposed upon the recognition of KLH by murine T-cell clones

Data presented in this paper show that the recognition of keyhole limpet hemocyanin by murine T-cell clones is restricted by products of the I region. These data have been obtained by genetic mapping studies as well as by the use of monoclonal la-specific antibodies which inhibit the ability of antigen-presenting cells to effectively present antigen to such T-cell clones. Use of heterozygous antigen-presenting cells derived from crosses between B6.C-H-2 ^bm12 and B10.A(4R) mice have allowed us to show that both trans-complementing I-A products are used for restriction of recognition of KLH. These data were confirmed using monoclonal Ia antibodies to inhibition recognition of KLH by the same T-cell clones. Thus, we have shown that there exist hybrid molecules formed by free combinatorial association of products encoded within the I-A subregion which restrict the recognition of soluble antigen. Additionally, we have shown that the molecule formed by complementation between the alpha chain encoded within the I-E region and a beta chain encoded within the A region (A_e) can function effectively in presenting KLH to certain murine T-cell clones. These results support the hypothesis that the recognition of individual antigenic epitopes within large multideterminant antigens is under the control of Ir genes.     

Dissection of the poly(glu60ala30tyr10) (GAT)-specific T-cell repertoire in H-21 k mice

We examined the antigen recognition of the class II major histocompatibility complex (MHC) of 45 poly(glu60 ala30 tyr10) (GAT)-reactive T-cell clones isolated by limiting dilution cloning of a pool of in vivo-primed and in vitro-restimulated A.TL lymph-node T cells. Each clone expressed the Thy-1.2+, Lyt-1+, Lyt-2-, LFA-1+, Ia-, and H-2Dd+ cell-surface phenotype and exhibited strict specificity for GAT on syngeneic antigen-presenting cells (APCs). The monitoring of the proliferative responses of these clones in the presence or absence of GAT, using APCs from strains with 11 independent H-2 haplotypes, revealed several distinct specificity patterns: (i) most (31 of 45, 73%) T-cell clones recognized GAT in a self-I-Ak-restricted manner; (ii) other alloreactive clones (5 of 45, 11%) were stimulated to proliferate, irrespective of the presence of GAT, in response to allodeterminants expressed on H-2s, H-2d, H-2f or H-2u spleen cells; (iii) a third T-cell clone subset (4 of 45, 9%) was activated by GAT in the context of not only self-I-Ak but also nonself restriction Ia determinants; and (iv) three clones (7%) exhibited a triple specificity, i.e., they recognized GAT in the context of self and nonself Ia determinants and were alloreactive. One of the latter clones responded to GAT in an apparently non-MHC-restricted manner and recognized an I-Ab allodeterminant. These data provide direct evidence that the antigen-specific and alloreactive T-cell repertoires overlap and that the self-MHC restriction of GAT-specific T-cell responses is not absolute in A.TL mice.     

The expression of Ia antigenic determinants on macrophages required for the in vitro antibody response.

A subpopulation of antigen-presenting macrophages required for an in vitro antibody response to burro erythrocytes was deleted by pretreating the splenic macrophages with anti-Ia serum and complement (C). The in vitro response of the macrophage depleted T-B cell population could not be restored by the addition of macrophages resistant to anti-Ia antibodies and C (Ia-). The response of Ia- macrophages and the macrophage-depleted T-B cells was only reconstituted by the addition of Ia+ macrophages. Macrophages pretreated with anti-Ia antibodies restricted to react with determinants of one I subregion could not support the in vitro antibody response when added to cultures whose macrophages were pretreated with anti-Ia serum and C specific for the I-J subregion. These results confirmed that Ia determinants of the I-A, the I-E, and the I-C subregions were all expressed on the I-J+ macrophage required for an in vitro antibody response.     

Genetic regulation of the immune response to hepatitis B surface antigen (HBsAg). V. T cell proliferative response and cellular interactions

We previously demonstrated that in vivo antibody production to HBsAg in the mouse is regulated by at least two immune response (Ir) genes mapping in the I-A (HBs-Ir-1) and I-C (HBs-Ir-2) subregions of the H-2 locus. To confirm that H-2-linked Ir genes regulate the immune response to HBsAg at the T cell level and to determine if the same Ir genes function in T cell activation as in B cell activation, the HBsAg-specific T cell responses of H-2 congenic and intra-H-2 recombinant strains were analyzed. HBsAg-specific T cell proliferation, IL 2 production, and the surface marker phenotype of the proliferating T cells were evaluated. Additionally, T cell-antigen-presenting cell (APC) interactions were examined with respect to genetic restriction and the role of Ia molecules in HBsAg presentation. The HBsAg-specific T cell proliferative responses of H-2 congenic and intra-H-2 recombinant strains generally paralleled in vivo anti-HBs production in terms of the Ir genes involved, the hierarchy of responses status among H-2 haplotypes, antigen specificity, and kinetics. However, the correlation was not absolute in that several strains capable of producing group-specific anti-HBs in vivo did not demonstrate a group-specific T cell proliferative response to HBsAg. The proliferative responses to subtype- and group-specific determinants of HBsAg were mediated by Thy-1+, Lyt-1+2- T cells, and a possible suppressive role for Lyt-1-2+ T cells was observed. In addition to T cell proliferation, HBsAg-specific T cell activation could be measured in terms of IL 2 production, because anti-HBs responder but not nonresponder HBs-Ag-primed T cells quantitatively produced Il 2 in vitro. Finally, the T cell proliferative response to HBsAg was APC dependent and genetically restricted in that responder but not nonresponder parental APC could reconstitute the T cell response of (responder X nonresponder)F1 mice, and Ia molecules encoded in both the I-A and I-E subregion are involved in HBsAg-presenting cell function.     

Isolation of twelve monoclonal antibodies against Ia and H-2 antigens. Serological characterization and reactivity with B and T lymphocytes

The cell hybridization technique was used for the production of 12 monoclonal antibodies against H-2K^k, H-2D^b, I-A^k and I-E^k antigens. The strain distribution pattern indicated that three antibodies reacted with new H-2 and Ia determinants, respectively, while the majority of determinants defined by the monoclonal antibodies showed good correlation with H-2 and Ia determinants described by conventional alloantisera.Monoclonal Ia antibodies showed strong reactivity with about 90% of surface IgM positive B cells, but not with T cells. In double fluorescence studies, both I-A and I-E determinants were always found to be coexpressed on the same B cells. When the high sensitivity of the fluorescence activated cell sorter was utilized, about 30 to 40% of purified lymph node T cells were found to carry both I-A and I-E antigens, although in a much lower density than B cells. In conclusion, monoclonal Ia antibodies appear to display the same serological and cellular reactivity pattern as do conventional antisera.     

Cellular immune response to acetylcholine receptors in murine experimental autoimmune myasthenia gravis: inhibition with monoclonal anti-I-A antibodies

Gene(s) at the I-A subregion of the murine major histocompatibility complex influence susceptibility to experimental autoimmune myasthenia gravis. C57Bl/6 mice immunized with acetylcholine receptors (AChR) in complete Freund's adjuvant demonstrated cellular and humoral immune responses to AChR. They developed muscle weakness characteristic of myasthenia gravis and demonstrated a reduction in the muscle AChR content. The kinetics of AChR-specific lymphocyte proliferation generally correlate with anti-AChR antibody response. AChR-specific lymphocyte proliferation was also observed in C57Bl/6 splenocytes after secondary immunization with AChR. The in vitro cellular reactivity to AChR in experimental autoimmune myasthenia gravis (EAMG) mice (C57Bl/6) was suppressed by monoclonal anti-I-Ab antibodies directed against private (Ia20) or public (Ia8) specificities, suggesting a critical role for these Ia determinants in the cellular immune response to AChR in murine EAMG.     

Detection of at least two distinct mouse I-E antigen molecules by the use of a monoclonal antibody

In an attempt to determine whether the expression of more than a single Ia antigen is determined by the I-E subregion of the mouse major histocompatibility complex (MHC), sequential immunoprecipitation analyses were performed by using a monoclonal antibody and alloantisera reactive with I-E subregion products. 3H-leucine-labeled glycoprotein preparations obtained from H-2d spleen cells were precleared with the monoclonal antibody 14-4-4S and then examined for residual Ia activity precipitable by an alloantiserum detected by SDS-polyacrylamide gel electrophoresis. Residual Ia activity was observed for all three strains of the H-2d haplotype tested. The residual Ia activity could be detected by sera absorbed with B10.A spleen cells, indicating that products of the I-E subregion rather than of the I-C subregion were responsible for this activity. No separable I-Ek molecules were detected in products of B10.A cells with the use of combinations of two monoclonal antibodies (including 14-4-4S) and several appropriate alloantisera. These findings indicate the presence of at least two similar but distinct I-E antigens encoded by the H-2d haplotype.     

Expression of I-A and I-E,C region-coded Ia antigens on functional B cell subpopulations.

Ia antigens from specific subregions have been examined on functional B cell populations. Expression of both I-A and I-E,C region antigens was demonstrated on cells required for both lipopolysaccharide mitogenesis and polyclonal activation. Similar I-A and I-E,C subregion expression was found on cells required for response to the T-independent antigen, polyvinylpyrrolidone. TNP-specific IgM and hen egg lysozyme-specific IgG plaque-forming cells also express I-A and I-E,C region antigens. No evidence was found for an Ia- population responsive in the systems tested. Further, no evidence of preferential expression of I-A or I-E,C region antigens was observed in any system examined. Therefore, it appears that B cells express both I-A and I-E,C region-coded Ia antigens.