T helper and T suppressor cells are restricted by the A and E molecules,respectively, in the F antigen system

The role of the A and E molecules as restriction elements was examined in the F antigen system. In the mouse the only responder haplotype known to date isk, and blocking studies with a monoclonal antibody show that in vitro T-cell proliferation is restricted by the A^k molecule. The (CBA × DBA/2) F₁ hybrid, which is a responder x nonresponder cross, is itself a nonresponder in terms of E-specific antibody production. Up to 10 days after priming, (CBA × DBA/2) F₁ T cells exhibited an E-specific proliferative response, but this diminished rapidly at later times. This diminution could be blocked with an E-specific monoclonal antibody, suggesting that suppression is restricted by the E molecule.     

T cell proliferative responses to a mitogen derived from Mycoplasma arthritidis are controlled by the accessory cell

Cell-free supernatants from broth cultures of Mycoplasma arthritidis (MAS) induce vigorous proliferative responses in thymus-derived T lymphocytes from H2k or H2d strains of mice. Populations of lymphoid cells from mice of H2b, H2q, or H2s haplotypes do not respond to this stimulus. Previous studies with lymphoid cells from congenic and recombinant strains of mice indicate that the T cell proliferative response induced by MAS is controlled by a gene(s) that maps to the I-E/C subregion of the murine major histocompatibility complex (MHC). The T cell proliferative response induced by MAS is dependent upon the presence of a population of la+, radioresistant accessory cells (AC). Data presented here demonstrates that responder strain AC that have been pulsed with MAS (followed by extensive washing) induced vigorous proliferative responses in subsequently added T cell populations. Pulsing of T cells with MAS, followed by the addition of AC, however, did not result in T cell proliferation. MAS was found to stimulate (responder X nonresponder) F1 T cells to proliferate if the MAS was presented in the context of either responder or (responder X nonresponder) F1 AC; nonresponder strain AC were ineffective in this regard. Nonresponder strain T cells were found to be capable of responding to MAS if it was presented in the context of responder strain AC, even if the T cells and AC were completely allogeneic. Thus, nonresponder strain T cells mounted vigorous proliferative responses if the MAS was presented in the context of responder strain AC. Conversely, responder strain T cells did not respond to MAS presented in the context of nonresponder strain AC. In addition, lymphoid cells from a B10 leads to B6AF1 radiation bone marrow chimera were also found to be capable of responding to MAS, but only in the presence of AC that expressed cell surface determinants controlled by the I-E/C subregion. The data presented here indicate that MAS-induced T cell proliferative responses are controlled at the level of the AC by a gene(s) that maps to the I-E/C subregion of the MHC.     

Normal HBsAg presentation and T-cell defect in the immune response of nonresponders

Lymphocytes from nonresponders to HBsAg fail to proliferate in vitro in the presence of HBsAg-pulsed antigen presenting cells. We studied four pairs of major histocompatibility complex (MHC)-matched, mixed lymphocyte reaction-negative individuals discordant for HBsAg response. For each pair, responder lymphocytes proliferated in the presence of nonresponder antigen-pulsed antigen presenting cells. Respondera nd nonresponder antigen presenting cells were equally effective. There was no evidence for inhibition of responder T-cell proliferation by nonresponder lymphocytes or antigen presenting cells. The defect is thus in the helper T cells of nonresponders and not in the antigen processing or binding of processed peptides to MHC molecules on antigen presenting cells.     

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.     

The nature of the interaction between suppressor and helper T cells in the response to LDHB

Purified Lyt-1+2+ T cells were depleted of alloreactive cells by BUdR and light treatment, and then were primed in vitro against LDHB presented on allogeneic APC. Such cells could be restimulated by LDHB on the same allogeneic APC, but not by LDHB on APC syngeneic with the T cells. The restimulated T cells suppressed the proliferative response of Lyt-1+2- T cells primed and restimulated by the same antigen. The suppression, which was antigen specific, occurred after a 6-hr co-culture of the suppressor (Tse) and proliferating helper (Th) cells. The successful interaction (as measured by suppression) between allogeneic Th and Tse cells was found to be determined by the restriction specificity but not the MHC haplotype of Th cells, and the MHC haplotype but not the restriction specificity of Tse cells. Thus, suppression occurred only when the Tse cells carried genes controlling the MHC molecules that served as restriction elements for antigen recognition by the Th cells. No evidence could be obtained for the participation of APC in the Tse-Th interaction. The data suggest the interaction is based on the recognition by the Th cell of the antigen presented in the context of MHC molecules controlled by the Tse cell.     

Insulin-specific suppressor T cell factors

Murine antibody responses to heterologous insulins are controlled by MHC-linked immune response genes. Although nonresponder mice fail to make antibody when injected with nonimmunogenic variants of insulin, we have recently shown that nonimmunogenic variants stimulate radioresistant, Lyt- 1+2- helper T cells that support secondary antibody responses. However, the helper activity can not be detected unless dominant, radiosensitive Lyt-1-2+, I-J+ suppressor T cells are removed. In this paper we report that extracts of primed Lyt-2+ suppressor T cells contain insulin-specific suppressor factors (TsF) that are capable of replacing the activity of suppressor T cells in vitro. The activity of these factors is restricted by MHC-linked genes that map to the I-J region, and immunoadsorption studies indicated that they bind antigen and bear I-J-encoded determinants. Insulin-specific TsF consists of at least two chains, one-bearing I-J and the other the antigen-binding site. Furthermore, mixing of isolated chains from different strains of mice indicates that the antigenic specificity is determined by the antigen-binding chain and the MHC restriction by the H-2 haplotype of the source of the non-antigen-binding, I-J+ chain. Moreover, mixtures containing antigen-binding chain from allogeneic cell donors and I-J+ chain from responder cell donors have activity in cultures containing responder lymphocytes. This suggests that preferential activation of suppressor T cells, rather than differential sensitivity to suppression, results in the nonresponder phenotype to insulin.     

Is genetically related macrophage factor (GRF) a soluble immune response (Ir) gene product?

The possibility that the antigen-presenting "macrophages" interacting with helper cells either directly or via the intermediary action of a soluble factor consisting of Ia antigen and a fragment of immunogen, termed GRG (genetically related factor), are a site of Ir gene action was investigated by using the synthetic polypeptide antigen (T,G)-A--L. It was found that T cells from (responder x nonresponder) F1 mice were stimulated by responder "macrophages" or GRF derived from these cells but not by the nonresponder macrophages of GRF from these cells. This suggests that the defect in helper cell induction in nonresponders is at the level of the presenting cell and that the macrophage factor GRF is a soluble Ir gene product. This conclusion was supported by the observation that there was normal presenting cell and GRF function in nonresponders, mouse strains such as CBA that yield helper cells and helper factor with (T,G)-A--L and have defects elsehwere.     

Macrophage-lymphocyte clusters in the immune response to soluble protein antigen in vitro. VII. Genetically restricted and nonrestricted physical interactions.

We have assessed the genetic restrictions on physical interactions between macrophages and central lymphocytes and between central and peripheral lymphocytes in antigen-specific macrophage-lymphocyte clusters with respect to I-region differences of inbred strains 2 and 13 guinea pigs. When using lymphocytes from guinea pigs immunized with DNP-OVA or DNP-GL in CFA, the antigen-specific interaction between central lymphocyte and macrophage requires that both cells be derived from animals syngeneic at the I-region of the major histocompatibility complex. In studies using antigens, the responses to which is under the control of MHC-linked Ir genes, macrophages from the responder, but not from the nonresponder parental strain support cluster formation with responder x nonresponder F1(2 X 13) T cells. In contrast, the physical interactions between central and peripheral T lymphocytes are not restricted by the I-region of the MHC and the peripheral lymphocyte need not be from an animal immune to the antigen used to drive macrophage central lymphocyte interactions.     

Differences in cellular immune response to l-glutamic acid60, l-alanine30, l-tyrosine10 (GAT) in full sibling embryo transfer cattle: Examination of requirements for cell proliferation

Pairs of full sibling embryo transfer cattle that expressed identical MHC class I and II products were tested for their in vitro proliferative response to GAT. Peripheral blood mononuclear cells from these cattle were either high or low responders to GAT. Cells from certain pairs of MHC identical siblings gave opposite responses. Low responder animals were further tested to determine if they might respond to GAT with different kinetics, with secondary in vitro restimulation, or with exogenous help provided by interleukin 2. Also, the role of antigen presenting cells and suppressor T cells from low responder animals was investigated. Using appropriate in vitro conditions, cells from all animals tested could respond to GAT. However, MHC identical animals tested under similar conditions exhibited differences in their response to GAT which suggests the proliferative immune response was influenced by factors in addition to MHC coded products.     

Specific absorption of T lymphocytes committed to soluble protein antigens by incubation on antigen-pulsed macrophage monolayers.

Monolayers of macrophages (Mphi) pulsed with antigen were used as immunosorbents for T lymphocytes from guinea pigs primed to soluble protein antigens. T lymphocytes were cultured on the Mphi monolayers for 4 hr, then aspirated and reincubated on a fresh monolayer pulsed with the same antigen for a second and a third step. T lymphocytes so treated were selectively deprived of cells responding in assay for antigen-dependent proliferation against the antigen used for pulsing the absorbing monolayer, but maintained their response to other antigens. The lymphocytes adhering to the Mphi of the absorbing monolayer were capable of giving a full response to the antigen used for pulsing the Mphi of the monolyers. The proliferative response of F1 T lymphocytes to antigen in association with Mphi of either parental strain could be absorbed leaving the response to antigen in association with Mphi of the other parental strain. The absorption of the proliferative response was not inhibited by addition of excess soluble antigen to the medium of the absorption culture. Our results indicate that specific guinea pig T lymphocytes responding by proliferation to soluble protein antigens recognize and bind specifically to a complex of Ia antigen and protein antigen at the surface of the Mphi.     

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.     

T cell subsets in (responder x nonresponder)F1 mice regulating antibody responses to L-glutamic acid60-L-alanine30-L-tyrosine (GAT)

Immune responses to GAT are controlled by H-2-linked Ir genes; soluble GAT stimulates antibody responses in responder mice (H-2b) but not in nonresponder mice (H-2q). In nonresponder mice, soluble GAT stimulates suppressor T cells that preempt function of helper T cells. After immunization with soluble GAT, spleen cells from (responder x nonresponder: H-2b X H-2q)F1 mice develop antibody responses to responder H-2b GAT-M phi but not to nonresponder H-2q GAT-M phi. This failure of immune F1 spleen cells to respond is due to an active suppressor T cell mechanism that is activated by H-2q, but not H-2b, GAT-M phi and involves two regulatory T cell subsets. Suppressor-inducer T cells are immune radiosensitive Lyt-1 +2-, I-A-, I-J+, Qa-1+ cells. Suppressor-effector T cells can be derived from virgin or immune spleens and are radiosensitive Lyt-1-2+, I-A-, I-J+, Qa-1+ cells. This suppressor mechanism can suppress responses of virgin or immune F1 helper T cells and B cells. Helper T cells specific for H-2b GAT-M phi are easily detected in F1 mice after immunization with soluble GAT; helper T cells specific for H-2q GAT-M phi are demonstrated after elimination of the suppressor-inducer and -effector cells. These helper T cells are radioresistant Lyt-1+2-, I-A+, I-J-, Qa-1- cells. These data indicate that the Ir gene defect in responses to GAT is not due to a failure of nonresponder M phi to present GAT and most likely is not due to a defective T cell repertoire, because the relevant helper T cells are primed in F1 mice by soluble GAT and can be demonstrated when suppressor cells are removed. These data are discussed in the context of mechanisms for expression of Ir gene function in responses to GAT, especially the balance between stimulation of helper vs suppressor T cells.     

Specific binding of T lymphocytes to macrophages. V. The role of Ia antigens on Mphi in the binding.

The effect of anti-Ia alloantiserum on the capacity of selected peritoneal exudate lymphocytes (selected PEL) to bind to antigen-pulsed F1 (responder x nonresponder) macrophages was investigated. With the use of selected PEL for antigens under Ir gene control, it was shown that anti-Ia serum to the responder haplotype blocked adherence of selected PEL to antigen-pulsed macrophages whereas anti-Ia serum to the nonresponder haplotype did not. The target cell of the anti-Ia alloantiserum appeared to be the macrophage because anti-13 Ia in contrast to anti-2 Ia did not inhibit binding of F1 (2 x 13) DNP-GL selected PEL to DNP-GL pulsed strain-2 Mphi (responder strain). Taken together with previous experiments that indicate that an antibody to the native protein antigen employed is unable to block specific binding, the present results suggest that T cells may recognize fragments of exogenous antigen in association with Ia molecules.     

Class I MHC molecules rather than other mouse genes dictate influenza epitope recognition by cytotoxic T cells

Influenza nucleoprotein (NP) is an important target antigen for influenza A virus cross-reactive cytotoxic T cells (Tc). Here we examine the NP epitope recognized by cloned and polyclonal BALB/c Tc and the genetics of this recognition pattern. We can define NP residues 147–161 as the epitope seen in conjunction with K ^d , the only H-2^d class I responder allele for NP restriction. H-2 ^d /H-2 ^b F₁ mice (C57BL × DBA/2) primed by influenza infection lyse only H-2^d target cells treated with peptide 147–161 while H-2^b targets are recognized only after treatment with NP residues 365–379 (previously found to be recognized by D^b restricted Tc cells). Tc cell recognition of NP peptide 147–161 is entirely dictated by expression of K ^d and not by other B10 or OH background genes of congenic mice. Restriction of a unique NP sequence by each responder class I major histocompatibility complex (MHC) allele suggests that antigen and class I MHC interact for Tc recognition.     

Recessive T cell response to poly (Glu50Tyr50) possibly caused by self tolerance

The proliferative T cell response of inbred mouse strains to the random copolymer poly(Glu50Tyr50) (GT) was found to fall into two categories. Some strains responded only marginally (delta cpm values less than 10,000 and stimulation indices less than 3), whereas other strains mounted a substantial response (delta cpm 10,000 to 80,000, SI 3 to 30). The response is controlled by the A alpha and A beta loci of the major histocompatibility complex (MHC), as well as by genes not linked to the MHC. Because the response is selectively inhibited by monoclonal antibodies specific for the A alpha A beta molecule, we assume that its control by A loci is manifested as an A-restriction of the participating T (Ly-1high, Ly-2-) cells. It is of interest that the responsiveness is recessive in F1 hybrids of responder and nonresponder strains that are H-2-identical, but differ at their genetic background. Nonresponsiveness of these F1 mice is caused neither by a defect of antigen presentation, nor the result of immune suppression on priming or at the effector phase of the response. It is most likely the consequence of clonal deletion during the establishment of self-tolerance.     

Helper effects required during in vivo priming for a cytolytic response to the H-Y antigen in nonresponder mice

Induction of H-Y-specific cytotoxic T lymphocyte (CTL) responses in nonresponder female mice was attempted by i.v. injection of allogeneic male cells, followed by in vitro restimulation of recipient spleen cells with syngeneic male cells. Responses were obtained only in two strain combinations in which the recipients, although phenotypically nonresponders, carried responder alleles at class I major histocompatibility complex (MHC) loci, and the immunizing cells differed from the recipients at class II MHC loci. The two positive strain combinations were B10.A(2R) anti-B10.A(4R), and B10.GD anti-B10.D2(R101). In the first combination, both recipient and donor are nonresponders to H-Y, and the CTL are induced via a bystander effect of another CTL response to a previously undetected minor histocompatibility (H) antigen. This "carrier" antigen can only induce CTL against H-Y and itself when the immunizing cells express class II MHC molecules. Furthermore, the presence of H-Y and the carrier antigen on the same cell is a prerequisite for the generation of H-Y-specific CTL. In the second combination, the recipient is a nonresponder, whereas the donor is a responder. The two strains differ at only E alpha and E beta class II MHC loci. For the induction of CTL, H-Y and the foreign E molecule must be expressed on the same cells. Thus, the B10.D2(R101) cells that express E molecules on their surface probably provide the E-nonexpressor B10.GD recipients with a stimulus for the generation of H-Y-specific T helper cells. The data are consistent with the notion that antigen-specific class II MHC-restricted T helper cells are involved in the initiation of CTL responses to minor H antigens.     

The functional helper T cell repertoire specific for L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT)

Experiments have been carried out to examine the potential helper T cell repertoire specific for the random terpolymer GAT on responder, nonresponder, and (responder x nonresponder)F1 murine strains. The ability of GAT-MBSA immunized T cells to collaborate with DNP-specific primary and secondary B lymphocytes of each strain in response to the antigen DNP-GAT was tested with the splenic fragment culture system. The results of these experiments show that there are GAT-specific T lymphocytes in the responder, nonresponder, and F1 strains but that these 3 GAT-specific T cell populations differ in their collaborative potential. In sum, these findings present new evidence that the nonresponder status to the terpolymer GAT is due, in part, to a functional deletion of helper T cells capable of recognizing the antigen in the context of the nonresponder haplotype. Further, a new responsive phenotype is evidenced when F1 secondary B cells are stimulated in nonresponder GAT-MBSA-primed recipients. In this case, rather than the IgG1 responses observed in such strain combinations to other antigens such as DNP-Hy or DNP-Gl phi 9, only IgM responses were obtained. This new phenotype may be the result of GAT-specific suppression of isotype switching by B cells bearing the nonresponder cell surface alloantigens.     

Genetically restricted immune responses in guinea pigs primed in vivo with antigen-bearing macrophages.

Guinea pigs injected intradermally with antigen pulsed macrophages generate a population of immune T cells that proliferate in vitro on second exposure to antigen. T cells from F1 (2 X 13) guinea pigs immunized with DNP-OVA on one parental macrophage respond in vitro only to DNP-OVA on macrophages identical to those used for immunization and not to DNP-OVA associated with the other parental macrophages. These results demonstrate that the immunogenicity of antigen is dependent upon the macrophages used for priming in that, with this approach, strain 2 or 13 guinea pigs immunized with allogeneic macrophages pulsed with antigen do not respond to either allogeneic or syngeneic antigen-bearing macrophages. However, lysates of antigen-pulsed macrophages can still immunize either allogeneic or syngeneic recipient via their own macrophages. F1 (2 X 13) guinea pigs are immunized by insulin B chain pulsed strain 13 macrophages (responder) but not by strain 2 macrophages (nonresponder) suggesting that whether a F1 (nonresponder X responder) guinea pig recognizes antigen bound to a parental macrophage is genetically restricted before immunization to the same extent as the donor parental macrophages used for immunization.     

Antiviral protection by CD8+ versus CD4+ T cells. CD8+ T cells correlating with cytotoxic activity in vitro are more efficient in antivaccinia virus protection than CD4-dependent IL.

T cell-mediated protection against a recombinant vaccinia virus was evaluated in mice with respect to the relative contributions of CTL vs that of T cell-dependent IL and of CD4+ cells. H-2b mice primed with the wildtype of vesicular stomatitis virus serotype Indiana (VSV-IND wt) mount an in vitro measurable cytotoxic response against the nucleoprotein (NP) of VSV-IND and are protected against a challenge infection with a vaccinia-VSV recombinant virus expressing the NP of VSV-IND (vacc-IND-NP). Their protective mechanism was highly susceptible to in vivo depletion of CD8+ T cells, but resistant to CD4+ depletion or treatment with anti-IFN-gamma and anti-TNF-alpha. Surprisingly, also VSV-CTL nonresponder H-2k mice were protected against a challenging infection with vacc-IND-NP when primed with VSV-IND wt. In contrast to the CTL responder H-2b mice, this protection was highly susceptible to CD4+ T cell depletion and to anti-IFN-gamma or anti-TNF-alpha treatment, but resistant to CD8+ T cell depletion. Antibodies were not responsible because they failed to transfer protection; in contrast CD4+ T cells conferred significant protection. VSV-CTL responder H-2b and nonresponder H-2k mice were protected almost equally well against a challenge dose of 10(3) pfu vacc-IND-NP inoculated intracerebrally. However, after intracerebral challenge with 5 x 10(6) pfu vacc-IND-NP, the CTL nonresponder mice died, whereas the CTL responder mice eliminated the virus by day 5. These results collectively show that CD4+ T cell-dependent IL may mediate antiviral protection, but their efficiency is relatively weak compared with CD8-mediated protection correlating with cytotoxic activity in vitro.     

Inverse Ir gene control of the antibody and T cell proliferative responses to human basement membrane collagen

The immune response to pepsin-soluble human basement membrane-derived type IV collagen in mice has been characterized. Both T cell proliferative and antibody responses have been shown to be under major histocompatibility complex (MHC)-linked Ir gene control in inbred and MHC congenic mice. However, unlike previous examples studied, this response shows a separation of these two types of immunologic responsiveness. Only mice having I-As give potent in vitro T cell proliferative responses to type IV collagen whereas all mice except those having I-As give high antibody responses to this antigen. In (I-As X I-Anon-s) F1 mice, the T cell proliferative response was dominant, whereas antibody responses were markedly reduced compared with the responder parent. Given the recent demonstration that class II MHC-restricted, L3T4+ T cells can be divided into two sets, one of which helps for antibody responses and the other of which produces interleukin 2 and can also suppress such responses, it seems likely that these data can be accounted for on the basis of differential activation by this antigen of these two cell sets in mice of different MHC genotypes.