Major histocompatibility complex-restricted self-recognition in responses to trinitrophenyl-Ficoll. A novel cell interaction pathway requiring self-recognition of accessory cell H-2 determinants by both T cells and B cells

In vitro primary antibody responses to limiting concentrations of trinitrophenyl (TNP)-Ficoll were shown to be T cell dependent, requiring the cooperation of T helper (TH) cells, B cells, and accessory cells. Under these conditions, TH cells derived from long-term radiation bone marrow chimeras were major histocompatibility complex (MHC) restricted in their ability to cooperate with accessory cells expressing host-type MHC determinants. The requirement for MHC-restricted self-recognition by TNP-Ficoll-reactive B cells was assessed under these T-dependent conditions. In the presence of competent TH cells, chimeric B cells were found to be MHC restricted, cooperating only with accessory cells that expressed host-type MHC products. In contrast, the soluble products of certain monoclonal T cell lines were able to directly activate B cells in response to TNP-Ficoll, bypassing any requirement for MHC-restricted self-recognition. These findings demonstrate the existence of a novel cell interaction pathway in which B cells as well as TH cells are each required to recognize self-MHC determinants on accessory cells, but are not required to recognize each other. They further demonstrate that the requirement for self-recognition by B cells may be bypassed in certain T-dependent activation pathways.

Role of the major histocompatibility complex in T cell activation of B cell subpopulations: antigen-specific and H-2-restricted monoclonal TH cells activate Lyb-5+ B cells through an antigen-nonspecific and H-2-unrestricted effector pathway

Monoclonal T helper (TH) cell populations were employed to study the mechanism of activation of the Lyb-5+ B cell subpopulation in T cell-dependent antibody responses in vitro. It was demonstrated that monoclonal T cell populations were sufficient to help rigorously T-depleted unprimed (B + accessory) cells for direct plaque-forming cell responses to trinitrophenyl- (TNP) conjugated keyhole limpet hemocyanin (KLH). The activation of several lines of cloned (H-2b X H-2k)F1 TH cells was antigen (KLH) specific and H-2 restricted. Individual clones were restricted to H -2b, H-2k, or unique (H-2b X H-2k)F1 encoded determinants. Under the experimental conditions employed, responses mediated by cloned TH cells were found to result in the activation of the Lyb-5+ B cell subpopulation. The activation of Lyb-5+ B cells by cloned TH cells did not require covalent linkage of carrier and hapten, and responses could be stimulated in the presence of free KLH plus TNP conjugated to an irrelevant carrier. The H-2 restriction of TH cell function was shown to reflect a requirement for T cell recognition of determinants expressed by accessory cells, whereas no requirement existed for restricted T cell recognition of B cells. These findings suggest that the help provided by monoclonal TH cells, once activated, was both antigen nonspecific and H-2 unrestricted. Consistent with this interpretation, it was found that the supernatant of antigen-stimulated TH cells provided antigen-nonspecific help to T-depleted spleen cells. Thus, these results demonstrate that the activation of Lyb-5+ B cells by antigen-specific and H-2-restricted monoclonal TH cell populations is itself antigen nonspecific and H-2 unrestricted.

Role of the major histocompatibility complex in T cell activation of B cell subpopulations: antigen-specific and H-2-restricted monoclonal TH cells activate Lyb-5+ B cells through an antigen-nonspecific and H-2-unrestricted effector pathway

Monoclonal T helper (TH) cell populations were employed to study the mechanism of activation of the Lyb-5+ B cell subpopulation in T cell-dependent antibody responses in vitro. It was demonstrated that monoclonal T cell populations were sufficient to help rigorously T-depleted unprimed (B + accessory) cells for direct plaque-forming cell responses to trinitrophenyl- (TNP) conjugated keyhole limpet hemocyanin (KLH). The activation of several lines of cloned (H-2b X H-2k)F1 TH cells was antigen (KLH) specific and H-2 restricted. Individual clones were restricted to H -2b, H-2k, or unique (H-2b X H-2k)F1 encoded determinants. Under the experimental conditions employed, responses mediated by cloned TH cells were found to result in the activation of the Lyb-5+ B cell subpopulation. The activation of Lyb-5+ B cells by cloned TH cells did not require covalent linkage of carrier and hapten, and responses could be stimulated in the presence of free KLH plus TNP conjugated to an irrelevant carrier. The H-2 restriction of TH cell function was shown to reflect a requirement for T cell recognition of determinants expressed by accessory cells, whereas no requirement existed for restricted T cell recognition of B cells. These findings suggest that the help provided by monoclonal TH cells, once activated, was both antigen nonspecific and H-2 unrestricted. Consistent with this interpretation, it was found that the supernatant of antigen-stimulated TH cells provided antigen-nonspecific help to T-depleted spleen cells. Thus, these results demonstrate that the activation of Lyb-5+ B cells by antigen-specific and H-2-restricted monoclonal TH cell populations is itself antigen nonspecific and H-2 unrestricted.

T cells from fully H-2 allogeneic (A replaced by B) radiation bone marrow chimeras are functionally competent and host restricted but are alloreactive against hybrid Ia determinants expressed on (A x B)F1 cells

In this communication it is demonstrated that T cells from fully allogeneic A replaced by B radiation bone marrow chimeras are alloreactive against the hybrid Ia molecules expressed on the surface of heterozygous A X B cells. These results suggested that previous failures to generate cytotoxic T lymphocyte (CTL) responses from fully allogeneic chimeras by sensitizing the chimeric T cells to antigen in an (A X B)F1-priming environment might have been confounded by an ongoing alloreaction against determinants created by hybrid Ia molecules expressed on F1 cells. Consequently, the ability to generate CTL responses from fully allogeneic chimeras was re-examined by sensitizing the chimeric T cells to antigen presented by homozygous rather that F1 stimulator cells. It was found that T cells of donor bone marrow origin that mediate cytotoxic responses to trinitrophenyl-modified self determinants do differentiate into functional competence in an H-2-incompatible host environment and are restricted to the host H-2 haplotype.

Self recognition in allogeneic thymic chimeras. Self recognition by T helper cells from thymus-engrafted nude mice is restricted to the thymic H-2 haplotype

To examine the possibility that the thymus determines the I region-restricted self-recognition repertoire expressed by T helper (TH) cells, thymic chimeras were constructed by transplanting allogeneic neonatal thymic lobes into congenitally athymic nude mice. Spleen TH cells from the thymic chimeras were themselves of nude host origin but only cooperated with B+ accessory cells of the thymic haplotype for primary in vitro responses to sheep erythrocytes and trinitrophenyl conjugate of keyhole limpet hemocyanin. Thus, these experiments demonstrate that the self-recognition repertoire expressed by TH cells is determined by the H-2 phenotype of the intrathymic environment in which the TH cells had differentiated.

Functional helper activity of monoclonal T cell populations: antigen-specific and H-2 restricted cloned T cells provide help for in vitro antibody responses to trinitrophenyl-poly(LTyr,Glu)-poly(DLAla)--poly(LLys)

The ability of long-term cultured and monoclonal T cell populations to provide antigen-specific help was assessed in a system of Ir gene-controlled in vitro antibody responses to soluble antigens. T-cell colonies and monoclonal T-cell lines were generated which proliferated specifically in response to poly(LTyr,Glu)-poly(DLAla)--poly(LLys) [(T,G)-A--L] and were I-A restricted in these proliferative responses. These (T,G)-A--L-specific T-cell populations were evaluated for their ability to help unprimed and T-cell depleted spleen cell populations in the generation of antibody responses to trinitrophenyl (TNP)-(T,G)-A--L in vitro. It was found that long-term T-cell lines, including monoclonal T-cell populations derived by limiting dilution, were highly efficient helper cells for IgM responses to TNP-(T,G)-A--L. These helper T cells were both antigen-specific and I-A restricted in their ability to be activated and to cooperate with T-cell depleted spleen cell populations. Once specifically activated, however, these clones provided help that was antigen nonspecific. These studies have thus demonstrated the ability of antigen-specific and H-2-restricted monoclonal T-cell populations to provide help for responses to soluble antigens in vitro.

Role of the major histocompatibility complex in T cell activation of B cell subpopulations Lyb-5+ and Lyb-5- B cell subpopulations differ in their requirement for major histocompatibility complex-restricted T cell recognition

This report has examined the requirements for T helper (T(H)) cell recognition of major histocompatibility complex (MHC) determinants expressed by B cells for the activation of unprimed Lyb-5(+) and Lyb-5(-) B cell subpopulations . The generation of primary T(H) cell-dependent plaque-forming cell responses in vitro microculture required the presence of Lyb-5(+) B cells because B cell populations that were deprived, either genetically or serologically, of the Lyb-5(+) subpopulation were not activated in these responses. Cell-mixing experiments in which A X B {arrow} A chimeric T(H) cells were mixed with purified populations of parental accessory cells and parental B cells demonstrated that the in vitro activation of Lyb-5(+) B cells did not require T(H) cell recognition of B cell MHC determinants, although it did require T(H) cell recognition of accessory cell MHC determinants . In contrast to the failure of Lyb-5(-) B cells to be activated in primary T(H) cell-dependent responses in vitro microculture, isolated populations of Lyb-5(-) B cells were triggered by T(H) cells in vivo in short-term adoptive transfer experiments . By the use of A X B {arrow} A chimeric T(H) cells and parental strain B adoptive hosts, it was possible in vivo to distinguish genetically restricted T(H) cell recognition of B cells from genetically restricted T(H) cell recognition of accessory cells. Similar to the results obtained in vitro, the activation in vivo of unfractionated (Lyb-5(+) plus Lyb-5(-)) B cell populations did not require T(H) cell recognition of B cell MHC determinants . In contrast, in the same in vivo responses activation of isolated populations of Lyb-5(-) B cells did require T(H) cell recognition of B cell MHC determinants. The most straightforward interpretation of these experiments is that T(H) cell recognition of B cell MHC determinants is required for the activation of Lyb-5(-) B cells but is not required for the activation of Lyb-5(+) B cells . To better understand why T(H) cell activation of one B cell subpopulation is genetically restricted, whereas activation of another subpopulation is not, the response of Lyb-5(+) and Lyb-5(-) B cells to the soluble activating factors present in concanavalin A-induced spleen cell supernates (Con A SN) was examined. It was observed that Lyb-5(-) B cells, as opposed to Lyb-5(+) B cells, were unable to respond in microculture to the nonspecific T(H) cell- activating factors present in Con A SN, even though they were able to nonspecifically respond under the same conditions to trinitrophenyllipopolysaccharide. It was observed that the ability of B cell subpopulations to respond to nonspecific soluble T cell factors paralleled their ability to be activated by T(H) cells in a genetically unrestricted manner. Thus, the present experiments demonstrate that activation by T(H) cells of Lyb-5(-) B cells is MHC restricted, whereas activation of Lyb-5(+) B cells is not. These experiments suggest that one possible explanation for such differences is that activation of Lyb-5(+) B cells does not require direct interaction with T(H) cells because they can be activated by soluble activation signals that T(H) cells secrete.

Self recognition in allogeneic radiation bone marrow chimeras. A radiation-resistant host element dictates the self specificity and immune response gene phenotype of T-helper cells

The specificity of the self-recognition repertoire in fully allogeneic (A {arrow} B), semiallogeneic (A {arrow} A x B and A x B {arrow} A), and double donor (A + B {arrow} A) radiation bone marrow chimeras was assessed by the ability of their spleen cells to generate in vitro primary plaque-forming cell (PFC) responses to trinitrophenyl- keyhole limpet hemocyanin. In contrast to spleen cells from semiallogeneic and double donor chimeras, intact spleen cells from fully allogeneic BI0 {arrow} B10.A and B10.A {arrow} B10 chimeras were not capable of generating responses to trinitrophenyl (TNP)-keyhole limpet hemocyanin. However, cultures containing a mixture of both B10 {arrow} B10.A and B10.A {arrow} B10 spleen cells did respond, demonstrating that all the cell populations required for the in vitro generation of T-dependent PFC responses were able to differentiate into functional competence in a fully allogeneic major histocompatibility complex (MHC) environment. The self recognition repertoire of T-helper cells from fully allogeneic A {arrow} B chimeras was determined to be specific for the recognition of host, not donor, MHC determinants in that they were able to collaborate with cells expressing only host MHC determinants but not with cells expressing only donor MHC determinants, even though the functional lymphocytes in these chimeras were shown to be of donor origin. Experiments utilizing double donor A + B {arrow} A chimeras further demonstrated that the ability of chimeric T cells to recognize allogeneic MHC determinants as self structures was a function of a radiation-resistant host element and not simply a consequence of the tolerization of T cell precursors to allogeneic MHC determinants, because strain A lymphocytes isolated from A + B {arrow} A chimeras were tolerant to both A and B MHC determinants but were restricted to the self recognition of syngeneic host type A MHC determinants. Finally, the Ir gene phenotype expressed by B10 {arrow} B10.A and B10.A {arrow} B10 chimeric lymphocytes was determined by their ability to function in the Ir gene controlled response to TNP-poly-L-(Tyr,Glu)-poly-D,L-Ala-poly- L-Lys [(T,G)-A--L]. The ability of lymphocytes to function in TNP-(T,G)-A--L responses was not determined by their genotype but rather paralleled the specificity of their self recognition repertoire for high responder (H-2 (b)) determinants. The possible degeneracy of the MHC-specific self recognition repertoire is discussed, and a model is proposed for Ir gene regulation in which expression of Ir gene function by lymphocytes is an antigen-nonspecific consequence of the specificity and cross-reactivity of their self recognition repertoire.

Major histocompatibility complex-restricted self recognition. A monoclonal anti-I-Ak reagent blocks helper T cell recognition of self major histocompatibility complex determinants

The functional role of cell surface Ia antigens has been studied for in vitro antibody responses, using as a probe the ability of anti-Ia reagents to inhibit these responses. A hybridoma monoclonal anti-Ia reagent specific for a product of I-Ak (Ia.17) profoundly inhibited in vitro antibody responses to TNP-KLH by spleen cells of the I-Ak but not I-Ab haplotype. This inhibition by anti-I-Ak product, but not by interaction with T or B cell product, in spite of the fact that functional B cells as well as accessory cells could be shown to express the determinant detected by this hybridoma reagent. These results suggest that the Ia expressed by accessory cells in of unique functional importance in these responses. To further characterize the function of Ia antigens in this response system, the mechanism of anti-I-Ak inhibition was determined. The inhibition resulting from interaction of anti-I-Ak with accessory cell Ia was not mediated by nonspecific suppressor cells, nor was there nonspecific interference with accessory cell function as a result of the binding of anti-Ia antibody. The relationship between anti-Ia inhibition and T helper cell recognition of self determinations on accessory cells was analyzed using T cells from radiation bone marrow chimeras. It was demonstrated that (B10 X B10.A)F1 leads to B10 (F1 leads to B10) chimera T cells were able to cooperate with B10 (H-2b and I-Ab) but not B10.A (H-2a and I-Ak) accessory cells for responses to TNP-KLH; F1 leads to B10.A T cells were able to cooperate with B10.A but not B10 accessory cells; and both chimera populations were able to cooperate with (B10 X B10.A)F1 (F1) accessory cells. Monoclonal anti-I-Ak inhibited the cooperation of F1 leads to B10.A T cells with the same F1 accessory cells. Thus, inhibition by anti-I-Ak is dependent upon active helper T cell recognition of I-Ak-encoded determinants expressed on accessory cells. These findings demonstrate that T cells recognize self Ia determinants expressed on accessory cells, and that such recognition is required for the generation of T cell-dependent antibody responses.

Cellular and genetic control of antibody responses. VIII. MHC restricted recognition of accessory cells, not B cells, by parent-specific subpopulations of normal F1 T helper cells

Normal F1 helper T cell populations were analyzed under conditions in which the number of T cells was strictly limiting so that anti-hapten PFC responses to TNP-KLH increased linearly with increasing numbers of T cells. The slope of the response line is a measure of the helper activity of the added T cell population and reflects the number of helper T cells activated. In this way, the recognition and activation requirements of normal F1 helper T cell subpopulations were analyzed. It was determined that the F1 helper T cells specific for recognizing antigen in the context of the MHC determinants of only one parent specifically recognized those determinants expressed on accessory cells, but not on B cells. Indeed, each parent-specific F1 T cell subpopulation was only triggered by one parent's accessory cells; but once triggered, each parent-specific F1 T cell subpopulation was capable of activating either parent's B cells.

Cellular and genetic control of antibody responses. VIII. MHC restricted recognition of accessory cells, not B cells, by parent-specific subpopulations of normal F1 T helper cells

Normal F1 helper T cell populations were analyzed under conditions in which the number of T cells was strictly limiting so that anti-hapten PFC responses to TNP-KLH increased linearly with increasing numbers of T cells. The slope of the response line is a measure of the helper activity of the added T cell population and reflects the number of helper T cells activated. In this way, the recognition and activation requirements of normal F1 helper T cell subpopulations were analyzed. It was determined that the F1 helper T cells specific for recognizing antigen in the context of the MHC determinants of only one parent specifically recognized those determinants expressed on accessory cells, but not on B cells. Indeed, each parent-specific F1 T cell subpopulation was only triggered by one parent's accessory cells; but once triggered, each parent-specific F1 T cell subpopulation was capable of activating either parent's B cells.

Cellular and Genetic Control of Antibody Responses

The expression of Ir gene function has been analyzed in the in vitro primary anti-hapten response to TNP-(T,G)-A—L. Helper T cells, B cells, and accessory cells of responder H-2b and nonresponder H-2a origin were individually assessed for the ability to support PFC responses to TNP-(T,G)-A—L.

For the normal B10.A (H-2a) nonresponder, it was found that the accessory cell function of the spleen adherent cell (SAC) population was under H-2 linked Ir gene control, since B10 (H-2b) but not B10.A (H-2a) SAC were capable of reconstituting responses to TNP-(T,G)-A—L by responder T cells and B cells. In contrast, the function of nonresponder B10.A B cells was not under H-2 linked Ir gene control, since nonresponder B10.A B cells were as effective as responder B10 B cells in responding to TNP-(T,G)-A—L.

The ability of B10.A → (B10 × B10.A)F1 (B10.A → F1) chimera spleen cells to respond to TNP-(T,G)-A—L was also investigated. It was demonstrated that B10.A → F1 spleen cells, which were greater than 95% donor (H-2a) origin, were nonresponders, i.e., the overall nonresponder phenotype of B10.A cells was not altered by maturation in the chimera environment. In experiments designed to examine the competence of each cell subpopulation of B10.A → F1 chimera spleen, it was found that nonresponder (H-2a) B10.A → F1 T cells provided effective help for the response to TNP-(T,G)-A—L when cooperating with responder B10 (but not nonresponder B10.A) SAC accessory cells; and that nonresponder (H-2a) B10.A → F1 B cells were similarly effective in supporting the response to TNP-(T,G)-A—L in the presence of responder accessory cells. Finally, B10, but neither B10.A nor B10.A → F1, SAC were competent to reconstitute the response of accessory cell-depleted (B10 × B10.A) F1 spleen cells to TNP-(T,G)-A—L.

Taken together, these results demonstrated that the Ir gene-controlled unresponsiveness of H-2a nonresponder spleen cells was expressed only at the level of the accessory cell, and not by either T cells or B cells. Thus, H-2a spleen cells are nonresponders to TNP-(T,G)-A—L, because the only accessory cells normally available for presenting TNP-(T,G)-A—L are nonresponder H-2a. Whether the defect is intrinsic to nonresponder accessory cells or is due to the functional absence of helper T cells (or B cells) capable of responding to antigen presented by nonresponder accessory cells has not been established.

Cellular and genetic control of antibody responses. V. Helper T-cell recognition of H-2 determinants on accessory cells but not B cells

Requirements for helper T-cell recognition of H-2 determinants expressed on adherent accessory cells and on B cells was individually assessed in the anti-hapten PFC responses to TNP-KLH. Complicating allogeneic effects were minimized or avoided by the use of helper T cells from normal F1 hybrids, parent leads to F1 chimeras, and F1 leads to parent chimeras. The results of both in vitro and in vivo experiments demonstrated that: (a) helper T cells are not required to recognize the identical H-2 determinants on both accessory cells and B cells; (b) helper T cells are required to recognize K or I-A region-encoded determinants expressed on accessory cells; (c) no requirement was observed in vitro or in vivo for helper T-cell recognition of B-cell-expressed H-2 determinants; and (d) no requirement was observed for H-2 homology between accessory cells and B cells. The absence of required helper T-cell recognition of the identical H-2 determinants on both accessory cells and B cells was demonstrated in two ways: (a) naive of KLH-primed (A x B)F1 hybrid helper T cells collaborated equally well with B cells from either parentA or parentB in the presence of accessory cells from either parent; (b) A leads to (A x B)F1 chimeric spleen cells depleted of accessory cells collaborated equally well with accessory cells from either parentA or parentB, even though the B cells only expressed the H-2 determinants of parentA. A requirement for helper T-cell recognition of K or I-A region-encoded H-2 determinants on accessory cells was also demonstrated in two ways: (a) (A x B)F1 leads to parentA chimeric spleen cells depleted of accessory cells collaborated with accessory cells from parentA but not parentB; and (b) (A x B)F1 leads to parentA chimeric helper T cells collaborated with normal F1 B cells only in the presence of parental or recombinant accessory cells that expressed the K or I-A region-encoded determinants of parentA. Although restricted in their ability to recognize H-2 determinants on accessory cells, it was demonstrated both in vitro and in vivo that (A x B)F1 leads to parentA chimeric helper T cells were able to collaborate with B cells from either parentA or parentB. In vitro in the presence of accessory cells from parentA, (A x B)F1 leads to parentA chimeric helper T cells collaborated equally well with B cells from either parent. In addition, the inability of (A x B)F1 leads to parentA chimeric helper T cells to collaborate with (B + accessory) cells from parentB was successfully reversed by the addition of parentA SAC as added accessory cells. In vivo, upon the addition of parentA accessory cells, (A x B)F1 leads to parentA chimeric helper T cells collaborated with parentB B cells in short-term adoptive transfer experiments.

Cellular and Genetic Control of Antibody Responses in Vitro

The expression of Ia antigens by accessory cells participating in in vitro primary antibody responses, including responses under Ir gene control, has been investigated. The ability of radiationresistant, non-T, non-B, spleen-adherent cells to function as accessory cells has been demonstrated both for the Ir gene-controlled response to TNP-(T,G)-A--L and for the response to TNP-KLH, for which no Ir gene control has been demonstrated. Pretreatment of these adherent accessory cells with anti-Ia reagents and complement totally abrogated the accessory function of this population for responses to both antigens. This abrogation of accessory function resulted from the elimination of accessory cells, rather than from any active suppression of responsiveness, either by carry-over of anti-Ia antibody or antigen-antibody complexes. By using appropriate recombinant strains and specific anti-Ia reagents, it has further been demonstrated that Ia determinants encoded in at least two I subregions, I-A and I-E/C, are expressed on accessory cells for responses to TNP-KLH and TNP-('T,G)-A--L; and that determinants encoded in at least two I subregions are expressed simultaneously on the same cells. The data presented also suggest that the functionally active Ia-positive accessory cell is itself either nonphagocytic or constitutes a subpopulation of phagocytes expressing Ia antigens encoded in at least two I subregions.

Cellular and genetic control of antibody responses in vitro. III. Immune response gene regulation of accessory cell function

The possibility was investigated that Ir genes regulate the function of cells other than T or B cells in the primary IgM responses to the synthetic antigens trinitrophenylated poly-L-(Tyr,Glu)-poly-D,L-Ala--poly-L-Lys [TNP-(T,G)-A--L]and trinitrophenylated poly-,-(His,Glu)-poly-D, L-Ala--poly-L-Lys [TNP-(H,G)-A--L]. The primary responses of (B10 x B10.A)F(1) spleen cells to both antigens were abrogated by Sephadex G-10 passage, and restored by the addition of spleen adherent cells. The cell type in the spleen adherent cell population active in reconstituting the responses to TNP-(T,G)-A--L and TNP-(H,G)-A--L was a non-T, non-B, radiation-resistant, glass-adherent spleen cell. The responses of Sephadex G-10-passed (responder x nonresponder)F(1) spleen cells to TNP-(T,G)-A--L or TNP-(H,G)-A--L were reconstituted by spleen adherent cells from only responder strains. Spleen adherent cells from F(1) mice reconstituted the responses to both antigens. Spleen adherent cells from each of the strains tested reconstituted the non- Ir gene-controlled response to a third antigen, TNP-keyhole limpet hemocyanin. The inability of spleen adherent cells from nonresponder strains to reconstitute the responses to either TNP-(T,G)-A--L or TNP-(H,G)-A--L was not a result of active suppression induced by the presence of nonresponder adherent cells, since a mixture of responder and nonresponder spleen adherent cells reconstituted the responses to both antigens. The use of spleen adherent cells from recombinant strains demonstrated that the autosomal dominant genes controlling the ability of spleen adherent cells to function as accessory cells in the responses to TNP-(T,G)-A--L and TNP-(H,G)-A--L are located in the K or I-A regions of the responder H-2 complex, the same region(s) of H-2 as the Ir genes controlling overall in vitro and in vivo responsiveness to these antigens.

Regulatory Mechanisms in Cell-Mediated Immune Responses

Mouse spleen cells which had been primed to alloantigens by in vitro mixed lymphocyte culture (MLC) were found to actively suppress the in vitro generation of cell-mediated cytotoxicity (CML) by freshly explanted syngeneic spleen cells. Both antigen-specific and antigen-nonspecific suppressor activities were identified. Antigen-nonspecific suppression was active in suppressing CML sensitizations to alloantigens, including those to which the suppressor population had not been previously exposed. This form of suppression was radiation sensitive (abrogated by 1000 R). It was T cell dependent, appeared after 2 days of priming in MLC, and was effective in suppressing CML only when added during the first 2 days of sensitization culture. In contrast, antigen-specific suppression affected only those CML snesitizations in which the relevant sensitizing antigen either: 1) was the same antigen to which the suppressor population was previously primed, or 2) was presented on the same cell as that alloantigen. Antigen-specific suppression was resistant to 1000 R. It was T cell-dependent, first appeared after 3 days of priming in MLC, and suppressed CML only when added during the first 2 days of sensitization. These data are discussed in terms of regulatory mechanisms which may be active in determing the kinetics and antigen specificity of primary and secondary CML responses.