Cytotoxic T cells generated in the autologous mixed lymphocyte reaction. I. Primary autologous mixed lymphocyte reaction

In the course of the culture of an autologous mixed lymphocyte reaction (AMLR), T cells proliferated in response to autologous non-T cells, and differentiated to cytotoxic T cells (AMLR killers). DNA synthesis was necessary to generate AMLR killers, as the elimination of autoreactive proliferating cells with BUdR and UV light completely abrogated AMLR killer cytolysis. Amlr killers lysed various lymphoid cell lines, including autologous B cell lines, autologous or allogeneic mitogen blasts stimulated by Con A, PHA, or pokeweed mitogen, variious nonlymphoid cell lines derived from human, mouse, or rat, and weakly normal autologous or allogeneic non-T cells. KMT-17, methylcholanthrene-induced rat fibrosarcoma, was the only resistant cell line to have been tested. AMLR killers had characteristics similar to NK cells, Major histocompatibility antigens were not the target antigens for AMLR killers. AMLR killers distinguished the blasts stimulated by alloantigens as self from the blasts stimulated by mitogens as non-self.     

Stimulation of autologous and allogeneic human T cells by B cells occurs through separate B-cell antigen systems.

Studies were performed to determine the cell types involved in autologous mixed lymphocyte reactions (AMLR). Separation of peripheral blood leucocytes from normal donors into T-cell and B-cell enriched populations and subsequent co-culture of T cells and mitomycin-C inactivated non-T cells resulted in increased DNA synthesis by the responding T cells. Procedures such as removal of plastic-adherent or phagocytic cells enriched the B-cell content of the stimulator population and also enhanced stimulation of both autologous and allogeneic T cells. Velocity sedimentation separation of peripheral blood leucocytes into large and small lymphocyte fractions showed that cells which stimulate in MLR and AMLR are found in the small cell fraction and that large cells, morphologically monocytes, do not stimulate either reaction. Studies with anti-B-cell antisera obtained from multiparous women showed that the antigens of B cells which stimulate AMLR are distinct from those which stimulate MLR.     

Autologous responses to human leukaemic cells in mixed leucocyte culture

Summary Cryopreserved leukaemic blasts and remission non-T cells from 22 patients with acute leukaemia (15 lymphocytic, 7 non-lymphocytic) were tested as stimulators of autologous remission T cells and normal allogeneic T cells in primary and secondary MLC. In most cases the autologous response elicited by leukaemic cells was less than or equal to that elicited by remission non-T cells. However, T cells from 2 patients in long-standing first remission from ANLL displayed greater proliferation in response to leukaemic blasts than to remission non-T cells in both primary and secondary MLC. The results are suggestive of sensitization of these 2 patients to leukaemia-specific antigens, but other possible explanations are discussed. Abbreviations used: MLC, mixed leucocyte culture; ANLL, acute non-lymphocytic leukaemia; ALL, acute lymphoblastic leukaemia; AMLR, autologous mixed lymphocyte reaction; NK cells, natural killer cells; MNC, mononuclear cells     

Activated human T cells can present alloantigens but cannot present soluble antigens

Human T cells, when activated by antigen or mitogen, express Ia antigens. We have examined the capacity of activated T cells to stimulate autologous and allogeneic T cells and their ability to present soluble antigen. Interleukin 2-dependent T-cell lines (TCL), free of accessory cells, were used for antigen-presenting cells. These activated T cells were potent stimulators in an autologous mixed lymphocyte reaction (AMLR), more so than autologous irradiated non-T mononuclear cells. Activated T cells were also able to stimulate proliferation of allogeneic T cells in the absence of any other accessory cells, and this stimulation was blocked by anti-Ia antibodies. Resting unstimulated T cells were unable to stimulate autologous or allogeneic responses. Thus, activated T cells were able to present self antigens and alloantigens. However, activated T cells could not present soluble antigens to autologous T cells or to antigen-specific TCL even if exogenous interleukin 1 was added to cultures. The ability of activated T cells to stimulate an AMLR in vitro may reflect an important immunologic amplification mechanism in vivo. The ability of activated T cells to present alloantigens but not soluble antigens suggests an inability to process antigen, and this may provide further insights into the complexities of antigen presentation.     

T cell proliferation induced by autologous non-T cells is a response to apoptotic cells processed by dendritic cells

Self-reactive T cells are present in the mature immune repertoire as demonstrated by T cell proliferation induced by autologous non-T cells in the autologous mixed lymphocyte reaction. This reaction generates regulatory T cells in vitro and may reflect immune regulatory pathways in vivo, but the antigenic peptides recognized remain uncharacterized. We revisited this issue in light of the importance of apoptosis in immune regulation. We found that apoptosis among peripheral blood non-T stimulator cells is associated with augmented induction of autologous T cell proliferation. Our data show that caspase activity in the non-T stimulator population is essential for induction of autologous T cell proliferation, suggesting that cellular components in the non-T cell fraction are enzymatically modified, most likely by effector caspases, and have a direct or indirect effect on autoreactive T cell activation. Furthermore, exposure of macrophage-derived dendritic cells to apoptotic non-T cells augments autologous T cell proliferation, and blockade of alpha(v)beta(5) integrin, but not alpha(v)beta(3), inhibits the capacity of irradiated non-T cells or dendritic cells to stimulate autologous T cell proliferation. These experiments, using an entirely autologous system, suggest the interpretation that autoreactive T cells may recognize self-Ags modified through the actions of caspases and presented to T cells by dendritic cells. Induction of an in vivo autologous mixed lymphocyte reaction by caspase-modified self-Ags present in apoptotic cells may represent a mechanism to maintain peripheral immune tolerance.     

The human autologous mixed lymphocyte reaction. I. Suppression by macrophages and T cells

The autologous mixed lymphocyte reaction (AMLR) is a proliferative response of T cells to signals from autologous non-T cells. The AMLR has been an enigma to immunologists because spontaneous proliferation of cells removed from the body is usually substantially less than that observed with a strong AMLR. However, the AMLR is thought to represent an important in vitro function, since it has the attributes of other immune responses, and it is abnormal in a variety of disease states thought to have an immune basis. We reasoned that if the AMLR represented a fundamental immune phenomenon, it should be subject to regulation. In the present study, we present evidence for suppression of the AMLR by macrophages and by T cells. Macrophages inhibited the T cell proliferation to (B + null) cells in a dose-dependent fashion and throughout the time course of the AMLR. Elimination of suppressor T cells by a specific antiserum led to an increase in the AMLR, which was again suppressed in a dose-dependent way by addition of the suppressive T cells. It may be concluded that the AMLR itself is subject to immune regulation and that the suppressive influences observed probably strongly inhibit the AMLR in vivo. Removal of the suppressive principles allows the maximal expression of the AMLR in vitro. We believe that our demonstration of regulation of the AMLR should remove the enigma associated with it and lead to a better understanding of normal cell-cell interactions as well as the basis for abnormalities in a variety of immune-mediated diseases.     

The alpha chain,not the beta chain of HLA-DR antigens participates in activation of T cells in autologous mixed lymphocyte reaction

Using antisera against the alpha, the beta or the complex of both chains of HLA-DR antigens, we have studied the role of individual chains of HLA-DR antigens in activation of T cells in autologous mixed lymphocyte reaction (AMLR). Alpha chain-specific antibody, not anti-beta chain serum prevented T cells from acquiring responsiveness to interleukin-2 (IL-2), suppressed the production of 1L-2, and inhibited the T cell proliferative response in both primary and secondary AMLR cultures. However, proliferation of already activated IL-2 reactive T cells supported by IL-2 was not affected by any of the four different types of anti-DR sera used. Fifty to sixty percent of T cells activated by AMLR or by PHA possessed DR antigens and functioned well as stimulator cells in secondary AMLR cultures. Moreover, the stimulatory activity of these DR-positive T cells was suppressed by the anti-alpha chain, not by the beta chain-specific antibody. Since continuous proliferation of T cells requires IL-2 and since nonactivated T cells are not sensitive to IL-2 and are unable to absorb this growth factor, we conclude the following: (1) The alpha, not the beta chain of HLA-DR antigens seems to be the structure responsible for enabling resting T cells to respond to IL-2 and induce production of IL-2 in AMLR. (2) Once T cells have acquired responsiveness to IL-2 and the growth factor has been produced, there is no further requirement for HLA-DR antigens, but the availability of IL-2 determines the level and extent of proliferation of IL-2 sensitive T cells.     

Regulation of the growth of Epstein-Barr virus-infected B cells. I. Growth regression by E rosetting cells from VCA-positive donors is a combined effect of autologous mixed leukocyte reaction and activation of T8+ memory cells

Regression of B cell proliferation in co-cultures of EBV-infected B cells (BEBV) and autologous T cells at 1:4 ratio was studied. 3H-TdR incorporation was used to measure proliferation by the participating lymphocyte populations and a 51Cr release assay was used to document the generation of cells capable of killing autologous EBV-transformed B lymphoblastoid cell lines (LCLEBV). EBV-infected B cells cultured alone transformed to blasts by culture day 10, and continued to proliferate throughout the 22 day observation period. When EBV-infected B cells were co-cultured with E rosetted cells from VCA-positive donors, there was a characteristic proliferative response on day 10 (an augmented autologous mixed lymphocyte reaction; AMLR), followed by the development of T8+ cells capable of killing autologous LCLEBV, as well as over 90% suppression of EBV growth by day 22 as assessed by 3H-TdR incorporation, and confirmed in a visual outgrowth assay. Negative and positive selection techniques were used to define the regulatory components in the T cell population. Depletion of T8+ cells from the blood lymphocytes of VCA-positive donors did not significantly reduce the 10 day proliferative response, but the subsequent development of cytotoxic cells and the regression of BEBV outgrowth was not observed. Thus, the circulating T8+ cells are required for the subsequent appearance of autologous LCLEBV cytotoxicity and BEBV growth regulation. However, when the responder population consisted only of T8+ cells, the augmented AMLR response was absent, cytotoxic cell development was weak or absent, and there was no regression of EBV outgrowth. Therefore, the cells participating in the AMLR, as well as T8+ memory cells from VCA-positive donors, are necessary for the control of the in vitro EBV infection. Growth regression is dependent on the proliferation of the regulatory T cells. Mitomycin C treatment of fresh E rosetting cells or those exposed to BEBV for up to 10 days in culture abrogates growth regression and the subsequent appearance of LCLEBV killer cells. However, E rosetting cells exposed to BEBV for 14 days or more already have developed the ability to kill LCLEBV and no longer need to proliferate to induce growth regression when cultured with newly infected BEBV. These results lend additional support to the view that the control of EBV-induced B cell expansion requires a AMLR-dependent clonal amplification of EBV-specific, T8+ cytotoxic cells.     

Helper cells in the autologous mixed lymphocyte reaction. I. Generation of cytotoxic T cells recognizing modified self H-2

The autologous mixed lymphocyte reaction (AMLR) in mice measures the proliferative response of T cells to determinants on syngeneic non-T spleen cells. Normally, cytotoxic T lymphocytes (CTL) are not generated in this reaction. However, the addition of trinitrophenyl-modified mitomycin C-treated syngeneic T cells (TNP-Tm) to the AMLR results in the generation of TNP-specific CTL but does not alter the proliferative response. Significant cytotoxic activity is not detectable against TNP in association with Ia unless TNP is present on cells bearing those determinants. Thus, if unselected spleen cells are TNP-modified and used as stimulators in the AMLR, the proliferative response is enhanced and CTL are generated that recognize TNP in association with K, D, and I region-encoded determinants. The CTL generated in the AMLR are H-2 restricted and dependent on the presence of adherent cells in the sensitization cultures. The experiments presented here suggest that the AMLR can provide the help necessary for generating cytotoxic T cells in vitro.     

Interferon gamma modulates the ability of autologous non-T cells to stimulate T cells to produce and respond to interleukin 2

The interactions of T-cell receptor with self-Ia antigen on non-T cells induced IL-2 production and IL-2 receptors on the cell surface and thus responsiveness to IL-2 of T cells in autologous mixed-lymphocyte reaction (AMLR). Four-day-cultured autologous non-T cells lost their ability to stimulate T cells to produce and respond to IL-2 with concurrent decrease of HLA-DR and HLA-DQ antigen expressed on the cell surface. Culturing of non-T cells with 500 U/ml of recombinant interferon gamma (IFN-gamma) maintained their stimulating ability which was otherwise lost. Treatment of non-T cells with monoclonal anti-HLA-DR or anti-HLA-DQ antibody before mixture with T cells abrogated their ability to induce IL-2 production and IL-2 responsiveness of T cells. The combined data suggested that Ia antigen expressed on non-T cells is modulated by IFN-gamma, which increases the ability of non-T cells to stimulate autologous T cells to produce and respond to IL-2.     

Possible regulation of autologous mixed lymphocyte reaction by OKM1+ NK cells

T cells are stimulated by autologous non-T cells and interleukin 2 (IL-2) is produced in the conventional autologous mixed lymphocyte reaction (AMLR) in young healthy controls. The role of cells with natural killer (NK) cell markers (OKM1+ cells or Leu 7+ cells) in the AMLR was studied. There were significant inverse correlations between the percentage of input OKM1+ cells minus monocyte (OKM1+ NK cells) and either AMLR proliferation (gamma = -0.9, P less than 0.001) or IL-2 production (gamma = -0.75, P less than 0.01) in the AMLR cultures after 7 days measured at 7 days. A statistically significant correlation was observed between the percentage of input Leu 7+ cells and AMLR proliferation (gamma = -0.64, P less than 0.05), but not IL-2 production. These results suggest that the AMLR is controlled by OKM1+ NK, perhaps acting through IL-2 regulation.     

Regulation of T cell proliferation by cloned interferon-alpha mediated by Leu-11b-positive cells

The autologous T lymphocyte proliferative response (AMLR) induced by a B lymphocyte-enriched non-T, nonadherent cell population (NT, NAC) and by a macrophage-enriched population were both suppressed by the addition of a cloned interferon-alpha (IFN-alpha Con1) directly to the cultures. Preincubation of the stimulating NT, NAC with IFN-alpha Con1 resulted in comparable suppression. In contrast, preincubation of the macrophages with IFN-alpha Con1 resulted in significant augmentation of T cell proliferation. Depletion of Leu-11b-positive cells from the NT, NAC exposed to IFN-alpha Con1 restored the autologous T cell response. Addition of IFN-alpha Con1 activated Leu-11b-positive cells, isolated from the NT, NAC population, was suppressive of the AMLR. Although NK cytotoxicity was irradiation sensitive, suppression of the AMLR by IFN-alpha Con1-activated NT, NAC was resistant, suggesting that different subsets of cells or mechanisms by the same cells may have been responsible. These observations may offer insights into the potential role of cells with the NK phenotype, Leu-11b, and IFN in contributing to immuno-regulatory changes observed in clinical states associated with elevated concentrations of IFN.     

HLA-DR antigens render resting T cells sensitive to interleukin-2 and induce production of the growth factor in the autologous mixed lymphocyte reaction

Monoclonal anti-HLA-DR (anti-Ia) antibodies inhibited autologous mixed lymphocyte reactions (AMLR) when added from the initiation of the cultures, but not 72 hr later. The suppressive principle was removed by the stimulator non-T cells, but not by the responding T cells. Antibody-treated non-T cells lost their ability to activate T cells, whereas antibody-treated T cells could still respond to untreated non-T cells. The anti-DR antibodies prevented T cells from acquiring responsiveness to Interleukin-2 (IL-2). However, T cells previously activated by AMLR responded to IL-2 even in the presence of the anti-DR antibodies. OKT4⁺ lymphocytes synthesized IL-2 in the AMLR while OKT8⁺ cells did not. Anti-DR antibodies caused OKT4⁺ cells to become unresponsive to Interleukin-1 stimulation and inhibited the production of IL-2. Interleukin-1 (IL-1) promoted the synthesis of IL-2 in non-anti-DR-treated AMLR cultures. Since resting T cells are unresponsive to IL-2 and resting OKT4⁺ lymphocytes are unable to produce IL-2 even in the presence of IL-1, it is concluded that HLA-DR antigens render resting T cells sensitive to IL-2 and enable OKT4⁺ lymphocytes to respond to IL-1 and subsequently, to produce Interleukin-2.     

Improvement of decreased interleukin 2 (IL-2) responsiveness and IL-2 production in autologous mixed-lymphocyte reaction of cord blood lymphocytes by interferon-gamma and IL-2 and the role of HLA-DQ antigen

Cord blood T cells did not produce interleukin 2 (IL-2) nor acquire responsiveness to it in autologous mixed-lymphocyte reaction (AMLR) as they do when activated by phytohemagglutinin (PHA). The ability of the cells to respond to IL-2 was restored either by the addition of recombinant IL-2 to the AMLR culture or by the preculture of non-T stimulator cells with recombinant interferon-gamma (IFN-gamma). IL-2 production was also induced when the T cells were added with recombinant IL-2 at the initiation of the AMLR culture, preceded by the treatment of non-T cells with recombinant IFN-gamma. IL-2-producing cells of cord blood induced in the above-mentioned condition were defined to be OKT4+ T cells, because the deletion of OKT4+ T cells from T-cell population abrogated the reaction, while that of OKT8+ T cells did not. Acquisition of IL-2 responsiveness and IL-2 production of T cells seemed to be mediated by HLA-DR and HLA-DQ molecules of non-T cells because these reactions were blocked by the treatment of non-T cells either with monoclonal anti-HLA-DR or with anti-HLA-DQ antibody. The HLA-DR and HLA-DQ densities of cord blood non-T cells were low as compared with those of adult, but the expression of HLA-DQ was remarkably improved by IFN-gamma treatment. In regard to IL-2, both IFN-gamma and IL-2 were needed to enable the lymphocytes to produce. This may suggest that some functional maturation by IL-2 of responder T cells is further required. These combined data suggested that cord blood non-T cells are defective as a stimulator in AMLR and this could be corrected by enhancing the expression of HLA-DQ antigen.     

Helper cells in the autologous mixed lymphocyte reaction (AMLR): IV. H-2 restriction and specificity of cytotoxic cells induced by AMLR helper factor

Supernatants from Day 3 cultures of T cells stimulated with syngeneic non-T cells in the autologous mixed lymphocyte reaction (AMLR) contained a T-cell factor distinct from interleukin 2 (IL-2). The AMLR factor could mediate the cytotoxic response of T cells to hapten-modified nonstimulatory syngeneic cells. The activity of this helper factor was H-2 unrestricted. However, the cytotoxic cells which developed in the presence of this factor were H-2 restricted and hapten specific.     

Immunosuppressive activity of T cell clones generated from human T cells stimulated with autologous TPHA cells

T cell clones were generated from human T cells stimulated with autologous phytohemagglutinin (PHA)-activated T (TPHA) cells. Characterization of three T cell clones originated from donor SF and one from donor JM showed that they proliferated when stimulated with autologous TPHA cells, non-T cells, and peripheral blood mononuclear cells, but did not proliferate when stimulated with allogeneic TPHA cells, non-T cells, and mononuclear cells, with autologous and allogeneic resting T cells, and with PHA. These results in conjunction with the blocking of the proliferation by anti-histocompatibility leukocyte antigen class II monoclonal antibodies indicate that these class II antigens are involved in the proliferation of T cell clones stimulated with autologous lymphoid cells. The four T cell clones are cytotoxic neither to autologous lymphoid cells nor to a panel of cultured human cell lines. The four T cell clones display immunosuppressive activity, since they inhibit the proliferation of autologous and allogeneic cells stimulated with antigens and mitogens and the secretion of immunoglobulin by B cells stimulated with pokeweed mitogen in presence of T cells. Furthermore, the four T cell clones display differential inhibitory activity on the proliferation of cultured human cell lines. The immunosuppressive activity is species-specific, since the T cell clones do not inhibit the proliferation of murine cells. The suppression is mediated by a factor(s) with an apparent m.w. of 13,000 to 16,000. The suppressor activity is labile at alkaline pH and is lost following incubation with pronase (100 U/ml) for 30 min at 37 degrees C.     

Activated B lymphocytes: stimulators of an augmented autologous mixed leukocyte reaction

The characteristics of the non-T cell(s) which stimulate T-lymphocyte proliferation in the autologous mixed leukocyte reaction (AMLR) have been at issue since this in vitro reaction was first described. Dendritic cells have been shown to be the most potent stimulator cells, but B cells, null cells, and macrophages have also been demonstrated to have the capacity to stimulate autologous T-cell proliferation. A cell preparation obtained from human peripheral blood was highly enriched for surface immunoglobulin-positive B cells. These cells were activated by brief culture with various B-cell mitogens and then compared to untreated B cells with regard to stimulatory activity in the AMLR. Mitogen-activated B cells were markedly augmented in their capacity to stimulate autologous T-cell proliferation when compared with untreated B cells. Fractionation of the B-cell preparation into high- and low-density subpopulations demonstrated that the high-density cells, enriched in resting B cells, had minimal stimulatory activity but could be activated to have increased AMLR-stimulatory capacity. Proliferation of the activated B lymphocytes was not required for the generation of the augmented AMLR. Response to both untreated and mitogen-activated B cells was a property of T4-positive T lymphocytes. The increase in stimulatory capacity was associated with a decrease in cell surface immunoglobulin, but no significant alteration in the percentage or fluorescence intensity of anti-Ia staining cells was detected. Activated B cells which are generated in vivo may acquire the capacity to generate T effector cells or factors important in the regulation of B-cell function.     

Inhibitory effect of a low dose of prednisone on PHA-induced Ia antigen expression by human T cells and on proliferation of T cells stimulated with autologous PHA-T cells

Administration of a small dose of prednisone markedly reduced (1) the PHA-induced expression of Ia antigens by T cells, (2) the stimulatory activity of Ia antigen-bearing T cells in autologous and allogeneic mixed lymphocyte reactions (MLRs), and (3) the proliferative response of T cells stimulated with autologous PHA-activated T cells or autologous or allogeneic non-T cells. The inhibitory effects of prednisone are reversible and are not detectable on T cells isolated from blood drawn 24 hr following prednisone administration. The kinetics of the prednisone-mediated inhibition of MLRs with autologous PHA-T cells is different from that of MLRs with autologous non-T cells. These data in conjunction with the information available in the literature suggest that the mechanisms underlying these two types of autologous MLRs are different.     

Autologous mixes lymphocyte reaction in human cord blood lymphocytes: decreased generation of helper and cytotoxic T-cell functions and increased proliferative response and induction of suppressor T cells

T lymphocytes from neonates proliferated significantly more than peripheral blood T lymphocytes from adults in autologous mixed lymphocyte reactions (AMLR). AMLR-activated cord, as compared to adult T lymphocytes, exerted significantly less nonspecific cytotoxic activity on PHA-stimulated adult mononuclear cells and Epstein-Barr virus-transformed target cells. The impaired generation of cytotoxicity of cord T cells was not corrected by Interleukin-2. Blood T lymphocytes from adults activated in AMLR synthesized a helper factor that supported PWM-induced proliferation and immunoglobulin production in both adult and cord B lymphocytes. In contrast, cord blood T lymphocytes failed to produce the helper factor for B lymphocytes. T cells from AMLR cultures established with neonatal lymphocytes showed suppressor activity, as assessed in PWM-stimulated immunoglobulin synthesis of adult peripheral-blood mononuclear cells, significantly higher than that exhibited by T cells from AMLR cultures performed with lymphocytes from adults. Finally, neonatal B lymphocytes could be activated to the production of IgM but not IgG by either adult AMLR-derived helper factor plus PWM or by Epstein-Barr virus, whereas adult B cells secreted both IgM and IgG under the same type of stimulation.     

Activation and function of an autoreactive T cell clone with dual immunoregulatory activity

Previously it was demonstrated that the human autoreactive CD4+ T cell clone MTC-4 is bifunctional, having the capacity to augment differentiation of autologous B cells into Ig-secreting cells in the absence of PWM and the capacity to suppress such differentiation in the presence of PWM. In the present study it was shown that these two functions of MTC-4 are mediated by distinctly different mechanisms. In the presence of autologous class II MHC Ag, MTC-4 releases one or more non-MHC-restricted soluble factors which stimulate B cell differentiation. The helper factors are different from IL-2, and act on both resting (small) and activated (large) B cells. The suppressor function of MTC-4 cells is elicited when MTC-4 cells are co-cultured with autologous non-T cells preincubated with PWM for 4 h, but not with non-T cells preincubated with PWM for 24 h; thus, activated autologous non-T cells have a transient capacity to induce MTC-4 suppressor function. Induction of MTC-4 suppressor activity is not associated with increased proliferation of MTC-4 and is mediated by low numbers of these cells. Unlike helper function, MTC-4 suppression of Ig synthesis can occur late in B cell cultures, and MTC-4 suppresses Ig production by autologous B cells, but not by allogeneic B cells. Finally, in co-cultures with activated autologous non-T cells and allogeneic B cells, MTC-4 can simultaneously produce helper factors that augment Ig synthesis by allogeneic B cells and suppress Ig synthesis by autologous B cells. In summary, exposure of MTC-4 to autologous non-T cells causes release of non-MHC-restricted factors which augment Ig production by both resting and activated autologous B cells, whereas exposure of MTC-4 to recently activated B cells causes MTC-4 to express the additional function of directly suppressing Ig production by differentiated autologous B cells. Thus autoreactive T cells may be uniquely suited to regulate Ig production.