Cytotoxic CD4+ Th1-cells induce cell death by triggering a Fas-dependent apoptotic pathway. Potential targets include activated B cells, but it is not known whether the mode of B-cell stimulation influences susceptibility to Th1-mediated cytotoxicity. Here we report that CD40-ligand-stimulated B cells were extremely sensitive, whereas anti-IgM-stimulated B cells were resistant, to Fas-mediated apoptosis. B cells stimulated by both CD40L and anti-IgM were not susceptible to cytolysis, demonstrating that anti-IgM-mediated protection is an active, dominant process. Resistance to Th1-mediated cytotoxicity was similarly observed in CD40L-stimulated 3-83 (anti-H-2Kk,b) transgenic B cells co-cultured with H-2Kk or H-2Kb (but not H-2Kd) splenocytes. These results indicate that B cells can participate in regulating their own destruction. Protection against Fas-dependent apoptosis afforded by immunoglobulin-receptor engagement may constitute a fail-safe mechanism that eliminates bystander B cells activated by CD40L-expressing T cells, but ensures survival of antigen-specific B cells.

No abstract

The effect of Fas ligand (FasL) cytotoxicity on T/B collaboration was examined in vitro using cloned T helper 1 cells and antigen-pulsed, activated B cells. We compared antigen-pulsed B cells that had been activated through different membrane receptors (IgM, CD14 and CD40) for their ability to induce T cell proliferation and to respond to T cell help. We also used a Fas-Ig fusion protein, an inhibitor of FasL-mediated cytotoxicity, to determine the effect of FasL cytotoxicity on the T and B cell proliferative responses. The data show that the extent of both T and B cell proliferative responses correlate with the relative resistance of activated B cell populations to FasL cytotoxicity. Moreover, both T and B cell proliferation could be enhanced by Fas-Ig. Our results demonstrate that FasL cytotoxicity is a negative regulatory mechanism for both T and B cell proliferative responses and that Fas-Ig can be an immunopotentiating agent for both T and B cell immunity.

Naive CD4+ T cells proliferate strongly in response to stimulation by superantigens such as staphylococcal enterotoxin B (SEB). However, when these same cells revert to a resting phenotype and are subjected to restimulation with either SEB or anti‐CD3, the majority of these SEB‐responsive cells undergo Fas ligand (FasL)‐mediated activation‐induced cell death (AICD). We investigated the impact of Fas expression on T cell AICD by utilizing B cell stimulators that lacked functional FasL and either expressed or did not express the Fas receptor. Our results indicate that B cells play an important role in modulating the level of T cell AICD via the Fas/FasL pathway. Activated B cells expressing high levels of Fas receptor can redirect the FasL expressed by T cells primed to undergo AICD away from the T cells and prevent the induction of AICD in these cells. Furthermore, B cells stimulated through both the CD40 receptor and membrane IgM appear to mediate a stronger protective effect on T cells by virtue of their resistance to FasL‐mediated cytolysis. These observations suggest a mechanism by which normal B cell and T cell responses to foreign antigen are maintained, while responses to self antigen are not.

Naive CD4+ T cells proliferate strongly in response to stimulation by superantigens such as staphylococcal enterotoxin B (SEB). However, when these same cells revert to a resting phenotype and are subjected to restimulation with either SEB or anti-CD3, the majority of these SEB-responsive cells undergo Fas ligand (FasL)-mediated activation-induced cell death (AICD). We investigated the impact of Fas expression on T cell AICD by utilizing B cell stimulators that lacked functional FasL and either expressed or did not express the Fas receptor. Our results indicate that B cells play an important role in modulating the level of T cell AICD via the Fas/FasL pathway. Activated B cells expressing high levels of Fas receptor can redirect the FasL expressed by T cells primed to undergo AICD away from the T cells and prevent the induction of AICD in these cells. Furthermore, B cells stimulated through both the CD40 receptor and membrane IgM appear to mediate a stronger protective effect on T cells by virtue of their resistance to FasL-mediated cytolysis. These observations suggest a mechanism by which normal B cell and T cell responses to foreign antigen are maintained, while responses to self antigen are not.

Homozygosity for either of the autosomal recessive mutations, lpr or gld, confers an autoimmune syndrome characterized by massive lymphoid hyperplasia and extensive autoantibody production. Despite the similarities in disease progression, functional distinctions in these genetic defects have been observed in bone marrow transplantation studies. To elucidate the mechanisms responsible for the aberrant immune phenotype of these strains, we analyzed interactions between normal T cells and T cells from the two autoimmune strains with regard to their in vitro responses to autologous and allogeneic stimuli and in an in vivo bone marrow transplantation model. Despite similar propensities for lpr and gld T cells to spontaneously proliferate in vitro in response to autologous class II Ag, a dramatic difference in their immunoregulatory properties was found when mixtures of normal and autoimmune CD4+ responder cells were challenged with an allogeneic stimulus. T cells from the lpr, but not gld, mice blocked the normal T cell component of the response. In vivo, the ability of lpr stem cells to trigger a wasting syndrome when transplanted into normal irradiated recipients could not be prevented by including normal stem cells in the inoculum; however, the ability of gld stem cells to transfer the gld-lymphoproliferative syndrome to normal recipients could be prevented with the addition of normal stem cells. These results support a model whereby the lpr and gld strains are defective in reciprocal components of a down-regulatory signaling pathway; failure to express either the functional receptor or ligand leads to a dysregulated immune system resulting in systemic autoimmunity. Based on the linkage between the lpr locus and Fas Ag expression, we propose that the failure of lpr mice to express Fas results in overproduction of Fas-ligand, whereas gld mice fail to make either the Fas-ligand or a functionally related protein, presumably belonging to the TNF family.