Fas ligand-mediated cytotoxicity is directly responsible for apoptosis of normal CD4+ T cells responding to a bacterial superantigen.

Exposure of naive CD4+ T lymphocytes to superantigens such as staphylococcal enterotoxin B (SEB) induces a strong proliferative response. Prolonged exposure or subsequent restimulation of the responding T cell population with SEB leads to the apoptotic events of activation-induced cell death (AICD). However, T cells derived from either Fas-deficient lpr or Fas ligand-deficient gld autoimmune mouse strains, fail to undergo AICD under these conditions. Instead, these autoimmune T cells mount a vigorous proliferative response, suggesting a critical role for Fas/FasL interactions in this form of autoapoptosis. In the current study, we found that SEB-induced AICD was tied to the rapid induction of FasL expression in cells constitutively expressing high levels of Fas. Furthermore, the addition of soluble Fas-IgG fusion protein to the SEB-restimulated cultures blocked AICD and resulted in a 2 degrees proliferative response that was comparable in magnitude and kinetics to that of the lpr and gld T cells. The rapid onset of apoptosis in normal T cells subsequent to restimulation with SEB was in direct contrast to the proliferative response of the initial cultures, even though comparable levels of Fas and FasL RNA were found in T cells after 1 degree and 2 degrees challenge. The clonal expansion of the normal T cells responding to the initial SEB stimulation was, however, dramatically compromised when the normal cells were cocultured with an MRL-lpr responder population; addition of soluble Fas-IgG rescued the normal component of the response. Together, these data demonstrate first, that Fas/FasL interactions are intimately tied to superantigen-induced AICD, a form of autocrine cell death, and second, that FasL-mediated cytotoxicity is responsible for the disappearance of normal CD4+ T cells in lpr cocultures.