Constitutive activation of the Fas/FasL and CD28/CTLA-4 pathways in scurfy mice: implications for peripheral tolerance

Scurfy is an X-linked recessive disorder in mice characterized by the accumulation of activated lymphocytes in the peripheral lymphoid organs, culminating in early lethality due to severe wasting and cytokine toxicity. The gene(s) responsible for the scurfy phenotype are unknown, but the massive lymphoproliferation implies a loss of immune tolerance. Although a defect in thymic selection has not been excluded, the phenotype is more consistent with a polyclonal activation of the peripheral immune system. We have examined two pathways that have been documented to maintain peripheral lymphocyte homeostasis, the Fas-FasL pathway and the CD28/CTLA-4/B7 pathway. The Fas pathway is responsible for the activation-induced cell death (AICD) of lymphocytes at the termination of an immune response. It plays a critical role in restoring lymphocyte homeostasis and limiting the potentially deleterious effects of prolonged cytokine secretion. Mice with defects in the Fas-FasL pathway exhibit severe lymphoproliferation and autoimmune disease. In addition to the apoptotic pathway mediated by Fas-FasL, signalling through the CTLA-4 T cell receptor provides a negative regulatory signal to T cells, blocking cell cycle progression. In the absence of CTLA-4, the peripheral immune system is massively activated, leading to a lymphoproliferative phenotype very similar to scurfy. Our data suggest that both the Fas-FasL and CD28/CTLA-4/B7 pathways are activated in scurfy mice. Although scurfy lymphocytes demonstrate evidence of previous and recent activation, have upregulated Fas mRNA, Fas cell surface expression, and FasL mRNA, we detect negligible levels of apoptosis in freshly isolated scurfy lymphocytes. It is unclear why activated lymphocytes that are phenotypically­ primed for apoptosis are accumulating in scurfy peripheral lymphoid organs. However, our data excludes an intrinsic defect in the Fas apoptotic pathway. If provided with an exogenous signal through the Fas receptor, scurfy lymphocytes rapidly and spontaneously undergo apoptosis.