Anti-Caspase-8 Autoantibody Response in Silicosis Patients is Associated with HLA-DRB1, DQB1 and DPB1 Alleles

Five unrelated children are described with a rare autoimmune lymphoproliferative syndrome (ALPS) characterized by massive nonmalignant lymphadenopathy, autoimmune phenomena, and expanded populations of TCR-CD3+CD4-CD8- lymphocytes. These findings, suggesting a genetic defect in the ability of T lymphocytes to respond to normal immunoregulatory mechanisms, prompted an evaluation of lymphocyte apoptosis. Each child had defective Fas-mediated T lymphocyte apoptosis associated with a unique, deleterious Fas gene mutation. One mutation appeared to cause a simple loss of function; however, four others had a dominant negative phenotype when coexpressed with normal Fas. Family studies demonstrated the inheritance of the mutant Fas alleles. The occurrence of Fas mutations together with abnormal T cell apoptosis in ALPS patients suggests an involvement of Fas in this recently recognized disorder of lymphocyte homeostasis and peripheral self-tolerance.

Upon activation, the apoptosis-inducing cell membrane receptor CD95 (APO-1/Fas) recruits a set of intracellular signaling proteins (CAP1-4) into a death-inducing signaling complex (DISC). In the DISC, CAP1 and CAP2 represent FADD/MORT1. CAP4 was identified recently as an ICE-like protease, FLICE, with two death effector domains (DED). Here we show that FLICE binds to FADD through its N-terminal DED. This is an obligatory step in CD95 signaling detected in the DISC of all CD95-sensitive cells tested. Upon prolonged triggering of CD95 with agonistic antibodies all cytosolic FLICE gets proteolytically activated. Physiological FLICE cleavage requires association with the DISC and occurs by a two-step mechanism. Initial cleavage generates a p43 and a p12 fragment further processed to a p10 fragment. Subsequent cleavage of the receptor-bound p43 results in formation of the prodomain p26 and the release of the active site-containing fragment p18. Activation of FLICE is blocked by the peptide inhibitors zVAD-fmk, zDEVD-fmk and zIETD-fmk, but not by crmA or Ac-YVAD-CHO. Taken together, our data indicate that FLICE is the first in a cascade of ICE-like proteases activated by CD95 and that this activation requires a functional CD95 DISC.