Background: Fas (CD95) triggers programmed cell death and is involved in shutting off the immune response. Inherited deleterious mutations hitting Fas or its signaling pathway cause autoimmune/lymphoproliferative syndrome (ALPS).

Objective: To assess the possibility that decreased Fas function plays a role in development of MS.

Methods: The authors evaluated Fas function in long-term T cell lines (21 days of culture) from 32 patients with relapsing-remitting MS (RRMS), 15 with secondary progressive MS (SPMS), and 15 with primary progressive MS (PPMS) by assessing cell survival upon Fas triggering by monoclonal antibodies (Mab).

Results: Fas-induced cell death was significantly lower in all patient groups than in controls, and lower in SPMS than in RRMS. Moreover, 8/15 patients with PPMS, 10/15 with SPMS, and 8/32 with RRMS were frankly resistant to Fas. Frequency of resistance to Fas-induced cell death was significantly higher in all patient groups than in controls (2/75), and higher in SPMS than in RRMS. The findings that the parents of two Fas-resistant patients were Fas-resistant and that fusion of T cells from two Fas-resistant patients with Fas-sensitive HUT78 cells gave rise to Fas-resistant hybrid lines suggest that Fas-resistance is due to inherited alterations of the Fas signaling pathway, with production of molecules exerting a dominant negative effect on a normal Fas system.

Conclusions: Defects of the immune response shutting-off system may be involved in the pathogenesis of MS, particularly in its progressive evolution.