Rac GTPases are required for restimulation-induced CD4+ T Cell Death (RICD) (87.20)

Abstract

Abstract In activated CD4 T cells, TCR restimulation or antigen leads to FasL mRNA synthesis, expression and eventually apoptosis through cis or trans FasL-Fas interactions, in the process known as restimulation-induced cell death (RICD). Apoptosis requires both FasL synthesis and secretion and a protein-synthesis independent ‘competency to die’ signal that sensitizes TCR-stimulated cells to apoptosis induced by FasL. Recent work in our lab correlated competency signals with translocation of Fas into specialized membrane micro-domains (lipid rafts) in TCR restimulated T cells. However, what signaling pathways underlie the TCR-induced ‘competancy to die’ signal is not known. We have now identified the small Rho GTPase Rac as a modulator of TCR signals that mediate competency. SiRNA knockdown of Rac1 in jurkats and human CD4 T cells, results in specific abrogation of TCR induced ‘competency to die’ via Fas. Transfection of constitutively active Rac1 rescues TCR mediated Fas apoptosis in Vav-1 deficient Jurkat cells, which are devoid of competency signals. Absence of Rac1 does not affect a number of other parameters of early T cell signaling, however, we observe reduction in dephoshorylation of the Ezrin-Radixin-Moesin (ERM) proteins. CD2Cre Rac1 flox/flox mouse T cells as well as Rac2 −/− T cells have deficient Restimulation Induced cell death (RICD), which is more severe in T cells lacking both Rac1 and Rac2 (Rac1−/+/ Rac2 −/−). Further, in Jurkat and mouse T cells, Rac1 deficiency inhibits FasL upregulation upon TCR stimulation. Therefore, Rho GTPase proteins are critically involved in two distinct aspects of restimulation-induced T cell apoptosis and may be a novel target for immunomodulatory therapy.