IFN-γ Acts Directly on Activated CD4+ T Cells during Mycobacterial Infection to Promote Apoptosis by Inducing Components of the Intracellular Apoptosis Machinery and by Inducing Extracellular Proapoptotic Signals

Abstract

Despite many studies, the regulation of CD4(+) T cell apoptosis during the shutdown of immune responses is not fully understood. We have investigated the molecular mechanisms of IFN-gamma in regulating apoptosis of CD4(+) T cells during bacillus Calmette-Guérin (BCG) infection of mice. Our data provide new insight into the regulation of CD4(+) T cell apoptosis by IFN-gamma. As CD4(+) T cells responded to BCG infection, there was a coordinated increase in IFN-gamma production by effector CD4(+) T cells and a coordinated IFN-gamma-dependent up-regulation of many diverse apoptosis-pathway genes in effector CD4(+) T cells. Unexpectedly, IFN-gamma up-regulated transcripts and protein expression of Bcl-2, Bax, Bim, Bid, Apaf-1, and caspase-9 in activated CD4(+) T cells--components of the apoptosis machinery that are involved in promoting mitochondrial damage-mediated apoptosis. Wild-type, but not IFN-gamma knockout, CD4(+) T cells underwent apoptosis that was associated with damaged mitochondrial membranes. IFN-gamma also up-regulated expression of cell-extrinsic signals of apoptosis, including TRAIL, DR5, and TNFR1. Cell-extrinsic apoptosis signals from TNF-alpha, TRAIL, and NO were capable of damaging the mitochondrial membranes in activated CD4(+) T cells. Moreover, activated CD4(+) T cells from BCG-infected DR5, TNFR1, and inducible NO synthase knockout mice had impaired caspase-9 activity, suggesting impaired mitochondria-pathway apoptosis. We propose that IFN-gamma promotes apoptosis of CD4(+) T cells during BCG infection as follows: 1) by sensitizing CD4(+) T cells to apoptosis by inducing intracellular apoptosis molecules and 2) by inducing cell-extrinsic apoptosis signals that kill CD4(+) effector T cells.