Interferon gamma eliminates responding CD4 T cells during mycobacterial infection by inducing apoptosis of activated CD4 T cells

In Mycobacterium bovis Bacille Calmette-Guerin (BCG)-infected wild-type mic e, there was a large expansion of an activated (CD44(hi)) splenic CD4 T cel l population followed by a rapid contraction of this population to normal n umbers. Contraction of the activated CD4 T cell population in wild-type mic e was associated with increased apoptosis of activated CD4 T cells. In BCG- infected interferon (IFN)-gamma knockout (KO) mice, the activated CD4 T cel l population did not undergo apoptosis. These mice accumulated large number s of CD4(+)CD44(hi) T cells that were responsive to mycobacterial antigens. Addition of IFN-gamma to cultured splenocytes from BCG-infected IFN-gamma KO mice induced apoptosis of activated CD4 T cells. IFN-gamma-mediated apop tosis was abolished by depleting adherent cells or Mac-1(+) spleen cells or by inhibiting nitric oxide synthase. Thus, IFN-gamma is essential to a reg ulatory mechanism that eliminates activated CD4 T cells and maintains CD4 T cell homeostasis during an immune response.