A role for apoptosis-inducing factor in T cell development

Apoptosis-inducing factor (Aif) is a mitochondrial flavoprotein that regulates cell metabolism and survival in many tissues. We report that aif-hypomorphic harlequin (Hq) mice show thymic hypocellularity and a cell-autonomous thymocyte developmental block associated with apoptosis at the β-selection stage, independent of T cell receptor β recombination. No abnormalities are observed in the B cell lineage. Transgenes encoding wild-type or DNA-binding–deficient mutant Aif rectify the thymic defect, but a transgene encoding oxidoreductase activity–deficient mutant Aif does not. The Hq thymic block is reversed in vivo by antioxidant treatment, and Hq T but not B lineage cells show enhanced oxidative stress. Thus, Aif, a ubiquitous protein, serves a lineage-specific nonredundant antiapoptotic role in the T cell lineage by regulating reactive oxygen species during thymic β-selection

Inducible nitric oxide synthase in T cells regulates T cell death and immune memory

The progeny of T lymphocytes responding to immunization mostly die rapidly, leaving a few long-lived survivors functioning as immune memory. Thus, control of this choice of death versus survival is critical for immune memory. There are indications that reactive radicals may be involved in this death pathway. We now show that, in mice lacking inducible nitric oxide synthase (iNOS), higher frequencies of both CD4 and CD8 memory T cells persist in response to immunization, even when iNOS(+/+) APCs are used for immunization. Postactivation T cell death by neglect is reduced in iNOS(–/–) T cells, and levels of the antiapoptotic proteins Bcl-2 and Bcl-xL are increased. Inhibitors of the iNOS-peroxynitrite pathway also enhance memory responses and block postactivation death by neglect in both mouse and human T cells. However, early primary immune responses are not enhanced, which suggests that altered survival, rather than enhanced activation, is responsible for the persistent immunity observed. Thus, in primary immune responses, iNOS in activated T cells autocrinely controls their susceptibility to death by neglect to determine the level of persisting CD4 and CD8 T cell memory, and modulation of this pathway can enhance the persistence of immune memory in response to vaccination

Apoptosis-inducing factor regulates death in peripheral T cells

Apoptosis-inducing factor (Aif) is a mitochondrial flavoprotein with multiple roles in apoptosis as well as in cellular respiration and redox regulation. The harlequin (Hq) mouse strain carries an aif locus modification causing reduced Aif expression. We demonstrate that activated CD4+ and CD8+ peripheral T cells from Hq mice show resistance to neglect-induced death (NID) triggered by growth factor withdrawal, but not to death induced by multiple agents that trigger DNA damage. Aif translocates to the nucleus in cells undergoing NID, and, in Hq T cell blasts, resistance to NID is associated with reduced cytosolic release of mitochondrial cytochrome c, implicating Aif in this event. In contrast, Hq T cell blasts express higher levels of CD95L, demonstrating increased susceptibility to activation-induced cell death (AICD) and apoptosis triggered by hydrogen peroxide. Superoxide scavenging protects from AICD in wild-type, but not Hq, T cell blasts, suggesting that Aif plays a crucial superoxide-scavenging role to regulate T cell AICD. Finally, the altered pattern of death susceptibility is reproduced by siRNA-mediated reduction of Aif expression in normal T cells. Thus, Aif serves nonredundant roles, both proapoptotic and antiapoptotic, in activated peripheral T cells

Naive CD4 T cells from aged mice show enhanced death upon primary activation

Poor T cell immunity is one of the many defects seen in elderly humans and aged (Ad) mice. We report that naive CD4 T cells from aged mice (ANCD4 cells) showed greater apoptosis upon primary activation than those from young (Yg) mice, with loss of mitochondrial membrane potential, poor activation of Rel family transcription factors and increased DNA damage. Their ability to enhance glycolysis, produce lactate and induce autophagy following activation was also compromised. ANCD4 cells remained susceptible to death beyond first cell division. Activated ANCD4 cells also showed poor transition to a ‘central memory’ (CM) CD44high, CD62Lhigh phenotype in vitro. This correlated with low proportions of CM cells in Ad mice in vivo. Functionally, too, IFN-gamma responses recalled from T cells of immunized Ad mice, poor to begin with, worsened with time as compared with Yg mice. Thus, ANCD4 cells handle activation-associated stress very poorly due to multiple defects, possibly contributing to poor formation of long-lasting memory