Apoptotic programs are determined during lineage commitment of CD4⁺ T effectors: selective regulation of T effector-memory apoptosis by inducible nitric oxide synthase

Abstract

Lineage-committed T effectors generated in response to Ag during the inflammatory phase are destined to die during termination of the immune response. We present evidence to suggest that molecular signatures of lineage commitment are reflected in apoptotic cascades activated in CD4⁺ T effectors. Exemplifying this, ablation of inducible NO synthase (iNOS) protected effector-memory T (TEM) cells, but not TNaive or central-memory T cells, activated in vitro, from apoptosis triggered by cytokine deprivation. Furthermore, attrition of T effectors generated in the secondary, but not the primary, response to Ag was substantially reduced in mice, which received iNOS inhibitors. Distinct patterns of iNOS expression were revealed in wild-type TEM effectors undergoing apoptosis, and ablation of iNOS protein in primary and TEM wild-type effectors confirmed observations made in iNOS^−/− cells. Describing molecular correlates of this dependence, mitochondrial damage, activation of the protein Bax, and release from mitochondria of the apoptosis-inducing factor were selectively abrogated in iNOS^−/− TEM effectors. Suggesting that iNOS dependence was linked to the functional identity of T cell subsets, both iNOS induction and apoptosis were compromised in IFN-γ^−/− TEM effectors, which mirrored the response patterns of iNOS^−/− TEM. Collectively, these observations suggest that programs regulating deletion and differentiation are closely integrated and likely encoded during lineage commitment of T effectors.