CTL-Mediated Killing of Intracellular Mycobacterium tuberculosis Is Independent of Target Cell Nuclear Apoptosis

Abstract

Two subsets of human CTL have been defined based upon phenotype and function: CD4− CD8− double-negative (DN) CTL lyse susceptible targets via Fas-Fas ligand interaction and CD8+ CTL via the granule exocytosis pathway. CD8+ CTL, but not DN CTL, can mediate an antimicrobial activity against Mycobacterium tuberculosis-infected target cells that is dependent on cytotoxic granules that contain granulysin. We investigated the role of nuclear apoptosis for the antimicrobial effector function of CD1-restricted CTL using the caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone. We found that DN CTL-induced target cell lysis was completely dependent on caspase activation, whereas the cytolytic activity of CD8+ CTL was caspase independent. However, both DN and CD8+ CTL-induced nuclear apoptosis required caspase activation. More important, the antimicrobial effector function of CD8+ CTL was not diminished by inhibition of caspase activity. These data indicate that target cell nuclear apoptosis is not a requirement for CTL-mediated killing of intracellular M. tuberculosis.