Involvement of the Fas/Fas Ligand Pathway in Activation-Induced Cell Death of Mycobacteria-Reactive Human γδ T Cells: A Mechanism for the Loss of γδ T Cells in Patients with Pulmonary Tuberculosis

Abstract

Abstract Although the identity of T cells involved in the protection against Mycobacterium tuberculosis (Mtb) in humans remain unknown, patients with pulmonary tuberculosis (TB) have reduced numbers of Mtb-reactive, Vγ9+/Vδ2+ T cells in their blood and lungs. Here we have determined whether this γδ T loss is a consequence of Mtb Ag-mediated activation-induced cell death (AICD). Using a DNA polymerase-mediated dUTP nick translation labeling assay, 5% or less of freshly isolated CD4+ αβ or γδ T cells from normal healthy individuals and TB patients were apoptotic. However, during culture Mtb Ags induced apoptosis in a large proportion of Vγ9+/Vδ2+ peripheral blood T cells from healthy subjects (30–45%) and TB patients (55–68%); this was increased further in the presence of IL-2. By contrast, anti-CD3 did not induce any significant level of apoptosis in γδ T cells from healthy subjects or TB patients. Mtb Ag stimulation rapidly induced Fas and Fas ligand (FasL) expression by γδ T cells, and in the presence of metalloproteinase-inhibitors >70% of γδ T cells were FasL+. Blockade of Fas-FasL interactions reduced the level of Mtb-mediated γδ T cell apoptosis by 75 to 80%. Collectively, these findings demonstrate that Mtb-reactive γδ T cells are more susceptible to AICD and that the Fas-FasL pathways of apoptosis is involved. AICD of γδ T cells, therefore, provides an explanation for the loss of Mtb-reactive T cells during mycobacterial infection.