Stimulation of human peripheral blood mononuclear cells with live Mycobacterium bovis BCG activates cytolytic CD8+ T cells in vitro

Abstract

Experimental data have shown that Mycobacterium tuberculosis can survive within the host cell and in doing so may release secreted antigen into the endogenous antigen‐processing pathway. If mycobacterial antigen can gain access to MHC class I molecules then CD8⁺ T cells may play a role in host defence against M. tuberculosis infection. To identify whether there is a role for the CD8⁺ T cell in mycobacterial infection we have stimulated peripheral blood mononuclear cells (PBMC) from bacillus Calmette–Guérin (BCG) vaccinated individuals with live M. bovis BCG. The activation state of the T cells was established by staining for the interleukin‐2 (IL‐2) receptor (CD25), HLA‐DR or the transferrin receptor (CD71). Using FACScan analysis we have shown that, in vitro, live M. bovis BCG activates significantly more CD8⁺ T cells in comparison to the soluble antigen purified protein derivative (PPD). In addition, live M. bovis BCG activates more CD8⁺ T cells than a non‐viable preparation of the same M. bovis BCG following irradiation. The function of the activated CD8⁺ T cells was addressed using positively selected cells in a cytotoxic T‐cell assay. CD8⁺ T cells isolated from a 7‐day M. bovis BCG‐stimulated PBMC culture were shown to be cytolytic against target cells infected with live M. bovis BCG, dead M. bovis BCG, and to a lesser extent, PPD. These results suggest that CD8⁺ T cells may be activated by stimulation with live mycobacteria, and that this subset can play a cytolytic role in the immune response to mycobacterial infections.