Mycobacterium chelonae sensitisation induces CD4⁺‐mediated cytotoxicity against BCG

Significant variability in efficacy of live Mycobacterium bovis BCG as a tuberculosis vaccine is observed globally. Effects of pre-vaccination sensitisation to non-tuberculous environmental mycobacteria (Env) are suspected to underlie this phenomenon, but the mechanisms remain unclear. We postulated that it could be due to Env-specific T cells exerting cytotoxicity against BCG-infected host cells. After murine sensitisation with heat-killed antigens of different Env species, splenocytes from M. chelonae (CHE)-sensitised mice exerted the strongest cytotoxicity against autologous BCG-infected macrophages. This cytotoxicity was correlated with reduced BCG viability. The cytotoxicity was reduced by the depletion of CD4(+), but not CD8(+) or CD56(+) cells, and CD4(+) cells showed higher percentage of cytotoxicity than CD4(-) cells, supporting a role for CD4(+) cells in CHE-induced, BCG-specific cytotoxicity. Additionally, this cytotoxicity was IFN-gamma, perforin and FasL dependent. After CHE-sensitisation and subsequent BCG intranasal infection, there was significant expansion of lung CD4(+) cells, the main cell type producing IFN-gamma. This was associated with 2- and 6-fold reductions in lung BCG counts 1 and 3 wk, respectively post- infection, relative to non-sensitised mice. This is the first report describing cytotoxicity against BCG-infected cells as a mechanism underlying the influence of Env sensitisation on subsequent BCG responses.