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‐γ, 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‐γ. 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.