Phagosomal processing of Mycobacterium tuberculosis antigen 85B is modulated independently of mycobacterial viability and phagosome maturation

Control of Mycobacterium tuberculosis infection requires CD4 T-cell responses and major histocompatibility complex class II (MHC-II) processing of M. tuberculosis antigens (Ags). We have previously demonstrated that macrophages process heat-killed (HK) M. tuberculosis more efficiently than live M. tuberculosis. These observations suggested that live M. tuberculosis may inhibit Ag processing by inhibiting phagosome maturation or that HK M. tuberculosis may be less resistant to Ag processing. In the present study we examined the correlation between M. tuberculosis viability and phagosome maturation and efficiency of Ag processing. Since heat treatment could render M. tuberculosis Ags more accessible to proteolysis, M. tuberculosis was additionally killed by antibiotic treatment and radiation. Processing of HK, live, radiation-killed (RadK), or rifampin-killed (RifK) M. tuberculosis in activated murine bone marrow macrophages was examined by using an I-A(b)-restricted T-cell hybridoma cell line (BB7) that recognizes an epitope derived from Ag 85B. Macrophages processed HK M. tuberculosis more rapidly and efficiently than they processed live, RadK, or RifK M. tuberculosis. Live, RadK, and RifK M. tuberculosis cells were processed with similar efficiencies for presentation to BB7 T hybridoma cells. Furthermore, phagosomes containing live or RadK M. tuberculosis expressed fewer M. tuberculosis peptide-MHC-II complexes than phagosomes containing HK M. tuberculosis expressed. Since only live M. tuberculosis was able to prevent acidification of the phagosome, our results suggest that regulation of phagosome maturation does not explain the differences in processing of different forms of M. tuberculosis. These findings suggest that the mechanisms used by M. tuberculosis to inhibit phagosomal maturation differ from the mechanisms involved in modulating phagosome Ag processing.