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.

Alternate class I MHC antigen processing is inhibited by Toll-like receptor signaling pathogen-associated molecular patterns: Mycobacterium tuberculosis 19-kDa lipoprotein, CpG DNA, and lipopolysaccharide.

Pathogen-associated molecular patterns (PAMPs) signal through Toll-like receptors (TLRs) to activate immune responses, but prolonged exposure to PAMPs from Mycobacterium tuberculosis (MTB) and other pathogens inhibits class II MHC (MHC-II) expression and Ag processing, which may allow MTB to evade CD4(+) T cell immunity. Alternate class I MHC (MHC-I) processing allows macrophages to present Ags from MTB and other bacteria to CD8(+) T cells, but the effect of PAMPs on this processing pathway is unknown. In our studies, MTB and TLR-signaling PAMPs, MTB 19-kDa lipoprotein, CpG DNA, and LPS, inhibited alternate MHC-I processing of latex-conjugated Ag by IFN-gamma-activated macrophages. Inhibition was dependent on TLR-2 for MTB 19-kDa lipoprotein (but not whole MTB or the other PAMPs); inhibition was dependent on myeloid differentiation factor 88 for MTB and all of the individual PAMPs. Inhibition of MHC-II and alternate MHC-I processing was delayed, appearing after 16 h of PAMP exposure, as would occur in chronically infected macrophages. Despite inhibition of alternate MHC-I Ag processing, there was no inhibition of MHC-I expression, MHC-I-restricted presentation of exogenous peptide or conventional MHC-I processing of cytosolic Ag. MTB 19-kDa lipoprotein and other PAMPs inhibited phagosome maturation and phagosome Ag degradation in a myeloid differentiation factor 88-dependent manner; this may limit availability of peptides to bind MHC-I. By inhibiting both MHC-II and alternate MHC-I Ag processing, pathogens that establish prolonged infection of macrophages (>16 h), e.g., MTB, may immunologically silence macrophages and evade surveillance by both CD4(+) and CD8(+) T cells, promoting chronic infection.

Inhibition of IFN-gamma-induced class II transactivator expression by a 19-kDa lipoprotein from Mycobacterium tuberculosis: a potential mechanism for immune evasion.

Mycobacterium tuberculosis (MTB) persists inside macrophages despite vigorous immune responses. MTB and MTB 19-kDa lipoprotein inhibit class II MHC (MHC-II) expression and Ag processing by a Toll-like receptor 2-dependent mechanism that is shown in this study to involve a defect in IFN-gamma induction of class II transactivator (CIITA). Exposure of macrophages to MTB or MTB 19-kDa lipoprotein inhibited IFN-gamma-induced MHC-II expression, but not IL-4-induced MHC-II expression, by preventing induction of mRNA for CIITA (total, type I, and type IV), IFN regulatory factor-1, and MHC-II. MTB 19-kDa lipoprotein induced mRNA for suppressor of cytokine signaling (SOCS)1 but did not inhibit IFN-gamma-induced Stat1 phosphorylation. Furthermore, the lipoprotein inhibited MHC-II Ag processing in SOCS1(-/-) macrophages. MTB 19-kDa lipoprotein did not inhibit translocation of phosphorylated Stat1 to the nucleus or Stat1 binding to and transactivation of IFN-gamma-sensitive promoter constructs. Thus, MTB 19-kDa lipoprotein inhibited IFN-gamma signaling independent of SOCS1 and without interfering with the activation of Stat1. Inhibition of IFN-gamma-induced CIITA by MTB 19-kDa lipoprotein may allow MTB to evade detection by CD4(+) T cells.