Mycobacterium tuberculosis Inhibits MHC Class II Antigen Processing in Murine Bone Marrow Macrophages

Infection of murine bone-marrow-derived macrophages with viable Mycobacterium tuberculosis (MTB) H37Ra inhibited surface expression of MHC class II (MHC-II) molecules and processing of exogenous antigens for presentation to CD4+ T hybridoma cells. The inhibition was not dependent on bacterial viability, since it was also produced by exposure to dead bacilli and MTB cytosol preparations, suggesting that it was initiated by a constitutively expressed bacterial component. Northern blot analysis demonstrated that MTB bacilli or cytosol decreased MHC-II mRNA, and immunoprecipitation of biosynthetically labeled molecules confirmed that MHC-II protein synthesis was diminished. Exposure to MTB or MTB cytosol also decreased expression of H2-DM, but H2-DM expression was still sufficient to catalyze conversion of MHC-II to SDS-stable dimers, a measure of MHC-II peptide loading. Thus, infection with MTB decreased both MHC-II and H2-DM expression, but diminished MHC-II synthesis provided the major limitation to antigen processing.