Role of phagosomes and major histocompatibility complex class II (MHC-II) compartment in MHC-II antigen processing of Mycobacterium tuberculosis in human macrophages.

Mycobacterium tuberculosis resides in phagosomes inside macrophages. In this study, we analyzed the kinetics and location of M. tuberculosis peptide-major histocompatibility complex class II (MHC-II) complexes in M. tuberculosis-infected human macrophages. M. tuberculosis peptide-MHC-II complexes were detected with polyclonal autologous M. tuberculosis-specific CD4+ T cells or F9A6 T hybridoma cells specific for M. tuberculosis antigen (Ag) 85B (96-111). Macrophages processed heat-killed M. tuberculosis more rapidly and efficiently than live M. tuberculosis. To determine where M. tuberculosis peptide-MHC-II complexes were formed intracellularly, macrophages incubated with heat-killed M. tuberculosis were homogenized, and subcellular compartments were separated on Percoll density gradients analyzed with T cells. In THP-1 cells, M. tuberculosis Ag 85B (96- 111)-DR1 complexes appeared initially in phagosomes, followed by MHC class II compartment (MIIC) and the plasma membrane fractions. In monocyte-derived macrophages, M. tuberculosis peptide-MHC-II complexes appeared only in MIIC fractions and subsequently on the plasma membrane. Although phagosomes from both cell types acquired lysosome-associated membrane protein 1 (LAMP-1) and MHC-II, THP-1 phagosomes that support formation of M. tuberculosis peptide-MHC-II complexes had increased levels of both LAMP-1 and MHC-II. Thus, M. tuberculosis phagosomes with high levels of MHC-II and LAMP-1 and MIIC both have the potential to form peptide-MHC-II complexes from M. tuberculosis antigens in human macrophages.