The viable Mycobacterium tuberculosis H37Ra strain induces a stronger mouse macrophage response compared to the heat-inactivated H37Rv strain

Macrophages are the target cells for Mycobacterium tuberculosis (M. tuberculosis) as well as key effector cells for clearance of this pathogen. The aim of the present study was to measure and compare the responses of mouse peritoneal macrophages following exposure to the live M. tuberculosis H37Ra and heat-inactivated H37Rv strains. In vitro phagocytosis assays indicated that the macrophages had a higher capacity to engulf the live H37Ra strain compared to the inactivated H37Rv strain. Enzyme-linked immunosorbent assay (ELISA) demonstrated that H37Ra-stimulated macrophages produced significantly increased concentrations of interleukin-12p40 (IL-12p40), tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) compared to the untreated control cells. However, H37Rv exposure induced little to no increase in the levels of the cytokines examined. The results from ELISA were confirmed by reverse transcription-polymerase chain reaction (RT-PCR) at the mRNA level. There was a dose-dependent increase in nitric oxide (NO) and hydrogen peroxide (H2O2) production from the H37Ra-stimulated macrophages compared to the H37Rv-stimulated ones. Confocal microscopy and flow cytometric analysis indicated that the IFN-gamma-stimulated macrophages from viable H37Ra-immunized mice had an enhanced surface expression of CD40 ligand (CD40L) compared to those from inactivated H37Rv-immunized mice. Our data collectively indicate that exposure to the viable H37Ra strain induces a stronger macrophage response compared to exposure to the heat-inactivated H37Rv strain, which may be associated with the increased surface expression of CD40L in activated macrophages.