Decreased expression of cytokines that induce type 1 helper T cell/interferon-gamma responses in genetically susceptible mice infected with Mycobacterium avium.

Mycobacterium avium is an intracellular microorganism that infects and multiplies within macrophages. Cell-mediated immunity plays an important role in host defense. Two cytokines, interleukin 12 (IL-12) and interferon-gamma-inducing factor (IGIF), produced mainly by macrophages, are critical for the development of type 1 helper T (Th1) cell/interferon-gamma (IFN-gamma) responses and cell-mediated immunity. In this report, by using a mouse model of disseminated M. avium infection, we demonstrate that genetically susceptible BALB/c mice show decreased expression of IL-12 and IGIF in association with a diminished IFN-gamma/Th1 response. Conversely, resistant DBA/2 mice exhibited increased expression of IL-12, IGIF, and IFN-gamma. In both strains of infected mice, the level of IL-4/Th2 response was similar. These results suggest that decreased expression of IL-12 and IGIF leads to a diminished Th1 response without reciprocal enhanced Th2 responses in susceptible mice.