Macrophages are a significant source of type 1 cytokines during mycobacterial infection

T-helper 1 (Th1) cells are believed to be the major producer of the type 1 cytokine interferon-gamma (IFN-gamma) in cell-mediated immunity against int racellular infection. We have investigated the ability of macrophages to re lease type 1 cytokines and their regulatory mechanisms using both in vivo a nd in vitro models of pulmonary mycobacterial infection. During pulmonary i nfection by live Mycobacterium bovis bacilli Calmette-Guerin (BCG) in wild- type mice, lung macrophages released interleukin-12 (IL-12), IFN-gamma, and tumor necrosis factor-alpha (TNF-alpha), and expressed surface activation markers. However, macrophages in infected IL-12(-/-) mice released TNF-alph a but not IFN-gamma and lacked surface activation makers. In freshly isolat ed lung macrophages from naive IL-2(-/-) mice, mycobacteria alone released TNF-alpha but not IFN-gamma, whereas exogenously added IL-12 alone released a minimum of IFN-gamma. However, these macrophages released large quantiti es of IFN-gamma upon stimulation with both mycobacteria and IL-12. In contr ast, mycobacteria and exogenous IFN-gamma released only a minimum of endoge nous IFN-gamma. Endogenous IL-18 (IFN-gamma-inducing factor) played little role in IFN-gamma responses by macrophages stimulated by mycobacteria and I L-12. Our data reveal that macrophages are a significant source of type 1 c ytokines during mycobacterial infection and that both IL-12 and intracellul ar pathogens are required for the release of IFN-gamma but neat TNF-alpha. These findings suggest that macrophages regulate cell-mediated immunity by releasing not only IL-12 and TNF-alpha but also IFN-gamma and that full act ivation of IFN-gamma response in macrophages is tightly regulated.