1Center for Pulmonary and Infectious Disease Control

To evaluate the immunologic factors that contribute to protection against Mycobacterium avium complex (MAC), cytokine production by peripheral blood mononuclear cells (PBMC) from human immunode®ciency virus±negative persons with pulmonary MAC (MAC patients) and healthy control subjects with a delayed hypersensitivity skin test response to M. avium sensitin (MAS-positive control subjects) was measured. In MAC patients, mycobacterium-stimulated PBMC produced higher concentrations of interleukin (IL)±10 but lower concen-trations of interferon (IFN)±g, IL-12, and tumor necrosis factor (TNF)±a, compared with PBMC from MAS-positive control subjects. Immunolabeling for intracellular IL-10 revealed that this cytokine was produced by both monocytes and T cells. Alveolar macrophages pro-duced TNF-a and IL-10 in response to MAC, which suggests that these cytokines are produced in the lungs of patients with pulmonary disease caused by this pathogen. Our ®ndings suggest that IFN-g, TNF-a, and IL-12 contribute to protection against MAC, whereas IL-10 is immunosuppressive. Mycobacterium avium complex (MAC) includes M. avium and M. intracellulare, which are widely distributed in soil and wate