Interferon-gamma (IFN-gamma) previously has been shown to inhibit the replication of Chlamydia psittaci in epithelial cells by inducing indoleamine 2,3-dioxygenase, the enzyme that decyclizes tryptophan to N-formylkynurenine. The role of indoleamine 2,3-dioxygenase in IFN-mediated inhibition of C. psittaci in human macrophages has now been examined. Peripheral blood monocytes from normal donors were isolated and cultivated 10-14 days to allow differentiation to macrophages. Cells were then treated with either IFN-gamma or IFN-beta for 48 h before infection with sufficient C. psittaci to infect approximately 30% of the cells. Infected cells were incubated 24 h, at which time coverslips were fixed, stained with Giemsa, and examined for development of C. psittaci inclusions by light microscopy. Complete inhibition of inclusion development was observed with IFN-gamma. In the absence of lipopolysaccharide, inhibition of C. psittaci by IFN-beta was variable; however, in the presence of lipopolysaccharide, IFN-beta also completely inhibited C. psittaci replication. The addition of excess tryptophan to the culture medium at the time of infection partially reversed the effect of IFN on the inhibition of C. psittaci growth in a concentration-dependent manner. Indoleamine 2,3-dioxygenase activity was determined by measurement of the concentrations of tryptophan and its metabolites in the culture medium after reversed-phase high-performance liquid chromatography. Significant indoleamine 2,3-dioxygenase activity was observed only in macrophages treated with IFN-gamma or combined IFN-beta plus lipopolysaccharide, and resulted in greater than 50% of available tryptophan being catabolized in a 4-h period.(ABSTRACT TRUNCATED AT 250 WORDS)