Interferon decreases the growth inhibition of Mycobacterium avium-intracellulare complex by fresh human monocytes but not by culture-derived macrophages.

Using a rapid radiolabel assay, monocytes derived from the peripheral blood of normal donors were found to kill 40%-92% of inoculated Mycobacterium avium-intracellulare complex (MAC), an opportunistic pathogen commonly found in AIDS patients. However, bactericidal activity was significantly lower in 4-day culture-derived macrophages compared with matched monocyte cultures. The addition of interferon-gamma (IFN-gamma) to monocytes was found to inhibit the bactericidal activity of fresh monocytes. The number of bacteria recovered from fresh monocytes exposed to IFN-gamma was significantly higher than that in control cultures with MAC alone, suggesting that intracellular MAC growth could be stimulated by IFN-gamma. This enhancement of MAC survival and growth by IFN-gamma was not observed when culture-derived macrophages were used. Similar results were obtained with IFN-alpha/A2. These results indicate, therefore, that the innate efficiency of mycobacterial killing by monocytes can be down-regulated by IFN, but macrophages are not significantly affected.