Alpha/beta interferon impairs the ability of human macrophages to control growth of Mycobacterium bovis BCG

Administration of alpha/beta interferon (IFN-alpha/beta) to mice infected with Mycobacterium tuberculosis has been shown to increase mycobacterial growth. Because IFN-alpha/beta has direct pleiotropic effects on the differentiation and functional activities of macrophages, we evaluated the effect of IFN-alpha/beta on mycobacterial growth in human monocytes/macrophages in vitro. Monocytes cultured at optimal cell density could control the growth of M. bovis BCG, as assessed both by measurement of luciferase activity expressed by a mycobacterial reporter strain and by counting of CFU. In contrast, unrestrained mycobacterial growth was observed when monocytes were treated with alpha interferon (IFN-alpha) 3 days prior to or concomitant with infection. This striking loss of mycobacteriostatic activity was observed with IFN-alpha and IFN-beta and was induced in both freshly isolated monocytes and culture-derived macrophages. Pretreatment of monocytes with IFN-alpha modified cellular morphology and reduced viability following culture, but neither was observed for culture-derived macrophages, indicating that the effects of IFN-alpha on mycobacteriostatic activity and cell differentiation and death could be dissociated. These results are compatible with the possibility that the secretion of IFN-alpha/beta could directly promote mycobacterial growth in patients harboring these organisms.