Effects of gamma interferon and nitric oxide on the interaction of Mycobacterium avium subsp. paratuberculosis with bovine monocytes

In this study, we examined the effects of recombinant bovine gamma interferon (rIFN-gamma) and nitric oxide (NO) on the interaction of M. avium subsp. paratuberculosisi with bovine monocytes. Monocytes pretreated with rIFN-gamma exhibited slightly increased phagocytosis of M. avium subsp. paratuberculosis and modest inhibition of the intracellular growth of this microorganism. The number of viable intracellular bacilli decreased earlier in rIFN-gamma-pretreated monocytes than in control monocytes. After infection with M. avium subsp paratuberculosis, NO was not constitutively released, but NO release from infected monocytes was induced by treatment with rIFN-gamma or with rIFN-gamma and lipopolysaccharide (LPS). Release of nitric oxide was Inhibited by addition of NG-monomethyl-L-arginine; however, inhibition of nitric oxide did not alter the pattern of intracellular survival of M. avium subsp.paratuberculosis in rIFN-gamma-treated bovine monocytes. Although chemically generated nitric oxide killed M. avium subsp. paratuberculosis in a cell-free system in vitro, the amount of nitric oxide required was far greater than that released from infected monocytes stimulated with rIFN-gamma and LPS. Our data suggest that rIFN-gamma activates M. avium subsp. paratuberculosis-infected bovine monocytes to release nitric oxide but only modestly increases antimycobacterial activity of monocytes against this organism. This may be due, in part, to the fact that the amount of nitric oxide produced by rIFN-gamma-activated bovine monocytes is insufficient to kill intracellular M. avium subsp.