Differing requirement for inducible nitric oxide synthase activity in clearance of primary and secondary Cryptococcus neoformans infection.

The role of nitric oxide in resistance to cryptococcal infection was investigated. Mice deficient in inducible nitric oxide synthase (INOS) did not survive a primary intratracheal infection as did INOS-replete control mice. Despite adequate recruitment of host cells and generation of interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha at the site of infection, INOS-deficient mice failed to clear yeast from their lungs by five weeks of infection, in contrast to wild-type mice. INOS-deficient mice also had higher yeast brain burdens than did control mice after a primary intracerebral infection. Therefore, generation of nitric oxide is required for resistance to primary cryptococcal infection. However, INOS-deficient mice vaccinated subcutaneously and rechallenged intravenously had lung and brain yeast burdens equivalent to those of vaccinated controls, and therefore expressed effective acquired immunity to Cryptococcus neoformans. Cells harvested from infected INOS-deficient mice by bronchoalveolar lavage acted as anti-cryptococcal effectors in vitro at an effector:target ratio of 100:1, provided IFN-gamma was present, but did not inhibit yeast proliferation at a 10:1 effector:target ratio as cells from wild-type mice did. Therefore, INOS activity is important for anti-cryptococcal function of effectors of immunity during the primary response, but not for the generation or expression of secondary immunity to C. neoformans.