While
laccase of Cryptococcus neoformans is implicated in the virulence of the organism, our recent studies showing absence of
melanin in the infected mouse brain has led us to a search for alternative roles for
laccase in
cryptococcosis. We investigated the role of
laccase in protection of C. neoformans against murine alveolar macrophage (AM)-mediated antifungal activity by using a pair of congenic
laccase-positive (2E-TUC) and
laccase-deficient (2E-TU) strains. The
laccase-positive cells with
laccase derepression were more resistant to the antifungal activity of AM than a
laccase-deficient strain ([28.9 +/- 1.2]% versus [40.2 +/- 2.6]% killing). Addition of
L-dopa to Cryptococcus to produce
melanin in a
laccase-positive strain resulted in a slight increase in protection of C. neoformans from the antifungal activity of macrophages ([25.4 +/- 3.4]% versus [28.9 +/- 1.2]% killing). Recombinant cryptococcal
laccase exhibited
iron oxidase activity in converting Fe(II) to Fe(III). Moreover, recombinant
laccase inhibited killing of C. neoformans by
hydroxyl radicals catalyzed by
iron in a cell-free system. Addition of the
hydroxyl radical scavenger
mannitol or
dimethyl sulfoxide to AMs prior to the introduction of cryptococcal cells decreased killing of both strains and reduced the difference in susceptibility between the
laccase-positive and
laccase-deficient strains. Furthermore,
laccase-mediated protection from AM killing was inhibited by the addition of Fe(II), presumably by overcoming the effects of the
iron oxidase activity of cryptococcal
laccase. These results suggest that the
iron oxidase activity of
laccase may protect C. neoformans from macrophages by oxidation of phagosomal
iron to Fe(III) with a resultant decrease in
hydroxyl radical formation.