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 cr
yptococcosis. We investigated the role of laccase in protection of C. neofo
rmans 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 s
train ([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 activ
ity of macrophages ([25.4 +/- 3.4]% versus [28.9 +/- 1.2]% killing). Recomb
inant cryptococcal laccase exhibited iron oxidase activity in converting Fe
(II) to Fe(III), Moreover, recombinant laccase inhibited killing of C. neof
ormans by hydroxyl radicals catalyzed by iron in a cell-free system. Additi
on of the hydroxyl radical scavenger mannitol or dimethyl sulfoxide to AMs
prior to the introduction of cryptococcal cells decreased killing of both s
trains and reduced the difference in susceptibility between the laccase-pos
itive and laccase-deficient strains. Furthermore, laccase-mediated protecti
on from AM killing was inhibited by the addition of Fe(II), presumably by o
vercoming the effects of the iron oxidase activity of cryptococcal laccase.
These results suggest that the iron oxidase activity of laccase may protec
t C. neoformans from macrophages by oxidation of phagosomal iron to Fe(III)
with a resultant decrease in hydroxyl radical formation.