Introduction to Fungi, Third Edition

664 BASIDIOMYCETE YEASTS
5-fluorocytosine over several weeks is effective if
the disease has been diagnosed sufficiently early
(Mitchell & Perfect, 1995). For AIDS sufferers, ‘the
therapeutic goal is to ablate the symptoms and
signs of cryptococcosis until the patient dies of
other causes’ (Kwon-Chung & Bennett, 1992).
Antifungal drugs, especially fluconazole, may
have to be administered for the rest of the
patient’s life because of the high risk of relapses.
Cryptococcosis, like certain other fungal
diseases, is an AIDS-defining illness, i.e. patients
diagnosed as HIV-positive and suffering from
cryptococcosis are considered to have developed
AIDS.
Virulence factors
Much work is being done to identify the properties
which enable C. neoformans to cause disease
(reviewed by Buchanan & Murphy, 1998;
Casadevall & Perfect, 1998; Perfect, 2004). The
most important and obvious virulence factor is
the capsule which surrounds actively growing
yeast cells of C. neoformans and protects them
from adverse environmental effects. It also seems
to inhibit the phagocytosis of yeast cells by
macrophages. The capsule can be visualized by
light microscopy of yeast cells mounted in India
ink and can be rather more substantial than that
shown for Rhodotorula in Fig. 24.1b. The major
capsule polysaccharide of C. neoformans is a linear
a-(1,3)-mannan chain, of which roughly every
third mannose moiety is substituted with a
single b-(1,2)-glucuronic acid unit. The presence
of xylose determines the antigenic properties of
the capsule; one (serotype D), two (A), three (B) or
four (C) xylose residues may be present for every
three mannose moieties (Cherniak & Sundstrom,
1994). The capsule polysaccharides are produced
in such profusion that they can be detected in
the blood serum and other body fluids of
infected patients, and this is an important
diagnostic tool (Kwon-Chung & Bennett, 1992).
Mutants unable to produce a capsule in the host
are apathogenic.
Both C. neoformans var. gattii and vars. grubii/
neoformans have been shown to undergo switches
in colony phenotype from smooth to mucoid,
wrinked or pseudohyphal. Switching between
smooth and mucoid appears to be readily
reversible in the mammalian host in var. gattii,
but less so in the latter two. The mucoid type is
characterized by an increase in the thickness of
the polysaccharide capsule. In C. neoformans var.
neoformans and var. grubii a thick capsule coincides
with an enhanced resistance to antimycotics,
and mucoid strains may be selected by
prolonged chemotherapy (Guerrero et al., 2006).
In mice infected with C. neoformans var. gattii, Jain
et al. (2006) have shown that mucoid forms are
associated preferentially with pulmonary infections,
presumably because of an enhanced
resistance to intracellular digestion by phagocytes.
Smooth cells with their thinner coat were
preferentially isolated from infected brain tissue.
In contrast to morphotype switching in Candida
albicans (p. 277), no sexual function has been
suggested for switching in C. neoformans.
A second important virulence factor is melanin.
The ability of C. neoformans to synthesize
melanin distinguishes this species from other
members of the genus, and the melanized cell
wall has been suggested to protect the cell against
oxidative stress such as that encountered during
the oxidative burst after ingestion by macrophages
(Wang et al., 1995). The pathway of
melanin biosynthesis in C. neoformans is different
from the dihydroxynaphthalene (DHN) route
found in most fungi (see Fig. 12.46). The precursor
molecule is 3,4-dihydroxyphenylalanine (DOPA)
which cannot be synthesized by C. neoformans
but, if present, can be oxidized to quinones by a
laccase-type enzyme, and these quinones spontaneously
polymerize to melanin (Salas et al., 1996).
Catecholamines such as DOPA are present at high
levels in the brain, and this may explain the
preferential accumulation of C. neoformans in
brain tissue (Polachek et al., 1990).
A third important factor is, of course, the
ability of C. neoformans to grow at 37 39°C,
which is unique among Cryptococcus spp. Several
gene products are required for growth at 37°C,
and prominent among them is calcineurin,
a Ca 2þ -regulated serine/threonine phosphatase
involved in eukaryotic cellular signalling
(Odom et al., 1997). Calcineurin is the target of
a complex formed between a cyclophilin protein
involved in protein folding, and cyclosporin A
(see Fig. 12.24a), an immunosuppressive drug