Intracellular Proliferation Rate Predicts Virulence of Cryptococcal
Strains in a Murine Model of Cryptococcosis. Several early studies
have highlighted the hypothesis that macrophages may exacerbate
cryptococcal infection in mice. Alveolar macrophage depletion was
associated with amelioration of disease in three murine strains as
measured by lung fungal burden (14), and decreased the dissemination
of a glucosylceramide-deficient mutant of C. neoformans in
immunodeficient mice (15). We therefore tested whether high
intracellular proliferation capacity is linked to the hypervirulence of
the VIO lineage. To do this, IPR values were compared with both
published and newly-generated mouse median survival times
(ST50). Remarkably, IPR and murine ST50 are highly significantly
correlated (P 0.00017, linear regression, Fig. 2): a correlation that
holds true for both C. neoformans and C. gattii and is independent
of the mouse model used. In other words, the ability to survive and
proliferate inside a host macrophage contributes significantly to
cryptococcal virulence in the murine model and presumably also in
infected humans, as suggested by the human primary macrophage
data. This previously undiscovered link between intracellular proliferation
rate and virulence also provides a possible explanation for
why ‘‘ancestral’’ AFLP6A strains share the VIO genotype and yet
do not lead to disease outbreaks. For example, CBS6956, which is
considered as the potential origin of the AFLP6A subtype (11),
shows only a moderate IPR value of 1.35 (Fig. 1A and Table S1),
indicating that it cannot exploit the intracellular macrophage niche
as successfully as the VIO isolates.