Intracellular Parasitism by Histoplasma capsulatum : Fungal Virulence and Calcium Dependence

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Abstract
Histoplasma capsulatum is an effective intracellular parasite of macrophages and causes the most prevalent fungal respiratory disease in the United States. A “dimorphic” fungus, H. capsulatum exists as a saprophytic mold in soil and converts to the parasitic yeast form after inhalation. Only the yeasts secrete a calcium-binding protein (CBP) and can grow in calcium-limiting conditions. To probe the relation between calcium limitation and intracellular parasitism, we designed a strategy to disrupt CBP1 in H. capsulatum using a telomeric linear plasmid and a two-step genetic selection. The resulting cbp1 yeasts no longer grew when deprived of calcium, and they were also unable to destroy macrophages in vitro or proliferate in a mouse model of pulmonary infection.