HISTOPLASMA ACQUISITION OF CALCIUM AND EXPRESSION OF CBP1 DURING INTRACELLULAR PARASITISM

A highly adapted parasite of macrophages, the yeast phase of Histoplas ma capsulatum, survives and proliferates within phagolysosomes, while the mycelial phase exists only as a saprophyte in the soil, We have sh own previously that these two phases of Histoplasma differ in their ca lcium requirements for growth and in the production of a released calc ium-binding protein (CBP). Cloning and sequencing the CBP1 gene reveal ed two introns, a putative signal peptide and potential calcium-bindin g sites. We also evaluated CBP1 expression by reverse transcription-po lymerase chain reaction (RT-PCR) of yeasts grown in broth culture and within two host cell types, a macrophage-like cell line and respirator y epithelial cells. H. capsulatum yeasts expressed CBP1 in all of thes e settings. Splenocytes from mice immunized with H. capsulatum yeasts responded to purified CBP in proliferation assays, providing evidence for the production of CBP during the infection of mammalian hosts. In addition, after H. capsulatum yeasts were subjected to a calcium-free shock, exogenously added CBP allowed yeasts to incorporate more calciu m than yeasts incubated without added CBP, These results suggest that CBP may function to provide yeasts with calcium when they are in a low -calcium environment, such as the phagolysosomal compartment within ma crophages.