Phagosome-lysosome fusion in P388D1 macrophages infected with Histoplasma capsulatum.

The issue of whether or not phagocytized Histoplasma capsulatum yeasts evade phagosome-lysosome fusion (P-LF) has been debated by several investigators. To resolve this problem, yet avoid drawbacks associated with the conventional assays of P-LF (electron microscopy and the acridine orange assay), we used fluorescein isothiocyanate-labeled dextran (FITC-dextran) to monitor P-LF in the macrophage-like cell line P388D1.D2. Controls indicated that FITC-dextran could be used to distinguish between evasion of P-LF by Toxoplasma gondii and phagolysosome formation following ingestion of Saccharomyces cerevisiae. Phagosomes containing H. capsulatum clearly fused with FITC-dextran-labeled lysosomes at a rate comparable to that observed for S. cerevisiae. This was true for several strains of H. capsulatum including two avirulent strains derived in this laboratory. Varying the dose of H. capsulatum did not alter the percentage of phagolysosomes formed. Our results indicate that H. capsulatum is one of a small number of organisms which is able to survive in phagolysosomes.