Histoplasma capsulatum yeasts are phagocytosed via very late antigen-5, killed, and processed for antigen presentation by human dendritic cells.

Histoplasma capsulatum (Hc) is a facultative, intracellular parasite of world-wide importance. As the induction of cell-mediated immunity to Hc is of critical importance in host defense, we sought to determine whether dendritic cells (DC) could function as a primary APC for this pathogenic fungus. DC obtained by culture of human monocytes in the presence of GM-CSF and IL-4 phagocytosed Hc yeasts in a time-dependent manner. Upon ingestion, the intracellular growth of yeasts within DC was completely inhibited compared with rapid growth within human macrophages. Electron microscopy of DC with ingested Hc revealed that many of the yeasts were degraded as early as 2 h postingestion. In contrast to macrophages, human DC recognized Hc yeasts via the fibronectin receptor, very late Ag-5, and not via CD18 receptors. DC stimulated Hc-specific lymphocyte proliferation in a concentration-dependent manner after phagocytosis of viable and heat-killed Hc yeasts, but greater proliferation was achieved after ingestion of viable yeasts. These data demonstrate that human DC can phagocytose and degrade a fungal pathogen and subsequently process the appropriate Ags for stimulation of lymphocyte proliferation. In vivo, such interactions between DC and Hc may facilitate the induction of cell-mediated immunity.