Digestion of Histoplasma capsulatum yeasts by human macrophages

The strategies used by Histoplasma capsulatum yeasts to survive and multiply within human macrophages (M phi) are unknown. To better understand these strategies we studied the intracellular fate of viable vs heat-killed (HK) yeasts in human monocyte-derived M phi. Initial studies demonstrated that phagolysosome fusion was present in M phi ingesting either viable or HK yeasts. Viable yeasts multiplied within M phi phagolysosomes, whereas M phi completely digested intracellular FITC-labeled HK yeasts within 24 h after ingestion. This observation was confirmed by electron microscopy. M phi that had ingested colloidal gold-labeled HK yeasts contained gold particles but no visible yeasts at 24 h. Digestion of HK yeasts was evident as early as 4 h after phagocytosis, and was complete by 24 h. M phi digestion of HK yeasts was blocked completely when M phi were cultured for 24 h in the presence of chloroquine. In M phi simultaneously ingesting both viable and HK yeasts, viable yeasts multiplied, but HK yeasts were digested within the same cell. M phi that had ingested viable yeasts digested them completely when M phi were cultured for 24 h in the presence of cycloheximide or amphotericin B. Coculture of infected M phi with nystatin or ketoconazole resulted in inhibition of growth, but the yeasts were not digested. These data indicate that: 1), HK Hc yeasts are easily digested by preformed M phi lysosomal hydrolases; 2), viable Hc yeasts survive and multiply within M phi phagolysosomes, but the yeasts do not secrete a factor(s) that affects the ability of other phagolysosomes within the same M phi to digest killed yeasts; and 3), inhibition of yeast protein synthesis or cell wall biosynthesis is sufficient to render viable yeasts susceptible to digestion by human M phi.