Further studies on the inhibition of Histoplasma capsulatum within macrophages from immunized animals.

Mononuclear phagocytes freshly harvested from immunized animals restrict the intracellular growth of Histoplasma capsulatum. The fate of the inhibited fungus is the subject of this report. Macrophages harvested from mice immunized by sublethal infection were parasitized in vitro with yeast cells of H. capsulatum. The subsequent fate of the fungus was assessed by staining characteristics, viability on subculture, and radioisotopic techniques. No tinctorial distinction could be made between yeasts within normal or immune phagocytes stained by the May Greenwald-Giemsa or by the Gridley techniques. Yeast cells recovered after 24 h of residence within the cytoplasm of immune monocytes excluded the dye eosin-Y, germinated on Casamino Acid agar incubated at 30 C, and initiated growth at a normal rate within phagocytes from normal animals. Thus even though the growth of H. capsulatum was inhibited within macrophages from immunized animals, the fungus was not killed by the encounter. However, autoradiographic studies established that protein synthesis by the fungus in situ was impaired.