Human macrophages do not require phagosome acidification to mediate fungistatic/fungicidal activity against Histoplasma capsulatum

Histoplasma capsulatum (He) is a facultative intracellular fungus that modulates the intraphagosomal environment to survive within macrophages (MO). In the present study, we sought to quantify the intraphagosomal pH under conditions in which He yeasts replicated or were killed. Human M phi that had ingested both viable and heat-killed or fixed yeasts maintained an intraphagosomal pH of similar to 6.4-6.5 over a period of several hours. These results were obtained using a fluorescent ratio technique and by electron microscopy using the 3-(2,4-dinitroanilo)-3'-amino-N-methyidipropylamine reagent. M phi that had ingested Saccharomyces cerevisae, a nonpathogenic yeast that is rapidly killed and degraded by M phi, also maintained an intraphagosomal pH of similar to 6.5 over a period of several hours. Stimulation of human M phi fungicidal activity by coculture with chloroquine or by adherence to type 1 collagen matrices was not reversed by bafilomycin, an inhibitor of the vacuolar ATPase. Human M phi cultured in the presence of bafilomycin also completely degraded heat-killed He yeasts, whereas mouse peritoneal M phi digestion of yeasts was completely reversed in the presence of bafilomycin. However, bafilomycin did not inhibit mouse M phi fungistatic activity induced by IFN-gamma. Thus, human M phi do not require phagosomal acidification to kill and degrade He yeasts, whereas mouse M phi do require acidification for fungicidal but not fungistatic activity.