HeLa cells and Histoplasma capsulatum; phagocytosis and subsequent intracellular growth.

In an earlier study (Shepard, 1955) it was found that tubercle bacilli were much more actively phagocytosed by HeLa cells when horse serum was incorporated into the tissue culture medium, in place of the more commonly used human or rabbit serum. By selection of a favorable horse serum it could be seen that a significant proportion of the tubercle bacilli in an inoculum could thus be introduced into the HeLa cells (Shepard, 1958a), where their subsequent intracytoplasmic growth took place rapidly and with characteristic morphology. In a study of Histoplasma capsulatum and HeLa cells, Larsh et al. (1956) found that the mycelial phase converted to the tissue phase more readily in HeLa cell cultures than in artificial medium. In view of these findings a joint study was undertaken of the cytological aspects of the growth of this fungus in HeLa cells, especially as concerned the relationship between phagocytosis and the serum constituent of the tissue culture medium. It was soon evident that the use of human serum in the tissue culture medium led to more active phagocytosis of the yeast phase of H. capsulatum than did horse serum with known ability to promote phagocytosis of tubercle bacilli. These observations led to a survey of other animal sera, and of those tried, guinea pig serum was much superior. Guinea pig serum also was found to be satisfactory for the nutrition of the HeLa cells and this made it possible to devise methods for the rapid intracellular growth of H. capsulatum from small inocula. In addition to the research referred to (Larsh et al., 1956), the other published studies of the yeast phase of H. capsulatum in tissue culture cells appear limited to those of Randall and co-workers (1951, 1953a, b).