The role of humoral factors in cellular resistance.

In vitro experiments were undertaken to determine the role of compliment, properdin and antibody in the phagocytosis and cytopepsis of Histoplasma capsulatum by peritoneal macrophages from normal and immunized mice. Two methods of approach were employed. The first involved the use of freely suspended infected macrophages while the second made use of macrophages attached to glass before infection with the organism. Phagocytic rates were measured by microscopic examination of stained smears in the former method, while supernatant sampling techniques for P-32 activity was employed in the second method. Both procedures involved sampling the medium during incubation to follow cytopepsis. The hypothesis involved was that the P-32 activity appearing in the medium with time was a function of cytopeptic activity of the macrophages on ingested P-32 labeled organisms. Under the experimental condition it was found that rate of phagocytosis were not altered by the presence of various humoral factors studied nor were any differences noted in the rates of ingestion by normal and "immune" populations of cells. The rates of cytopepsis, however, were definitely enhanced by the presence of heat labile factors (i.e., complement and the properdin system, which consists of complement, properdin and Mg++) and less so by antibody, although complement and specific antibody produced increased cytopeptic rates. Studies using complement reagents lacking one or more components failed to reveal which fractions of the heat labile factor might be responsible for enhanced cytopepsis. The "immune" population of cells was found to have increased cytopeptic activity over normal cells in a freely suspended phagocyte system, but not by a fixed macrophage system. This finding points to the importance of the technology involved in biological investigative models.