Modifications of the immune response induced by Histoplasma capsulatum products.

To understand the host-parasite relationship in histoplasmosis, mice were inoculated with histoplasmin (HP), the filtrate of aged cultures of Histoplasma capsulatum, and the immune response of these mice towards sheep red blood cells (SRBC) and to trinitrophenyl hapten was studied. The filtrate abrogated completely the primary antibody response to both antigens as measured by the number of spleen hemolytic plaque forming cells when HP was administered at doses greater than 200 micrograms two days before the antigen. Suppression was elicited by HP when it was injected either intravenously, intraperitoneally, or subcutaneously. Inoculation with HP and antigen on the same day did not result in suppression. The secondary antibody response was not modified at any dose. Variation of the response with time was determined by counting the number of rosette forming cells (RFC) to SRBC every two days for a total of 21 days. Antibody-mediated RFC ('B' rosettes) were depressed throughout the experiment, while the number of non 'B', presumably T, RFC did not vary from control values. In animals inoculated with HP alone, high number of RFC were detected on day 11 after HP inoculation, suggesting that HP may have polyclonal activation effects. These results support the possibility that H. capsulatum evades the immune defenses by induction of a suppressive phenomenon in which the afferent limb of the immune response is not involved. This effect appeared to be induced directly by a product of the fungus, instead of by factors generated during the immune response to this microorganism.