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Active Substance of Histoplasma capsulatum that Inhibits Blastogenic Response of Lymphocytes

A substance inhibiting blast transformation of murine spleen lymphocytes stimulated with various mitogens, such as LPS, PHA, and PWM, was obtained from yeast‐form cells of Histoplasma capsulatum. This active substance was partially purified from the cell‐free extract by DEAE‐Sepharose CL‐6B column chromatography. As a result of this partial purification, the inhibitory activity was 1.26 μg/ml in terms of ID50. Materials from H. capsulatum also inhibited blast transformation of murine spleen lymphocytes stimulated with the antigen PPD as well as mitogens LPS, PHA, and PWM. However, the con A‐induced proliferative response was only slightly affected. A similar result was observed for the MLR. These inhibitory activities were abolished by heating at 70 C for 30 min. These results suggest that the heat‐labile active substance produced by H. capsulatum might directly affect the lymphocytes, leading to inhibition of their blast transformation.