Cryptococcal skin test antigen: preparation variables and characterization.

Antigen capable of eliciting delayed hypersensitivity reactions in the skin of sensitized guinea pigs could be extracted from Cryptococcus neoformans cells by stirring the cells from 3 to 5 days in concentrated urea or guanidine. Hydrolysis of urea to ammonia by cryptococcal urease accompanied urea extraction, but alkalinity appeared neither necessary nor sufficient for extraction. Antigen from live cells gave larger delayed skin reactions than did antigen from Formalin-killed cells. Peak skin test reactivity appeared to reside in protein-rich fraction having an elution volume on Sephadex G50 corresponding to a molecular weight of 10(4). Activity precipitated with half-saturated ammonium sulfate and could be detected in a single, narrow, rapidly migrating band on disc electrophoresis. Dialyzable proteinaceous antigen and high-molecular-weight, serologically active polysaccharide were present in the antigen, but not active in the delayed hypersensitivity reactions.