Immune response to Cryptococcus neoformans soluble polysaccharide. I. Serological assay for antigen and antibody

Chromium chloride was used as a coupling agent for the conjugation of purified cryptococcal polysaccharide to sheep erythrocytes. Sensitized erythrocytes were used in a passive hemagglutination (PHA) assay for antibody to cryptococcal polysaccharide and a passive hemagglutination inhibition (PHI) assay for antigen. The PHA assay was more sensitive than complement fixation, agglutination, or precipitation tests for antibody. The PHI assay could detect submicrogram quantities of soluble polysaccharide. Antigen or antibody could be detected in serum or spinal fluid from seven of eight patients with cryptococcosis. Tests for antigen or antibody were negative with sera from patients with histoplasmosis, blastomycosis, coccidioidomycosis, aspergillosis, or allescheriosis. A low frequency (3%) of positive reactors for antibody was found among sera from normal persons and from persons with unrelated diseases; whereas, all tests for antigen were negative. The assay showed a high degree of sensitivity for immunoglobulins of the immunoglobulin M class; however, cryptococcal antibody of the immunoglobulin G class was also detected. The immunological specificity of the polysaccharide preparation was due to carbohydrate rather than to protein associated with the polysaccharide.