A 55 kDa antigen of Pneumocystis carinii: analysis of the cellular immune response and characterization of the gene.

Rat-derived Pneumocystis carinii contains a major antigen complex of 45-55 kDa. The fusion protein of a cDNA encoding the 3' portion of the 55kDa antigen, which had previously been shown to be recognized by serum antibodies of exposed subjects, was investigated for its ability to stimulate a cellular immune response. Rats exposed to P. carinii via the environment exhibited a vigorous proliferative response to the antigen whereas unexposed rats did not. The full-length cDNA for a 55kDa antigen was cloned and found to contain a 1245bp open reading frame capable of encoding a 414-amino-acid peptide. The gene encoding this protein contained a single 39bp intron and transcribed a 1.45kb RNA message. The cloning and characterization of the 55kDa antigen gene will allow production of the specific immunological reagents necessary to characterize this molecule and study its role in the biology and pathogenesis of P. carinii.