Isolation and characterization of a recombinant antigen of Pneumocystis carinii

Pneumocystis carinii contains a major group of antigens which migrates as a broad band of 45 to 55 kDa and 35 to 45 kDa in organisms derived from rats and humans, respectively. This complex is among the most common P. carinii antigens found in the respiratory tract and is recognized by serum antibodies of infected individuals. We have isolated a cDNA clone encoding the 3' portion of a 45- to 55-kDa antigen of rat-derived P. carinii. The predicted protein encoded by this cDNA contains a distinctive domain composed of 10 copies of a 7-amino-acid sequence motif rich in glutamic acid residues. Affinity-purified antibodies to this peptide reacted with the 45- to 55-kDa band of rat-derived P. carinii and with the 35- to 45-kDa band of human-derived P. carinii, indicating shared epitopes. The fusion protein was recognized by serum antibodies from rats and humans with natural exposure to P. carinii and by human immunodeficiency virus patients with P. carinii pneumonia. The production of this recombinant protein should allow more detailed studies of the host-parasite relationship of this important opportunistic infection.