Immunization with recombinant Pneumocystis carinii p55 antigen provides partial protection against infection: characterization of epitope recognition associated with immunization.

Many therapeutic options exist for the treatment of Pneumocystis carinii pneumonia, a common fungal opportunistic pulmonary pathogen, but treatment is often complicated by side effects and toxicity and, more recently, markers of drug resistance have been described. The development of immunotherapetic modalities such as active immunization or passive immunotherapy may play an increasing important role in the prevention and treatment of infection. Passive immunotherapy with polyclonal anti-P. carinii reagents, such as serum or T cells, and monospecific reagents reactive with the major surface glycoprotein (MSG or gpA), such as monoclonal antibodies or MSG primed T cells, reduce the severity or eradicate infection. Active immunization with whole P. carinii, P. carinii extracts or MSG has afforded partial protection against the subsequent development of P. carinii pneumonia in some animal models. Identification of additional antigens with protective benefits will aid in the development of vaccines or other reagents. The p55 antigen of rat-derived P. carinii is well recognized by animals following natural exposure to the organism. This 414 amino acid residue antigen found within the cell wall of P. carinii contains 7 repeats of a glutamic acid-rich motif in the carboxyl portion of the molecule. Both humoral and cellular immune responses reactive with this repeated domain are present following natural infection while, the amino terminal portion of the molecule is immunologically silent. In this study, immunization with recombinant p55 elicited significant humoral and cellular immune responses which persisted during 10 weeks of immunosupression in corticosteroid treated rats; rp55 immunization resulted in a significant reduction in organism burden, improved histological score, lower lung weight to body weight ratio (a marker of infection or lung inflammation) and improved survival (P < 0.01). Greater protection was afforded by immunization with a peptide containing amino acid residues 1-200, than by the entire rp55 molecule. Epitope recognition by serum from animals immunized with rp55 differed from that of naturally exposed animals with oligoclonal responses to residues 22-92 and residues 196-218. This study demonstrates that protection against P. carinii can be afforded by immunization with antigen preparations other than whole extracts of P. carinii or the major surface antigen, MSG. This antigen moiety will likely be most useful as a vaccine candidate in combination with other immunogens which provide similar partial protection.