Pneumocystis carinii as a cause of pneumonia in the immunocompromised host, has become increasingly important since the onset of the AIDS epidemic. Characterization of specific antigens to which the host immune response is mounted, may result in the development of new therapeutic modalities. A prominent class of antigens of rat-derived P. carinii migrate between 45 and 55 kDa. Recently I have cloned a gene that encodes an antigen of the 45 - 55 kDa class of rat-derived P. carinii. Infected animals mount a vigorous humoral and cell mediated response to this 55 kDa antigen but nothing is known of its location or cellular function. I hypothesize that the 55 kDa antigen : i) is functionally important to P. carinii and ii) may play an important role in the pathogenesis of infection and the host immune response to the organism. This project will attempt to define the cellular function of the 55 kDa antigen and to characterize the host response to this antigen. The specific aims of the project will be: (1) To study the expression of the 55 kDa antigen in rat- derived P. carinii and examine its role in promoting adherence to lung cells; (2) To examine interaction of the 55 kDa antigen with other P. carinii and host proteins by molecular approaches; (3) To characterize the humoral and cellular immune response to the 55 kDa antigen; and (4) To clone the homologous gene from human-derived P. carinii. I will identify the cellular localization of the 55 kDa antigen and characterize its expression by Indirect Fluorescent Antibody (IFA) techniques, Immuno- Electron Microscopy (IEM). If shown to be a surface moiety, a cellular approach will test specific peptide sequences of the 55 kDa antigen, such as an Arg-Gly-Asp (RGD) sequence located at position 80, for adherence to eukaryotic cells. To examine the functional interaction with other P. carinii and host proteins, molecular approaches utilizing (i) the two hybrid system in yeast, (ii) the random peptide expression library will be used. The humoral and cellular immune responses to the 55 kDa antigen will be characterized. B- and T-cell determinants will be identified and responses to these epitopes examined after natural infection and following immunization. Work on this recombinant antigen of P. carinii will identify the role of this antigen in the biology of P. carinii and in the pathogenesis of P. carinii infection. An understanding of the function of this antigen and its role in the immune response might provide information enabling modulation of the growth and metabolism of the organism or alteration of the immune response.