CELLULAR-RESPONSES TO A 55-KILODALTON RECOMBINANT PNEUMOCYSTIS-CARINII ANTIGEN

The host-parasite interaction in Pneumocystis carinii pneumonia is poo rly understood. In recent years, two major groups of P. carinii antige ns have been identified. One class of antigens is characterized by a b road band of immunoreactivity bet ween 45 and 55 kDa in P. carinii der ived from rats. This antigen complex is the P. carinii antigen most co mmonly found in respiratory tract specimens and most frequently recogn ized by the host immune response. The availability of a recombinant an tigen has permitted studies focusing on the cellular and humoral respo nses to a single antigen within this class, p55. In this study, we hav e demonstrated that the p55 antigen elicits a cell-mediated immune res ponse in animals previously exposed to P. carinii. Under conditions of natural exposure, the 5' portion of the molecule, p55(1-200), appears immunologically silent, failing to elicit lymphocyte proliferation or cytokine secretion. Following active immunization, the 5' portion is capable of stimulating lymphocyte proliferation. The 3' portion, p55(2 68-414), has at least one immunodominant region which contains a 7-ami no-acid repeat motif. The cells responding to p55 include a CD4(+) T c ell which secretes a Th1 cytokine pattern. A detailed understanding of the host-parasite interaction will facilitate the development of immu noprophylaxis and immunotherapy for P. carinii infection.