EXPRESSION OF VARIANTS OF THE MAJOR SURFACE GLYCOPROTEIN OF PNEUMOCYSTIS-CARINII

Previously, we have shown that a multicopy family of related but uniqu e genes encodes the major surface glycoprotein (MSG) of Pneumocystis c arinii. To examine whether different members of this gene family are e xpressed by P. carinii, antisera were prepared against peptides whose sequences were determined from the deduced amino acid sequences of var iants of rat-derived MSG. Immunohistochemical staining of serial secti ons of rat lungs of infected infected animals showed that at least thr ee variants of MSG were expressed in an individual lobe, that there wa s a focal expression of these variants within the lung, and that the r elative numbers of these foci were different. Indirect immunofluoresce nt staining of purified P. carinii organisms using these antisera reve aled that at least three variants of MSG were present in organisms iso lated from an individual rat and that both cysts and trophozoites reac ted with each antiserum. A substantial difference in the fraction of o rganisms reacting with a specific antipeptide antiserum was seen when comparing organisms isolated from rats raised in a single colony over a period of two years as well as organisms isolated at one time point from rats raised in different colonies. This demonstration of antigeni c variation in P. carinii supports the hypothesis that P. carinii util izes such variation for evading host defense mechanisms.