Expression of a putative immunosuppressive protein in human tumors and tissues

Tumor-induced immunosuppression by murine retrovirus-induced tumors and nonviral murine and human tumors has been shown to be mediated by the transmembrane (TM) envelope (env) protein p15E. This in vitro activity is inhibitable by anti-(murine)p15E antibodies, implying that a TM-like protein is produced by such tumors. The leading candidate genes that might encode such proteins in human tumors are human endogenous retroviral (HERV) sequences. We have utilized immunohistochemistry to determine what tissues may express HERV env proteins. We subcloned a restriction fragment from the putative TM human env gene of a type C-related HERV (clone-4-1) into a fusion protein gene construct. Using a rabbit polyclonal antiserum against the fusion protein, we observed staining in a variety of human tumor and nontumor tissues.