Immunohistochemical and biochemical characterization of retroviral-related antigens expressed in normal baboon placenta and other tissues
The genomes of all eukaryotes contain multiple copies of DNA sequences that are related to sequences found in infectious retroviruses . Endogenous retroviruses (ERV s) are generally non-pathogenic except in mice where they have been found to induce tumours and immunological disorders. The ERV s have morphological features consistent with type-C retroviral particles and are expressed in normal placental tissues in most mammals. They have antigenic similarity with exogenous retroviruses such as HIV -1 and may have a role in the regulation of cellular gene expression, syncytiotrophoblast formation or pregnancy-related immunosuppression. Baboon endogenous virus (BaEV) is the only endogenous retrovirus so far which has been shown to be infective in vitro. The entire nucleotide sequence of BaEV has been determined. It has been shown to have a chimeric genomic structure of about 8 kb long. BaEV particle expression in placental tissues has been demonstrated using electron microscopy. However, to date, very little work has been done to evaluate the expression of retroviral-related antigens in normal baboon tissues. In this project, a panel of antibodies (polyclonal and monoclonal antibodies) against viral proteins (anti-HIV and anti-SfV) were assessed for their cross-reactivity (using immunohistochemistry) with normal placental and other baboon tissues. Retroviral particles were further characterized by immunoblotting and ELISA. Mouse polyclonal antibodies were produced against isolated baboon placental ERV particles and were also characterized as described above. Some of the antibodies (anti-HIV-1 gp41, anti-HIV-2 gp120, anti-HIV-1 RT, and anti-BERV) displayed specific immunoperoxidase staining on placental syncytiotrophoblast and cross-reacted with exogenous retroviral proteins on immunoblot analyses. Four monoclonal antibodies (anti-ERV3 env, anti-HERV-K RT, and HIV-1 gp120) consistently recognized three retroviral cross-reactive proteins of molecular weights 38, 58 and 64 kDa present on placental tissues. Minimal reverse transcriptase (RTase) activity associated with sucrose gradient-purified placental retroviral-like particles was also demonstrated'. These studies confirm the specific expression. ,of endogenous retroviral particles in placental villous tissue and suggest retroviral proteins may play an immunomodulatory role at the maternal-fetal interface.