Immunohistochemical localization of retroviral-related antigens expressed in normal baboon placental villous tissue

: Endogenous retroviral particles (ERVs) have been detected in the genome of all eukaryotes. They are generally non‐pathogenic except in mice where they have been found to induce tumors and immunological disorders. The ERVs have morphological features consistent with type‐C retroviral particles and are commonly expressed in normal placental villous tissues. ERVs may have a role in the regulation of placental gene expression, syncytiotrophoblast formation, or pregnancy‐related immunosuppression. In this study, well‐characterized antibodies (monoclonal and polyclonal antibodies) raised against retroviral proteins (anti‐HIV and anti‐SIV) and endogenous retroviral (ERV) particles were assessed for their cross‐reactivity (by using immunohistochemistry) with normal baboon placental and other adult tissues. The monoclonal antibodies to exogenous retroviral proteins (anti‐HIV‐2 gp120, anti‐HIV‐1 gp41, anti‐SIVmac p27, anti‐HIV‐1 RT, and anti‐HIV‐2 core protein) showed specific immunohistochemical reactivity with the syncytiotrophoblast. Antibodies to endogenous retroviral gene products (anti‐ERV3 env, anti‐HERV‐K RT, and anti‐HERV‐K env) also reacted in a similar manner and did not cross‐react with other adult tissues. These studies have shown that retroviral‐cross‐reactive proteins are expressed in baboon placental syncytiotrophoblast and may have a role to play at the feto‐maternal interface.