Characterisation of retroviral-related antigens expressed in normal baboon placental tissues

The genomes of all eukaryotes contain multiple copies of DNA sequences that are related to sequences found in infectious retroviruses. Endogenous retroviruses (ERVs) are generally non-pathogenic although they have been implicated in the induction of tumours and immunological disorders. The ERVs have morphological features consistent with type-C retroviral particles and are expressed in normal placental tissue in most mammals. They have antigenic similarity with exogenous retroviruses such as HIV-1 and may have a role to play in the regulation of cellular gene expression, syncytiotrophoblast formation or pregnancy-related immunosuppression. Some of the human endogenous retroviruses have been well-characterised. Among the non-human primates, the baboon endogenous virus (BaEV) is the only endogenous retrovirus so far which has been shown to be effective 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 study, mouse polyclonal antibodies were produced against isolated baboon placental ERV particles and characterised using immunohistochemistry and immunoblotting techniques. Most of the anti-BERV antibodies displayed specific immunoperoxidase staining on placental syncytiotrophoblast and cross-reacted with exogenous retroviral proteins on immunoblot analyses. Reverse transcriptase (RTase) activity associated with sucrose gradient-purified placental retroviral-like particles were also demonstrated. These studies indicate that endogenous retroviral particles are expressed in baboon placental villous tissue and suggest retroviral proteins may play an immunomodulatory role at the maternal-foetal interface.