Ovine trophoblast protein-1 (oTP-1), the major product secreted by the trophectoderm of the sheep conceptus between days 13 and 21 of pregnancy, is considered to mediate maternal recognition of pregnancy by maintaining the function of the corpus luteum. Its amino acid sequence has 40–55% identity with various mammalian interferons-α (IFN-α), and it has been shown to have antiviral activity. The present results confirm that oTP-1, which at days 15–17 of pregnancy is produced by a single embryo at more than 100 μg (>1 million antiviral units) per day, is a functional IFN. A preparation of purified oTP-1 was made. Its amino-terminal sequence suggested that it consisted of a single homogeneous protein, so that its antiviral activity probably was not due to a contaminant. In a cytopathic effect inhibition assay with GBK-2 bovine cells challenged with vesicular stomatitis, its specific activity was 1.3 × 107 end point units/mg protein. It also protected GBK-2 cells against four other viruses, and A549 human cells against encephalomyocarditis virus. The antiviral activity was neutralized by an antiserum to human leukocyte IFN. Like human IFN-α, oTP-1 at concentrations as low as 10−9 M inhibited the growth of GBK cells in culture and suppressed mitogen-stimulated incorporation of [3H]thymidine into ovine lymphocytes. Possible roles for oTP-1, functioning as an IFN-α during early pregnancy, are discussed.
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