A monoclonal antibody which inhibits the biological activity of rat prolactin, but not prolactin from other species

Monoclonal antibodies generated to ovine prolactin were screened for their ability to neutralize the biological activity of prolactin from several species. By Western blot analysis, antibody 6F11 cross-reacted strongly with prolactin in homogenates of anterior pituitary glands from squirrel monkey, sheep and rat. In addition, this antibody (1 μg IgG/ml) completely inhibited the lactogenic activity of serum or purified prolactin (0.5 ng/ml) from rat, but not prolactin from any other source, in the Nb2 lymphoma bioassay. 6F11 cross-reacted with purified ovine and rat prolactin by enzyme-linked immunosorbentassay (ELISA) and Western blot analysis with similar affinities, suggesting that the 6F11 epitope was common to these peptides. Another monoclonal antibody (17D9, 35 ng IgG/ml) showed the opposite selectivity, completely inhibiting the activity of 0.3 ng/ml ovine prolactin, but not 0.5 ng/ml rat prolactin, in the Nb2 assay. Thus, we have identified monoclonal antibodies which cross-react with both ovine and rat prolactin, but selectively neutralize the lactogenic activity of prolactin from only one species.