Differential biological activities between mono- and bivalent fragments of anti-prolactin receptor antibodies.

Previous studies have established that antibodies against PRL receptors can mimic PRL effects on casein gene expression and on thymidine incorporation into DNA in the mammary gland. In the present work, bivalent F(ab')2 and monovalent Fab' fragments of the anti-PRL receptor antibodies were prepared. Both inhibited the binding of 125I-labeled PRL to rabbit mammary gland membranes. F(ab')2 as well as the unmodified antibodies were able to enhance casein synthesis and thymidine incorporation into DNA in cultured rabbit mammary gland explants. Moreover, when added to isolated membranes, both were able to induce the generation of the PRL relay which specifically stimulates caseins gene transcription in isolated mammary nuclei. In contrast, monovalent fragments were totally devoid of any of these PRL-like activities. However, bivalent and monovalent antibodies were equipotent in inducing a down-regulation of PRL receptors in mammary explants. These data indicate that the biological PRL-like activity of antibodies against PRL receptors is strictly related to their bivalent structure. This fact indicates a possible crucial role of a microaggregation of PRL receptors in the transmission of the PRL message across the membranes. In addition, these experiments reinforce the idea that internalization and down-regulation are not directly related to PRL action on casein or DNA synthesis in mammary gland.