Prolactin and the Immune System

Prolactin (PRL) Is involved In a wide range of physiological effects in several species and its immunoregulatory role has already been well documented. The PRL receptor has been cloned from various species. There are at least two receptor isoforms (short and long) in rats and mice, which differ only in their cytoplasmic domains, generated by alternative splicing of a single gene, although in human only the long form exists. Using the reverse transcriptase-polymerase chain reaction (RT-PCR), we detected transcripts encoding both forms of PRL receptor in all lymphoid tissues examined in human, mouse, and rat, but in mouse and rat the ratio between the two forms was variable from animal to animal. Concerning the transcript encoding the PRL itself, a clear signal was always found in human lymphocytes and occasionally in rat thymus. We also developed a quantitative PCR (Q-PCR) in order to measure the absolute number of transcripts in thymus and spleen from rats at two stages of estrous cycle. The level of expression of the two forms was about equal. Finally, we identified the tyrosine kinase JAK2, which is constitutively associated with the PRLR, using the Nb2 rat lymphoma cell line as a model system with which to study the action of PRL on cell mitogenesis. We also showed that, after stimulation by PRL, the dimerization process is a prerequisite step for the phosphorylatlon of the PRLR and JAK2, which represents the earliest event in the signal transduction pathway.