Immunolocalization of a novel protein (P19) in the endometrium of fertile and subfertile mares.

One of the major progesterone-dependent endometrial proteins in the mare is a novel 19 kDa lipocalin (P19). This protein is secreted by the endometrial glands and is readily detectable in uterine secretions during the luteal phase of the oestrous cycle and early pregnancy. The function of P19 is unknown, but since most lipocalins act as carriers of small hydrophobic molecules, it probably transports a maternal factor to the conceptus during pregnancy. In this study, a high titre antiserum raised against recombinant-derived P19 was used to detect by immunohistochemistry the protein in endometrial biopsies from normal healthy mares and mares with endometrosis. Immunoreactive P19 was undetectable in prepubertal fillies and in anoestrous mares and was barely detectable in oestrous mares. However, it was present in large amounts during dioestrus and very early pregnancy, and in moderate amounts after day 20 of gestation. In six subfertile mares with endometrosis, an abnormal pattern was found at all stages of the reproductive cycle, especially in the 'gland nests'. These showed either an absence of P19 or an accumulation of the protein due to a lack of secretion, thus indicating asynchronous glandular activity. These results identified a high proportion of abnormal, asynchronous endometrial glands in a group of subfertile mares and indicated that abnormal secretion of P19 may be one of the reasons for the reduced fertility rates observed in aged mares with endometrosis.