A monoclonal antibody raised against rat ovarian antigen recognizes Leydig cell surface: an immunocytochemical study

Using a monoclonal antibody (Mab) against constituents of rat ovarian granulosa cells, we found a 59-kDa protein located on the plasma membrane of a number of granulosa cells in follicles at different stages of development. This Mab (5G5) was found to bind to Leydig cells in rat male gonads. The localization of the antigen, recognized by Mab 5G5 in rat testis, was studied by light and electron microscopic immunocytochemistry (ABC method and IGS technique). Even though Sertoli cells in male gonads are regarded as the counterpart of granulosa cells in ovaries, the results of the experiments described here do not allow such an interpretation because staining with this antibody was restricted to the Leydig cell surface. The immunoreactivity in testicular sections from immature rats was similar to that found in adult testicular tissue. Our immunocytochemical results indicate that the plasma membrane of Leydig cells in rat male gonads shares certain biochemical and molecular properties with rat ovarian granulosa cells. On the basis of the immunocytochemical studies reported here, we suggest that the antigen recognized by Mab 5G5 may be common to all rat steroidogenic organs. Further studies are needed to establish the identity of the antigen in Leydig cells as well as its site of synthesis and its site of action. Thus, Mab 5G5 appears to hold significant potential as a powerful tool for future investigations.