Detection of Fas ligand in the bovine oviduct

Presence of a Fas-Fas ligand (FasL) system defines the immune-privileged status of certain tissues such as placenta. This study examined the fluids and tissue(s) of the bovine oviduct, where both spermatozoa and early embryos escape elimination by the female immune system, for the presence and the distribution of Fas and FasL, which might provide an explanation for the immune-privileged site of this organ. In the present study, the immunolocalisation of FasL and Fas, as well as the gene expression of FasL, were determined in the uterotubal junction (UTJ), isthmic (I) and ampullar (A) segments of the oviduct during oestrus and the luteal phase of the oestrous cycle. The degree of apoptosis of oviductal epithelium was examined by the TUNEL method. Oviductal fluid (ODF), collected chronically via indwelling catheters from the I or A segments during both non-luteal and luteal phases of the cycle, was analysed for the presence of FasL. The Fas immunostaining was scattered along the epithelium of all regions of the oviduct and cycle stages investigated, whereas FasL immunolabelling was more conspicuous in oestrous samples. This staining disappeared during the luteal phase, which was particularly evident in the sperm reservoir (UTJ and I). There were fewer TUNEL-positive cells than Fas- or FasL-positive cells in the oviductal epithelium, suggesting that tubal Fas and FasL are not directly involved in epithelial apoptosis. Western blot analyses detected FasL in ODF collected from both I and A, most conspicuously as a 24-27kDa band but also at a 40-45kDa band level. FasL mRNA was expressed in the epithelial cells from the sperm reservoir and A during both non-luteal and luteal phases. However, the level of expression differed significantly between segments during the luteal phase. The results provide novel evidence that the Fas-FasL system is present in the bovine oviduct and could be involved in mediating survival of spermatozoa and early embryos.