Macrophage migration inhibitory factor in the human epididymis and semen

During epididymal transit, mammalian spermatozoa acquire new proteins involved in the acquisition of motility and of male gamete fertilising ability. We have previously shown that membranous vesicles called epididymosomes are involved in the transfer of epididymal-originating proteins to spermatozoa. The cytokine macrophage migration inhibitory factor (MIF) is one of these proteins but the role played by MIF in relation to epididymal sperm maturation still remains unclear. As this protein has already been shown to bear different functions depending on its location, we investigated its distribution along the epididymis and in different compartments of human semen. Northern and Western blot analysis as well as immunohistochemical studies show that MIF is expressed all along the epididymis with a higher level of transcript in the proximal segment. MIF is associated with two types of membranous vesicles, i.e. epididymosomes and prostasomes, the latter being prostate-originating membranous vesicles present in the semen. In semen, MIF is associated with spermatozoa, prostasomes as well as the soluble fraction. The amount of MIF in the seminal fluid varies from one individual to another but does not correlate with the amount of MIF associated with ejaculated spermatozoa. There is a negative correlation between the amount of sperm-associated MIF and the percentage of motility in different semen samples. Sperm separation using discontinuous Percoll gradient centrifugation shows a higher amount of MIF associated with poorly motile spermatozoa compared to highly motile spermatozoa present in the lower Percoll fraction. These results are discussed with regards to the possible involvement of MIF in sperm motility acquisition during the epididymal transit.