Human seminal plasma is a potent inhibitor of natural killer cell activity in vitro.

The effect of human seminal plasma on the natural killer (NK) activity of human peripheral blood leukocytes in vitro has been investigated. Small concentrations (as low as 0.01% v/v) of seminal plasma were found to inhibit the ability of human peripheral blood leukocytes to lyse K562 target cells. These inhibitory effects were observed with seminal plasma donations from normal and infertile individuals, including those who had been vasectomised. The inhibition noted appeared to be independent of the presence of antisperm antibodies which could conceivably cross-react with NK cells. Furthermore, the factors responsible survived boiling for 15 min and could not be absorbed out with K562 target cells. These profound inhibitory effects might be relevant to malignancy of the urogenital tract and the defence against sexually transmitted disease.