Purified porcine seminal plasma protein enhances in vitro immune activities of porcine peripheral lymphocytes.

The porcine seminal plasma protein (PSP) accounts for much more than 50% of the total proteins in seminal plasma. PSP has been previously purified and its biochemical properties characterized. However, the biological functions of PSP remain to be elucidated. We hypothesize that PSP is involved in the regulation of uterine immune activity. In the current study, effects of PSP on in vitro lymphocyte activities and the presence of PSP binding sites on lymphocytes were examined. In mitogen-induced proliferation assay, lymphocytes from peripheral blood of gilts were cultured with pokeweed mitogen (PWM), phytohemagglutinin (PHA), or concanavalin A (Con A) in the presence or absence of PSP. PSP at 50, 125, and 250 ng/well augmented PWM-induced [3H]thymidine uptake in a dose-responsive manner by 152.8 +/- 8.1%, 225.9 +/- 35.2%, and 274.8 +/- 53.6%, respectively, compared with that of control. PSP did not alter lymphocyte proliferation in the absence of PWM. Similarly, PSP had little or no effect on PHA- or Con A-induced lymphocyte proliferation. In one-way mixed lymphocyte reactions, PSP at 50, 125, and 250 ng/well enhanced [3H]thymidine uptake in a dose-responsive manner by 181.5 +/- 16.5%, 339.9 +/- 48.2%, and 600.1 +/- 84.8% of control, respectively. Using biotinylated PSP-I, PSP binding sites were localized on approximately 3-5% of the lymphocyte population. In summary, we have demonstrated that PSP itself is not a mitogen/antigen to porcine lymphocytes but that it has a stimulatory effect on lymphocyte activities initiated by PWM or surface antigens of lymphocytes. PSP may exert its functions by interacting with PSP binding sites on a subpopulation of porcine lymphocytes. The high potency of PSP on lymphocyte activities and the abundance of PSP in seminal plasma have suggested that PSP may play an important role in regulating immune responses in the porcine uterine environment.