Human seminal plasma prolactin-inducible protein is an immunoglobulin G-binding protein

Antisperm antibodies (ASA) are present in 20% of couples seeking treatment for infertility. Antibody-binding proteins in seminal plasma may protect sperm from ASA-induced damage. We have previously isolated several IgG-binding proteins from human seminal plasma using IgG affinity chromatography. Here, we report another such protein which we have identified by amino acid sequencing and confirmed by western blotting to be prolactin-inducible protein (PIP). PIP binds via the Fc fragment of IgG. We have determined the level of PIP in normal seminal plasma to be 3.4 mg/ml (interquartile range 2.0-4.4 mg/ml). We have found there is no difference in the mean level of PIP in seminal plasma from fertile or infertile men regardless of ASA status. PIP was shown to exist in several isoforms in seminal plasma by Western blot. There is a complex pattern of PIP isoform variability in seminal plasma from fertile and infertile men but one multimeric form of PIP was absent from the seminal plasma of men with ASA who were fertile. This may reflect consumption of PIP in these men. The physiological function of PIP remains unknown, but the ability of PIP to bind IgG-Fc suggests PIP may have an immunomodulatory role.