Sperm-bound antisperm antibodies prevent capacitation of bovine spermatozoa

It was hypothesized here that sperm-bound antisperm antibodies (ASAs) impair the ability of bovine spermatozoa to undergo capacitation, bind to the zona pellucida, and complete the acrosome reaction. The effect of ASA binding on these functions was evaluated in frozen/thawed spermatozoa from four bulls before and after induction of ASAs. Ejaculates were divided into ASA negative (<10% immunoglobulin [Ig]G- and IgA-bound spermatozoa) or ASA positive (≥20% IgG and/or IgA-bound spermatozoa). The percentage of capacitated (Merocyanine 540 positive) live spermatozoa in response to heparin was lower in ASA-positive than ASA-negative ejaculates (P < 0.0001). Treatment with heparin resulted in a higher percentage of capacitated spermatozoa compared with control treatments in ASA-negative but not ASA-positive ejaculates. The percentage of capacitated spermatozoa after heparin treatment was negatively correlated with IgA (P = 0.02, R² = -0.48) but not IgG binding. Sperm binding to the zona pellucida was lower in IgA-positive (six spermatozoa/oocyte; 3-10 spermatozoa/oocyte) than IgA-negative ejaculates (seven spermatozoa/oocyte; 4-13 spermatozoa/oocyte) (P = 0.019). Zona binding was negatively correlated with the percentage of IgA-bound spermatozoa (P = 0.04; R² = -0.24) but not IgG-bound spermatozoa. The percentage of acrosome-reacted spermatozoa was higher in calcium ionophore A23187-treated than control aliquots in both ASA-negative and ASA-positive ejaculates (P < 0.0001). However, the percentage of acrosome-reacted spermatozoa did not differ between ASA-positive and ASA-negative samples, and no correlation was identified with IgG or IgA binding. It was concluded that sperm-bound IgA affected the ability of bovine spermatozoa to undergo capacitation. ASAs inhibited the changes in plasma membrane fluidity associated with capacitation and binding of spermatozoa to the zona pellucida.