A novel sperm FcR binding assay that assesses sperm fertilizing potential

Freshly ejaculated mammalian sperm have poor fertilizing ability, with fertility only gained after sperm undergo capacitation and the acrosome reaction. To visualize exposed Fc receptors (FcRs), which occur during the acrosome reaction and whose absence has been related to infertility, a novel sperm FcR binding assay (FcR assay) was developed to assess fertilizing potential of sperm in proof-of-concept studies. A competition binding assay between sperm FcR and exogenously added FcR was used to assess whether the FcR was a functioning ligand in bull sperm. Once FcR was confirmed as a functional ligand, time-based expression of FcR was then evaluated in bull and human sperm using the FcR assay. This FcR assay was then used to evaluate fertility outcomes in cattle with cryopreserved intrauterine insemination (IUI) sperm, and to evaluate sperm FcR expression in patients undergoing IUI treatment in a prospective observational study. Time-based analyses of ejaculates from bull and human sperm demonstrated characteristic, reproducible sinusoidal patterns of FcR expression that corresponded to high and low periods of fertility potential in each species. The pregnancy rate in cattle approached statistical significance using the FcR assay results to inform optimum insemination timing windows versus conventional untimed methods (73.0% vs. 68.4%, respectively; <i>p</i> = 0.06; 95% confidence interval [CI]: 0.98, 1.57) with a 4.4% increase in the overall pregnancy rate. In patients undergoing IUI treatment, FcR expression patterns were identified where sperm were at their optimal fertilizing state, with overall pregnancy rates increasing from 21% to 42% (<i>p</i> = 0.01) when inseminations occurred during the windows where the fertilizing potential of the sperm was deemed optimal. These results suggest that sperm fertility potential is quantifiable in semen samples using our novel sperm FcR assay. Importantly, the FcR assay has the ability to identify optimal fertility windows in real-time, and also in the procedure ejaculates.