Pre‑stimulation follicle aspiration enhances ovarian response and oocyte competence via elevated intrafollicular interleukin-17 in dairy cows

In brief: This study reveals a novel mechanism behind the common practice of follicle aspiration prior to ovarian stimulation in cattle. We demonstrate that the procedure not only relieves dominant follicle suppression but also is associated with a beneficial local immune response characterized by elevated interleukin-17 (IL-17). IL-17 is correlated with the promotion of granulosa cell proliferation, steroidogenesis, and survival, and shows an association with enhanced oocyte developmental competence, leading to the production of higher-quality blastocysts. Our findings suggest IL-17 as a potential key mediator linking mechanical intervention to improved in vitro embryo production outcomes. Abstract: The presence of a dominant follicle (DF) during ovarian stimulation (OS) for ovum pick-up (OPU) and in vitro embryo production in cattle suppresses subordinate follicle growth, limiting oocyte yield and quality. While DF aspiration prior to OS is known to improve outcomes, its mechanism remains unclear. This study demonstrates that pre-stimulation follicle aspiration enhances OS efficacy not only by removing the DF's inhibition but also through a potentially beneficial procedural effect. We found that aspirating ovaries, with or without a DF, significantly increased the subsequent follicular response and was associated with improved oocyte developmental competence, which corresponded to higher cleavage and blastocyst rates. From a mechanistic perspective, the aspiration procedure was associated with elevated intra-follicular interleukin-17 (IL-17) concentration. In vitro, 40 ng/ml IL-17 promoted granulosa cell proliferation, enhanced steroid hormone synthesis including estradiol and progesterone, and inhibited Apoptosis via regulating key genes: upregulating PCNA, CYP19A1, CYP11A1, and BCL2, while downregulating P53, Bax, and CASPASE3. Furthermore, supplementing IL-17 during oocyte maturation significantly enhanced blastocyst formation rate and quality, resulting in blastocysts with a higher inner cell mass ratio, increased total cell number, and reduced Apoptosis. We conclude that pre-stimulation aspiration improves in vitro embryo production outcomes by combining the relief of follicular dominance with an IL-17-associated enhancement of follicular function and oocyte developmental potential, suggesting IL-17 as a potential mechanistic link.