Immunological aspects of ovarian follicle ovulation and corpus luteum formation in cattle

Abstract

The intricate involvement of the innate immune system during the transition of the preovulatory follicle to a corpus luteum was investigated. The first study examined the contribution and timing of the cellular immune system to ovulation and corpus luteum formation, and the second study assessed the comprehensive biochemical composition of follicular fluid during pre- peri-ovulatory period, finally the third study investigated the expression profile of a set of miRNAs in the differentiating (pre-ovulatory), luteinizing (periovulatory) and ruptured (post-ovulatory) dominant follicle and in the early corpus luteum. In Chapter 2, immunohistochemical analysis revealed that ovulation is characterized by an influx of neutrophils prior to, or coincident with, the LH surge, followed by an expansion of macrophage and dendritic cell populations in theca and granulosa tissues. The actions of this expanded immune cell repertoire are tightly regulated, as evidenced by the contemporaneous expression of both inflammatory and anti-inflammatory cytokines, resulting in a temporary sterile inflammatory response within the ovulatory follicle. Within 24 h of ovulation, inflammation starts to resolve and early corpus luteum formation is characterized by the expression of tissue growth promoting and modelling factors such as VEGFA, MMP9, TIMP1. In Chapter 3, global metabolomic profiling of follicular fluid in the final 24h period before ovulation highlighted a flux in the concentration of several major metabolites which are associated with inflammation and immunomodulation. In particular, the concentrations of Tryptophan, BCAA and Sphingomyelin metabolites, which are mainly related to inflammatory/immune responses by modulating inflammation and contributing to cellular homeostasis, were increased. Here for the first time, the presence of xenobiotics metabolites in bovine follicular fluid from pre- and peri-ovulatory follicle was reported, and their increased level at the time of ovulation could be hypothesized to be a result of a detoxification and protection process at ovulation. In addition, the concentration of metabolites of the nucleotide pathway were significantly depleted in peri-ovulatory follicular fluid compared to pre-ovulatory follicular fluid; hypoxanthine, xanthine, and inosine have been associated with maintenance of oocyte meiotic arrest. In Chapter 4, the spatio-temporal expression of a panel of miRNAs was identified in the ovulatory follicle and early corpus luteum tissues. The findings suggest a potential role for miRNAs in the regulation of genes involved in bovine dominant follicle development, ovulation and corpus luteum formation. While the number of differentially expressed miRNAs was higher in granulosa cells compared to thecal cells, the observed elevated miR-155 and miR-142 expression in the theca tissue at ovulation is likely associated with dendritic cell recruitment and function in the ovulatory follicle, supporting the concept that ovulation is an inflammatory event triggered by the LH surge. The findings of the work detailed in this thesis provide new knowledge on the sterile inflammatory response to the LH surge in the ovulatory follicle which is rapidly resolved in advance of corpus luteum formation.