Impact of pituitary pars intermedia dysfunction on inflammation within the equine reproductive tract of the mare

Pituitary pars intermedia dysfunction (PPID) is an age-related endocrinopathy associated with elevated systemic inflammation, and specifically an upregulation of interleukin-8 (IL-8). It is unknown if PPID in concomitant with reproductive tract inflammation. This is a pertinent question, as chronic inflammation of the endometrium and ovary would impede fertility. Therefore, the objective of this study was to evaluate the impact of PPID on the reproductive tract of the mare.

PPID was diagnosed via thyrotropin releasing-hormone (TRH) stimulation test, where PPID was diagnosed as ACTH>120 pg/mL post-stimulation, and controls were diagnosed as ACTH<60 pg/mL. In the first study, seven PPID mares and four age-matched control mares had endometrial biopsies taken when in diestrus. In the second study, seven PPID mares and ten age-matched control mares had follicular fluid aspirated from preovulatory follicles using transvaginal aspirations. Analysis included qPCR analysis of select targets associated with endometrial inflammation in addition to immunochemistry for leukocytes. Finally, immunoassay was used to assess the production of systemic and follicular fluid cytokines. Statistics were performed using SAS 9.4®. The impact of PPID on the expression of transcripts, production of cytokines, and number of leukocytes was evaluated using an unequal variances t-test. The correlation between ACTH and number of leukocytes was assessed using a Pearson's correlation test. Significance was set to p < 0.05, with trends noted at p < 0.1.

Only endometrial IL-8 was found to increase in expression in the PPID population (p = 0.02). There was a positive correlation between ACTH and the endometrial expression of IL-8 (p < 0.001; R² = 0.80). A weak correlation was also noted between ACTH and expression of IL-6 (p = 0.04; R² = 0.41) and IFNγ (p < 0.01; R² = 0.63). PPID mares had more endometrial leukocytes than control animals (p = 0.03), which was also positively correlated with ACTH (p = 0.03; R² = 0.47). An increase in IL-8 was also noted in the follicular fluid (p < 0.01) of PPID mares.

The systemic inflammation previously reported in the PPID animal was also observed within the reproductive tract of the mare, and this was found as both expression and production of pro-inflammatory cytokines in addition to presence of leukocytes. Furthermore, this inflammation was noted within the uterus in addition to the preovulatory follicle. Future research is warranted to determine if this increase in inflammation of the reproductive tract is detrimental to the fertility of PPID mares.