214 In utero heat stress alters testis development and circulating cytokines profiles in pre-pubertal boars

Abstract

In utero heat stress (IUHS) reduces boar semen quality and represents an emerging threat to swine reproductive efficiency. However, the cellular/molecular mechanisms driving this effect are poorly characterized. Additionally, genomic selection for improved heat tolerance may be a novel IUHS mitigation strategy. The study objective was to compare cytokine profiles, anti-müllerian hormone (AMH) concentrations, and testis composition in pre-pubertal boars gestated in IUHS or in utero thermoneutral (IUTN) conditions from dams divergently selected for greater heat tolerance (TOL) or sensitivity (SEN), resulting in four treatment combinations (IUHS+SEN, IUHS+TOL, IUTN+SEN, IUTN+TOL). Dams were exposed to either thermoneutral (21.46 ± 2.92 °C) or cyclic heat stress (26-36 ºC) conditions from d6.3±0.9 to d65.3±0.9 of gestation. Thereafter, dams remained in thermoneutral conditions. After birth, a subset of boars (IUHS+SEN, n=7; IUHS+TOL, n=8; IUTN+SEN, n=8; IUTN+TOL, n=10) were castrated at d2 of age. Testes were trimmed, weighed, and fixed for histological analyses. Additional animals (IUHS+SEN, n=9; IUHS+TOL, n=9-10; IUTN+SEN, n=9; IUTN+TOL, n=9-11) remained intact, and blood samples were collected at d21 (weaning) and d70 of age for cytokine and AMH analyses. Testis weight was increased (P< 0.05) in IUTN+SEN boars compared to all other treatments, suggesting an effect on Sertoli cells. For testis histology, IUTN+TOL boars had a greater number of seminiferous tubules/field (P< 0.05) compared with IUHS+SEN and IUTN+SEN boars. Additionally, seminiferous tubule number/field was increased (P=0.05) in testes of IUHS+TOL boars compared with IUHS+SEN boars. Average seminiferous tubule area tended to be greater (P=0.07) in IUHS+TOL boars compared with IUTN+TOL boars. There was a tendency for IUTN+SEN boars to have greater (P=0.10) AMH (Sertoli cell product) concentrations versus IUHS+TOL boars at d70. Regarding cytokines, only interferon (IFN)-g differed between treatments at weaning; specifically, IUHS+SEN boars had 2-fold greater concentrations than IUTN+SEN boars (P=0.05), suggesting that IUHS promotes systemic inflammation in the neonatal boar. Likewise, IUHS+SEN boars tended (P=0.06) to have 2-fold elevated concentrations of IFNg compared to IUTN+TOL boars at weaning. Interestingly, numerous cytokines differed based upon treatment at d70. Both interleukin (IL)-2 (P=0.05) and IL-18 (P=0.05) concentrations were elevated in the IUHS+TOL treatment compared to IUTN+SEN. Furthermore, IUHS+TOL boars had 10-fold greater (P=0.05) IL-2 concentrations than IUHS+SEN boars. Other cytokines which tended (P< 0.10) to be greater in IUHS+TOL compared with IUTN+SEN boars included: IL-1a, IL-4, and IL-10. When comparing IUHS+TOL boars to IUHS+SEN boars, tendencies (P< 0.10) were detected for IL-10 (6-fold greater in IUHS+TOL) and IL-18 (4-fold greater in IUHS+TOL). These findings indicate that IUHS disrupts early testis development and promotes a pro-inflammatory environment in young boars. These pre-pubertal alterations may contribute to IUHS-induced subfertility in adult boars. Moreover, genomic selection for heat tolerance is a promising approach to mitigate the negative consequences of IUHS on boar fertility.