The anti-inflammatory effect of exogenous lactoferrin on breeding-induced endometritis when administered post-breeding in susceptible mares

Abstract

The deposition of semen into the uterus of the horse induces a transient innate immune response that lasts 24-36 h in the normal mare. There exists a subset of mares that are unable to resolve this inflammation in a timely manner, and are classified as susceptible to the disease of persistent breeding-induced endometritis (PBIE). Lactoferrin is a protein of interest as a potential therapeutic for this persistent inflammation due to its anti-inflammatory and bactericidal properties. The addition of human recombinant lactoferrin (hrLF) to the insemination dose was previously shown to suppress mRNA expression of the pro-inflammatory cytokine tumor necrosis factor (TNF)-α at 6 h after insemination, but no studies have shown the effect of lactoferrin when infused post-breeding. Therefore, the objectives of this study were to (1) assess the safety of intra-uterine infusion of hrLF, (2) evaluate the effect of intrauterine infusion of hrLF post-breeding as a modulator of the immune response to breeding in the susceptible mare, and (3) determine the most effective concentration of hrLF. For the first experiment four normal mares received an intrauterine infusion of 500 μg/mL hrLF resuspended in 10 mL lactated Ringer's solution (LRS) and heart rate, rectal temperature, respiration, and endometrial quality were evaluated. For the second experiment, six mares classified as susceptible to PBIE were bred during estrus with 500 × 10⁶ progressively motile sperm comprised of the ejaculates from two stallions, which were centrifuged over Androcoll-E to remove seminal plasma. Each insemination dose was resuspended in 30 mL LRS. Six hours after breeding, a 1L LRS uterine lavage was performed prior to treatments. Four treatments were administered over four consecutive estrous cycles in randomized order of: 10 mL LRS (vehicle control), 50 μg/mL hrLF resuspended in 10 mL LRS, 250 μg/mL hrLF resuspended in 10 mL LRS, and 500 μg/mL hrLF resuspended in 10 mL LRS. Twenty-four hours after breeding the mares were evaluated via transrectal ultrasonography for fluid retention. A low volume uterine lavage (250 mL LRS) was performed and the effluent was evaluated for polymorphonuclear neutrophils (PMNs). Finally, an endometrial biopsy was obtained for qPCR analysis of selected inflammatory cytokines. Lactoferrin had no significant overall effect on vital signs or endometrial quality. The addition of hrLF (50 μg/mL, 250 μg/mL, 500 μg/mL) did not significantly affect the amount of fluid detected post-breeding, but suppressed the ratio of PMNs to epithelial cells at all three concentrations compared to controls. In addition, all three concentrations of hrLF increased the mRNA expression of the anti-inflammatory cytokine interleukin-1 receptor antagonist (IL-1RN), while the 50 μg/mL dose significantly suppressed mRNA expression of the pro-inflammatory cytokine interferon gamma (IFNγ). In conclusion, the infusion of hrLF post-breeding was found to modulate the inflammatory response to breeding in the mare, and appears to be most effective at the 50 μg/mL concentration.

Keywords: Cytokines; Endometritis; Inflammation; Lactoferrin; Uterus.