Defining postpartum uterine disease and the mechanisms of infection and immunity in the female reproductive tract in cattle

Abstract

Uterine microbial disease affects half of all dairy cattle after parturition, causing infertility by disrupting uterine and ovarian function. Infection with Escherichia coli, Arcanobacterium pyogenes, and bovine herpesvirus 4 causes endometrial tissue damage. Toll-like receptors on endometrial cells detect pathogen-associated molecules such as bacterial DNA, lipids, and lipopolysaccharide (LPS), leading to secretion of cytokines, chemokines, and antimicrobial peptides. Chemokines attract neutrophils and macrophages to eliminate the bacteria, although persistence of neutrophils is associated with subclinical endometritis and infertility. Cows with uterine infections are less likely to ovulate because they have slower growth of the postpartum dominant follicle in the ovary, lower peripheral plasma estradiol concentrations, and perturbation of hypothalamic and pituitary function. The follicular fluid of animals with endometritis contains LPS, which is detected by the TLR4/CD14/LY96 (MD2) receptor complex on granulosa cells, leading to lower aromatase expression and reduced estradiol secretion. If cows with uterine disease ovulate, the peripheral plasma concentrations of progesterone are lower than those in normal animals. However, luteal phases are often extended in animals with uterine disease, probably because infection switches the endometrial epithelial secretion of prostaglandins from the F series to the E series by a phospholipase A2-mediated mechanism, which would disrupt luteolysis. The regulation of endometrial immunity depends on steroid hormones, somatotrophins, and local regulatory proteins. Advances in knowledge about infection and immunity in the female genital tract should be exploited to develop new therapeutics for uterine disease.

Figure 1. The incidence of uterine bacterial infection and disease in postpartum dairy cattle

Bacteria can be isolated from the uterus of most cows during the postpartum period; each marker (●) indicates the percent of animals with bacteria isolated from the uterine lumen [10-14]. The shaded areas represent estimates of the proportion of animals with metritis (), clinical endometritis (),or a normal uterus (); the remainder of animals have sub-clinical endometritis [15].

Figure 2. Grading scheme for clinical endometritis

A) Vaginal mucus character is graded as 0 = clear or translucent mucus; 1 = mucus containing flecks of white or off-white pus; 2 = exudate containing < 50% white or off-white mucopurulent material; and 3 = exudate containing > 50% purulent material, usually white or yellow, but occasionally sanguineous [2]. B) Endometritis grades reflect the number of pathogenic (■) but not opportunist non-pathogenic (□) bacteria isolated from the uterus of cattle [11]; data are presented as semi-quantitative scores of the number of colony forming units (CFU) from uterine swabs, where CFU score 0 = no growth; 1 = < 10 CFU; 2 = 10 to 100 CFU; 3 = 101 to 500 CFU; and 4 = > 500 CFU. Values differ from endometritis grade 0, ** P < 0.01, *** P < 0.001. C) Endometritis grade is prognostic for treatment success [22]; treatment success rates were determined as the percent of animals (n = 300) with normal vaginal mucus 2 weeks after initial endometritis grading and treatment 21-28 days post partum. Values differ between endometritis grade, * P < 0.05, ** P < 0.01.

Figure 3. The mechanisms underlying infertility associated with uterine disease

A. Bacterial infection with E. coli and A. pyogenes is common after parturition [10]. The innate immune system is alerted by endometrial cell Toll-like receptors (TLRs) detecting pathogen-associated molecules (PAMPs) such as bacterial DNA and lipids, and E. coli lipopolysaccharide (LPS), which is bound to LPS binding protein (LBP) [4, 5]. The bovine endometrial cells secrete cytokines and chemokines to direct the immune response, increase the expression of anti-microbial peptides (AMPs), and secrete principally prostaglandin E₂ (PGE) rather than prostaglandin F_2α (PGF) [9, 46]. Bacterial infection causes endometrial damage and inflammation, reducing the chance of conception. B. Cytokines and chemokines direct the immune response. Chemokines attract neutrophils (PMNs) and macrophages (MØs) to eliminate the bacteria. However, neutrophil function is often compromised in cattle around the time of parturition [47]. Persistence of PMNs in the endometrium in the absence of bacteria is thought to be the primary characteristic of subclinical endometritis [15, 24]. C. It is thought that viral replication may be stimulated in macrophages that are persistently infected with BoHV-4 by PGE and LPS [34]. The BoHV-4 can then infect the endometrial stromal and epithelial cells causing further tissue damage [45]. D. Follicle stimulating hormone (FSH) concentrations from the pituitary are unaffected be uterine disease and so waves of ovarian follicles emerge in the first weeks after parturition [10]. However, the release of gonadotrophin releasing hormone from the hypothalamus and luteinising hormone (LH) from the pituitary can be suppressed by LPS – reducing the ability to ovulate a dominant follicle [48, 49]. E. Cows with endometritis have slower growth of dominant follicles in the ovary and lower peripheral plasma estradiol concentrations and so are less likely to ovulate [10]. Follicular fluid contains LPS in animals with endometritis, granulosa cells express the TLR4/CD14/LY96 (MD2) complex required to detect LPS, and LPS perturbs estradiol secretion from granulosa cells by reducing aromatase expression [7]. F. If cows with endometritis ovulate, they form a corpus luteum secreting progesterone, and reinitiate ovarian cycles. However, the peripheral plasma concentrations of progesterone are lower than in normal fertile animals [32]. Cytokines may perturb luteal cell steroidogenesis [50]. Luteolysis is probably disrupted and luteal phases are often extended because bacteria switch the endometrial epithelial secretion of prostaglandins from the F to the E series [9].