Inflammation pro-resolving mechanisms associated with puerperal metritis in dairy cows

In healthy dairy cows, a puerperal transient inflammation of the endometrium is necessary for the clearance of placental debris and bacteria and to achieve complete uterine involution. However, metabolic, immune and inflammatory disturbances leading to uterine dysbiosis may disturb the control of this physiological inflammation, and allow the establishment of metritis. There is a gap in knowledge, regarding the pro-resolving mechanisms controlling the puerperal endometrial physiologic inflammation, and their putative disturbance behind the establishment of metritis. This observational retrospective cohort study investigated the involvement of macrophages, adipokines and Specialized Pro-Resolving Mediators (SPM) in postpartum dairy cows with strict phenotypes (clinical, metabolic, fertility and milk yield) of healthy puerperium (n = 20) or puerperal metritis (n = 20). Diagnosis of puerperal metritis occurred at 9 ± 4 (Mean ± SD) days postpartum (DPP). Blood (at 2, 10, 21, 30 and 42 DPP) and uterine fluid (at 21 DPP) samples were collected for measurement of SPM and adipokines, and endometrial cytologic parameters were evaluated (at 21 and 42 DPP). Results below are expressed as Mean ± SEM. In early postpartum (2-10 DPP), cows with puerperal metritis had greater plasma concentrations (pg/mL) of PGE2 (2 DPP - 13.0 ± 1.7 vs. 5.3 ± 1.7) and lesser concentrations of SPM (5-HETE: 2 DPP - 65.4 ± 13.3 vs. 120.9 ± 13.3; 10 DPP - 75.6 ± 12.7 vs. 109.7 ± 12.7; LXA4: 2 DPP - 0.6 ± 0.1 vs. 1.0 ± 0.1; 10 DPP - 0.5 ± 0.1 vs. 0.8 ± 0.1; LXB4: 10 DPP - 2.1 ± 0.5 vs. 4.8 ± 0.5; 18-HEPE: 10 DPP - 35.6 ± 8.1 vs. 78.3 ± 8.1) than healthy cows. At the end of clinical puerperium (21 DPP), compared with cows with healthy puerperium, cows with puerperal metritis had greater uterine fluid concentrations (pg/mL) of pro-inflammatory mediators (chemerin: 1.7 ± 0.1 vs. 1.2 ± 0.1; PGE2: 13230.5 ± 2477.6 vs. 4016.2 ± 2335.9), lesser concentrations of pro-resolving mediators (adiponectin: 0.8 ± 0.1 vs. 1.1 ± 0.1; 17-HDHA: 64.8 ± 27.7 vs. 154.7 ± 27.7; MaR2: 0.5 ± 0.2 vs. 0.9 ± 0.1; RvE2: 0.9 ± 0.2 vs. 1.8 ± 0.2), greater abundance of M1-polarized macrophage protein markers and a higher M1:M2 ratio in the uterine lumen. Cytological endometritis sequelae were present in 16 (80%) cows with puerperal metritis, whereas it was not identified in cows with healthy puerperium. However, at this stage (21 DPP) cows with puerperal metritis also had greater concentrations (pg/mL) of pro-resolving mediators in plasma (adiponectin: 6.6 vs. 5.0) and uterine fluid (apelin: 1.0 ± 0.1 vs. 0.8 ± 0.1; RvE1: 2.1 ± 0.2 vs. 1.4 ± 0.2) than healthy cows. This persisted later in postpartum (42 DPP), when cows with puerperal metritis also showed greater plasma concentrations (pg/mL) of LXA4 (1.0 ± 0.1 vs. 0.6 ± 0.1), 18-HEPE (74.4 ± 8.5 vs. 45.6 ± 8.5), RvE1 (4.3 ± 0.4 vs. 3.1 ± 0.4), adiponectin (6.5 ± 0.5 vs. 5.2 ± 0.5) and apelin (5.5 ± 0.3 vs. 4.7 ± 0.3) than healthy cows. This delayed pro-resolving response was associated with the recovery of the endometritis sequelae of metritis. In conclusion, results of this study support the concept that pro-resolving mechanisms, combining the polarization of macrophages to the M2 phenotype, adipokines and SPM, must start early in postpartum, and its lower magnitude or delayed response is associated with establishment of puerperal metritis. Cows with puerperal metritis show a subsequent high rate of endometritis sequelae, which highlights the relevance of preventive management strategies.