International Journal of Molecular Sciences Review Persistent Breeding-Induced Endometritis in Mares—A Multifaceted Challenge: From Clinical Aspects to Immunopathogenesis and Pathobiology Igor F. Canisso 1,*, Lorenzo G.T.M. Segabinazzi 1,2 and Carleigh E. Fedorka 3 1 Department of Veterinary Clinical Medicine, College of Veterinary Medicine, University of Illinois Urbana-Champaign, Champaign, IL 61802, USA;
[email protected] 2 Department of Animal Reproduction and Veterinary Radiology, Faculty of Veterinary Medicine and Animal Science, São Paulo State University (UNESP), Botucatu 18618-000, São Paulo, Brazil 3 The Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, KY 40503, USA;
[email protected] * Correspondence:
[email protected]; Tel.: +1-2172809040 Received: 20 December 2019; Accepted: 7 February 2020; Published: 20 February 2020 Abstract: Post-breeding endometritis (i.e., inflammation/infection of the endometrium), is a physiological reaction taking place in the endometrium of mares within 48 h post-breeding, aimed to clear seminal plasma, excess sperm, microorganisms, and debris from the uterine lumen in preparation for the arrival of an embryo. Mares are classified as susceptible or resistant to persistent breeding-induced endometritis (PBIE) based on their ability to clear this inflammation/infection by 48 h post-breeding. Mares susceptible to PBIE, or those with difficulty clearing infection/inflammation, have a deficient immune response and compromised physical mechanisms of defense against infection. Molecular pathways of the innate immune response known to be involved in PBIE are discussed herein. The role of the adaptive uterine immune response on PBIE remains to be elucidated in horses. Advances in the pathobiology of microbes involved in PBIE are also revised here.