How one researcher is using her background in reproductive immunology to better understand the leading cause of equine abortion

The phenomenon of pregnancy has fascinated researchers for centuries. How does the developing fetus, a foreign body that expresses antigens from the father, survive undetected by the mother’s immune system? Initially, pregnancy was believed to survive due to a suppressed maternal immune system; and yet pregnant women can battle infection and mount responses to vaccines and any number of pathogens, leading to the realization that the female immune system wasn’t suppressed, but rather altered.

But, what about the horse?

This topic has been the primary focus of recent research by Carleigh Fedorka, PhD, postdoctoral scholar in the Gluck Equine Research Center laboratory of Barry Ball, DVM, PhD, Dipl. ACT, Albert G Clay Endowed Chair in Equine Reproduction. While much can be inferred from the human, the pregnant mare differs physiologically from the pregnant woman in a variety of ways. Human gestation relies solely on the endocrine support of progesterone, while equine gestation is supported by metabolites of this hormone during much of gestation. The human fetus is surrounded by a single fetal fluid compartment (the amnion), while the equine fetus resides within two (the amnion and allantois). And while human placentation is a single layered hemochorial type (‘invasive’), equine placentation is a multi-layered epitheliochorial placenta (‘non-invasive’). Taken together, inferences made between the two may not be relevant. Therefore, Fedorka’s research has focused on an understanding of the normal immune response to pregnancy in the horse, in addition to evaluating alteration within this response following pregnancy-related complications, such as in the case of placentitis.

To begin, the adaptive immune response to pregnancy was evaluated, the findings of which were recently published in the article, “Alterations in T cell-related transcripts at the feto-maternal interface throughout equine gestation,” in the journal Placenta. This branch of the immune system dictates both response to pathogens (pro-inflammatory Th1 cells / anti-inflammatory Th2 cells) in addition to tolerance of the semi-allogeneic fetus (meaning its genetic make-up is not identical to the dam) by the regulatory T cells (Tregs). In the normal equine pregnancy, numerous aspects of the adaptive immune response were found to mimic what has been noted in the human, the most important of which was that the induction of tolerance towards the fetus was associated with a heightened Treg response. Additionally, it was noted that no alterations were seen in the pro-inflammatory Th1 transcripts within the semi-allogeneic placenta, indicating an evolved mechanism to remain undetected and not attacked by the maternal immune system.

Understanding the normal immune response to equine gestation is an imperative so that alterations within this normal response could be assessed for potential biomarkers, therapeutics and predictors of equine pregnancy-related complications, of which placentitis remains the largest.  In women, pregnancy loss coincides with an increase in Th17 cells alongside a dysregulation of the Treg response, and this can be improved through use of therapeutics that stimulate this arm of immunity, including NeupogenÒ (recombinant GM-CSF) or intravenous immunoglobulins (IVIG; plasma). Data from the laboratory suggests that ascending placentitis leads to an increase in Th17-related transcripts in the placenta, and this coincides with a decrease in Treg-related transcripts in both the endometrium and chorioallantois. It is unknown if a treatment such as IVIG can increase the number of Treg cells in circulation in the horse, but the group recently performed a preliminary study to assess this and hope to have results soon. 

As the adaptive immune system and the innate immune system are tightly linked, the group has also investigated the cytokines that couple them. Fedorka and team recently published the article, “The feto-maternal immune response to equine placentitis,” in the American Journal of Reproductive Immunology to describe this. In this study, they found that the maternal response to ascending placentitis differs from that of the fetus, as the mare responded to the disease with a primarily pro-inflammatory response, while the fetus appeared to play a regulatory role within this disease and expressed many anti- and immuno-modulating cytokines. Additionally, an increase in many of these cytokines (IL-1b, IL-6, IL-10) could be noted in the fetal fluids, suggesting that ultrasound-guided fetal fluid sampling may be considered in detecting placentitis.

Unfortunately, due to the risk of abortion caused by medical examination or treatment), few practitioners will utilize fetal fluid sampling as a diagnostic for disease, and the team’s interests shifted towards less invasive sampling procedures for biomarker detection. Fedorka and her colleagues assessed the effect of ascending placentitis on specific cytokines in blood circulation and found interleukin-6 (IL-6) to be elevated.  A signaling molecule that can act as both pro- and anti-inflammatory, we also found that in the disease of placentitis, IL-6 activated a specific receptor and signaling pathway to stimulate anti-inflammatory, anti-apoptotic and pro-survival outcomes. IL-6 demonstrated both high sensitivity and specificity for detection of placentitis in the experimental model, and future research is needed to see if it is useful in the field. These results will be presented at the American Association of Equine Practitioners annual convention in December 2020.

Reproduction is a discipline that involves biomedical issues in several areas, including endocrinology, immunology and infectious disease. It is only with a grasp of the various topics within that researchers can make appropriate inferences into the diseases, disorders and problems that affect the cornerstone of our industry: the broodmare. It is through this conjunction of reproduction and immunology that Fedorka hopes to benefit the industry by improving the understanding of both normal and abnormal. While the normal is fascinating on a scientific level, it is with this information that future research can assess the abnormal, eventually utilizing this information to develop biomarkers for disease prediction, alter management for disease prevention and assess therapeutics for improvement of outcomes.

The topic of reproductive immunology paves a path in each of these directions, all of which target the main goal of UK’s reproduction laboratory: overall health of the horse.

Carleigh Fedorka, PhD, postdoctoral scholar in the Gluck Equine Research Center laboratory of Barry Ball, DVM, PhD, Dipl. ACT, Albert G Clay Endowed Chair in Equine Reproduction, provided this information.