Associations between immunological and hormonal parameters during healthy pregnancy in mares

Associations between the immune and endocrine systems during equine pregnancy remain poorly understood. Based on the hypothesis that distinct phases of the equine healthy gestation are characterized by specific associations between circulating immunological and hormonal parameters, contributing to pregnancy maintenance, this investigation aimed to: (i) evaluate how circulating immunological and hormonal parameters change across different phases of pregnancy; (ii) investigate associations between changes in circulating immunological and hormonal parameters; and (iii) propose potential hormonal drivers of immunological modulation during pregnancy. Peripheral blood samples were prospectively collected from mares (n = 8) before ovulation and during pregnancy at 30, 90, 150, 210, 240, 270, 300, and 330 days of their healthy gestations. An immunological panel included the distribution of circulating T cell (CD3, CD4, and CD8) and B cell subpopulations, complete blood counts (CBC), and serum protein profile. Hormonal analyses included equine chorionic gonadotropin (eCG), progestogens, androgens, estrogens, corticosteroids, and thyroid hormones. At 90 days, a statistically significant increase in peripheral blood CD4 T cell distribution was accompanied by a concomitant reduction in B cell distribution. This immunological modulation correlated positively with eCG, progesterone (P4), 5α-dihydroprogesterone (DHP), and estrone sulfate, and inversely with B cell levels. In contrast, at 210 days, B cell distribution peaked significantly while CD4 T cell distribution declined, concomitant with a rise in albumin levels. These changes positively correlated with cortisone and hematocrit. Mid-gestation was characterized by associations between different androgens and circulating T cell and B cell distributions. Consistent negative associations were observed between progesterone, DHP, and estradiol-17β with glucocorticoid metabolites throughout gestation. Estradiol-17β and IgM concentrations showed a positive correlation in late gestation. Immune-hormone and hormone-hormone associations were more pronounced during early and mid-gestation, while the final 100 days of pregnancy were characterized by relatively constant levels. Collectively, our findings suggest immune-hormone associations that potentially orchestrate immunomodulation, fetal development, successful pregnancy maintenance, and parturition in the mare.