The maternal leucocyte response to the endometrial cups in horses is correlated with the developmental stages of the invasive trophoblast cells.

Invading trophoblasts form endometrial cups in the endometrium of the pregnant mare. In the present study we characterized the maternal leucocyte response to endometrial cups from their formation to their regression. The maternal leucocyte response was correlated with the stages of trophoblast development. (1) Aggregates of CD4+ and CD8+ cells were present between the migrating and differentiating endometrial cup trophoblasts and surrounding the forming endometrial cups. (2) Numbers of CD4+ cells within the mature endometrial cups were much reduced. At the periphery of the endometrial cups CD4+ and CD8+ cells were found in patchy accumulations around endometrial glands; small clusters of CD79+ B lymphocytes were present as well. (3) Scattered CD4+ and CD8+ cells were found within dying endometrial cups; areas of cell death were infiltrated with neutrophils. Large aggregates of CD4+ cells and CD8+ cells, and small but numerous clusters of CD79+ cells and eosinophils, were found outside of the dying endometrial cups. The CD4+ or CD8+ cells were mostly CD3+ T cells; some were probably macrophages which can express both of these markers in horses. The correlation between the developmental stages of the endometrial cup trophoblast and the maternal leucocyte response suggests a complicated cytokine-mediated regulatory network.