Immune cell populations in the equine corpus luteum throughout the oestrous cycle and early pregnancy: an immunohistochemical and flow cytometric study

Recent evidence indicates that the cells of the immune system and their lar ge network of secretory products, or cytokines, play an active role in the ovary throughout the oestrous cycle. Ln the present study, immune cell popu lations (T and B lymphocytes, macrophages, granulocytes and eosinophils) an d expression of major histocompatibility complex (MHC) class II were invest igated in corpora lutea from mares in early (days 2-4), mid- (days 7-10) an d late (days 12-14) dioestrus, the post-luteolytic phase (days 16-17) and e arly pregnancy. The number of T lymphocytes within the corpus luteum increa sed in the late luteal phase. CD4+ cells did not increase until day 16, whe reas the number of CD8+ cells increased before functional luteolysis; an ap parently selective luteal infiltration of CD8+ cells was observed. MHC clas s II expression by nonsteroidogenic cells was increased in samples from day s 16-17, as was the number of infiltrating macrophages. Flow cytometry reve aled very low expression of MHC class II by large luteal cells at all stage s of the oestrous cycle. In early pregnancy, the number of CD4+ and CD8+ ce lls and macrophages decreased, as did MHC class II expression, compared wit h mid-dioestrous samples. B cells were present in very small numbers in all samples examined. Eosinophils were similarly sparsely distributed and numb ers decreased further in pregnancy. After exogenous PGF(2 alpha) administra tion, populations of CD4+ cells and non-specific esterase staining cells we re significantly smaller than after natural luteolysis, whereas eosinophil numbers were increased compared with samples from days 16-17. However, the number of CD8+ and CD5+ cells and MHC class II expression were not signific antly different from those observed after natural luteolysis. These finding s indicate that populations of immune cells in the equine corpus luteum var y during its lifespan. The selective increase in CD8+ cells before function al luteolysis indicates that they have a physiological role in the regressi on of the corpus luteum.