Influence of age and parity on the distribution of cells expressing major histocompatibility complex class II, CD4, or CD8 molecules in the endometrium of mares during estrus

Objective-To evaluate effect of age and parity on distribution and number o f cells expressing major histocompatibility complex (MHC) class II, CD4, or CD8 molecules in the endometrium of mares during estrus. Animals-52 gynecologically healthy mares, categorized as young (3 to 8 year s; n = 17) or old (9 to 16 years; 15) and nulliparous (n = 6), nulliparous embryo donors (16), or parous (10). Procedures-Endometrial specimens collected from the uterine body acid horns during estrus were stained by use of the avidin-biotin-peroxidase method, using monoclonal antibodies against equine MHC class II, CD4, and CD8 molec ules. Labeled cells in the stratum compactum within 5 randomly selected fie lds at 400X magnification (total area = 0.31 mm(2)) were counted, and numbe rs were compared among groups and between locations. Results-Age did not affect cell numbers within the 3 cell subsets examined. Numbers in each subset were higher in the uterine body than in the horns, although the difference was not significant for cells expressing MHC class ii. Significantly more cells expressing MHC class II molecules were detecte d in the uterine body of nulliparous and parous mares than in embryo donors , whereas in the horns, these cells were significantly higher in number onl y in parous mares. Parity did not affect number of CD4(+) or CD8(+) cells. Conclusions and Clinical Relevance-The increased likelihood for endometriti s to develop in mares as they age cannot be explained by a decrease in numb er of cells expressing MHC class II, CD4, or CD8 molecules within the endom etrium. However, greater number of cells within these 3 subsets detected in the uterine body, compared with the horns, during estrus suggests a local readiness to act against microorganisms or semen introduced during mating o r insemination.