Local versus systemic control of numbers of endometrial T cells during pregnancy in sheep

Pregnancy in sheep is associated with changes in numbers of specific T-lymp hocyte populations in the uterine endometrium. These changes probably contr ibute to evasion by the conceptus of maternal immunological rejection and i ndicate a possible role for T cells in placental growth, parturition and po st-parturient uterine defence against infection. The purpose of the present experiment was to evaluate the relative importance of systemic signals (i. e. those present throughout the uterus or from the circulation, including c onceptus hormones secreted into the maternal blood) versus locally acting c onceptus signals for regulating changes in numbers of endometrial lymphocyt es during pregnancy. The approach taken was to surgically confine pregnancy to one uterine horn and compare differences in lymphocyte numbers between the two uterine horns as well as between both horns of pregnant ewes with t hose of ovariectomized ewes. As compared with ovariectomized ewes, there wa s a decline in numbers of CD45R(+) lymphocytes within glandular epithelium and an increase in gamma delta T-cell number within the luminal epithelium. These changes occurred in both the pregnant and non-pregnant uterine horns of unilaterally pregnant ewes. Moreover, there were no significant differe nces in lymphocyte: numbers between the two uterine horns of unilaterally p regnant ewes. Expression of CD25 was absent in tissues from both uterine ho rns. In conclusion, changes in numbers of endometrial lymphocytes during pr egnancy, rather than due to locally acting signals of conceptus origin, are the result of hormonal signals of maternal or conceptus origin that either act directly on endometrial lymphocytes or stimulate the uterine endometri um to induce synthesis of regulatory molecules that affect lymphocyte dynam ics.