Immunosuppressive Activity Associated with Early Pregnancy in the Bovine1

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Abstract
Immunosuppressive activity was assessed in uterine flushings (UF) and uterine vein serum and plasma from nonpregnant and early-pregnant cows, and in media from the short-term culture of Day 18 bovine embryos. The preparations were tested for their ability to inhibit [3H]thymidine (3H-TdR) incorporation into phytohemagglutinin-stimulated bovine lymphocytes. On Days 2-3 (called Day 3), Days 9-10 (called Days 10), and Days 17-19 (called Days 18) of the estrous cycle (estrus = Day 0) and pregnancy, untreated and superovulated cows were anesthesized and jugular vein and uterine vein blood was collected. The uteri were removed and flushed to obtain UF and embryos. Uterine flushings were concentrated and tested for immunosuppressive activity at 400 .mu.g uterine protein/ml culture fluid. Uterine flushings from both Day 18 pregnant and Day 18 nonpregnant cows were immunosuppressive (8/8), whereas Day 10 UF were usually not immunosuppressive (7/10). Day 3 UF were usually stimulatory or only marginally suppressive (8/8). Uterine vein serum and plasma from Day 18 cows were not suppressive when compared to jugular vein serum or plasma from the same cow; neither were Day 18 uterine vein serum or plasma suppressive when compared to those same samples taken from Day 3 cows. Embryo culture media obtained from the 48-h culture of Day 18 embryos was consistently suppressive. The activity was lost after dialysis in 1000-MW cut-off tubing, removed by charcoal, and reduced by protease digestion. Two mechanisms whereby the embryo could escape immune rejection are suggested: the progesterone-induced secretion of a uterine immunosuppressive substance(s) and the production by the embryo of a low MW immunosuppressive substance(s).