Immunosuppressive Effect of Ovine Uterine Secretory Protein upon Lymphocytes in vitro12

The immunosuppressive effect of ovine uterine protein upon phytohemagglutinin (PHA)-stimulated peripheral blood lymphocytes was investigated in vitro. Uterine secretory protein was collected and concentrated in sheep uterine flushings (SUF) obtained from nonpregnant and pregnant Dorset ewes on Days 4, 9, and 14 of the cycle. Varying concentrations of protein (2.5–1600μg/ml) were added to cultures each containing 1 × 106 ovine lymphocytes and 0.6 μg of PHA. Lymphocyte response to PHA was suppressed in cultures containing SUF protein from Day 4, 9, and 14 nonpregnant and pregnant ewes. The regression of SUF protein concentration with lymphocyte suppression (as percent of control) for each day and reproductive stage was significant (P<0.001). Almost complete suppression occurred with much less SUF protein (100 μg/ml) obtained on Day 14 of pregnancy than was obtained at most other times (1600 μg/ml). Ovine serum and bovine serum albumin had no significant immunosuppressive effect. The immunosuppressive effect of SUF protein was not mediated by cytotoxicity. It is unknown whether lymphocyte suppression observed in vitro occurs in vivo. If a parallelism exists, data from this investigation strongly suggest that the ovine uterus may possess a continual immunosuppressive substance(s) during pregnancy and the estrous cycle and that the activity of this substance increases considerably by Day 14 of pregnancy.