¡¾Abstract¡¿ AIM: To have a progressive knowledge of the inhibitory capacity of placental villi on T cell response and its changes in the 3 trimesters of pregnancy. METHODS: Tissue extracts were prepared from early (10 weeks), middle (20 weeks) and late (37 weeks) placental villi, and their impacts on T cell activation were evaluated by detecting CD69 and CD25 expressions, impacts on proliferation were analyzed by method of WSTª²1, and impacts on Th1/Th2 cytokine repertoire were measured by cytometric bead array (CBA). Content of indoleamine 2,3ª²dioxygenase (IDO) in the tissue extracts was detected by Western blotting. RESULTS: All tissue extracts from the 3 trimesters showed inhibitory effects on activation, proliferation and cytokine secretion of human peripheral T cells stimulated by PHA, and also on activation of mouse T cell activation stimulated by ConA (P<0.01). In general, the 20 week extracts had the strongest immunosuppressive effects. The content of IDO was much more in 20 week extracts than in 37 week extracts, and there was only trace amount of IDO in 10 week extracts. CONCLUSION: Villous extracts from the 3 trimesters of pregnancy had different degrees of inhibitory effects on T cell response, and their inhibitions to different T cell behaviors were not consistent, indicating their different expression patterns (quality and quantity) of inhibitory factors. ......