Background BAFF,the B cell activation factor,is a member of the tumor necrosis factor(TNF)ligand family that binds to BCMA,TACI,and BAFF-R.Previous studies have shown that members of the TNF family are detected in human placental trophoblast cells,but the expression patterns of BAFF involved in human decidua and the differential expression of BAFF between normal pregnancy and miscarriage are still incompletely documented or unknown.This study was designed to investigate the expression of BAFF and BAFF-R in the trophoblast and decidua of normal early pregnant women and recurrent spontaneous abortion(RSA)patients.Methods Forty-five patients with RSA and 45 normal pregnant women were included in this study.By reverse transcriptase-polymerase chain reaction(RT-PCR),Western blotting and immunohistochemical experiments,we explored the expression of BAFF and BAFF-R in the maternal-fetal interface of normal early pregnant women and RSA patients.Results Analysis by RT-PCR and Western blotting revealed that BAFF was detected in both trophoblast and decidua of all the samples,and the expression level was higher in the tissues of normal early pregnant women(P＜0.05)than that of recurrent spontaneous abortion patients under the same gestational weeks.Messages for BAFF-R were absent.Immunohistochemical experiments showed that expression of BAFF was cell-specific which was localized to villous cytotrophoblast and syncytiotrophoblast cells in trophoblast and to stromal cells in decidua.Whereas BAFF was prominent on the trophoblast and decidua of normal early pregnant women,it was decreased in the tissues of RSA patients.Conclusions BAFF might steer maternal leukocytes away from a harmful immune response and toward a favorable one and play a potentially vital role for successful pregnancy.