The aim of this retrospective study was to compare the immune tolerance status of patients suffered from unexplained spontaneous abortion (URSA) before and after treatment with paternal lymphocyte induced immunization (PLII) four times, and its relationship to the pregnancy outcome. 168 URSA patients were included in the present study. Among 168 couples, 138 couples were conceived again, of whom 86 were successfully pregnant till 20 gestational weeks, 31 cases again failed in the first

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... , 21 cases were still under follow-up, another 30 cases still had not conceived. Both the level of one way mixed lymphocyte culture blocking efficiency (MLC-BE) and anti-idio blocking antibody (BE-Ab2) were markedly elevated in succeeded group after PLII. In contrast, although a significant increase could be observed in the failed group after treatment, the elevation of BE-Ab2 was much lower than that in successful group. PLII therapy significantly up-regulated the percentage of peripheral CD4 + CD25 + CD127-regulatory T cells (Tregs) in successfully pregnant women; however, there was no significant change of Tregs in pregnancy loss cases although receiving PLII therapy. These results suggested a positive correlation between higher frequency of Tregs and rate of successful pregnancies. The sensitivity and specificity of combination of Tregs with MLC-BE and BE-Ab2 were 81.8% and 81.3%, respectively. Therefore, the percentage of Tregs in peripheral blood may hopefully serve as a potential biomarker for monitoring the efficacy of therapy in URSA patients. Combination of Tregs with MLC-BE and BE-Ab2 may expect to better evaluate the efficacy of PLII in URSA patients.