Programmed cell death-1 (PD-1) and T-cell immunoglobulin mucin-3 (Tim-3) regulate CD4+ T cells to induce Type 2 helper T cell (Th2) bias at the maternal-fetal interface.

Study Question: Are the immune regulatory molecules programmed cell death-1 (PD-1) and T-cell immunoglobulin mucin-3 (Tim-3) involved in regulating CD4+ T cell function during pregnancy?

Summary Answer: PD-1 and Tim-3 promote Type 2 helper T cell (Th2) bias and pregnancy maintenance by regulating CD4+ T cell function at the maternal-fetal interface.

What Is Known Already: The maternal CD4+ T cell response to fetal antigens is thought to be an important component of maternal-fetal tolerance during pregnancy. PD-1 and Tim-3 are important for limiting immunopathology. The co-expression of PD-1 and Tim-3 on T cells identifies a T cell subset with impaired proliferation and cytokine production. Combined blockade of Tim-3 and PD-1 could restore T cell function to the greatest degree.

Study Design, Size, Duration: The expression of PD-1 and Tim-3 on CD4+ T cells was analyzed by flow cytometry, and in vitro and in vivo analyses were used to investigate the role of PD-1/Tim-3 signal in the regulation of CD4+ T cells function and pregnancy outcome.

Participants/ Materials, Setting, Methods: A total of 88 normal pregnant women, 37 women with recurrent spontaneous abortion, 36 normal pregnant mice and 45 abortion-prone mice were included. We measure the expression of PD-1 and Tim-3 on CD4+ T cells and their relationship to the function of CD4+ T cells and pregnancy outcome, as well as the effects of blocking PD-1 and Tim-3 pathways on decidual CD4+ T (dCD4+ T) cells during early pregnancy.

Main Results And The Role Of Chance: PD-1 and Tim-3, by virtue of their up-regulation on dCD4+ T cells during pregnancy, define a specific effector/memory subset of CD4+ T cells and promote Th2 bias at the maternal-fetal interface. Using in vitro and in vivo experiments, we also found that combined targeting of PD-1 and Tim-3 pathways results in decreased production of Th2-type cytokines by dCD4+ T cells and increased fetal resorption of normal pregnant murine models. Moreover, decreased PD-1 and Tim-3 on dCD4+ T cells may be associated with miscarriage.

Limitations And Limits Of Caution: Further study is required to examine the mechanism of PD-1 and Tim-3 effects on Th2 cytokine production by CD4+ T cells during pregnancy.

Wider Implications Of The Findings: These results have important implications for understanding the physiological mechanisms that promote maternal-fetal tolerance. Our study also indicates that targeting Tim-3 and PD-1 pathways may represent novel therapeutic strategies to prevent pregnancy loss.

Study Funding/competing Interests: This study was supported by the National Basic Research Program of China (2015CB943300); National Nature Science Foundation of China (81490744, 91542116, 31570920, 81070537, 31171437, 81370770, 31270969, 31570920, 91542116); the Key Project of Shanghai Municipal Education Commission (14ZZ013) and the Key Project of Shanghai Basic Research from Shanghai Municipal Science and Technology Commission (12JC1401600). None of the authors have any conflict of interest to declare.