To investigate the regulatory effect of CLEC2D-CD161 interaction on killing capacity of decidual natural killer (dNK) cells during early pregnancy and its association with the incidence of recurrent spontaneous abortion (RSA).

Decidua tissues were collected from normal pregnancies (n=16) and RSA cases (n=6) at 6-10 gestational weeks in the Department of Obstetrics and Gynecology of Peking University Third Hospital from October 2018 to May 2019. (1) Expressions of CLEC2D and CD161 in decidua from early pregnancy were detected using immunofluorescence. (2) Primary dNK cells were isolated from decidua from early pregnancy. dNK cells pre-treated with CD161 antibody (blocking CD161, B-CD161) were co-cultured with JEG-3 cells which were knocked-down by CLEC2D small interfering RNA (siCLEC2D), followed by killing capacity assessment of dNK cells by cytotoxicity assay and determination of expressions of related molecules by quantitive real-time polymerase chain reaction. (3) Western blot and flow cytometry were used to detect the expression of CLEC2D and CD161 in decidua tissues. Cytotoxicity assay was performed to analyze the killing capacity of dNK cells. T test was used for statistical analysis between normal and RSA cases.

(1) CLEC2D was mainly expressed in extravillous trophoblast (EVT) cells and CD161 was mainly detected in dNK cells. CD161-positive dNK cells and CLEC2D-positive EVT cells were adjacently located in decidua tissues allowing their interaction. (2) Cytotoxicity assay suggested that CD161 blocking in dNK cells or CLEC2D knockdown in JEG-3 cells could enhance the cytotoxicity of dNK cells. The target cell lysis rates at the effector-target ratios of 40∶1, 20∶1, 10∶1 and 5∶1 in B-CD161 group were (59.12±4.56)%, (25.96±5.44)%, (13.60±8.94)% and (12.53±8.94)%, and in IgG control group were (20.01±1.96)%, (8.51±1.32)%, (3.24±0.75)% and (3.82±1.92)%, respectively. There were significant differences between the two groups at the effector-target ratios of 40∶1 (t=13.922, P<0.01) and 20∶1 (t=5.403 P<0.05), but not at 10∶1 or 5∶1 (P>0.05). The target cell lysis rates at the effector-target ratios of 40∶1, 20∶1, 10∶1 and 5∶1 in si-CLEC2D group were (43.37±2.01)%, (32.99±2.08)%, (23.47±1.36)% and (11.48±0.37)%, and in the negative control (NC) group were (15.54±1.46)%, (13.84±1.68)%, (9.94±3.01) and (5.50±0.99)%, respectively. Differences between the two groups at all effector-target ratios were statistically significant (t=19.402, 12.400, 7.093 and 9.842, all P<0.01). Moreover, the expression of dNK killing-related factor granzyme B in the siCLEC2D group was higher than that in the NC group. (3) Compared with the normal pregnancy group, the RSA group showed decreased CD161 expression and increased killing capacity of dNK cells, but no significant difference in CLEC2D expression.

At early pregnancy, CLEC2D on EVT cells can interact with CD161 on dNK cells, which inhibits the cytotoxicity of dNK cells and induces immune tolerance at the fetal-maternal interface. Decreased expression of CD161 in decidua results in increased cytotoxicity of dNK cells, which may be one of the causes of immune rejection in RSA.

Abortion, habitual; Decidua; Killer cells, natural; Receptors, cell surface; Lectins, C-type; NK cell lectin-like receptor subfamily B