Objective:To investigate interleukin-37 (IL-37) expression in patients with diabetic kidney disease (DKD), and to assess the regulation of exogenous IL-37 on CD8 + T cell function in DKD patients. Methods:A cross-section study was carried out. Twenty healthy controls, thirty-six patients with diabetes mellitus type 2 (T2DM), and forty-seven DKD patients were enrolled in the study. Peripheral blood was collected. Plasma and peripheral blood mononuclear cells were isolated. IL-37 and soluble IL-1 receptor 8 (IL-1R8) levels in the plasma were measured by enzyme-linked immunosorbent assay (ELISA). IL-18 receptor α chain (IL-18Rα), IL-1R8 and immune checkpoint molecules levels in CD8 + T cells were measured by flow cytometry. CD8 + T cells were purified, and were stimulated with recombinant IL-37. CD8 + T cells were co-cultured with HEK293 cells in either direct contact or indirect contact manner. Levels of perforin, granzyme B, interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) were measured by ELISA. The proportion of target cell death was assessed by measuring lactate dehydrogenase level. Results:Plasma IL-37 levels in DKD patients [(63.42±23.30) ng/L] were significant lower than those in healthy controls [(143.02±50.67) ng/L] and T2DM patients [(87.88±40.62) ng/L] ( t=8.848, P<0.001; t=3.456, P<0.001). Plasma IL-37 level had good predictive values for T2DM in health individuals and for DKD in T2DM patients [the area under the curve was 0.797 (95% CI 0.676-0.917, P<0.001) and 0.691 (95% CI 0.576-0.807, P=0.003), respectively]. Plasma IL-37 level was negatively correlated with urea nitrogen ( r=-0.313, P=0.032) and creatinine ( r=-0.477, P<0.001), and positively correlated with estimated glomerular filtration rate (eGFR) ( r s=0.478, P<0.001) in DKD patients. IL-1R8 + CD8 + cell proportion in DKD patients (33.60%±9.47%) was significantly higher compared to healthy controls (16.29%±5.97%) and T2DM patients (17.13%±4.85%) ( t=7.545, 9.516, both P<0.001), but did not correlate with fast blood glucose, urea nitrogen, creatinine, or eGFR (all P>0.05). There were no statistical differences of IL-18Rα + CD8 + cell proportion, soluble IL-1R8 level, or immune checkpoint molecule proportion in CD8 + T cells among healthy controls, T2DM patients, and DKD patients (all P>0.05). Perforin and granzyme B secretions by CD8 + T cells were significantly elevated in DKD patients compared with healthy controls [(108.78±12.42) ng/L vs. (94.60±10.07) ng/L, t=3.096, P=0.005; (261.34±48.79) ng/L vs. (166.28±30.80) ng/L, t=3.387, P=0.002] and T2DM patients [(108.78±12.42) ng/L vs. (92.58±14.71) ng/L, t=3.263, P=0.003; (261.34±48.79) ng/L vs. (170.66±39.24) ng/L, t=2.627, P=0.014]. There were no significant differences of either IFN-γ or TNF-α secretions by CD8 + T cells among healthy controls, T2DM patients, and DKD patients (all P>0.05). In direct contact co-culture manner, CD8 + T cell-induced HEK293 cell death was down- regulated (13.03%±4.97% vs. 17.88%±5.19%, t=2.235, P=0.037). The levels of perforin [(222.02±25.79) ng/L vs. (294.30±25.58) ng/L, t=6.603, P<0.001], granzyme B [(416.27±90.24) ng/L vs. (524.71±115.53) ng/L, t=2.454, P=0.023], IFN-γ [(23.66±4.20) ng/L vs. (35.18±8.51) ng/L, t=4.026, P<0.001] and TNF-α [(1.62±0.29) μg/L vs. (2.09±0.57) μg/L, t=2.302, P=0.034] were also reduced as well. In indirect contact co-culture manner, there were no significant differences of CD8 + T cell-induced HEK293 cell death, perforin, or granzyme B levels between no stimulation and IL-37 stimulation (all P>0.05). IFN-γ and TNF-α levels in the supernatants were reduced in response to IL-37 stimulation [(23.56±6.24) ng/L vs. (32.56±9.90) ng/L, t=2.550, P=0.019; (1.41±0.31) μg/L vs. (2.10±0.44) μg/L, t=4.011, P<0.001]. Conclusion:IL-37 level is reduced in DKD patients.Exogenous IL-37 suppresses the cytotoxicity of CD8 + T cells in DKD patients.