Growth Factor Midkine Promotes T-Cell Activation through Nuclear Factor of Activated T Cells Signaling and Th1 Cell Differentiation in Lupus Nephritis

Activated T cells play crucial roles in the pathogenesis of autoimmune diseases, including lupus nephritis (LN). The activation of calcineurin/nuclear factor of activated T cells (NFAT) and STAT4 signaling is essential for T cells to perform various effector functions. Here, we identified the growth factor midkine (MK; gene name, Mdk) as a novel regulator in the pathogenesis of 2,6,10,14-tetramethylpentadecane-induced LN via activation of NFAT and IL-12/STAT4 signaling. Wild-type (Mdk^+/+) mice showed more severe glomerular injury than MK-deficient (Mdk^−/−) mice, as demonstrated by mesangial hypercellularity and matrix expansion, and glomerular capillary loops with immune-complex deposition. Compared with Mdk^−/− mice, the frequency of splenic CD69⁺ T cells and T helper (Th) 1 cells, but not of regulatory T cells, was augmented in Mdk^+/+ mice in proportion to LN disease activity, and was accompanied by skewed cytokine production. MK expression was also enhanced in activated CD4⁺ T cells in vivo and in vitro. MK induced activated CD4⁺ T cells expressing CD69 through nuclear activation of NFAT transcription and selectively increased in vitro differentiation of naive CD4⁺ T cells into Th1 cells by promoting IL-12/STAT4 signaling. These results suggest that MK serves an indispensable role in the NFAT-regulated activation of CD4⁺ T cells and Th1 cell differentiation, eventually leading to the exacerbation of LN.