Objectives: To explore roles of NKG2D in interactions of CD4 ⁺ T cells and dendritic cells (DCs) in juvenile idiopathic arthritis (JIA) patients Methods: Peripheral blood from active JIA patients and healthy controls were used for flow cytometry assessments of NKG2D ⁺CD4 ⁺ T cells and expressions of MICA and MICB on DCs. NKG2D genetically modified CD4 ⁺ T cells resulted from transfection with lentiviral vectors harboring NKG2D and NKG2D siRNA. ELISA measured supernatant cytokines in co-cultured CD4 ⁺ T cells and DCs. CD4 ⁺ T cell subgroups, MICA and MICB expression on DCs was determined by flow cytometry and transcription factors by real-time PCR. Results: All JIA patients had significantly higher content of CD4 ⁺NKG2D ⁺ T cells compared to healthy controls (P < 0.01). Expression of NKG2D on CD4 ⁺ T cells, and MICA and MICB on DCs were significantly greater in articular JIA than systemic JIA (P < 0.05). NKG2D induced IL-12 and suppressed IL-10 and TGF-β from CD4 ⁺ T cells, increased IFN-γ ⁺ CD4+ T and IL-17 ⁺ CD4 ⁺ T cells, RORc and T-bet, but reduced CD25 ⁺ Foxp3 ⁺ CD4 ⁺ T cells, IL-4 ⁺ CD4 ⁺ T cells, Foxp3, and GATA3 in JIA patients (P<0.05). NKG2D increased IL-12 and T-bet by CD4 ⁺ T cells in healthy controls (P<0.05). NKG2D decreased IL-10 and increased CD83, MICA, and MICB of DCs via interaction with CD4 ⁺ T cells in JIA and control patients (P<0.05). Conclusions: NKG2D regulates differentiation of CD4 ⁺ T cells directly and the maturation of DCs indirectly. Targeting NKG2D may be a potential therapy for JIA.