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To explore the expression of major histocompatibility complex class I chain-related gene A (MICA) and its ligand in colonic mucosa and the role of MICA-natural killer (NK) group 2D (NKG2D) interaction in activating NK cells in ulcerative colitis (UC) patients.Intestinal mucosal biopsies were obtained from patients with UC and the controls. The expression of major histocompatibility complex class I-related gene (MIC) genes was determined by a reverse transcription polymerase chain reaction (RT-PCR) and the imaging of MICA expressed on colonic mucosa was measured by confocal microscopy resonance scanning. NKG2D and intracellular interferon (IFN)-γ expressions on NK cells were assayed by flow cytometry.The relative amount of MICA mRNA in the colonic mucosa of UC patients was significantly higher than in that of the controls (3.5408 ± 2.6658 vs 1.0477 ± 0.7201, P = 0.001), as were the major histocompatibility complex class I chain-related gene B (MICB) (8.9879 ± 3.2893 vs 4.6293 ± 1.2616, P < 0.001) and NKG2D mRNA expression (2.4395 ± 0.8147 vs 1.1624 ± 0.3954, P < 0.001). Confocal microscopy resonance scanning had shown that MICA was localized predominantly on the basolateral membranes of the epithelium. Further flow cytometry confirmed that the percentage of IFN-γ producer NK cells that expressed NKG2D in peripheral blood lymphocytes was higher in UC patients than in the healthy controls (45.36% ± 12.47% vs 27.45% ± 9.30%, P < 0.001).MICA, MICB and NKG2D were upregulated in the colonic mucosa of UC and were associated with activating NK cells with promoted NKG2D and IFN-γ production.