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BACKGROUND
Lipocalin-2 (LCN2) is a potent bacteriostatic protein. We aimed to investigate its role as a disease activity marker in patients with inflammatory bowel disease (IBD) and its induction by the Th17 cytokines IL-17A, IL-22, and TNF-α in colonic epithelial cells. Moreover, we analyzed the influence of IBD-associated IL23R alleles on LCN2 serum levels in IBD patients.

METHODS
LCN2 serum levels were determined in 131 IBD patients (71 with Crohn's disease [CD], 60 with ulcerative colitis [UC]) and 63 healthy controls. IBD patients were genotyped for 10 IBD-associated IL23R polymorphisms. LCN2 expression after stimulation with IL-17A, IL-22, and TNF-α was measured in human colonic epithelial cell lines.

RESULTS
A significant upregulation of serum LCN2 in active IBD (median [IQR], 36.84 [21.17-73.74] ng/mL; P = 0.01) compared with healthy controls (24.22 [17.76-35.25] ng/mL) was confined to active UC (42.21 [28.97-73.74] ng/mL; P = 0.0006). LCN2 proved to be a marker of UC disease activity (area under the curve 0.75, sensitivity 0.83, specificity 0.63; P = 0.0002). IL-17A showed a synergistic effect with IL-22 and TNF-α in inducing colonic epithelial expression of LCN2 and its essential transcription factor IKBZ. In CD, LCN2 concentrations were significantly modulated by IL23R genotype status with homozygous carriers of IBD risk-increasing alleles showing particularly low LCN2 levels.

CONCLUSIONS
Serum LCN2 proves to be a biomarker of active UC. Lower LCN2 levels in CD patients carrying IBD risk-increasing IL23R variants may result from a restricted upregulation of LCN2 due to an impaired Th17 immune response.