Comparison of serum IL-33 levels among IL33 genotypes. The IL-33 levels were tested in 141 DMARDs-naïve RA patients and 115 healthy controls (HCs). Serum IL-33 concentrations were significantly higher in patients with RA than in healthy controls (P = 0.000 (a). Patients with the CC genotype showed significantly lower serum IL-33 levels than did patients with TC and TT genotypes (b). When we divided the patients into ACPA-positive and ACPA-negative groups, the same result was obtained in the ACPA-positive group (c). In the ACPA-negative group (n = 41), patients with the CC genotype also showed lower serum IL-33 levels than did patients with TC and TT genotypes (21.8 ± 40.5 versus 91.5 ± 209.5 versus 125.1 ± 331.0 ng/ml); however, no statistical difference was found. IL-33 expression was significantly higher in the dominant model (TT + TC) (P = 0.008 (d). TT, TC, and CC represent genotypes of IL33 rs7044343.