To investigate the possible role of a functional polymorphism in the soluble interleukin 6 receptor (sIL-6R) gene in the genetic background of rheumatoid arthritis (RA).

An association between disease status and the sIL-6R rs8192284 (A358D) variant was tested in 965 patients with RA and 988 unrelated healthy controls. Odds ratios (ORs) for disease were calculated with asymptotic 95% CI; p values <0.05 were considered statistically significant after adjustment for multiple testing. To determine the relationship between protein levels and IL-6R A358D genotype, the protein levels of sIL-6R in 100 plasma samples from healthy controls were measured using an ELISA and compared across the genotype groups.

The allele frequency of the C allele (alanine) was lower in cases than in controls (38.4% vs 41.7%, p=0.04, OR 0.9, 95% CI 0.8 to 1.0), as were the CC/AC genotypes compared with AA genotype frequencies (61.0% in RA cases vs 67.5% in controls, p=0.004, OR 0.8, 95% CI 0.6 to 0.9). Plasma levels of sIL-6R differed significantly according to genotype in the controls: 17.00±2.03 ng/ml for A/A, 20.08±1.83 ng/ml for A/C and 21.57±2.10 ng/ml for C/C (p=0.0001).

These data suggest a role for genetically determined lower sIL-6R levels as a risk factor for RA. The proinflammatory role of the IL6 system in established RA has been highlighted by the use of anti-sIL-6R antibodies. However, the findings of this study suggest a protective effect of IL6 on the risk of developing RA.