Different T cell subsets in the nodule and synovial membrane: Absence of interleukin‐17A in rheumatoid nodules

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Abstract
Objective To determine gene expression of the interleukin-17 (IL-17) family members (IL-17A–F) in rheumatoid subcutaneous nodules, and to assess the cytokines involved in regulating IL-17A expression. Methods Total RNA was isolated from 19 nodules obtained from 16 different patients with rheumatoid arthritis (RA). Reverse transcription–polymerase chain reaction (PCR) was used to screen for gene expression of the IL-17 subtypes (IL-17A–F) in all nodules. Quantitative real-time PCR was used to measure the expression of interferon-γ (IFNγ), IL-6, IL-23, IL-12, and transforming growth factor β (TGFβ), relative to GAPDH as control, in a subset of 10 nodules. Results IL-17A gene expression was present in only 1 of 19 nodules, IL-17B in 17 of 19 nodules, IL-17C in 18 of 19 nodules, IL-17D in 16 of 19 nodules, and IL-17E in 3 of 19 nodules. IL-17F was absent in all samples. Cytokines that stimulate IL-17A production (IL-6, IL-23) as well as those that inhibit IL-17A production (IL-12, IFNγ, TGFβ) were present in the majority of nodules. Quantitative real-time PCR showed a similar pattern of gene expression for the individual cytokines between the different nodules. The mean ± SD expression of IL-6 relative to GAPDH was 2.28 ± 2.2 ng, and that of TGFβ was 2.96 ± 1.14 ng. There was a lower relative expression of IL-23 (0.05 ± 0.05 ng), while the expression of IFNγ was 0.67 ± 0.68 ng and that of IL-12 was 0.48 ± 0.23 ng. Conclusion IL-17 family members are varyingly expressed in rheumatoid nodules. The paucity of IL-17A in nodules suggests an important difference from that observed in the synovium. The expression of IL-23 below a critical threshold level seems the most likely explanation for the virtual absence of IL-17A. The presence of tissue destruction within the nodule despite the absence of IL-17A suggests that IL-17A may be an important amplifier rather than an absolute requirement for inflammation in RA.