Background/Purpose:

Presence of a type I interferon (IFN) signature has been described for several autoimmune diseases, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA). For SLE, it was shown that the IFN signature is induced by IFNα from plasmacytoid dendritic cells via specific immune complexes. Although the IFN signature in RA seems to be comparable to SLE, the source of IFN induction is yet unknown. This study aims to investigate the mechanism by which components of RA serum contribute to the IFN signature.

Methods:

Healthy PBMCs were exposed to RA or SLE serum. IFNα protein production was measured in an immunoassay after 20h incubation with 5% patient serum. To study the involvement of immune complexes containing antinuclear antibodies, samples were co-cultured with necrotic or apoptotic cells (Lövgren et al., Arthritis Rheum. 2004 Jun;50(6):1861-72). Moreover, IFN response genes (IRG), IFNα and IFNβ mRNA induction were measured by quantitative PCR after 4h and 8h incubation with 25% patient serum. An IFN score is calculated as the average expression of 3 IRGs (RSAD2, IFI44L, MX1). To study the involvement of new protein synthesis on IRG induction, part of the samples were co-cultured with 2µg/ml cycloheximide (CHX).

Results:

Results
As expected, SLE serum induced more IFNα protein induction than healthy donor serum (NHS) (p=0.0006, Mann Whitney test), which even further increased with dead cell material (p=0.006, Mann Whitney test). RA serum did not induce significantly more IFNα protein production than NHS, although a small increase was observed in the absence of dead cell material (p=0.0516, Mann Whitney test).

With respect to IRG mRNA induction, both RA and SLE sera induced higher levels than NHS, though with different induction kinetics. SLE serum showed IRG mRNA induction already at 4h, which stayed present after 8h. CHX treatment even slightly increased IRG mRNA induction at 8h, suggesting that the IRG mRNA induction occurs independently of new protein synthesis and is probably mediated directly by IFNα, as described before. RA serum, on the other hand, showed only IRG mRNA induction after 8h. This IRG mRNA induction was downregulated upon CHX treatment, indicating an indirect IRG induction process. To study whether the IRG induction was preceded by IFN mRNA induction, IFNα en IFNβ mRNA expression was determined. The IRG induction by RA serum was positively correlated to IFNβ mRNA induction at 4h and 8h (Pearson correlations, 4h: r= 0.6873, p=0.0023; 8h: r=0.7173, p=0.013), but not to IFNα production.

Conclusion:

RA serum did not induce IFNα protein production, and mRNA induction kinetics differed from those of SLE serum. Furthermore, the IRG mRNA induction by RA serum was preceded by IFNβ mRNA induction. Altogether, this indicates a different source of IRG induction in SLE and RA serum and thus possibly a different mechanism behind their type I IFN gene signature.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/on-the-origin-of-the-type-i-interferon-signature-in-rheumatoid-arthritis/