Endogenous retroelement activation is implicated in IFN-α production and anti-CCP autoantibody generation in early Rheumatoid Arthritis.

Objectives: Endogenous retroelements (EREs) stimulate type 1 interferon (IFN-I) production but have not been explored as potential interferonogenic triggers in Rheumatoid Arthritis (RA). We investigated ERE expression in early RA (eRA), a period where IFN-I is increased.
Methods: ERE expression (LTR5, LINE1, SINE) in disease modifying treatment naïve eRA whole blood and bulk synovial tissue was examined by RT-PCR and Nanostring alongside IFN-α activity. Circulating lymphocyte subsets, including B cell subsets, from eRA patients and early psoriatic arthritis (ePsA), were flow cytometrically sorted and similarly examined. Existing established RA and osteoarthritis (OA) synovial single-cell sequencing data was re-interrogated to identify repeat elements, and associations explored.
Results: There was significant co-expression of all ERE classes and IFNA in eRA synovial tissue (n=22, p<0.0001) and significant positive associations between whole blood LINE1 expression (n=56) and circulating IFN-α protein (p=0.018) and anti-CCP titres (p<0.0001). ERE expression was highest in circulating eRA B-cells, particularly naïve B-cells compared with ePsA, with possible ERE regulation by SAMDH1 implicated and associations with IFNA again observed. Finally, in established RA synovium, LTRs, particularly ERVK, were most increased in RA compared with OA where, for all synovial subsets (monocytes, B-cells, T-cells and fibroblasts), ERE expression associated with increased IFN-I signalling (p<0.001).
Conclusions: Peripheral blood and synovial ERE expression is examined for the first time in eRA highlighting both a potential causal relationship between ERE and IFN-I production and an intriguing association with anti-CCP autoantibodies. This suggests EREs may contribute to RA pathophysiology with implications for future novel therapeutic strategies.
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