Objectives: Interferon regulatory factor 5 (IRF5) plays a central role in interferon-mediated inflammation and is implicated in autoimmune diseases, including systemic lupus erythematosus (SLE). Detecting IRF5 in plasma is challenging due to its low circulating levels, highlighting the need for a highly sensitive and quantitative assay. This study aimed to develop such an assay, validate the existence of previously identified IRF5 high and low subgroups in SLE, and support the potential of IRF5 as a biomarker in precision medicine.

Method: We established a solid-phase proximity ligation assay (SP-PLA) for IRF5 detection using a commercially available polyclonal antibody, which was benchmarked against two in-house recombinant antibodies. A linear calibration curve was generated using recombinant IRF5 protein (range 0.01-100 pg/μL), with a limit of detection between 0.01 and 0.05 pg/μL. EDTA-plasma samples from three SLE subgroups (n = 25 per group) were analysed.

Results: IRF5 was detectable in all SLE plasma samples using SP-PLA (mean 0.63 pg/μL; sd 1.92 pg/μL; maximum 13.54 pg/μL). The IRF5 high SLE subgroup showed significantly higher IRF5 plasma levels than the IRF5 low SLE subgroup (Dunn's post-hoc test, adjusted p < 0.05).

Conclusion: The SP-PLA enabled sensitive and specific detection of low-level IRF5 in plasma and confirmed the presence of IRF5 high and low subgroups in SLE using a quantitative method. These findings support the potential of IRF5 as a biomarker, but validation in independent cohorts is required. The assay may facilitate patient stratification in future research and precision medicine approaches targeting the interferon pathway.