An elevated polyclonal free light chain level reflects a strong interferon signature in patients with systemic autoimmune diseases

The work described has received support from the Innovative Medicines Initiative Joint Undertaking under grant agreement No. 115565, the resources for which are composed of a financial contribution from the European Union's Seventh Framework Programme (FP7/2007-2013) and the European Federation of Pharmaceutical Industries and Associations (EFPIA) companies'in-kind contribution. We thank all the members of PRECISESADS Consortium and INSERM U1227 for their effort in the sample recruitment, distribution and assessment of the samples used in this study. We are grateful to Dr Wesley H Brooks (Tampa, USA) for editorial assistance and to Valerie Le Troadec for secretarial assistance.High amount of polyclonal free light chains (FLC) are reported in systemic autoimmune diseases (SAD) and we took advantage of the PRECISESADS study to better characterize them. Serum FLC levels were explored in 1979 patients with SAD (RA, SLE, SjS, Scl, APS, UCTD, MCTD) and 614 healthy controls. Information regarding clinical parameters, disease activity, medications, autoantibodies (Ab) and the interferon α and/or γ scores were recorded. Among SAD patients, 28.4% had raised total FLC (from 12% in RA to 30% in SLE and APS) with a normal kappa/ lambda ratio. Total FLC levels were significantly higher in SAD with inflammation, active disease in SLE and SjS, and an impaired pulmonary functional capacity in SSc, while independent from kidney impairment, infection, cancer and treatment. Total FLC concentrations were positively correlated among the 10/17 (58.8%) autoantibodies (Ab) tested with anti-RNA binding protein Ab (SSB, SSA-52/60 kDa, Sm, U1-RNP), anti-dsDNA/nucleosome Ab, rheumatoid factor and negatively correlated with complement fractions C3/C4. Finally, examination of interferon (IFN) expression as a potential driver of FLC overexpression was tested showing an elevated level of total FLC among patients with a high IFNα and IFNγ Kirou's score, a strong IFN modular score, and the detection in the sera of B-cell IFN dependent factors, such as TNF-R1/TNFRSF1A and CXCL10/IP10. In conclusion, an elevated level of FLC, in association with a strong IFN signature, defines a subgroup of SAD patients, including those without renal affectation, characterized by increased disease activity, autoreactivity, and complement reduction.Innovative Medicines Initiative Joint Undertaking 115565European Commission FP7/2007-2013European Federation of Pharmaceutical Industries and Associations (EFPIA)Institut National de la Sante et de la Recherche Medicale (Inserm)European Commission U122