Additive immunosuppressive effect of leflunomide and hydroxychloroquine supports rationale for combination therapy for Sjögren's syndrome

Objective: Effective treatment for primary Sjögren's syndrome (pSS) is not available. pSS immunopathology involves a variety of immune-cells and dysregulated pathways; targeting several pathways instead of only one could therefore be effective. Treatment with leflunomide (LEF) and hydroxychloroquine (HCQ) might be successful given their unique immunosuppressive properties. We aimed to study the in vitro effects of LEF, HCQ and their combination on T- and B-cell proliferation, cytokine and immunoglobulin production by activated PBMCs. Methods: PBMCs of six healthy individuals and nine pSS patients were stimulated with superantigen and TLR9 Agonist to mimic the hallmark features. LEF, HCQ and their combinations were tested at clinically observed concentrations and proliferation, cytokine and immunoglobulin production were measured. Results: TCR/TLR9 activation of PBMCs induced strong proliferation of T and B-cells and production of CXCL13, IFN-α, IFN-γ, IgG and IgM. LEF dose-dependently inhibited all measured parameters, where HCQ potently and dose-dependently decreased B cell proliferation, CXCL13, IFN-α, IgG and IgM production. At different concentration combinations, HCQ and LEF inhibited several immune hallmark features more potently than each single compound. Conclusion: A combination of LEF and HCQ at clinically applicable concentrations additively inhibits immune activation, supporting a potential implementation of this drug combination in pSS treatment.