Background The dysregulation of germinal centers and ectopic follicles, resulting in hyperactive B cells, is thought to be a key mechanism leading to systemic lupus erythematosus (SLE). Interleukin-21 (IL-21) is a pro-inflammatory cytokine that is proposed to contribute to the pathogenesis of several autoimmune inflammatory disorders, including SLE. In mice, IL-21 is described as a regulator of T follicular helper (Tfh) cell differentiation and B cell maturation. However, there is limited published data evaluating the function of IL-21 in the human immune system or the relevance of IL-21 in SLE.
Objectives In the present study, our aim was to investigate IL-21 and IL-21 receptor (IL-21R) expression in SLE and the role of the IL-21 pathway in germinal center biology.
Methods We obtained peripheral blood from patients with active SLE (SLEDAI ≥4) and age/gender-matched healthy controls. Patients had been treated with less than 20 mg/day Prednisone and had not received B cell depleting therapy. Expression of IL-21 and IL-21R was analysed by flow cytometry or RT-PCR. For functional studies, Tfh and B cells were sorted from tonsils or peripheral blood from healthy individuals, stimulated with SEB, and proliferation and Ig production were measured by CFSE dilution and luminex, respectively.
Results In the periphery, IL-21 was produced by CD4+ and CD8+ cells, and IL-21 levels were significantly higher in SLE CD4+ cells compared to healthy controls as measured by intracellular cytokine staining. Furthermore, increased levels of IL-21 correlated with elevated IL-17 and IFN-γ production. IL-21R was expressed on CD4+ T cells, CD8+ T cells, NK cells, B cells, and mature DCs, and levels were not significantly altered in SLE samples. CD4+ Tfh cells from human tonsils or peripheral blood produced significantly more IL-21 than non-Tfh cells and blockade with a neutralising anti-IL-21 antibody resulted in a moderate decrease in Tfh proliferation. In Tfh-B cell co-culture assays from either tonsils or peripheral blood, anti-IL-21 inhibited B cell maturation, as measured by Ig production.
Conclusions Together, these data demonstrate elevated IL-21 production in SLE patients and provide evidence for an IL-21 mediated effect on Tfh proliferation and Tfh-induced B cell maturation in humans. Therefore, targeting IL-21 activity with a neutralising antibody has potential as a novel therapeutic approach for the treatment of autoimmune inflammatory disorders such as SLE.
Acknowledgements We acknowledge Dr. Jane Buckner, Melissa Peda, Gina Marchesini, Kevin Criste, Christine Chan, and Thien-Son Nguyen from the Benaroya Research Institute at Virginia Mason for their contributions to these studies.
Disclosure of Interest J. Cox Employee of: Novo Nordisk, S. Hussell Employee of: Novo Nordisk, E. Thomas Employee of: Novo Nordisk, R. Wu Employee of: Novo Nordisk, K. Valliant-Saunders Employee of: Novo Nordisk, P. Smith Employee of: Novo Nordisk, S. Peng: None Declared, H. Blumberg Employee of: Novo Nordisk, S. Levin Employee of: Novo Nordisk, D. Lundsgaard Employee of: Novo Nordisk