JAK Inhibitors and B Cell Function: A Comparative Study of Their Impact on Plasma Cell Differentiation, Cytokine Production, and Naïve B Cell Activation

B cells play a crucial role in autoimmune diseases, as evidenced by autoantibody responses and the effectiveness of B cell-targeted therapies. Janus kinase inhibitors (JAKi), which target downstream signaling of cytokine receptors, are potent rheumatic disease-modifying drugs. However, besides reducing inflammation, JAKi may impact the adaptive immune system. In this study, we examined the effects of JAKi on B-cell function using in vitro cultures and multiparameter flow cytometry. The results show a JAKi-mediated reduction in plasma cell differentiation, primarily by inhibition of memory B-cell stimulation and proliferation. JAKi exposure resulted in stalling R848, IL-2, and IL-21 stimulated B cells in an intermediate activated state with elevated naïve cells displaying increased expression of CXCR5, CD71, CD22, and CD20. In addition, the data demonstrate a moderate JAKi-mediated reduction of B cell TNF and IL-8 cytokine expression following stimulation. Importantly, the efficacy varied greatly between drugs; tofacitinib and upadacitinib (pan JAKi; JAK1i) exhibited the strongest impact, while baricitinib (JAK1/JAK2i) showed donor-dependent variation, and filgotinib (JAK1i) had no effect. All JAKi, except filgotinib, inhibited IL-2 or IL-21-induced STAT3 phosphorylation. Still, filgotinib demonstrated similar inhibition of phospho-STAT5 as other JAKi following IL-21. These findings underscore the therapeutic impact of JAKi through the modulation of B-cell functions.