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Type B insulin resistance syndrome (TBIR) is a rare autoimmune disease characterised by autoantibodies targeting insulin receptors. TBIR is often complicated by systemic lupus erythematosus (SLE). We describe the case of a 59-year-old Japanese man with TBIR complicated with lupus nephritis (LN), who presented with nephrotic syndrome and severe hypoglycaemia. Treatment with prednisolone (PSL), mycophenolate mofetil (MMF), and tacrolimus (TAC) resulted in improved SLE activity and glucose intolerance with the reduction of anti-insulin receptor autoantibodies. To the best of our knowledge, this is the first reported case of TBIR complicated with LN that was successfully treated using multitarget therapy with PSL, MMF, and TAC., (© Japan College of Rheumatology 2021. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Background: Innate immune cells play a crucial role in the pathophysiology of rheumatoid arthritis (RA) via release of cytokines. Small-molecule inhibitors of Janus kinases (JAKi) are clinically efficacious in patients with RA. However, the isoform-specific action of each JAKi is difficult to assess, since JAKs form heterodimeric complexes with cytokine receptors. We assessed the effects of several JAKi on GM-CSF-primed human innate immune cells., Results: Treatment with JAKi (tofacitinib, baricitinib, upadacitinib) prevented GM-CSF-induced JAK2/STAT5 phosphorylation at higher concentrations (400 nM) in THP-1 cells. Whereas compared with baricitinib or upadacitinib, the inhibitory effects of tofacitinib on the GM-CSF-induced JAK2/STAT5 phosphorylation were weak at lower concentrations (≤ 100 nM). All JAKi inhibited GM-CSF-induced IL-1β production by human neutrophils. However, the inhibitory effects of baricitinib on IL-1β production were larger compared to those of tofacitinib or upadacitinib at lower concentrations (≤ 100 nM). Similarly, all JAKi inhibited GM-CSF-induced caspase-1(p20) production by human neutrophils., Conclusion: We conclude that incubation with JAKi prevents GM-CSF-mediated JAK2/STAT5 activation in human innate immune cells. Although baricitinib and upadacitinib almost completely blocked GM-CSF-mediated JAK2/STAT5 signaling, the inhibitory effects of tofacitinib were weaker at lower concentrations suggesting that variation exists among these JAKi in the inhibition of JAK2 signaling pathways.