Distinct interleukin-6 production in IPL and TAFRO subtypes of idiopathic multicentric Castleman disease

Idiopathic multicentric Castleman disease (iMCD) is a rare lymphoproliferative disorder characterized by systemic inflammation and lymphadenopathy. Two major clinical subtypes, idiopathic plasmacytic lymphadenopathy (iMCD-IPL) and iMCD with thrombocytopenia, anasarca, fever, renal dysfunction/reticulin fibrosis, and organomegaly (iMCD-TAFRO), have distinct pathophysiological mechanisms. While interleukin-6 (IL-6) is known to be elevated in iMCD, differences in the sources of IL-6 production between subtypes remain unclear. We examined the source of IL-6 production and its transcriptional regulation across iMCD subtypes using immunohistochemistry, in situ hybridization, and gene expression profiling. Immunohistochemistry and in situ hybridization revealed that plasma cells were the predominant IL-6-expressing cells in iMCD-IPL, whereas vascular endothelial cells expressed IL-6 in iMCD-TAFRO. Plasma cells exhibited stronger IL-6 protein expression in iMCD-IPL than in iMCD-TAFRO. Gene expression analysis revealed upregulation of XBP1, MZB1, DERL3, SSR4, FKBP11, FKBP2, PIM2, RABAC1, and SDF2L1 in iMCD-IPL, implicating endoplasmic reticulum stress and plasma cell differentiation in IL-6 dysregulation. Our findings suggest that XBP1-mediated IL-6 production may contribute to the pathogenesis of iMCD-IPL, potentially explaining its favorable responses to IL-6 blockade therapy. In contrast, IL-6 production in iMCD-TAFRO may be predominantly from vascular endothelial cells, suggesting that elevated serum IL-6 is a secondary phenomenon of the cytokine storm in this subtype. Future studies should clarify how proteomics and gene expression profiling could inform subtype-specific therapeutic strategies in iMCD.