The involvement of PI3K–Akt signaling in the clinical and pathologic findings of iMCD-TAFRO and NOS subtypes.

Idiopathic multicentric Castleman disease is a rare lymphoproliferative disorder that is clinically classified into idiopathic plasmacytic lymphadenopathy (IPL); thrombocytopenia, anasarca, fever, reticulin fibrosis, and organomegaly (TAFRO); and not otherwise specified (NOS). Although each subtype shows varying degrees of hypervascularity, no statistical data on the degree of vascularization have been reported. Additionally, the mechanisms underlying vascularization in each clinical subtype are poorly understood. Here, we aimed to clarify these mechanisms by evaluating the histopathological characteristics of each clinical subtype across 37 patients and performing a whole-transcriptome analysis focusing on angiogenesis-related gene expression. Histologically, TAFRO and NOS exhibited a significantly higher degree of vascularization than IPL (IPL vs TAFRO: p < 0.001, IPL vs NOS: p = 0.002). In addition, the germinal centers (GCs) were significantly more atrophic in TAFRO than in IPL. In TAFRO and NOS, "whirlpool vessels" in GCs were seen in most cases (TAFRO: 9/9, 100%, NOS: 6/8, 75%), but not in IPL (IPL vs TAFRO: p < 0.001, IPL vs NOS: p = 0.007). Likewise, immunostaining for Ets-related gene revealed higher levels in endothelial cells of GCs in TAFRO than in IPL (p = 0.014), and TAFRO and NOS were associated with a significantly higher number of endothelial cells in interfollicular areas compared to that in IPL (TAFRO vs IPL: p < 0.001, NOS vs IPL: p = 0.002). Gene expression analysis revealed that the PI3K-Akt signaling pathway was significantly enriched in the TAFRO and NOS (TAFRO/NOS) groups. This pathway, which may be activated by vascular endothelial growth factor A and some integrins, is known to affect angiogenesis by increasing vascular permeability, which may explain the clinical manifestations of anasarca and/or fluid retention in TAFRO/NOS. These results suggest that the PI3K-Akt pathway plays an important role in the pathogenesis of TAFRO/NOS.