Pi3k/Akt/Mtor Pathway Activity Is Increased in Lymph Node Tissue from Idiopathic Multicentric Castleman Disease Patients with Tafro Syndrome

Castleman disease (CD) describes a group of heterogeneous diseases defined by shared lymph node histopathology, including atrophic or hyperplastic germinal centers, prominent follicular dendritic cells, hypervascularization, polyclonal lymphoproliferation, and/or polytypic plasmacytosis. Unicentric CD (UCD) involves a solitary enlarged lymph node that displays CD histopathology, and patients rarely experience systemic symptoms. In contrast, multicentric CD (MCD) involves multiple regions of enlarged lymph nodes, systemic inflammation, cytopenias, and vital organ dysfunction due to a cytokine storm often including interleukin-6. MCD is caused by uncontrolled infection with Kaposi sarcoma-associated/human herpesvirus-8 (HHV-8) in ~50% of cases. The etiology of the remaining HHV-8-negative MCD cases is idiopathic (iMCD). In iMCD patients, blockade of IL-6 signaling with siltuximab, the only FDA-approved iMCD treatment, induced responses in 34% of cases in the phase II registrational trial. The large proportion of non-responders suggest that alternative pathways are responsible for driving disease pathogenesis in some patients. For these individuals, identification of molecular and cellular abnormalities for therapeutic targeting is urgently needed, particularly for those with the most severe clinical presentations. In fact, a clinical subgroup of iMCD was recently described with a very severe presentation: thrombocytopenia, anasarca, myelofibrosis, renal dysfunction, and organomegaly (iMCD-TAFRO). We previously reported increased phosphoinositide 3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) signaling--a central pathway downstream of multiple cell surface receptors, implicated in both autoimmune and oncologic disorders--in a treatment refractory iMCD-TAFRO case that experienced an extended remission on treatment with an mTOR inhibitor. To extend these findings, herein we report immunohistochemistry for phosphorylated ribosomal protein S6 (phospho-S6), a marker of mTOR activation, in lymph node tissue from additional iMCD-TAFRO cases (n=10) and sentinel lymph nodes from breast cancer patients without evidence of metastasis (n=5). Anti-phospho-S6 (Ser235/236, Clone D57.2.2E) was used following standard protocols, and Aperio ImageScope and Image Analysis Toolkit software (color deconvolution v9 algorithm) were used to quantify pixel staining intensity in the germinal center, mantle zone, follicular and interfollicular regions. This analysis identified an increased number of pixels staining weak, medium, and strong for phospho-S6 in the interfollicular region of iMCD-TAFRO cases (p<0.005 for all comparisons) and an increased number of pixels staining weak for phospho-S6 in the germinal center (p<0.05) compared to control cases. Given that T cells are largely represented in the interfollicular region and mTOR signaling is critical to T cell proliferation, we hypothesized that the observed increase in phospho-S6 signal would occur in CD3⁺ cells. However, co-immunofluorescence assays for phospho-S6 and CD3 (Dako, A0452) across iMCD-TAFRO cases identified 0.08 ± 0.16% (mean ± standard deviation; n = 4 cases) of phospho-S6-positive cells as expressing CD3. In contrast, co-immunofluorescence for phospho-S6 and CD138 (Dako, Clone MI15) revealed 17.89 ± 11.26% of phospho-S6-positive cells as plasma cells. iMCD is considered an IL-6 driven disorder; however, anti-IL-6 therapy is effective in only a portion of cases. Alternative signaling pathways driving CD pathogenesis are poorly understood. This study provides the largest quantification to-date of aberrant PI3K/Akt/mTOR activity in iMCD-TAFRO, the first systematic study demonstrating increased mTOR activation in iMCD-TAFRO, and the first to identify a cell type, plasma cells. These findings are key to advancing our understanding of the pathological cell types and disrupted signaling pathways in iMCD.