A Paracrine Circuit Regulates Vascular Endothelial Growth Factor Production By Bone Marrow Plasma Cells in Patients with POEMS Syndrome

Background: POEMS syndrome is a rare paraneoplastic disorder, characterized by polyneuropathy, organomegaly, osteosclerosis and extravascular volume overload, due to an underlying plasma cell dyscrasia. Vascular endothelial growth factor (VEGF), a potent angiogenic cytokine that can increase vascular permeability, plays a critical role in its pathogenesis. Despite extensive VEGF studies concerning its role in disease diagnosis, monitoring and response assessment, little is known about its cellular source and mechanisms governing the production of VEGF in POEMS patients.

Methods and Materials: Patients with POEMS syndrome, 62 newly diagnosed and 46 of them after lenalidomide-dexamethasone (LenDex) treatment, admitted to Peking Union Medical College Hospital between February 2014 and April 2015, were included in the current study. Clinical information, serum and bone marrow samples were collected with the Institutional approval. Serum levels of VEGF were measured by ELISA. VEGF levels in bone marrow plasma cells were quantified via real-time quantitative PCR and multiparameter flow cytometry, respectively. In addition, the phenotype, clonality and intracellular interleukin-6 (IL-6) expression of bone marrow plasma cells were also analyzed by flow cytometry. Furthermore, immunohistochemical staining was performed to study the plasma cell distribution, clonality and expression of VEGF, IL-6 and hypoxia-inducible factor-1α (HIF-1α) in bone marrow biopsies. Statistical analyses were conducted using SPSS 22, and a p < 0.05 was considered as significant.

Results: Serum levels of VEGF were dramatically elevated in patients with newly diagnosed POEMS syndrome (median 5958 pg/mL), significantly higher than both disease and healthy controls (p < 0.001), and can be used as a diagnostic marker (area under curve 0.988, p < 0.001). A cut-off value of 2000 pg/mL had a specificity of 97.7% with a sensitivity of 91.9% in support of the diagnosis. After treatment, VEGF levels decreased gradually (6-cycle after LenDex, median 1184 pg/mL, p < 0.001; 12-cycle after LenDex, median 832 pg/mL, p < 0.001). Bone marrow plasma cells showed remarkable VEGF expression, validated in both mRNA and protein levels, which also decreased in response to LenDex therapy. More importantly, a statistically linear correlation was observed between serum and bone marrow plasma cell VEGF levels (newly diagnosed patients, rho = 0.33, p = 0.01; post-therapeutic patients, rho = 0.53, p < 0.001), supporting that bone marrow plasma cells were the source of circulating VEGF. Intriguingly, immunophenotying revealed that bone marrow plasma cells were polyclonal in most newly diagnosed cases. Monoclonal population, co-existing with polyclonal plasma cells, was observed only in 11 patients (18%). Further analyses showed that the relative amounts of these two populations were similar (41% vs. 59%) and they also had comparable intracellular VEGF expression (mean fluorescent intensity, 2009 vs. 2367, p = 0.594). However, monoclonal plasma cells had significantly higher intracellular IL-6 expression (mean fluorescent intensity, 1635 vs. 865, p = 0.006). In 46 newly diagnosed cases with bone marrow biopsies available, immunohistochemical staining was performed, and plasma cells were typically distributed in a scattered manner, with focal aggregates observed in 21 cases (46%). Intracellular κ and λ light chain staining demonstrated that the background scattered plasma cells were polyclonal, while both monoclonal and polyclonal fractions were observed in the focal area, which showed similar nuclear and intracellular staining of HIF-1α and VEGF, respectively. In contrast, IL-6 was mainly expressed in λ-restricted plasma cells.

Conclusion: Bone marrow plasma cells are the potential source of VEGF production in patients with POEMS syndrome. Monoclonal plasma cells, aggregates focally in the bone marrow, may secret IL-6 to stimulate polyclonal plasma cells proliferation, and consequent VEGF production in a paracrine circuit.