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Monocyte-derived fibrocytes recently garnered attention because the novel pathogenesis of myelofibrosis (MF), and suppression of fibrocyte differentiation by serum amyloid P remarkably improved MF. We previously revealed that human fibrocytes highly expressed signaling lymphocytic activation molecule F7 (SLAMF7) compared with macrophages and that SLAMF7high monocytes in the peripheral blood (PB) of MF patients were significantly elevated relative to those in healthy controls (HCs). In this study, we evaluated SLAMF7high monocyte percentage in the PB of HCs, myeloproliferative neoplasm (MPN) patients with MF, and MPN patients without MF by using a cross-sectional approach. We found that MPN patients with MF who harbored JAK2V617F had a significantly elevated SLAMF7high monocyte percentage, which correlated positively with the JAK2V617F allele burden. In addition, the serum concentration of interleukin-1ra (IL-1ra) was significantly correlated with the SLAMF7high monocyte percentage and JAK2V617F allele burden. These findings suggest that both SLAMF7high monocytes and IL-1ra could be useful noninvasive markers of MF onset. Furthermore, the JAK2V617F allele burden of SLAMF7high monocytes was significantly higher than that of SLAMF7low monocytes and could be a potential target of elotuzumab (Elo), an anti-SLAMF7 antibody used for treating multiple myeloma. Elo independently inhibited differentiation of fibrocytes derived not only from HCs but also from MF patients in vitro. Elo also ameliorated MF and splenomegaly induced by romiplostim administration in humanized NOG mice. In conclusion, an increase of SLAMF7high monocytes with higher JAK2V617F allele burden was associated with the onset of MF in MPN patients harboring JAK2V617F, and Elo could be a therapeutic agent for MPN patients with MF who harbor JAK2V617F.

Myelofibrosis (MF) may be caused by various pathogenic mechanisms such as elevation in circulating cytokine levels, cellular interactions and genetic mutations. However, the underlying mechanism of MF still remains unknown. Recent studies have revealed that fibrocytes, the spindle-shaped fibroblast-like hematopoietic cells, and the thrombopoietin (TPO)/myeloproliferative leukemia protein (MPL; TPO receptor) signaling pathway play a certain role in the development of MF. In the present study, we aimed to investigate the relationship between fibrocytes and MPL activation. We showed that TPO or a TPO receptor agonist directly induces fibrocyte differentiation using murine fibrocyte cell lines and a murine MF model. Conversely, elimination of macrophages expressing MPL by clodronate liposomes reversed the MF phenotype of the murine model, suggesting that fibrocyte differentiation induced by MPL activation contributes to the progression of MF. Furthermore, we revealed that SLAMF7 MPL monocytes in human peripheral blood mononuclear cells were possible fibrocyte precursors and that these cells increased in number in MF patients not treated with ruxolitinib. Our findings confirmed a link between fibrocytes and the TPO/MPL signaling pathway, which could result in a greater understanding of the pathogenesis of MF and lead to the development of novel therapeutic interventions.

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Acute graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in allogeneic hematopoietic cell transplantation (allo-HCT). Majority of the current immunosuppressive strategies targeting donor T cells to prevent or treat acute GVHD are only partially effective, and often require escalated immunosuppressive therapy. Recent studies have revealed that activation of antigen-presenting cells in the proinflammatory milieu is important for the priming and promotion of GVHD. This activation is mediated by innate immune signaling pathways, which therefore potentially represent new targets in addressing GVHD. Using gene expression analysis of peripheral monocytes from patients' post-allo-HCT, we detected an upregulation of TGF-β-activated kinase 1 (TAK1), a key regulator of the toll-like receptor signaling pathway. 5Z-7-oxozeaenol, a selective inhibitor of TAK1, reduced proinflammatory cytokine production by activated monocytes under lipopolysaccharide stimulation and T cell proliferation in allogeneic-mixed leukocyte reactions with monocyte-derived dendritic cells. In an experimental mouse model of GVHD, 5Z-7-oxozeaenol administration after allo-HCT ameliorated GVHD severity and mortality, with significant reduction in serum TNFα, IL-1β, and IL-12 levels. Our findings suggest that altering the activation status of innate immune cells by TAK1 inhibition may be a novel therapeutic approach for acute GVHD.