SLAMF7high CD16negative Monocytes Increase in Peripheral Blood of Patients with Myelofibrosis in Correlation with JAK2V617F Mutation
Authors: Maekawa Takaaki , Kato Shoichiro , Okada Yosuke , Tachi Noriaki , Teramoto Masahiro , Kawamura Toshikuni , Osawa Yukiko , Kobayashi Shinichi , Sato Ken , Hashimoto Michihiro , Suzu Shinya , Usuki Kensuke , Morishita Soji , Araki Marito , Komatsu Norio , Kimura Fumihiko 
Published: October 2019

Abstract
Introduction
Myelofibrosis (MF) occurrence can be attributed to various pathogenic mechanisms. A recent study showed that the neoplastic clone of fibrocytes (spindle shaped, fibroblast-like blood cells derived from monocyte lineage) was essential in primary MF pathogenesis; moreover, serum amyloid P (PRM-151), which suppresses fibrocyte differentiation, markedly improved survival and MF in a murine xenograft model (J Exp Med 2016; 213: 1723). Using a romiplostim-induced murine MF model, direct induction of fibrocyte differentiation using TPO receptor activation, leading to MF progression, was demonstrated (Leukemia 2017; 31: 2709).
Using DNA microarray analysis, we previously showed that compared with macrophages, human fibrocytes highly express SLAMF7 (J Immunol 2015; 195: 4341). Elotuzumab (Elo) is an anti-SLAMF7 antibody recently developed for treating multiple myeloma. We showed that Elo inhibits human fibrocyte differentiation in vitro and relieves MF using the mouse model in vivo, identifying Elo as a potential therapeutic agent (manuscript in preparation). Additionally, monocytes with high SLAMF7 expression and negative CD16 expression were significantly elevated in the peripheral blood (PB) of MF patients compared with that of healthy controls (HCs). We hypothesized that this monocyte subset reflects fibrocyte activation and is the target of Elo.
For the clinical study of Elo, we evaluated SLAMF7high CD16− monocyte percentage in PB of HCs, myeloproliferative neoplasm (MPN) patients with or without MF, and non-MPN patients with MF in a cross-sectional manner.
Methods
Six HCs, 12 non-MPN patients with MF, and 44 MPN patients (18 without and 26 with MF) were enrolled; their blood samples were collected. All MPN patients underwent bone marrow biopsy in 2016-2018 and were diagnosed by the 2016 WHO classification and diagnostic criteria. Non-MPN patients underwent bone marrow biopsy in 2015-2018, and MF was confirmed over MF-1 according to the European Consensus Criteria.
SLAMF7high CD16- monocyte percentage in PB monocytes of HCs and patients was calculated using flow cytometry. Further, all patients were tested for JAK2V617F, CALR, or MPL genetic mutations. The allele burden of JAK2V617F was measured. Moreover, fibrocytes were differentiated from PB mononuclear cells of patients with genetic mutation and were verified for genetic mutation in their fibrocytes.
Results
Patients were classified in three groups (MPN without MF, MPN with MF, and non-MPN with MF). The median percentage of SLAMF7high CD16− monocytes in PB of HCs, MPN patients without MF, MPN patients with MF, and non-MPN patients with MF were 1.66%, 2.48%, 27.4%, and 19.8%, respectively. Compared with HCs and MPN patients without MF, SLAMF7high CD16− monocyte percentage of MPN and non-MPN patients with MF significantly increased (p < 0.05 and p < 0.01, respectively) (Figure A).
MPN patients with MF harboring JAK2V617F (n = 17) had a significantly higher SLAMF7high CD16− monocyte percentage than those without MF harboring JAK2V617F (n = 8) (median: 43.70% vs. 7.00%, p < 0.001). While a similar trend was observed in patients not harboring JAK2V617F (median: 1.15% vs. 3.26%, p = 0.05), the contrast between patients with and without MF patients was significant for those with JAK2V617F (Figure B). More than 25% of SLAMF7high CD16− monocytes in PB indicates MF in MPN patients harboring JAK2V617F (sensitivity: 82.4%, specificity: 87.5%).
In addition, the JAK2V617F allele burden of fibrocytes was strongly correlated with the SLAMF7high CD16− monocyte percentage in PB (p = 0.0003) (Figure C). In all patients, fibrocytes and PB harbored the same genetic mutation.
Conclusion
The percentage of SLAMF7high CD16− monocytes in PB elevated in MF patients regardless of MPN, whereas it was not elevated in MPN patients without MF. In MPN patients, those with MF harboring JAK2V617F presented a high SLAMF7high CD16-monocyte percentage in PB, positively correlating with the JAK2V617F allele burden of fibrocytes. In conclusion, SLAMF7high CD16- monocyte levels are closely related with MF and neoplastic clones of fibrocytes harboring JAK2V617F, indicating fibrocyte activation and representing a non-invasive marker of MF progression and a potential target for Elo treatment.
Figure. Figure.

Disclosures
Usuki: Sumitomo Dainippon Pharma: Research Funding, Speakers Bureau; Celgene Corporation: Research Funding, Speakers Bureau; Daiichi Sankyo: Research Funding; Boehringer-Ingelheim Japan: Research Funding; Sanofi K.K.: Research Funding; Shire Japan: Research Funding; SymBio Pharmaceuticals Limited.: Research Funding; Chugai Pharmaceutical: Speakers Bureau; Takeda Pharmaceutical: Speakers Bureau; Ono Pharmaceutical: Speakers Bureau; Novartis: Speakers Bureau; Janssen Pharmaceutical K.K: Research Funding; Pfizer Japan: Research Funding, Speakers Bureau; GlaxoSmithKline K.K.: Research Funding; Kyowa Hakko Kirin Co., Ltd.: Research Funding; Otsuka Pharmaceutical Co., Ltd.: Research Funding; Astellas Pharma Inc.: Research Funding; Nippon Shinyaku: Speakers Bureau; Mochida Pharmaceutical: Speakers Bureau; MSD K.K.: Speakers Bureau.