PS1435 SLAMF7 HIGH CD16 NEGATIVE MONOCYTES INCREASE IN PERIPHERAL BLOOD OF PATIENTS WITH MYELOFIBROSIS IN CORRELATION WITH JAK2V617F MUTATION
Authors: Kato S. , Maekawa T. , Ogata K. , Osawa Y. , Nagao S. , Takano K. , Okada Y. , Tachi N. , Teramoto M. , Kawamura T. , Horiuchi T. , Kobayashi A. , Kobayashi S. , Sato K. , Hashimoto M. , Suzu S. , Usuki K. , Morishita S. , Araki M. , Komatsu N. , Kimura F. 
Published: June 2019

Background:Monocyte‐derived fibrocytes are vital in the pathogenesis of primary myelofibrosis (MF) (J Exp Med 2016;213:1723). We established the direct induction of fibrocyte differentiation using TPO receptor activation in a mouse model (Leukemia 2017;31:2709). Moreover, we demonstrated that elotuzumab (Elo; anti‐SLAMF7 antibody) inhibited human fibrocyte differentiation in vitro and relieved MF in the mouse model, determining Elo as a potential therapeutic agent (manuscript in preparation). In the peripheral blood (PB) of patients with MF, SLAMF7high CD16− monocytes were significantly increased than that of healthy controls (HCs). Thus, we hypothesize that SLAMF7high CD16− monocytes reflected fibrocyte activation and were the target of Elo.Aims:Through this cross‐sectional study, we aimed to identify suitable candidates for a clinical trial of Elo by evaluating SLAMF7high CD16− monocyte percentage in the PB of HCs, myeloproliferative neoplasm (MPN) patients with or without MF, and non‐MPN patients with MF.Methods:We enrolled 6 HCs, 12 non‐MPN patients with MF, and 44 patients with MPN (18 without and 26 with MF) and collected their blood samples. All patients with MPN underwent a bone marrow biopsy in 2016–2018 and were diagnosed per the 2016 WHO classification and diagnostic criteria. Non‐MPN patients underwent a bone marrow biopsy in 2015–2018, and MF was established over MF‐1 per the European Consensus Criteria. We evaluated the SLAMF7high CD16– monocyte percentage in PB monocytes of HCs and patients using flow cytometry. All PB samples were tested for JAK2V617F, CALR, or MPL genetic mutations, and the allele burden of the JAK2V617F mutation was measured in PB samples and cultured fibrocytes. Furthermore, we collected SLAMF7high/low CD16– monocytes separately of 2 patients harboring the JAK2V617F mutation and assessed the JAK2V617F allele burden of that fraction and the ratio of fibrocytes in culture.Results:The SLAMF7high CD16− monocyte percentage significantly increased in MPN and non‐MPN patients compared with HCs and MPN patients without MF (P < 0.001 and P < 0.01, respectively). JAK2V617F MPN patients with MF exhibited a significantly higher SLAMF7high CD16− monocyte percentage than those without MF (P < 0.001). We noted a similar trend in patients not harboring the JAK2V617F mutation (P = 0.05). The ROC analysis revealed that >25% of SLAMF7high CD16− monocytes in PB estimates MF in patients with MPN harboring the JAK2V617F mutation (sensitivity: 82.4%, specificity: 87.5%). Moreover, the JAK2V617F allele burden of fibrocytes markedly correlated with the SLAMF7high CD16− monocyte percentage in the PB (P = 0.0003). In CD16– monocytes of 2 patients, the JAK2V617F allele burden of the SLAMF7high fraction was high (87.96% and 94.75%) compared with the SLAMF7low fraction (49.09% and 45.64%, respectively). Furthermore, the culture assay of isolated SLAMF7high/low monocytes revealed that the frequency of fibrocytes derived from SLAMF7high monocytes markedly increased compared with SLAMF7low monocytes.Summary/Conclusion:SLAMF7high CD16– monocytes are neoplastic cells with homozygous JAK2V617F, and the levels closely correlate with MF and a neoplastic clone of fibrocytes harboring the JAK2V617F mutation; this fraction indicates fibrocyte activation and denotes a noninvasive marker of MF progression and a potential target for Elo treatment.