PS1435 SLAMF7 HIGH CD16 NEGATIVE MONOCYTES INCREASE IN PERIPHERAL BLOOD OF PATIENTS WITH MYELOFIBROSIS IN CORRELATION WITH JAK2V617F MUTATION
Authors: S. Kato, T. Maekawa, K. Ogata, Y. Osawa, S. Nagao, K. Takano, Y. Okada, N. Tachi, M. Teramoto, T. Kawamura, T. Horiuchi, A. Kobayashi, S. Kobayashi, K. Sato, M. Hashimoto, S. Suzu, K. Usuki, S. Morishita, M. Araki, N. Komatsu, F. Kimura
Journal: HemaSphere (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, SLAMF7 high CD16 − monocytes were significantly increased than that of healthy controls (HCs). Thus, we hypothesize that SLAMF7 high 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 SLAMF7 high 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 SLAMF7 high 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 SLAMF7 high/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 SLAMF7 high CD16 − monocyte percentage significantly increased in MPN and non‐MPN patients compared with HCs and MPN patients without MF ( P &lt; 0.001 and P &lt; 0.01, respectively). JAK2V617F MPN patients with MF exhibited a significantly higher SLAMF7 high CD16 − monocyte percentage than those without MF ( P &lt; 0.001). We noted a similar trend in patients not harboring the JAK2V617F mutation ( P = 0.05). The ROC analysis revealed that &gt;25% of SLAMF7 high 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 SLAMF7 high CD16 − monocyte percentage in the PB ( P = 0.0003). In CD16 – monocytes of 2 patients, the JAK2V617F allele burden of the SLAMF7 high fraction was high (87.96% and 94.75%) compared with the SLAMF7 low fraction (49.09% and 45.64%, respectively). Furthermore, the culture assay of isolated SLAMF7 high/low monocytes revealed that the frequency of fibrocytes derived from SLAMF7 high monocytes markedly increased compared with SLAMF7 low monocytes. Summary/Conclusion: SLAMF7 high 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.