Thrombocytopenia in myelofibrosis is characterized by inflammatory megakaryocytes with reduced G6B expression

The megakaryocytic (MK)-specific immunoreceptor G6b-B plays an essential role in MK development. Because germ line loss-of-function mutations of G6b-B in humans and its deletion in mouse models lead to thrombocytopenia and a myelofibrosis-like clinical phenotype (MF-MPIG6B), we explored the role of G6b-B in patients with myelofibrosis (MF) due to a myeloproliferative neoplasm (MPN) with thrombocytopenia (MPN-MF-T). We demonstrated that MKs generated from mononuclear cells (MNCs) from a patient with MF-MPIG6B as well as patients with MPN-MF-T failed to express GATA binding protein 1 and G6B and possessed a protein pattern expression characteristic of MKs primed for inflammation rather than platelet production. MNCs from patients with MPN-MF-T also generated fewer MK-biased hematopoietic stem cells and greater numbers of small cytoplasmic immature MKs (CD41+CD42-G6B-) as compared with MNCs from patients with nonthrombocytopenic MPN-MF (MPN-MF-NT). Plasma levels of transforming growth factor β1 (TGFβ1) and chitinase-3-like protein (CHI3L1) also known as YKL-40, which were shown to arrest normal MK maturation, were elevated in the patients with MF-MPIG6B. Although TGFβ1 plasma levels were similarly elevated in patients with MPN-MF-T and MPN-MF-NT, tumor necrosis factor α (TNFα) and YKL-40 levels were upregulated to a greater extent in patients with MPN-MF-T than those with MPN-MF-NT. Moreover, we identified a reciprocal positive regulatory loop involving TGFβ1 and YKL-40 in MF MKs. These findings indicate that impaired MK maturation, and reduced G6B expression lead to the predominance of proinflammatory MKs, which produce factors that further arrest MK development in patients with MF-MPIG6B and MPN-MF-T patients. This trial was registered at www.clinicaltrials.gov as #NCT03895112.