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. Since germline loss-of-function mutations of G6b-B in man and its deletion in mouse models leads to thrombocytopenia and a myelofibrosis-like clinical phenotype (MF-MPIG6B), we explored the role of G6b-B in patients with 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 GATA1 and G6B and possessed a protein pattern expression characteristic of MKs primed for inflammation rather than platelet production. MNCs from MPN-MF-T patients also generated fewer MK biased hematopoietic stem cells (HSCs) and greater numbers of small cytoplasmic immature MKs (CD41+CD42-G6B-) as compared to MNCs from non-thrombocytopenic MPN-MF patients (MPN-MF-NT). Plasma levels of TGFβ1 and YKL-40 which were shown to arrest normal MK maturation were elevated in the MF-MPIG6B patient. Although TGFβ1 plasma levels were similarly elevated in MPN-MF-T and MPN-MF-NT patients, TNFα and YKL-40 levels were upregulated to a greater extent in MPN-MF-T than MPN-MF-NT patients. 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 pro-inflammatory MKs which produce factors that further arrest MK development in MF-MPIG6B and MPN-MF-T patients. NCT03895112.