Adhesion to fibronectin via α5β1 integrin supports expansion of the megakaryocyte lineage in primary myelofibrosis

Excessive accumulation of extracellular matrix (ECM) is a hallmark of bone marrow (BM) milieu in primary myelofibrosis (PMF). Because cells have the ability to adhere to the surrounding ECM through integrin receptors, we examined the hypothesis that an abnormal ECM-integrin receptor axis contributes to BM megakaryocytosis in JAK2(V617F+) PMF. Secretion of ECM protein fibronectin (FN) by BM stromal cells from PMF patients correlates with fibrosis and disease severity. Here, we show that Vav1-hJAK2(V617F) transgenic mice (JAK2(V617F+)) have high BM FN content associated with megakaryocytosis and fibrosis. Further, megakaryocytes from JAK2(V617F+) mice have increased cell surface expression of the alpha 5 subunit of the alpha 5 beta 1 integrin, the major FN receptor in megakaryocytes, and augmented adhesion to FN compared with wild-type controls. Reducing adhesion to FN by an inhibitory antibody to the alpha 5 subunit effectively reduces the percentage of CD41(+) JAK2(V617F+) megakaryocytes in vitro and in vivo. Corroborating our findings in mice, JAK2(V617F+) megakaryocytes from patients showed elevated expression of a5 subunit, and a neutralizing antibody to a5 subunit reduced adhesion to FN and megakaryocyte number derived from CD341 cells. Our findings reveal a previously unappreciated contribution of FN-alpha 5 beta 1 integrin to megakaryocytosis in JAK2(V617F+) PMF.