Adhesion to fibronectin via a5b1 integrin supports expansion of the megakaryocyte lineage in primary myelofibrosis

Abstract

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 JAK2V617F1 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-hJAK2V617F transgenic mice (JAK2V617F1) have high BM FN content associated with megakaryocytosis and fibrosis. Further, megakaryocytes from JAK2V617F1 mice have increased cell surface expression of the a5 subunit of the a5b1 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 a5 subunit effectively reduces the percentage of CD411 JAK2V617F1 megakaryocytes in vitro and in vivo. Corroborating our findings in mice, JAK2V617F1 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-a5b1 integrin to megakaryocytosis in JAK2V617F1 PMF.