A Role for Angiogenesis in Hemophilic Synovitis.

Hemarthrosis, or bleeding into a joint, is the most common manifestation of hemophilia. Recurrent hemarthroses and synovial proliferation leads to the formation of a thickened synovial pannus, or synovitis, followed by destructive infiltration into the adjacent cartilage and bone, a condition known as hemophilic arthropathy. While the clinical sequence of events in hemophilic joint disease is understood, the pathogenesis of synovitis has not yet been elucidated. As such, there is scant objective evidence for the selective application of strategies like prophylaxis or isotopic synovectomy (IS) to prevent imminent joint deterioration. In other fibroproliferative disorders such as rheumatoid arthritis, osteoarthritis, psoriatic arthritis, and tumor growth, angiogenesis and the concomitant recruitment of bone marrow-derived (BMD) progenitors has been implicated in disease progression. Here, for the first time, we report direct evidence for the involvement of angiogenesis in the development and sustenance of hemophilic synovitis. Histological sections of synovial tissue from patients with hemophilic synovitis and arthropathy were highly vascularized and contained a high number of inflammatory cells, predominantly CD68+ macrophages, which co-localized with secreted pro-angiogenic factors including vascular endothelial growth factor (VEGF), basic fibroblastic growth factor (bFGF), matrix metalloproteinase-9 (MMP-9), tumor necrosis factor-alpha (TNF-a) and cyclooxygenase-2 (COX-2). Hemophiliacs with joint disease exhibited a five-fold increase in 12 circulating angiogenesis-related cytokines compared to individuals with a bleeding disorder without joint disease and to healthy controls, as profiled by a protein array. Likewise, plasma VEGF and MMP-9, critical mediators of BMD progenitor mobilization and important pro-angiogenic factors, were significantly elevated in hemophiliacs with joint disease compared to the non-joint disease groups (Table 1). Accordingly, hemophiliacs with joint disease (n=9) exhibited an increased percentage of circulating CD34+ progenitor cells as compared to control groups (n=6). Finally, in a functional in vitro assay, plasma from hemophiliacs with joint disease showed a markedly enhanced capacity to elicit an angiogenic response from human umbilical vein endothelial cells (HUVECs) as evidenced by the induction of morphogenetic changes resembling capillary-like structure tube formation and cord-like processes connecting cellular nodes. These data collectively suggest that angiogenesis may play an important role in the development of synovial proliferation and synovitis in hemophilia. Indexing circulating angiogenic growth factors and CD34+ precursors may provide effective means of detecting early synovial proliferation and identifying candidates for early intervention with prophylaxis or IS. Ultimately, anti-angiogenic agents may provide a novel preventative treatment strategy for hemophilic arthropathy.

Table 1. Plasma concentrations of pro-angiogenic factors.