The role of CX₃CL1/CX₃CR1 in pulmonary angiogenesis and intravascular monocyte accumulation in rat experimental hepatopulmonary syndrome

Background & Aims: Hepatopulmonary syndrome (HPS), classically attributed to intrapulmonary vascular dilatation, occurs in 15-30% of cirrhotics and causes hypoxemia and increases mortality. In experimental HPS after common bile duct ligation (CBDL), monocytes adhere in the lung vasculature and produce vascular endothelial growth factor (VEGF)-A and angiogenesis ensues and contribute to abnormal gas exchange. However, the mechanisms for these events are unknown. The chemokine fractalkine (CX₃CL1) can directly mediate monocyte adhesion and activate VEGF-A and angiogenesis via its receptor CX ₃CR1 on monocytes and endothelium during inflammatory angiogenesis. We explored whether pulmonary CX₃CL1/CX₃CR1 alterations occur after CBDL and influence pulmonary angiogenesis and HPS. Methods: Pulmonary CX₃CL1/CX₃CR1 expression and localization, CX₃CL1 signaling pathway activation, monocyte accumulation, and development of angiogenesis and HPS were assessed in 2- and 4-week CBDL animals. The effects of a neutralizing antibody to CX₃CR1 (anti-CX ₃CR1 Ab) on HPS after CBDL were evaluated. Results: Circulating CX₃CL1 levels and lung expression of CX₃CL1 and CX ₃CR1 in intravascular monocytes and microvascular endothelium increased in 2- and 4-week CBDL animals as HPS developed. These events were accompanied by pulmonary angiogenesis, monocyte accumulation, activation of CX₃CL1 mediated signaling pathways (Akt, ERK) and increased VEGF-A expression and signaling. Anti-CX₃CR1 Ab treatment reduced monocyte accumulation, decreased lung angiogenesis and improved HPS. These events were accompanied by inhibition of CX₃CL1 signaling pathways and a reduction in VEGF-A expression and signaling. Conclusions: Circulating CX ₃CL1 levels and pulmonary CX₃CL1/CX₃CR1 expression and signaling increase after CBDL and contribute to pulmonary intravascular monocyte accumulation, angiogenesis and development of experimental HPS.