Hypoxia Induces Adaptive Lymphangiogenesis via Cd74 and Vegfr3 to Modulate Pulmonary Hypertension

Pulmonary arterial hypertension (PAH) is characterized by excessive remodeling of the proximal and distal arterioles, driven by endothelial cell apoptosis and uncontrolled mural cell proliferation. Increasing evidence suggests an important role of inflammation in PAH, but 1 crucial part of the immune system, the pulmonary lymphatics, has been largely overlooked. Patients with idiopathic PAH often develop abnormal tertiary lymphoid structures adjacent to remodeled arteries, yet the role of lymphatic vessels in PAH pathogenesis and vascular remodeling remains unclear. Using a mouse model to specifically label lymphatic endothelial cells (Prox1-CreERT2::Rosa26-LSL-tdT), we found that pulmonary lymphatic vessels proliferated and dilated in response to hypoxia. We further showed that Vegfr3 inhibition using MAZ51 reduced hypoxia induced lymphangiogenesis and was associated with severe pulmonary hypertension. Additionally, lymphatic endothelial cell-specific deletion of Vegfr3 (Prox1-CreERT2::Vegfr3 fl/fl ) prevents adaptive lymphangiogenesis and exacerbates pulmonary hypertension, resulting in right ventricular hypertrophy. Comparative single-cell RNA sequencing analysis of human idiopathic PAH and hypoxia-induced pulmonary hypertension mouse lungs revealed upregulation of CD74/Cd74 in lymphatic endothelial cell clusters, and in humans this was associated with enhanced expression of FLT4/VEGFR3 and downstream mediators in the MEK/ERK signaling pathway. Consistently, increased CD74 expression was observed in lymphatic vessels in human idiopathic PAH lung sections, and CD74 overexpression in human lymphatic cells was associated with impaired barrier permeability and coexpression of VEGFR3. Together, these findings provide novel insights into the role of lymphatic Cd74/CD74 activation in pulmonary hypertension development and suggest that therapies aimed at augmenting lymphatic function may improve outcomes in patients with PAH.