RAMP1 signaling in immune cells regulates inflammation-associated lymphangiogenesis

Calcitonin gene-related peptide (CGRP) regulates inflammation via signaling through receptor activity-modifying protein (RAMP) 1. Here, we investigated the role of RAMP1 signaling in growth of lymphatic vessels during inflammation. Lymphangiogenesis in the diaphragm of RAMP1-deficient (^-/-) mice or their wild-type (WT) counterparts was induced by repeated intraperitoneal injection of lipopolysaccharide (LPS). Compared with WT mice, LPS-induced lymphangiogenesis in RAMP1^-/- mice was suppressed. This was accompanied by the reduced expression of vascular endothelial growth factor (VEGF)-C and VEGF-D. The number of CD4⁺ cells in diaphragm tissue from WT mice was greater than RAMP1^-/- mice. Removing CD4⁺ cells attenuated lymphangiogenesis and expression of VEGF-C and VEGF-D. CD4⁺ cells isolated from RAMP1^-/- mice exhibited reduced expression of VEGF-C and VEGF-D. The number of CD11b⁺ cells from RAMP1^-/- mice was higher than WT mice and was associated with the upregulated expression of genes related to pro-inflammatory macrophage phenotype and downregulation of reparative macrophage phenotype-related expression. When fluorescein isothiocyanate (FITC)-dextran was injected into the peritoneal cavity, the amount of residual FITC-dextran in WT mice was lower than that in RAMP1^-/- mice. The present results suggest that RAMP1 signaling in immune cells plays a critical role in inflammation-related lymphangiogenesis; therefore, it represents a novel target for controlling lymphangiogenesis.