Myeloid-specific blockade of Notch signaling alleviates murine pulmonary fibrosis through regulating monocyte-derived Ly6c lo MHCII hi alveolar macrophages recruitment and TGF-β secretion

Macrophages in lung, including resident alveolar macrophages (AMs) and interstitial macrophages (IMs), and monocyte-derived macrophages, play important roles in pulmonary fibrosis (PF), but mechanisms underlying their differential regulation remain unclear. Recombination signal-binding protein Jκ (RBP-J)-mediated Notch signaling regulates macrophage development and phenotype. Here, using bleomycin-induced fibrosis model combined with myeloid-specific RBP-J disruption (RBP-J ) mouse, we investigated the role of Notch signaling in macrophages during PF. Compared with the control, RBP-J mice exhibited alleviated lung fibrosis as manifested by reduced collagen deposition and inflammation, and decreased TGF-β production. FACS analysis suggested that decreased Ly6c MHCII AMs might make the major contribution to attenuated fibrogenesis in RBP-J mice, probably by reduced inflammatory factor release and enhanced matrix metalloproteinases expression. Using clodronate-mediated macrophage depletion in RBP-J mice, we demonstrated that embryonic-derived AMs play negligible role in lung fibrosis, which was further supported by adoptive transfer experiments. Moreover, on CCR2 knockout background, the effect of RBP-J deficiency on fibrogenesis was not elicited, suggesting that Notch regulated monocyte-derived AMs. Co-culture experiment showed that monocyte-derived AMs from RBP-J mice exhibit reduced myofibroblast activation due to decreased TGF-β secretion. In conclusion, monocyte-derived Ly6c MHCII AMs, which are regulated by RBP-J-mediated Notch signaling, play an essential role in lung fibrosis.