RBP-J regulates homeostasis and function of circulating Ly6C(lo) monocytes.

Notch-RBP-J signaling plays an essential role in the maintenance of myeloid homeostasis. However, its role in monocyte cell fate decisions is not fully understood. Here, we showed that conditional deletion of transcription factor RBP-J in myeloid cells resulted in marked accumulation of blood Ly6Clo monocytes that highly expressed chemokine receptor CCR2. Bone marrow transplantation and parabiosis experiments revealed a cell-intrinsic requirement of RBP-J for controlling blood Ly6CloCCR2hi monocytes. RBP-J-deficient Ly6Clo monocytes exhibited enhanced capacity competing with wildtype counterparts in blood circulation. In accordance with alterations of circulating monocytes, RBP-J deficiency led to markedly increased population of lung tissues with Ly6Clo monocytes and CD16.2+ interstitial macrophages. Furthermore, RBP-J deficiency-associated phenotypes could be genetically corrected by further deleting Ccr2 in myeloid cells. These results demonstrate that RBP-J functions as a crucial regulator of blood Ly6Clo monocytes and thus derived lung-resident myeloid populations, at least in part through regulation of CCR2.

Kou T ，Kang L ，Zhang B ，Li J ，Zhao B ，Zeng W ，Hu X ... -  《eLife》

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-JcKO ) mouse, we investigated the role of Notch signaling in macrophages during PF. Compared with the control, RBP-JcKO mice exhibited alleviated lung fibrosis as manifested by reduced collagen deposition and inflammation, and decreased TGF-β production. FACS analysis suggested that decreased Ly6clo MHCIIhi AMs might make the major contribution to attenuated fibrogenesis in RBP-JcKO mice, probably by reduced inflammatory factor release and enhanced matrix metalloproteinases expression. Using clodronate-mediated macrophage depletion in RBP-JckO 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-JcKO mice exhibit reduced myofibroblast activation due to decreased TGF-β secretion. In conclusion, monocyte-derived Ly6clo MHCIIhi AMs, which are regulated by RBP-J-mediated Notch signaling, play an essential role in lung fibrosis.

Zhang N ，Yang K ，Bai J ，Yi J ，Gao C ，Zhao J ，Liang S ，Wei T ，Feng L ，Song L ，Han H ，Qin H ... -  《-》

CCR2 Deficiency Impairs Ly6C(lo) and Ly6C(hi) Monocyte Responses in Orientia tsutsugamushi Infection.

Orientia (O.) tsutsugamushi, the causative agent of scrub typhus, is a neglected, obligate intracellular bacterium that has a prominent tropism for monocytes and macrophages. Complications often involve the lung, where interstitial pneumonia is a typical finding. The severity of scrub typhus in humans has been linked to altered plasma concentrations of chemokines which are known to act as chemoattractants for myeloid cells. The trafficking and function of monocyte responses is critically regulated by interaction of the CC chemokine ligand 2 (CCL2) and its CC chemokine receptor CCR2. In a self-healing mouse model of intradermal infection with the human-pathogenic Karp strain of O. tsutsugamushi, we investigated the role of CCR2 on bacterial dissemination, development of symptoms, lung histology and monocyte subsets in blood and lungs. CCR2-deficient mice showed a delayed onset of disease and resolution of symptoms, higher concentrations and impaired clearance of bacteria in the lung and the liver, accompanied by a slow infiltration of interstitial macrophages into the lungs. In the blood, we found an induction of circulating monocytes that depended on CCR2, while only a small increase in Ly6Chi monocytes was observed in CCR2-/- mice. In the lung, significantly higher numbers of Ly6Chi and Ly6Clo monocytes were found in the C57BL/6 mice compared to CCR2-/- mice. Both wildtype and CCR2-deficient mice developed an inflammatory milieu as shown by cytokine and inos/arg1 mRNA induction in the lung, but with delayed kinetics in CCR2-deficient mice. Histopathology revealed that infiltration of macrophages to the parenchyma, but not into the peribronchial tissue, depended on CCR2. In sum, our data suggest that in Orientia infection, CCR2 drives blood monocytosis and the influx and activation of Ly6Chi and Ly6Clo monocytes into the lung, thereby accelerating bacterial replication and development of interstitial pulmonary inflammation.

Petermann M ，Orfanos Z ，Sellau J ，Gharaibeh M ，Lotter H ，Fleischer B ，Keller C ... -  《Frontiers in Immunology》

Spatio-temporal profile, phenotypic diversity, and fate of recruited monocytes into the post-ischemic brain.

Garcia-Bonilla L ，Faraco G ，Moore J ，Murphy M ，Racchumi G ，Srinivasan J ，Brea D ，Iadecola C ，Anrather J ... -  《Journal of Neuroinflammation》

Occurrences and Functions of Ly6C(hi) and Ly6C(lo) Macrophages in Health and Disease.

Macrophages originating from the yolk sac or bone marrow play essential roles in tissue homeostasis and disease. Bone marrow-derived monocytes differentiate into Ly6Chi and Ly6Clo macrophages according to the differential expression of the surface marker protein Ly6C. Ly6Chi and Ly6Clo cells possess diverse functions and transcriptional profiles and can accelerate the disease process or support tissue repair and reconstruction. In this review, we discuss the basic biology of Ly6Chi and Ly6Clo macrophages, including their origin, differentiation, and phenotypic switching, and the diverse functions of Ly6Chi and Ly6Clo macrophages in homeostasis and disease, including in injury, chronic inflammation, wound repair, autoimmune disease, and cancer. Furthermore, we clarify the differences between Ly6Chi and Ly6Clo macrophages and their connections with traditional M1 and M2 macrophages. We also summarize the limitations and perspectives for Ly6Chi and Ly6Clo macrophages. Overall, continued efforts to understand these cells may provide therapeutic approaches for disease treatment.

Li YH ，Zhang Y ，Pan G ，Xiang LX ，Luo DC ，Shao JZ ... -  《Frontiers in Immunology》