Fibronectin leucine-rich transmembrane protein 2 drives monocyte differentiation into macrophages via the UNC5B-Akt/mTOR axis.

Upon migrating into the tissues, hematopoietic stem cell (HSC)-derived monocytes differentiate into macrophages, playing a crucial role in determining innate immune responses towards external pathogens and internal stimuli. However, the regulatory mechanisms underlying monocyte-to-macrophage differentiation remain largely unexplored. Here we divulge a previously uncharacterized but essential role for an axon guidance molecule, fibronectin leucine-rich transmembrane protein 2 (FLRT2), in monocyte-to-macrophage maturation. FLRT2 is almost undetectable in human monocytic cell lines, human peripheral blood mononuclear cells (PBMCs), and mouse primary monocytes but significantly increases in fully differentiated macrophages. Myeloid-specific deletion of FLRT2 (Flrt2ΔMyel ) contributes to decreased peritoneal monocyte-to-macrophage generation in mice in vivo, accompanied by impaired macrophage functions. Gain- and loss-of-function studies support the promoting effect of FLRT2 on THP-1 cell and human PBMC differentiation into macrophages. Mechanistically, FLRT2 directly interacts with Unc-5 netrin receptor B (UNC5B) via its extracellular domain (ECD) and activates Akt/mTOR signaling. In vivo administration of mTOR agonist MYH1485 reverses the impaired phenotypes observed in Flrt2ΔMyel mice. Together, these results identify FLRT2 as a novel pivotal endogenous regulator of monocyte differentiation into macrophages. Targeting the FLRT2/UNC5B-Akt/mTOR axis may provide potential therapeutic strategies directly relevant to human diseases associated with aberrant monocyte/macrophage differentiation.

Pristane attenuates atherosclerosis in Apoe-/- mice via IL-4-secreting regulatory plasma cell-mediated M2 macrophage polarization.

Atherosclerosis, an inflammatory progressive vascular disease, causes heart disease and stroke worldwide. B cells with immune suppressive functions have been implicated in autoimmune, inflammatory, and cardiovascular diseases. However, the precise role of regulatory B cells and the interaction with macrophages in atherosclerosis remains undefined. In our study, eight-week-old female apolipoprotein E null (Apoe-/-) mice were treated with a single dose of vehicle or pristane and then placed on an atherogenic diet for 12 weeks. We found that pristane decreased atherosclerotic lesion formation and increased stability of atherosclerotic plaques in Apoe-/- mice. We also observed lower frequencies of CD19+ B cells but higher frequencies of CD138+ plasma cells and CD206+ M2 macrophages in Apoe-/- mice treated with pristane. Importantly, pristane inhibited immune cell infiltration into the vascular wall. The upregulation of IL-4 in bone-marrow CD138+ plasma cells from pristane-treated Apoe-/- mice was demonstrated by RNA-sequencing (RNA-seq). Consistently, oxidized low-density lipoprotein (oxLDL) directly induced IL-4-secreting plasma cell generation in vitro. In a co-culture system incubating an anti-IL-4 neutralizing antibody, the results showed that oxLDL-induced CD138+ plasma cells could boost M2 macrophage polarization via IL-4 secretion. Our data demonstrate an unexpected role that pristane induces IL-4-producing CD138+ regulatory plasma cell generation and M2 polarization to protect atherosclerosis development.