Monocytes fundamentally contribute to immune surveillance and the inflammatory response in immunoinflammatory diseases like atherosclerosis. Recruitment of these cells to the site of injury requires their trafficking across the blood vessel wall. A series of events, including capture, rolling, slow rolling, arrest, adhesion strengthening, and lateral locomotion, precede monocyte transmigration. Recent investigations have revealed new aspects of this cascade. This article revisits some conventional paradigms and selectively highlights new findings, including novel insights into monocyte differentiation and recently identified functional mediators, signalling pathways, and new structural aspects of monocyte extravasation. The emerging roles of endothelial junctional molecules like vascular endothelial-cadherin and the junctional adhesion molecule family, adhesion molecules such as intercellular adhesion molecule-1, molecules localized to the lateral border recycling compartment like cluster of differentiation 99, platelet/endothelial cell adhesion molecule-1, and poliovirus receptor (CD155), as well as other cell surface molecules such as cluster of differentiation 146 and ephrins in transendothelial migration are discussed.

Background: Throughout the inflammatory response that accompanies atherosclerosis, autoreactive CD4 + T-helper cells accumulate in the atherosclerotic plaque. Apolipoprotein B 100 (apoB), the core protein of low-density lipoprotein, is an autoantigen that drives the generation of pathogenic T-helper type 1 (T H 1) cells with proinflammatory cytokine secretion. Clinical data suggest the existence of apoB-specific CD4 + T cells with an atheroprotective, regulatory T cell (T reg ) phenotype in healthy individuals. Yet, the function of apoB-reactive T regs and their relationship with pathogenic T H 1 cells remain unknown. Methods: To interrogate the function of autoreactive CD4 + T cells in atherosclerosis, we used a novel tetramer of major histocompatibility complex II to track T cells reactive to the mouse self-peptide apo B 978-993 (apoB + ) at the single-cell level. Results: We found that apoB + T cells build an oligoclonal population in lymph nodes of healthy mice that exhibit a T reg -like transcriptome, although only 21% of all apoB + T cells expressed the T reg transcription factor FoxP3 (Forkhead Box P3) protein as detected by flow cytometry. In single-cell RNA sequencing, apoB + T cells formed several clusters with mixed T H signatures that suggested overlapping multilineage phenotypes with pro- and anti-inflammatory transcripts of T H 1, T helper cell type 2 (T H 2), and T helper cell type 17 (T H 17), and of follicular-helper T cells. ApoB + T cells were increased in mice and humans with atherosclerosis and progressively converted into pathogenic T H 1/T H 17-like cells with proinflammatory properties and only a residual T reg transcriptome. Plaque T cells that expanded during progression of atherosclerosis consistently showed a mixed T H 1/T H 17 phenotype in single-cell RNA sequencing. In addition, we observed a loss of FoxP3 in a fraction of apoB + T regs in lineage tracing of hyperlipidemic Apoe –/– mice. In adoptive transfer experiments, converting apoB + T regs failed to protect from atherosclerosis. Conclusions: Our results demonstrate an unexpected mixed phenotype of apoB-reactive autoimmune T cells in atherosclerosis and suggest an initially protective autoimmune response against apoB with a progressive derangement in clinical disease. These findings identify apoB autoreactive T regs as a novel cellular target in atherosclerosis.

Integrin-based therapeutics have garnered considerable interest in the medical treatment of inflammation. Integrins mediate the fast recruitment of monocytes and neutrophils to the site of inflammation, but are also required for host defense, limiting their therapeutic use. Here, we report a novel monoclonal antibody, anti-M7, that specifically blocks the interaction of the integrin Mac-1 with its pro-inflammatory ligand CD40L, while not interfering with alternative ligands. Anti-M7 selectively reduces leukocyte recruitment in vitro and in vivo. In contrast, conventional anti-Mac-1 therapy is not specific and blocks a broad repertoire of integrin functionality, inhibits phagocytosis, promotes apoptosis, and fuels a cytokine storm in vivo. Whereas conventional anti-integrin therapy potentiates bacterial sepsis, bacteremia, and mortality, a ligand-specific intervention with anti-M7 is protective. These findings deepen our understanding of ligand-specific integrin functions and open a path for a new field of ligand-targeted anti-integrin therapy to prevent inflammatory conditions.