Circadian Expression of Migratory Factors Establishes Lineage-Specific Signatures that Guide the Homing of Leukocyte Subsets to Tissues.

The number of leukocytes present in circulation varies throughout the day, reflecting bone marrow output and emigration from blood into tissues. Using an organism-wide circadian screening approach, we detected oscillations in pro-migratory factors that were distinct for specific vascular beds and individual leukocyte subsets. This rhythmic molecular signature governed time-of-day-dependent homing behavior of leukocyte subsets to specific organs. Ablation of BMAL1, a transcription factor central to circadian clock function, in endothelial cells or leukocyte subsets demonstrated that rhythmic recruitment is dependent on both microenvironmental and cell-autonomous oscillations. These oscillatory patterns defined leukocyte trafficking in both homeostasis and inflammation and determined detectable tumor burden in blood cancer models. Rhythms in the expression of pro-migratory factors and migration capacities were preserved in human primary leukocytes. The definition of spatial and temporal expression profiles of pro-migratory factors guiding leukocyte migration patterns to organs provides a resource for the further study of the impact of circadian rhythms in immunity.

CD4+c-Met+Itgα4+ T cell subset promotes murine neuroinflammation.

OBJECTIVE: c-Met, a tyrosine kinase receptor, is the unique receptor for hepatocyte growth factor (HGF). The HGF/c-Met axis is reported to modulate cell migration, maturation, cytokine production, and antigen presentation. Here, we report that CD4+c-Met+ T cells are detected at increased levels in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS). METHODS: c-Met expression by CD4+ T cells was analyzed mostly by flow cytometry and by immunohistochemistry from mice and human PBMCs. The in vivo role of CD4+c-Met+ T cells was assessed in EAE. RESULTS: CD4+c-Met+ T cells found in the CNS during EAE peak disease are characterized by a pro-inflammatory phenotype skewed towards a Th1 and Th17 polarization, with enhanced adhesion and transmigration capacities correlating with increased expression of integrin α4 (Itgα4). The adoptive transfer of Itgα4-expressing CD4+Vα3.2+c-Met+ T cells induces increased disease severity compared to CD4+Vα3.2+c-Met- T cells. Finally, CD4+c-Met+ T cells are detected in the brain of MS patients, as well as in the blood with a higher level of Itgα4. These results highlight c-Met as an immune marker of highly pathogenic pro-inflammatory and pro-migratory CD4+ T lymphocytes associated with neuroinflammation.

A Potent Leukocyte Transmigration Blocker: GT-73 Showed a Protective Effect against LPS-Induced ARDS in Mice.

We recently developed a molecule (GT-73) that blocked leukocyte transendothelial migration from blood to the peripheral tissues, supposedly by affecting the platelet endothelial cell adhesion molecule (PECAM-1) function. GT-73 was tested in an LPS-induced acute respiratory distress syndrome (ARDS) mouse model. The rationale for this is based on the finding that the mortality of COVID-19 patients is partly caused by ARDS induced by a massive migration of leukocytes to the lungs. In addition, the role of tert-butyl and methyl ester moieties in the biological effect of GT-73 was investigated. A human leukocyte, transendothelial migration assay was applied to validate the blocking effect of GT-73 derivatives. Finally, a mouse model of LPS-induced ARDS was used to evaluate the histological and biochemical effects of GT-73. The obtained results showed that GT-73 has a unique structure that is responsible for its biological activity; two of its chemical moieties (tert-butyl and a methyl ester) are critical for this effect. GT-73 is a prodrug, and its lipophilic tail covalently binds to PECAM-1 via Lys536. GT-73 significantly decreased the number of infiltrating leukocytes in the lungs and reduced the inflammation level. Finally, GT-73 reduced the levels of IL-1ß, IL-6, and MCP-1 in bronchoalveolar lavage fluid (BALF). In summary, we concluded that GT-73, a blocker of white blood cell transendothelial migration, has a favorable profile as a drug candidate for the treatment of ARDS in COVID-19 patients.

Novel inhibitors of leukocyte transendothelial migration.

Leukocyte transendothelial migration is one of the most important step in launching an inflammatory immune response and chronic inflammation can lead to devastating diseases. Leukocyte migration inhibitors are considered as promising and potentially effective therapeutic agents to treat inflammatory and auto-immune disorders. In this study, based on previous trioxotetrahydropyrimidin based integrin inhibitors that suboptimally blocked leukocyte adhesion, twelve molecules with a modified scaffold were designed, synthesized, and tested in vitro for their capacity to block the transendothelial migration of immune cells. One of the molecules, namely, methyl 4-((2-(tert-butyl)-6-((2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene) methyl) phenoxy) methyl) benzoate, (compound 12), completely blocked leukocyte transendothelial migration, without any toxic effects on immune or endothelial cells (IC50 = 2.4 µM). In vivo, compound 12 exhibited significant therapeutic effects in inflammatory bowel disease (IBD)/Crohn's disease, multiple sclerosis, fatty liver disease, and rheumatoid arthritis models. A detailed acute and chronic toxicity profile of the lead compound in vivo did not reveal any toxic effects. Such a type of molecule might therefore provide a unique starting point for designing a novel class of leukocyte transmigration blocking agents with broad therapeutic applications in inflammatory and auto-immune pathologies.

Sushi repeat protein X-linked 2, a novel mediator of angiogenesis.

On appropriate stimuli, quiescent endothelial cells start to proliferate and form de novo blood vessels through angiogenesis. To further define molecular mechanisms accompanying the activation of endothelial cells during angiogenesis, we identified genes that were differentially regulated during this process using microarray analyses. In this work, we established a regulatory role for Sushi repeat protein X-linked 2 (Srpx2) in endothelial cell remodeling during angiogenesis. In particular, silencing of Srpx2 using small interfering RNAs (siRNAs) specifically attenuated endothelial cell migration and delayed angiogenic sprout formation. In vivo, Srpx2 expression was detected in de novo formation of blood vessels in angiogenic tissues by in situ mRNA hybridization and immunostaining. Pulldown experiments identified Srpx2 as a ligand for vascular uPAR, a key molecule involved in invasive migration of angiogenic endothelium. Immunostaining revealed coexpression of the Srpx2 and uPAR on vascular endothelium. These findings suggest that Srpx2 regulates endothelial cell migration and tube formation and provides a new target for modulating angiogenesis.

LPS differentially regulates adhesion and transendothelial migration of human monocytes under static and flow conditions.

One of the key components of the innate immune response is the recognition of microbial products such as LPS by Toll-like receptors on monocytes and neutrophils. We show here that short-term stimulation of primary human monocytes with LPS led to an increase in adhesion of monocytes to endothelial cells and a dramatic decrease in transendothelial migration under static conditions. In contrast, under normal physiological flow, monocyte adhesion and migration across a human umbilical vein endothelial cell monolayer appeared to be unaffected by LPS treatment. LPS stimulation of monocytes activated beta(1) and beta(2) integrins, but did not increase their surface expression levels. During septic shock, reduction in blood flow as a result of vasodilation and vascular permeability leads to adhesion and accumulation of LPS-stimulated circulating monocytes onto the blood vessel walls. The different findings of monocyte migration under static and flow conditions in our study may offer one explanation for this phenomenon. The rapid engagement of LPS-activated monocytes preventing transendothelial migration could represent a novel mechanism of bacterial exclusion from the vasculature. This occurs during the early stages of sepsis, and in turn may modulate the severity of the pathophysiology.

JAM family and related proteins in leukocyte migration (Vestweber series).

Exploring the role of junctional adhesion molecules (JAMs) has proven to be varied and controversial. The purpose of this review is to discuss the new and exciting roles of these IgSF molecules and how they have evolved to contribute to diverse functions from development to inflammation. In particular, recent research has focused on JAM subfamily members JAM-A, -B, and -C with newly described roles in leukocyte trafficking during inflammation and angiogenesis. However, research on all JAM family members has demonstrated recurring themes with striking similarities in the many diverse processes they are now known to regulate.

Angiogenic factor-driven inflammation promotes extravasation of human proangiogenic monocytes to tumours.

Recruitment of circulating monocytes is critical for tumour angiogenesis. However, how human monocyte subpopulations extravasate to tumours is unclear. Here we show mechanisms of extravasation of human CD14dimCD16+ patrolling and CD14+CD16+ intermediate proangiogenic monocytes (HPMo), using human tumour xenograft models and live imaging of transmigration. IFNγ promotes an increase of the chemokine CX3CL1 on vessel lumen, imposing continuous crawling to HPMo and making these monocytes insensitive to chemokines required for their extravasation. Expression of the angiogenic factor VEGF and the inflammatory cytokine TNF by tumour cells enables HPMo extravasation by inducing GATA3-mediated repression of CX3CL1 expression. Recruited HPMo boosts angiogenesis by secreting MMP9 leading to release of matrix-bound VEGF-A, which amplifies the entry of more HPMo into tumours. Uncovering the extravasation cascade of HPMo sets the stage for future tumour therapies.

Divergent JAM-C Expression Accelerates Monocyte-Derived Cell Exit from Atherosclerotic Plaques.

Atherosclerosis, caused in part by monocytes in plaques, continues to be a disease that afflicts the modern world. Whilst significant steps have been made in treating this chronic inflammatory disease, questions remain on how to prevent monocyte and macrophage accumulation in atherosclerotic plaques. Junctional Adhesion Molecule C (JAM-C) expressed by vascular endothelium directs monocyte transendothelial migration in a unidirectional manner leading to increased inflammation. Here we show that interfering with JAM-C allows reverse-transendothelial migration of monocyte-derived cells, opening the way back out of the inflamed environment. To study the role of JAM-C in plaque regression we used a mouse model of atherosclerosis, and tested the impact of vascular JAM-C expression levels on monocyte reverse transendothelial migration using human cells. Studies in-vitro under inflammatory conditions revealed that overexpression or gene silencing of JAM-C in human endothelium exposed to flow resulted in higher rates of monocyte reverse-transendothelial migration, similar to antibody blockade. We then transplanted atherosclerotic, plaque-containing aortic arches from hyperlipidemic ApoE-/- mice into wild-type normolipidemic recipient mice. JAM-C blockade in the recipients induced greater emigration of monocyte-derived cells and further diminished the size of atherosclerotic plaques. Our findings have shown that JAM-C forms a one-way vascular barrier for leukocyte transendothelial migration only when present at homeostatic copy numbers. We have also shown that blocking JAM-C can reduce the number of atherogenic monocytes/macrophages in plaques by emigration, providing a novel therapeutic strategy for chronic inflammatory pathologies.

The role of leukocyte-stromal interactions in chronic inflammatory joint disease.

Rheumatoid arthritis (RA) is a debilitating, chronic, persistent inflammatory disease that is characterised by painful and swollen joints. The aetiology of RA is unknown, however whereas past research has concentrated on the role of immune or inflammatory infiltrating cells in inflammation, it is becoming clear that stromal cells play a critical part in regulating the quality and duration of an inflammatory response. In this review we assess the role of fibroblasts within the inflamed synovium in modulating immune responses; in particular we examine the role of stromal cells in the switch from resolving to persistent inflammation as is found in the rheumatoid synovium.

Vascular and epithelial junctions: a barrier for leucocyte migration.

Rapid mobilization of leucocytes through endothelial and epithelial barriers is key in immune system reactivity. The underlying mechanisms that regulate these processes have been the basis for many recent studies. Traditionally, leucocyte extravasation had been believed to occur through a paracellular route, which involves localized disruption of endothelial cell junctions. However, more recently, a transcellular route has been described involving the passage through the endothelial cell body. Leucocytes are also able to migrate through epithelium to monitor mucosal tissues and microenvironments. A number of adhesion molecules are known to regulate transmigration of leucocytes through epithelial and endothelial layers. Paracellular and transcellular leucocyte transmigration are regulated by adhesion molecules such as PECAM-1 (platelet-endothelial cell adhesion molecule 1), CD99, VE-cadherin (vascular endothelial cadherin) and JAM (junctional adhesion molecule) proteins. The purpose of this review is to discuss the role of these molecules in leucocyte transmigration and how they contribute to the different mechanisms that regulate leucocyte trafficking.

JAM-C regulates unidirectional monocyte transendothelial migration in inflammation.

Monocyte recruitment from the vasculature involves sequential engagement of multiple receptors, culminating in transendothelial migration and extravasation. Junctional adhesion molecule-C (JAM-C) is localized at endothelial intercellular junctions and plays a role in monocyte transmigration. Here, we show that blockade of JAM-B/-C interaction reduced monocyte numbers in the extravascular compartment through increased reverse transmigration rather than by reduced transmigration. This was confirmed in vivo, showing that an anti-JAM-C antibody reduced the number of monocytes in inflammatory tissue and increased the number of monocytes with a reverse-transmigratory phenotype in the peripheral blood. All together, our results suggest a novel mechanism of reducing accumulation of monocytes at inflammation sites by disruption of JAM-C-mediated monocyte retention.

Neutrophil transmigration under shear flow conditions in vitro is junctional adhesion molecule-C independent.

Endothelial cell junctional adhesion molecule (JAM)-C has been proposed to regulate neutrophil migration. In the current study, we used function-blocking mAbs against human JAM-C to determine its role in human leukocyte adhesion and transendothelial cell migration under flow conditions. JAM-C surface expression in HUVEC was uniformly low, and treatment with inflammatory cytokines TNF-alpha, IL-1beta, or LPS did not increase its surface expression as assessed by FACS analysis. By immunofluorescence microscopy, JAM-C staining showed sparse localization to cell-cell junctions on resting or cytokine-activated HUVEC. Surprisingly, staining of detergent-permeabilized HUVEC revealed a large intracellular pool of JAM-C that showed little colocalization with von Willebrand factor. Adhesion studies in an in vitro flow model showed that functional blocking JAM-C mAb alone had no inhibitory effect on polymorphonuclear leukocyte (PMN) adhesion or transmigration, whereas mAb to ICAM-1 significantly reduced transmigration. Interestingly, JAM-C-blocking mAbs synergized with a combination of PECAM-1, ICAM-1, and CD99-blocking mAbs to inhibit PMN transmigration. Overexpression of JAM-C by infection with a lentivirus JAM-C GFP fusion protein did not increase adhesion or extent of transmigration of PMN or evoke a role for JAM-C in transendothelial migration. These data suggest that JAM-C has a minimal role, if any, in PMN transmigration in this model and that ICAM-1 is the preferred endothelial-expressed ligand for PMN beta(2) integrins during transendothelial migration.

Rheumatoid fibroblast-like synoviocytes overexpress the chemokine stromal cell-derived factor 1 (CXCL12), which supports distinct patterns and rates of CD4+ and CD8+ T cell migration within synovial tissue.

OBJECTIVE: A characteristic feature of the inflammatory infiltrate in rheumatoid arthritis is the segregation of CD4 and CD8 T lymphocyte subsets into distinct microdomains within the inflamed synovium. The aim of this study was to test the hypothesis that chemokines in general and stromal cell-derived factor 1 (SDF-1; CXCL12) in particular are responsible for generating this distinctive microcompartmentalization. METHODS: We examined how synovial CD4/CD8 T cell subsets interacted in coculture assays with fibroblasts derived from chronic inflammatory synovial lesions and normal synovial tissue as well as from fetal lung and adult skin. We used the ability of T cells to migrate beneath fibroblasts (a process called pseudoemperipolesis) as an in vitro marker of T cell accumulation within synovial tissue. RESULTS: Rheumatoid fibroblast-like synoviocytes (FLS) displayed a unique ability to support high levels of CD4 and CD8 T cell pseudoemperipolesis. Nonrheumatoid FLS as well as fetal lung fibroblasts supported low levels of pseudoemperipolesis, while skin-derived fibroblasts were unable to do so. CD8 T cells migrated under fibroblasts more efficiently and at a higher velocity than CD4 T cells, a feature that was intrinsic to CD8 T cells. Rheumatoid fibroblasts constitutively produced high levels of SDF-1 (CXCL12), which was functionally important, since blocking studies showed reductions in T cell pseudoemperipolesis to levels seen in nonrheumatoid FLS. Rheumatoid fibroblasts also constitutively produced high levels of vascular cell adhesion molecule 1 (VCAM-1; CD106), but this did not contribute to T cell pseudoemperipolesis, unlike the case for B cells, which require SDF-1 (CXCL12)-CXCR4 and CD49d-VCAM-1 (CD106) interactions. Importantly, only combinations of rheumatoid FLS and rheumatoid-derived synovial fluid T cells supported pseudoemperipolesis when examined ex vivo, confirming the in vivo relevance of these findings. CONCLUSION: These studies demonstrate that features intrinsic to both fibroblasts (the production of SDF-1) and CD8/CD4 T cells (the expression of CXCR4) are responsible for the characteristic pattern of T lymphocyte accumulation seen in the rheumatoid synovium. These findings suggest that the SDF-1/CXCR4 ligand/receptor pair is likely to play an important functional role in T lymphocyte accumulation and positioning within the rheumatoid synovium.