CX3CL1-Fc treatment prevents atherosclerosis in Ldlr KO mice

OBJECTIVE

Atherosclerosis is a major cause of cardiovascular disease. Monocyte-endothelial cell interactions are partly mediated by expression of monocyte CX3CR1 and endothelial cell fractalkine (CX3CL1). Interrupting the interaction between this ligand-receptor pair should reduce monocyte binding to the endothelial wall and reduce atherosclerosis. We sought to reduce atherosclerosis by preventing monocyte-endothelial cell interactions through use of a long-acting CX3CR1 agonist.

METHODS

In this study, the chemokine domain of CX3CL1 was fused to the mouse Fc region to generate a long-acting soluble form of CX3CL1 suitable for chronic studies. CX3CL1-Fc or saline was injected twice a week (30 mg/kg) for 4 months into Ldlr knockout (KO) mice on an atherogenic western diet.

RESULTS

CX3CL1-Fc-treated Ldlr KO mice showed decreased en face aortic lesion surface area and reduced aortic root lesion size with decreased necrotic core area. Flow cytometry analyses of CX3CL1-Fc-treated aortic wall cell digests revealed a decrease in M1-like polarized macrophages and T cells. Moreover, CX3CL1-Fc administration reduced diet-induced atherosclerosis after switching from an atherogenic to a normal chow diet. In vitro monocyte adhesion studies revealed that CX3CL1-Fc treatment caused fewer monocytes to adhere to a human umbilical vein endothelial cell monolayer. Furthermore, a dorsal window chamber model demonstrated that CX3CL1-Fc treatment decreased in vivo leukocyte adhesion and rolling in live capillaries after short-term ischemia-reperfusion.

CONCLUSION

These results indicate that CX3CL1-Fc can inhibit monocyte/endothelial cell adhesion as well as reduce atherosclerosis.

Macrophage Polarization: Different Gene Signatures in M1(LPS+) vs. Classically and M2(LPS-) vs. Alternatively Activated Macrophages

Macrophages are found in tissues, body cavities, and mucosal surfaces. Most tissue macrophages are seeded in the early embryo before definitive hematopoiesis is established. Others are derived from blood monocytes. The macrophage lineage diversification and plasticity are key aspects of their functionality. Macrophages can also be generated from monocytes in vitro and undergo classical (LPS+IFN-γ) or alternative (IL-4) activation. In vivo, macrophages with different polarization and different activation markers coexist in tissues. Certain mouse strains preferentially promote T-helper-1 (Th1) responses and others Th2 responses. Their macrophages preferentially induce iNOS or arginase and have been called M1 and M2, respectively. In many publications, M1 and classically activated and M2 and alternatively activated are used interchangeably. We tested whether this is justified by comparing the gene lists positively [M1(=LPS+)] or negatively [M2(=LPS-)] correlated with the ratio of IL-12 and arginase 1 in transcriptomes of LPS-treated peritoneal macrophages with in vitro classically (LPS, IFN-γ) vs. alternatively activated (IL-4) bone marrow derived macrophages, both from published datasets. Although there is some overlap between in vivo M1(=LPS+) and in vitro classically activated (LPS+IFN-γ) and in vivo M2(=LPS-) and in vitro alternatively activated macrophages, many more genes are regulated in opposite or unrelated ways. Thus, M1(=LPS+) macrophages are not equivalent to classically activated, and M2(=LPS-) macrophages are not equivalent to alternatively activated macrophages. This fundamental discrepancy explains why most surface markers identified on in vitro generated macrophages do not translate to the in vivo situation. Valid in vivo M1/M2 surface markers remain to be discovered.

A Single-Step Chemoenzymatic Reaction for the Construction of Antibody-Cell Conjugates

Employing live cells as therapeutics is a direction of future drug discovery. An easy and robust method to modify the surfaces of cells directly to incorporate novel functionalities is highly desirable. However, genetic methods for cell-surface engineering are laborious and limited by low efficiency for primary cell modification. Here we report a chemoenzymatic approach that exploits a fucosyltransferase to transfer bio-macromolecules, such as an IgG antibody (MW∼ 150 KD), to the glycocalyx on the surfaces of live cells when the antibody is conjugated to the enzyme's natural donor substrate GDP-Fucose. Requiring no genetic modification, this method is fast and biocompatible with little interference to cells' endogenous functions. We applied this method to construct two antibody-cell conjugates (ACCs) using both cell lines and primary cells, and the modified cells exhibited specific tumor targeting and resistance to inhibitory signals produced by tumor cells, respectively. Remarkably, Herceptin-NK-92MI conjugates, a natural killer cell line modified with Herceptin, exhibit enhanced activities to induce the lysis of HER2+ cancer cells both ex vivo and in a human tumor xenograft model. Given the unprecedented substrate tolerance of the fucosyltransferase, this chemoenzymatic method offers a general approach to engineer cells as research tools and for therapeutic applications.

Neutrophils: New insights and open questions

Neutrophils are the first line of defense against bacteria and fungi and help combat parasites and viruses. They are necessary for mammalian life, and their failure to recover after myeloablation is fatal. Neutrophils are short-lived, effective killing machines. Their life span is significantly extended under infectious and inflammatory conditions. Neutrophils take their cues directly from the infectious organism, from tissue macrophages and other elements of the immune system. Here, we review how neutrophils traffic to sites of infection or tissue injury, how they trap and kill bacteria, how they shape innate and adaptive immune responses, and the pathophysiology of monogenic neutrophil disorders.

Atherosclerosis in the single-cell era

PURPOSE OF REVIEW

The immune system plays a critical role in the development and modulation of atherosclerosis. New high-parameter technologies, including mass cytometry (CyTOF) and single-cell RNA sequencing (scRNAseq), allow for an encompassing analysis of immune cells. Unexplored marker combinations and transcriptomes can define new immune cell subsets and suggest their functions. Here, we review recent advances describing the immune cells in the artery wall of mice with and without atherosclerosis. We compare technologies and discuss limitations and advantages.

RECENT FINDINGS

Both CyTOF and scRNAseq on leukocytes from digested aortae show 10-30 immune cell subsets. Myeloid, T, B and natural killer cells were confirmed. Although cellular functions can be inferred from RNA-Seq data, some subsets cannot be identified based on current knowledge, suggesting they may be new cell types. CyTOF and scRNAseq each identified four B-cell subsets and three macrophage subsets in the atherosclerotic aorta. Limitations include cell death caused by enzymatic digestion and the limited depth of the scRNAseq transcriptomes.

SUMMARY

High-parameter methods are powerful tools for uncovering leukocyte diversity. CyTOF is currently more powerful at discerning leukocyte subsets in the atherosclerotic aorta, whereas scRNAseq provides more insight into their likely functions.

Regulatory CD4+ T Cells Recognize Major Histocompatibility Complex Class II Molecule-Restricted Peptide Epitopes of Apolipoprotein B

BACKGROUND

CD4+ T cells play an important role in atherosclerosis, but their antigen specificity is poorly understood. Immunization with apolipoprotein B (ApoB, core protein of low density lipoprotein) is known to be atheroprotective in animal models. Here, we report on a human APOB peptide, p18, that is sequence-identical in mouse ApoB and binds to both mouse and human major histocompatibility complex class II molecules.

METHODS

We constructed p18 tetramers to detect human and mouse APOB-specific T cells and assayed their phenotype by flow cytometry including CD4 lineage transcription factors, intracellular cytokines, and T cell receptor activation. Apolipoprotein E-deficient ( Apoe-/-) mice were vaccinated with p18 peptide or adjuvants alone, and atherosclerotic burden in the aorta was determined.

RESULTS

In human peripheral blood mononuclear cells from donors without cardiovascular disease, p18 specific CD4+ T cells detected by a new human leukocyte antigen-antigen D related-p18 tetramers were mostly Foxp3+ regulatory T cells (Tregs). Donors with subclinical cardiovascular disease as detected by carotid artery ultrasound had Tregs coexpressing retinoic acid-related orphan receptor gamma t or T-bet, which were both almost absent in donors without cardiovascular disease. In Apoe-/- mice, immunization with p18 induced Tregs and reduced atherosclerotic lesions. After peptide restimulation, responding CD4+ T cells identified by Nur77-GFP (green fluorescent protein) were highly enriched in Tregs. A new mouse I-Ab-p18 tetramer identified the expansion of p18-specific CD4+ T cells on vaccination, which were enriched for interleukin-10-producing Tregs.

CONCLUSIONS

These findings show that APOB p18-specific CD4+ T cells are mainly Tregs in healthy donors, but coexpress other CD4 lineage transcription factors in donors with subclinical cardiovascular disease. This study identifies ApoB peptide 18 as the first Treg epitope in human and mouse atherosclerosis.

A clinically applicable adjuvant for an atherosclerosis vaccine in mice

Vaccination with MHC-II-restricted peptides from Apolipoprotein B (ApoB) with complete and incomplete Freund's adjuvant (CFA/IFA) is known to protect mice from atherosclerosis. This vaccination induces antigen-specific IgG1 and IgG2c antibody responses and a robust CD4 T cell response in lymph nodes. However, CFA/IFA cannot be used in humans. To find a clinically applicable adjuvant, we tested the effect of vaccinating Apoe-deficient mice with ApoB peptide P6 (TGAYSNASSTESASY). In a broad screening experiment, Addavax, a squalene-based oil-in-water adjuvant similar to MF59, was the only adjuvant that showed similar efficacy as CFA/IFA. This was confirmed in a confirmation experiment for both the aortic arch and whole aorta analyzed by en face analysis after atherosclerotic lesion staining. Mechanistically, restimulated peritoneal cells from mice immunized with P6 in Addavax released significant amounts of IL-10. Unlike P6 in CFA/IFA, vaccination with P6 in Addavax did not induce any detectable IgG1 or IgG2c antibodies to P6. These data suggest that squalene-based adjuvants such as MF59 are good candidate adjuvants for developing a clinically effective atherosclerosis vaccine.

Publisher Correction: Oxidized phospholipids are proinflammatory and proatherogenic in hypercholesterolaemic mice

In this Letter, affiliation number 1 was originally missing from the HTML; the affiliations were missing for author Ming-Yow Hung in the HTML; and the Fig. 4 legend erroneously referred to panels a-h, instead of a-g. These errors have been corrected online.

Oxidized phospholipids are proinflammatory and proatherogenic in hypercholesterolaemic mice

Oxidized phospholipids (OxPL) are ubiquitous, are formed in many inflammatory tissues, including atherosclerotic lesions, and frequently mediate proinflammatory changes ¹ . Because OxPL are mostly the products of non-enzymatic lipid peroxidation, mechanisms to specifically neutralize them are unavailable and their roles in vivo are largely unknown. We previously cloned the IgM natural antibody E06, which binds to the phosphocholine headgroup of OxPL, and blocks the uptake of oxidized low-density lipoprotein (OxLDL) by macrophages and inhibits the proinflammatory properties of OxPL^2-4. Here, to determine the role of OxPL in vivo in the context of atherogenesis, we generated transgenic mice in the Ldlr^-/- background that expressed a single-chain variable fragment of E06 (E06-scFv) using the Apoe promoter. E06-scFv was secreted into the plasma from the liver and macrophages, and achieved sufficient plasma levels to inhibit in vivo macrophage uptake of OxLDL and to prevent OxPL-induced inflammatory signalling. Compared to Ldlr^-/- mice, Ldlr ^-/- E06-scFv mice had 57-28% less atherosclerosis after 4, 7 and even 12 months of 1% high-cholesterol diet. Echocardiographic and histologic evaluation of the aortic valves demonstrated that E06-scFv ameliorated the development of aortic valve gradients and decreased aortic valve calcification. Both cholesterol accumulation and in vivo uptake of OxLDL were decreased in peritoneal macrophages, and both peritoneal and aortic macrophages had a decreased inflammatory phenotype. Serum amyloid A was decreased by 32%, indicating decreased systemic inflammation, and hepatic steatosis and inflammation were also decreased. Finally, the E06-scFv prolonged life as measured over 15 months. Because the E06-scFv lacks the functional effects of an intact antibody other than the ability to bind OxPL and inhibit OxLDL uptake in macrophages, these data support a major proatherogenic role of OxLDL and demonstrate that OxPL are proinflammatory and proatherogenic, which E06 counteracts in vivo. These studies suggest that therapies inactivating OxPL may be beneficial for reducing generalized inflammation, including the progression of atherosclerosis, aortic stenosis and hepatic steatosis.

Comprehensive assessment of immune cells in mouse and human atherosclerosis by single cell RNA sequencing and mass cytometry

Atherosclerosis, an inflammatory disease of large arteries, is – through its clinical manifestations stroke and myocardial infarction – globally the leading cause of morbidity and mortality. Although cells of the innate and adaptive immune system are found in atherosclerotic plaques, their phenotypic and functional diversity is poorly understood. Here, we applied single cell RNA-sequencing (scRNAseq) and mass cytometry (CyTOF) to assess leukocyte diversity in depth, thus defining an immune cell atlas in atherosclerosis.

Single cell transcriptional profiling of aortic leukocytes from 20-week old chow (CD) and western diet (WD) fed Apoe−/− and Ldlr−/− mice revealed 11 phenotypically different leukocyte clusters, whilst WD changed their abundance. Gene set enrichment analysis of single cells established that multiple pathways, e.g. for lipid metabolism or cytokine secretion, pertained to particular leukocyte clusters. Applying a novel 35-marker CyTOF panel confirmed the phenotypic diversity of aortic leukocytes. Among lymphocytes, we detected three principal B-cell subsets defined by scRNAseq, CyTOF, and flow cytometry. These B cell subsets harbor distinct surface marker expression, functional gene pathways, and ex vivo cytokine production. Finally, we used leukocyte cluster gene signatures to assess leukocyte frequencies in 121 human plaques by a transcriptomic deconvolution strategy revealing similar immune cell complexity.

The definition of leukocyte diversity by high-dimensional analyses enables a fine-grained analysis of aortic leukocyte subsets, reveals new immunological mechanisms and cell-type specific pathways, and may result in novel diagnostic risk stratification tools.

Abstract 004: Comprehensive Assessment of Immune Cells in Mouse and Human Atherosclerosis by Single-cell RNA-sequencing and Mass Cytometry

Atherosclerosis, an inflammatory disease of large arteries, is - through its clinical manifestations stroke and myocardial infarction - globally the leading cause of morbidity and mortality. The interplay of pro- and anti-inflammatory leukocytes in the aorta modulates and drives atherosclerosis. Although cells of the innate and adaptive immune system are found in atherosclerotic plaques, their phenotypic and functional diversity is poorly understood. Here, we applied single cell RNA-sequencing (scRNAseq) and mass cytometry (CyTOF) to assess leukocyte diversity in depth, thus defining an immune cell atlas in atherosclerosis. Single cell transcriptional profiling of aortic leukocytes from 20-week old chow (CD) and western diet (WD) fed Apoe -/- and Ldlr -/- mice revealed 11 phenotypically different leukocyte clusters. Atherosclerotic aortas exhibited enhanced leukocyte diversity, whilst WD further changed the abundance of leukocyte subpopulations. Gene set enrichment analysis of single cells established that multiple pathways, e.g. for lipid metabolism, proliferation, and cytokine secretion, pertained to particular leukocyte clusters. Applying a novel 35-marker CyTOF panel with metal-labelled antibodies confirmed the phenotypic diversity of aortic leukocytes. Among lymphocytes, we detected three principal B-cell subsets defined by scRNAseq, CyTOF, and flow cytometry. These B cell subsets harbor distinct surface marker expression, functional gene pathways, and ex vivo cytokine production. Finally, we used leukocyte cluster gene signatures to assess leukocyte frequencies in 121 human plaques by a transcriptomic deconvolution strategy. This approach revealed a similar immune cell complexity in human carotid plaques with a higher percentage of monocytes and macrophages. In addition, the frequency of genetically defined leukocyte populations in carotid plaques predicted cardiovascular events in patients. The definition of leukocyte diversity by high-dimensional analyses enables a fine-grained analysis of aortic leukocyte subsets, reveals new immunological mechanisms and cell-type specific pathways, and may result in novel diagnostic risk stratification tools.

Abstract 410: Statin Use May Alleviate Coagulation- and Inflammation-associated Gene Expression in Circulating Classical Monocytes in Women With Chronic HIV-1 Infection

During virally-suppressed chronic human immunodeficiency virus (HIV) infection, persistent inflammation contributes to the development of cardiovascular disease, a major comorbidity in people living with HIV. Monocytes play a key role in atherosclerotic plaque development, inflammation, and stability, but their contribution to the CVD under viral suppression in people living with HIV remains unknown. Here, we investigated the transcriptomes of classical (CD14 ++ CD16 - ) blood monocytes from 92 women with and without chronic HIV infection and subclinical cardiovascular disease (sCVD), defined as the presence of focal carotid artery plaque, from the Women's Interagency HIV Study (WIHS). Differential gene expression, based on four two-way comparisons among participant groups (HIV-sCVD-, HIV+sCVD-, HIV-sCVD+, and HIV+sCVD+, 23 subjects each), identified large pro-inflammatory gene signatures for both sCVD and virally-suppressed HIV. These findings were further corroborated by Ingenuity Pathway Analysis. We found that classical monocytes persistently express common CVD-related markers of inflammation, including IL6, IL1β, and IL12B; overlapping with many transcripts identified in sCVD+ participants. In comorbid disease (HIV+sCVD+), those reporting statin use showed dramatically reduced pro-coagulant tissue factor (F3) and tissue factor pathway inhibitor (TFPI and TFPI2) gene expression to a level comparable with healthy participants. Cytokine expression profiles associated with the tissue factor pathway were also modified in participants on statins, suggesting that statins may benefit women with chronic HIV infection by limiting pro-coagulation and inflammation pathways in classical monocytes.

Abstract 031: A Ligand-specific Blockade of the Integrin Mac-1 Selectively Targets Pathologic Vascular Inflammation While Maintaining Protective Host-defense

Background: Integrins, such as Mac-1 (alphaMbeta2, CD11b/CD18), drive myeloid cell recruitment and inflammation in cardiovascular disease. Although integrins have generated interest as therapeutic targets in acute and chronic inflammation, they also contribute to host defense, regeneration, and haemostasis. To overcome these limitations, we have designed a novel antibody that targets a distinct region within the Mac-1 major ligand-binding (I-) domain required for binding of its ligand CD40L. Here, we describe the consequences of a ligand-specific inhibition and its efficacy in mice.

Methods and Results: To generate an antibody specifically targeting the interaction of Mac-1 with CD40L, mice were immunized with a peptide containing the CD40L-binding motif EQLKKSKTL. Antibody clones were screened and tested for specificity. One clone, termed anti-M7, showed specific peptide binding and blocked the binding of the activated, open conformation of Mac-1 to CD40L, but not to alternative Mac-1 ligands suggesting ligand specificity. Anti-M7 highly effectively limited leukocyte recruitment in vitro and to the mesentery and the omentum in intravital confocal microscopy. While conventional anti-Mac-1 antibodies induced outside-in activation of the integrin, anti-M7 did not provoke MAP-kinase dependent activation and expression of inflammatory cytokines in macrophages. Following cecal-ligation and puncture (CLP) sepsis in mice, a conventional anti-Mac-1 antibody inhibited leukocyte accumulation, but potentiated bacteremia and limited overall survival. In contrast, anti-M7 limited peritoneal leukocyte accumulation, reduced bacterial titers, and improved survival during CLP.

Conclusion: We present proof-of-concept demonstrating selective inhibition of the interaction between the leukocyte integrin Mac-1 and CD40L by a novel monoclonal antibody. This antibody shows several advantages over conventional anti-integrin therapy, and prevented inflammation and sepsis-related mortality in mice. This novel approach gives new insight into ligand-specific integrin pathways and merits further evaluation as a selective anti-inflammatory therapy in cardiovascular disease.

Single-Cell RNA-Seq Reveals the Transcriptional Landscape and Heterogeneity of Aortic Macrophages in Murine Atherosclerosis

RATIONALE

It is assumed that atherosclerotic arteries contain several macrophage subsets endowed with specific functions. The precise identity of these subsets is poorly characterized as they have been defined by the expression of a restricted number of markers.

OBJECTIVE

We have applied single-cell RNA sequencing as an unbiased profiling strategy to interrogate and classify aortic macrophage heterogeneity at the single-cell level in atherosclerosis.

METHOD AND RESULTS

We performed single-cell RNA sequencing of total aortic CD45⁺ cells extracted from the nondiseased (chow fed) and atherosclerotic (11 weeks of high-fat diet) aorta of low-density lipoprotein receptor-deficient (Ldlr^-/-) mice. Unsupervised clustering singled out 13 distinct aortic cell clusters. Among the myeloid cell populations, resident-like macrophages with a gene expression profile similar to aortic resident macrophages were found in healthy and diseased aortas, whereas monocytes, monocyte-derived dendritic cells, and 2 populations of macrophages were almost exclusively detectable in atherosclerotic aortas, comprising inflammatory macrophages showing enrichment in Il1b and previously undescribed TREM2^hi (triggered receptor expressed on myeloid cells 2) macrophages showing enrichment in Trem2. Differential gene expression and gene ontology enrichment analyses revealed specific gene expression patterns distinguishing these 3 macrophage subsets and monocyte-derived dendritic cells and uncovered putative functions of each cell type. Notably, TREM2^hi macrophages seemed to be endowed with specialized functions in lipid metabolism and catabolism and presented a gene expression signature reminiscent of osteoclasts, suggesting a role in lesion calcification. TREM2 expression was moreover detected in human lesional macrophages. Importantly, these macrophage populations were present also in advanced atherosclerosis and in Apoe^-/- aortas, indicating relevance of our findings in different stages of atherosclerosis and mouse models.

CONCLUSIONS

These data unprecedentedly uncovered the transcriptional landscape and phenotypic heterogeneity of aortic macrophages and monocyte-derived dendritic cells in atherosclerotic and identified previously unrecognized macrophage populations and their gene expression signature, suggesting specialized functions. Our findings will open up novel opportunities to explore distinct myeloid cell populations and their functions in atherosclerosis.

Atlas of the Immune Cell Repertoire in Mouse Atherosclerosis Defined by Single-Cell RNA-Sequencing and Mass Cytometry

RATIONALE

Atherosclerosis is a chronic inflammatory disease that is driven by the interplay of pro- and anti-inflammatory leukocytes in the aorta. Yet, the phenotypic and transcriptional diversity of aortic leukocytes is poorly understood.

OBJECTIVE

We characterized leukocytes from healthy and atherosclerotic mouse aortas in-depth by single-cell RNA-sequencing and mass cytometry (cytometry by time of flight) to define an atlas of the immune cell landscape in atherosclerosis.

METHODS AND RESULTS

Using single-cell RNA-sequencing of aortic leukocytes from chow diet- and Western diet-fed Apoe^-/- and Ldlr^-/- mice, we detected 11 principal leukocyte clusters with distinct phenotypic and spatial characteristics while the cellular repertoire in healthy aortas was less diverse. Gene set enrichment analysis on the single-cell level established that multiple pathways, such as for lipid metabolism, proliferation, and cytokine secretion, were confined to particular leukocyte clusters. Leukocyte populations were differentially regulated in atherosclerotic Apoe^-/- and Ldlr^-/- mice. We confirmed the phenotypic diversity of these clusters with a novel mass cytometry 35-marker panel with metal-labeled antibodies and conventional flow cytometry. Cell populations retrieved by these protein-based approaches were highly correlated to transcriptionally defined clusters. In an integrated screening strategy of single-cell RNA-sequencing, mass cytometry, and fluorescence-activated cell sorting, we detected 3 principal B-cell subsets with alterations in surface markers, functional pathways, and in vitro cytokine secretion. Leukocyte cluster gene signatures revealed leukocyte frequencies in 126 human plaques by a genetic deconvolution strategy. This approach revealed that human carotid plaques and microdissected mouse plaques were mostly populated by macrophages, T-cells, and monocytes. In addition, the frequency of genetically defined leukocyte populations in carotid plaques predicted cardiovascular events in patients.

CONCLUSIONS

The definition of leukocyte diversity by high-dimensional analyses enables a fine-grained analysis of aortic leukocyte subsets, reveals new immunologic mechanisms and cell-type-specific pathways, and establishes a functional relevance for lesional leukocytes in human atherosclerosis.

Transmission of integrin β7 transmembrane domain topology enables gut lymphoid tissue development

Sun et al. establish the importance of transmission of changes in β-integrin transmembrane domain (TMD) topology in physiological integrin affinity modulation and biological function. Introduction of a flexible kink in the β7 integrin TMD blocks talin-mediated agonist-induced α4β7 integrin activation and function in gut lymphoid tissue development.

Integrin activation regulates adhesion, extracellular matrix assembly, and cell migration, thereby playing an indispensable role in development and in many pathological processes. A proline mutation in the central integrin β3 transmembrane domain (TMD) creates a flexible kink that uncouples the topology of the inner half of the TMD from the outer half. In this study, using leukocyte integrin α4β7, which enables development of gut-associated lymphoid tissue (GALT), we examined the biological effect of such a proline mutation and report that it impairs agonist-induced talin-mediated activation of integrin α4β7, thereby inhibiting rolling lymphocyte arrest, a key step in transmigration. Furthermore, the α4β7(L721P) mutation blocks lymphocyte homing to and development of the GALT. These studies show that impairing the ability of an integrin β TMD to transmit talin-induced TMD topology inhibits agonist-induced physiological integrin activation and biological function in development.

A ligand-specific blockade of the integrin Mac-1 selectively targets pathologic inflammation while maintaining protective host-defense

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.

Inflammatory Pathways Regulated by Tumor Necrosis Receptor-Associated Factor 1 Protect From Metabolic Consequences in Diet-Induced Obesity

RATIONALE

The coincidence of inflammation and metabolic derangements in obese adipose tissue has sparked the concept of met-inflammation. Previous observations, however, suggest that inflammatory pathways may not ultimately cause dysmetabolism.

OBJECTIVE

We have revisited the relationship between inflammation and metabolism by testing the role of TRAF (tumor necrosis receptor-associated factor)-1, an inhibitory adapter of inflammatory signaling of TNF (tumor necrosis factor)-α, IL (interleukin)-1β, and TLRs (toll-like receptors).

METHODS AND RESULTS

Mice deficient for TRAF-1, which is expressed in obese adipocytes and adipose tissue lymphocytes, caused an expected hyperinflammatory phenotype in adipose tissue with enhanced adipokine and chemokine expression, increased leukocyte accumulation, and potentiated proinflammatory signaling in macrophages and adipocytes in a mouse model of diet-induced obesity. Unexpectedly, TRAF-1^-/- mice were protected from metabolic derangements and adipocyte growth, failed to gain weight, and showed improved insulin resistance-an effect caused by increased lipid breakdown in adipocytes and UCP (uncoupling protein)-1-enabled thermogenesis. TRAF-1-dependent catabolic and proinflammatory cues were synergistically driven by β3-adrenergic and inflammatory signaling and required the presence of both TRAF-1-deficient adipocytes and macrophages. In human obesity, TRAF-1-dependent genes were upregulated.

CONCLUSIONS

Enhancing TRAF-1-dependent inflammatory pathways in a gain-of-function approach protected from metabolic derangements in diet-induced obesity. These findings identify TRAF-1 as a regulator of dysmetabolism in mice and humans and question the pathogenic role of chronic inflammation in metabolism.

Structure-function characterization of three human antibodies targeting the vaccinia virus adhesion molecule D8

Vaccinia virus (VACV) envelope protein D8 is one of three glycosaminoglycan adhesion molecules and binds to the linear polysaccharide chondroitin sulfate (CS). D8 is also a target for neutralizing antibody responses that are elicited by the smallpox vaccine, which has enabled the first eradication of a human viral pathogen and is a useful model for studying antibody responses. However, to date, VACV epitopes targeted by human antibodies have not been characterized at atomic resolution. Here, we characterized the binding properties of several human anti-D8 antibodies and determined the crystal structures of three VACV-mAb variants, VACV-66, VACV-138, and VACV-304, separately bound to D8. Although all these antibodies bound D8 with high affinity and were moderately neutralizing in the presence of complement, VACV-138 and VACV-304 also fully blocked D8 binding to CS-A, the low affinity ligand for D8. VACV-138 also abrogated D8 binding to the high-affinity ligand CS-E, but we observed residual CS-E binding was observed in the presence of VACV-304. Analysis of the VACV-138- and VACV-304-binding sites along the CS-binding crevice of D8, combined with different efficiencies of blocking D8 adhesion to CS-A and CS-E allowed us to propose that D8 has a high- and low-affinity CS-binding region within its central crevice. The crevice is amenable to protein engineering to further enhance both specificity and affinity of binding to CS-E. Finally, a wild-type D8 tetramer specifically bound to structures within the developing glomeruli of the kidney, which express CS-E. We propose that through structure-based protein engineering, an improved D8 tetramer could be used as a potential diagnostic tool to detect expression of CS-E, which is a possible biomarker for ovarian cancer.

Rolling neutrophils form tethers and slings under physiologic conditions in vivo

Human and mouse neutrophils are known to form tethers when rolling on selectins in vitro. Tethers are ∼0.2 μm thin, ∼5-10 μm-long structures behind rolling cells that can swing around to form slings that serve as self-adhesive substrates. Here, we developed a mouse intravital imaging method, where the neutrophil surface is labeled by injecting fluorescently labeled mAb to Ly-6G. Venules in the cremaster muscle of live mice were imaged at a high frame rate using a confocal microscope equipped with a fast resonant scanner. We observed 270 tethers (median length 3.5 μm) and 31 slings (median length 6.9 µm) on 186 neutrophils of 15 mice. Out of 199 tether break events, 123 were followed by immediate acceleration of the rolling cell, which shows that tethers are load-bearing structures in vivo. In venules with a high wall shear stress (WSS; > 12 dyn/cm² ), median rolling velocity was higher (19 μm/s), and 43% of rolling neutrophils had visible tethers. In venules with WSS < 12 dyn/cm² , only 26% of rolling neutrophils had visible tethers. We conclude that neutrophil tethers are commonly present and stabilize rolling in vivo.

Patrolling Mechanics of Non-Classical Monocytes in Vascular Inflammation

Non-classical monocytes have emerged as the preeminent vascular housekeepers. Continuous intravascular screening is enabled by slow patrolling on the endothelium and allows a rapid response to local perturbations. Intravital imaging has been crucial to elucidate the molecular mechanisms and migratory phenotype of patrolling. In this review, we discuss technical requirements of intravital microscopy such as imaging modalities, labeling strategies, and data analysis. We further focus on patrolling kinetics and adhesion receptors in different organs and vascular beds including arteries during homeostasis and vascular inflammation and define pertinent questions in the field.

Developing Neutrophils Must Eat…Themselves!

In this issue of Immunity, Riffelmacher et al. (2017) show that autophagy is necessary for the release of free fatty acids from intracellular stores within neutrophil precursor cells. This limits glycolysis, increases oxidative phosphorylation, and is essential for neutrophil maturation.

Scavenger Receptor CD36 Directs Nonclassical Monocyte Patrolling Along the Endothelium During Early Atherogenesis

OBJECTIVE

Nonclassical monocytes (NCM) function to maintain vascular homeostasis by crawling or patrolling along the vessel wall. This subset of monocytes responds to viruses, tumor cells, and other pathogens to aid in protection of the host. In this study, we wished to determine how early atherogenesis impacts NCM patrolling in the vasculature.

APPROACH AND RESULTS

To study the role of NCM in early atherogenesis, we quantified the patrolling behaviors of NCM in ApoE^-/- (apolipoprotein E) and C57BL/6J mice fed a Western diet. Using intravital imaging, we found that NCM from Western diet-fed mice display a 4-fold increase in patrolling activity within large peripheral blood vessels. Both human and mouse NCM preferentially engulfed OxLDL (oxidized low-density lipoprotein) in the vasculature, and we observed that OxLDL selectively induced NCM patrolling in vivo. Induction of patrolling during early atherogenesis required scavenger receptor CD36, as CD36^-/- mice revealed a significant reduction in patrolling activity along the femoral vasculature. Mechanistically, we found that CD36-regulated patrolling was mediated by a SFK (src family kinase) through DAP12 (DNAX activating protein of 12KDa) adaptor protein.

CONCLUSIONS

Our studies show a novel pathway for induction of NCM patrolling along the vascular wall during early atherogenesis. Mice fed a Western diet showed increased NCM patrolling activity with a concurrent increase in SFK phosphorylation. This patrolling activity was lost in the absence of either CD36 or DAP12. These data suggest that NCM function in an atheroprotective manner through sensing and responding to oxidized lipoprotein moieties via scavenger receptor engagement during early atherogenesis.

Natural variation of macrophage activation as disease-relevant phenotype predictive of inflammation and cancer survival

Although mouse models exist for many immune-based diseases, the clinical translation remains challenging. Most basic and translational studies utilize only a single inbred mouse strain. However, basal and diseased immune states in humans show vast inter-individual variability. Here, focusing on macrophage responses to lipopolysaccharide (LPS), we use the hybrid mouse diversity panel (HMDP) of 83 inbred strains as a surrogate for human natural immune variation. Since conventional bioinformatics fail to analyse a population spectrum, we highlight how gene signatures for LPS responsiveness can be derived based on an Interleukin-12β and arginase expression ratio. Compared to published signatures, these gene markers are more robust to identify susceptibility or resilience to several macrophage-related disorders in humans, including survival prediction across many tumours. This study highlights natural activation diversity as a disease-relevant dimension in macrophage biology, and suggests the HMDP as a viable tool to increase translatability of mouse data to clinical settings.

The inter-individual variation of the immune system broadly impacts pathophysiology. Here, the authors use the hybrid mouse diversity panel as a surrogate for human natural immune variation and derive a macrophages gene signature robustly correlating with susceptibility to macrophage-related disorders in humans.

Differential DARC/ACKR1 expression distinguishes venular from non-venular endothelial cells in murine tissues

BACKGROUND

Intravascular leukocyte recruitment in most vertebrate tissues is restricted to postcapillary and collecting venules, whereas capillaries and arterioles usually support little or no leukocyte adhesion. This segmental restriction is thought to be mediated by endothelial, rather than hemodynamic, differences. The underlying mechanisms are largely unknown, in part because effective tools to distinguish, isolate, and analyze venular endothelial cells (V-ECs) and non-venular endothelial cells (NV-ECs) have been unavailable. We hypothesized that the atypical chemokine receptor DARC (Duffy Antigen Receptor for Chemokines, a.k.a. ACKR1 or CD234) may distinguish V-ECs versus NV-ECs in mice.

METHODS

We generated a rat-anti-mouse monoclonal antibody (MAb) that specifically recognizes the erythroid and endothelial forms of native, surface-expressed DARC. Using this reagent, we characterized DARC expression and distribution in the microvasculature of murine tissues.

RESULTS

DARC was exquisitely restricted to post-capillary and small collecting venules and completely absent from arteries, arterioles, capillaries, veins, and most lymphatics in every tissue analyzed. Accordingly, intravital microscopy showed that adhesive leukocyte-endothelial interactions were restricted to DARC+ venules. DARC was detectable over the entire circumference of V-ECs, but was more concentrated at cell-cell junctions. Analysis of single-cell suspensions suggested that the frequency of V-ECs among the total microvascular EC pool varies considerably between different tissues.

CONCLUSIONS

Immunostaining of endothelial DARC allows the identification and isolation of intact V-ECs from multiple murine tissues. This strategy may be useful to dissect the mechanisms underlying segmental microvascular specialization in healthy and diseased tissues and to characterize the role of EC subsets in tissue-homeostasis, immune surveillance, infection, inflammation, and malignancies.

MHC class II tetramers identify apolipoprotein B-specific CD4 T cells in mice and humans

Immunization with atherosclerotic plaque antigens including MHC-II-restricted peptides from apolipoprotein B (ApoB, core protein of low density lipoprotein, LDL) is known to be atheroprotective in animal models. Herein we report a human ApoB peptide whose sequence is identical to mouse ApoB and which binds to both mouse (I-Ab) and human MHC class II. Immunization with this peptide reduced atherosclerotic plaque in the aortas of Apoe−/− mice fed with a Western diet and induced IL-10 producing CD25+FoxP3+ regulatory T cells (Tregs). After peptide restimulation in vivo, responding CD4+ T cells (by Nur77-GFP) were highly enriched in CD25+FoxP3+ Tregs. Using a novel I-Ab tetramer reagent, we further showed that peptide-specific CD4+ T cells expand upon vaccination, ~50% express FoxP3, and were 10-fold enriched for IL-10 producers. In human peripheral blood mononuclear cells (PBMCs) from individuals assessed for subclinical cardiovascular disease, ApoB-specific CD4+ T cells were detected using novel HLA-DR1/ApoB peptide tetramers. Peptide-specific CD4+ T cells from donors with carotid artery plaques showed more RORγt expression and less FoxP3 expression compared to those without. This is the first demonstration of ApoB-specific CD4+ T cells by tetramer in humans and the first animal model evidence of atheroprotective vaccination with a human ApoB peptide.

Abstract 44: Failure of Protective Autoimmunity in Mouse and Human Atherosclerosis

Background: In atherosclerosis, CD4 + T helper cells recognize auto-antigens including ApoB, the main protein in low-density lipoprotein (LDL). However, atherosclerosis-specific, auto-reactive CD4 + T cells have not been detected in vivo , and their function is unknown.

Methods and Results: We have previously identified peptides derived from mouse ApoB that bind with high affinity to the MHC class II molecule of C57BL/6 mice (I-A b ). We designed and validated a new multimer of a recombinant MHC-II molecule fused to one ApoB auto-epitopes, P6 (TGAYSNASSTESASY, P6:I-A b ), that enabled detection of low-affinity, P6-reactive CD4 + T cells. Using this P6:I-A b multimer, we identified ApoB-reactive CD4 + T cells in healthy, young C57BL/6 mice that were predominately differentiated T-regulatory cells (T regs ) and expressed IL-10, a known atheroprotective cytokine. This population was detectable in lymph nodes and already showed a memory phenotype in young animals without atherosclerosis. In Apoe -/- mice, adoptively transferred ApoB P6-specific T regs accumulated in the aorta and draining lymph nodes and gave rise to pathogenic T H 1 and T H 17 cells. This phenotypic switch was caused by enhanced plasticity of antigen-specific T regs as evidenced by multiple clusters of intermediate T reg -T eff phenotypes in single cell RNA sequencing of 4485 antigen-specific CD4 + T cells. In the plaque, many T cells were ex-T regs as identified by a FoxP3 lineage tracker mouse, suggesting that atherosclerosis-specific CD4 + T cells lost their regulatory capacity. Vaccination with P6 maintained a protective phenotype in antigen-specific T regs and protected from atherosclerosis. In humans, ApoB-specific CD4 + T cells from atherosclerotic patients showed the same cytokine patterns found in mouse CD4 + T cells, suggesting that autoimmunity to ApoB is protective first, but later gives rise to a pathogenic CD4 + T cell response that aggravates atherosclerosis.

Conclusion: Protective T-regulatory cells recognizing peptide antigens of ApoB exist in naïve mice, protect against atherosclerosis, but convert into pathogenic T H 1 and -17 cells during the natural course of disease in mice and humans. These results call for immunomodulatory therapies to maintain protective autoimmunity.

CD27 co-stimulation increases regulatory T cell survival and limits progression of atherosclerosis

Co-stimulatory signals are important shaping adaptive immunity which is pivotal to all stages of atherosclerosis. The interaction of the co-stimulatory molecules CD27 and CD70 modulates Treg development. Furthermore, T cell activation, proliferation, and also differentiation are affected by CD27-CD70 interactions. Thus, we hypothesized that the deficiency of CD27 will lead to exacerbated atherosclerosis.

Cd27−/− mice were crossed with Apoe−/− mice. Cd27−/−Apoe−/− and littermate controls (Cd27+/+Apoe−/−) were sacrificed at the age of 18 and 28 weeks. Bone marrow of Cd27−/−Apoe−/− and littermate controls was transplanted into Apoe−/− recipient mice. Lesion size, histology and cellular composition were analyzed in the ascending aorta.

Human and murine T cells co-localize with CD27 in atherosclerotic lesions. At early stages of atherosclerosis, Cd27−/−Apoe−/− mice displayed bigger and more complex atherosclerotic lesions. Flow cytometry revealed a significant decrease in abundance of splenic and aortic Tregs and increased apoptosis of Treg in the thymus of Cd27−/−Apoe−/− mice. In contrast, at later stages of atherosclerosis (28 weeks), splenic and lesional Treg content and atherosclerotic burden were similar. Transplantation of Cd27−/−Apoe−/− bone marrow, in contrast to control, induced increased atherosclerotic plaque size and a profound pro-inflammatory plaque phenotype accompanied by reduced frequency of aortic and splenic Tregs. Furthermore, the number of circulating leukocytes was increased.

Taken together, CD27 deficiency impairs thymic Treg development which exacerbates early atherogenesis. However, the role of CD27–CD70 interactions does not affect later stages of atherosclerosis.

Defects in DNA replication hit NK cells and neutrophils

Patients who present with unique immunological phenotypes provide an opportunity to better understand defect-driving mutations. In this issue of the JCI, Cottineau and colleagues characterize 5 individuals who exhibited growth restriction, facial deformities, and a history of bacterial and viral infection. Further characterization revealed that these patients were neutropenic and NK cell deficient. These phenotypes were unexpectedly linked to mutations in the gene encoding a subunit of the Go-Ichi-Ni-San (GINS) complex, which is essential for DNA replication prior to cell division. Together, the results of this study lay the groundwork for future studies to explore the role of DNA replication in immune cell generation and function.

ATVB Distinguished Scientist Award: How Costimulatory and Coinhibitory Pathways Shape Atherosclerosis

OBJECTIVE

Immune cells play a critical role in atherosclerosis. Costimulatory and coinhibitory molecules of the tumor necrosis factor receptor and CD28 immunoglobulin superfamilies not only shape T-cell and B-cell responses but also have a major effect on antigen-presenting cells and nonimmune cells.

APPROACH AND RESULTS

Pharmacological inhibition or activation of costimulatory and coinhibitory molecules and genetic deletion demonstrated their involvement in atherosclerosis. This review highlights recent advances in understanding how costimulatory and coinhibitory pathways shape the immune response in atherosclerosis.

CONCLUSIONS

Insights gained from costimulatory and coinhibitory molecule function in atherosclerosis may inform future therapeutic approaches.

Endothelial Protective Monocyte Patrolling in Large Arteries Intensified by Western Diet and Atherosclerosis

Supplemental Digital Content is available in the text.

Rationale:

Nonclassical mouse monocyte (CX3CR1^high, Ly-6C^low) patrolling along the vessels of the microcirculation is critical for endothelial homeostasis and inflammation. Because of technical challenges, it is currently not established how patrolling occurs in large arteries.

Objective:

This study was undertaken to elucidate the molecular, migratory, and functional phenotypes of patrolling monocytes in the high shear and pulsatile environment of large arteries in healthy, hyperlipidemic, and atherosclerotic conditions.

Methods and Results:

Applying a new method for stable, long-term 2-photon intravital microscopy of unrestrained large arteries in live CX3CR1-GFP (green fluorescent protein) mice, we show that nonclassical monocytes patrol inside healthy carotid arteries at a velocity of 36 μm/min, 3× faster than in microvessels. The tracks are less straight but lead preferentially downstream. The number of patrolling monocytes is increased 9-fold by feeding wild-type mice a Western diet or by applying topical TLR7/8 (Toll-like receptor) agonists. A similar increase is seen in CX3CR1^+/GFP/apoE^−/− mice on chow diet, with a further 2- to 3-fold increase on Western diet (22-fold over healthy). In plaque conditions, monocytes are readily captured onto the endothelium from free flow. Stable patrolling is unaffected in CX3CR1-deficient mice and involves the contribution of LFA-1 (lymphocyte-associated antigen 1) and α₄ integrins. The endothelial damage in atherosclerotic carotid arteries was assessed by electron microscopy and correlates with the number of intraluminal patrollers. Abolishing patrolling monocytes in Nr4a1^−/− apoE^−/− mice leads to pronounced endothelial apoptosis.

Conclusions:

Arterial patrolling is a prominent new feature of nonclassical monocytes with unique molecular and kinetic properties. It is highly upregulated in hyperlipidemia and atherosclerosis in a CX3CR1-independent fashion and plays a potential role in endothelial protection.

Atheroprotective vaccination with MHC-II-restricted ApoB peptides induces peritoneal IL-10-producing CD4 T cells

Although immunization with major histocompatibility complex (MHC) class II-restricted apolipoprotein B (ApoB) peptides has been shown to be atheroprotective, the mechanism is unclear. Here, we investigated CD4⁺ T cell populations in immunized atherosclerotic mice. Peptides (16-mers) from mouse ApoB, the core protein of low-density lipoprotein (LDL), were screened for binding to I-A^b by computer prediction and confirmed by radiolabeled peptide competition. Three new peptides, P101 (FGKQGFFPDSVNKALY, 5.5 nM IC₅₀), P102 (TLYALSHAVNSYFDVD, 6.8 nM), and P103 (LYYKEDKTSLSASAAS, 95 nM), were tested in an atherosclerosis model (Apoe^-/- mice on Western diet). Immunization with each of the three peptides (1 time in complete Freund's adjuvant subcuntaneously and 4 time in incomplete Freund's adjuvant intraperitoneally) but not with adjuvant alone showed significantly reduced atherosclerotic plaques in the aortic root by serial sections and in the whole aorta by en face staining. There were no differences in body weight, LDL cholesterol, or triglycerides. Peritoneal leukocytes from ApoB peptide-immunized mice, but not control mice, secreted significant amounts of IL-10 (150 pg/ml). Flow cytometry showed that peptide immunization induced IL-10 in 10% of peritoneal CD4⁺ T cells, some of which also expressed chemokine (C-C motif) receptor 5 (CCR5). Vaccination with ApoB peptides expanded peritoneal FoxP3⁺ regulatory CD4⁺ T cells and more than tripled the number of CCR5⁺FoxP3⁺ cells. Similar trends were also seen in the draining mediastinal lymph nodes but not in the nondraining inguinal lymph nodes. We conclude that vaccination with MHC class II-restricted autologous ApoB peptides induces regulatory T cells (Tregs) and IL-10, suggesting a plausible mechanism for atheroprotection.NEW & NOTEWORTHY Vaccination against apolipoprotein B (ApoB), the protein of LDL, attracts attention as a novel approach to prevent atherosclerosis. We discovered major histocompatibility complex class II-restricted ApoB peptides, which reduce atherosclerosis and induce IL-10-producing CD4⁺ T cells and chemokine (C-C motif) receptor 5 expression on regulatory T cells, suggesting that immunization with ApoB peptides inhibits atherosclerosis by inducing anti-inflammatory cytokines.

MISTICA: Minimum Spanning Tree-Based Coarse Image Alignment for Microscopy Image Sequences

Registration of an in vivo microscopy image sequence is necessary in many significant studies, including studies of atherosclerosis in large arteries and the heart. Significant cardiac and respiratory motion of the living subject, occasional spells of focal plane changes, drift in the field of view, and long image sequences are the principal roadblocks. The first step in such a registration process is the removal of translational and rotational motion. Next, a deformable registration can be performed. The focus of our study here is to remove the translation and/or rigid body motion that we refer to here as coarse alignment. The existing techniques for coarse alignment are unable to accommodate long sequences often consisting of periods of poor quality images (as quantified by a suitable perceptual measure). Many existing methods require the user to select an anchor image to which other images are registered. We propose a novel method for coarse image sequence alignment based on minimum weighted spanning trees (MISTICA) that overcomes these difficulties. The principal idea behind MISTICA is to reorder the images in shorter sequences, to demote nonconforming or poor quality images in the registration process, and to mitigate the error propagation. The anchor image is selected automatically making MISTICA completely automated. MISTICA is computationally efficient. It has a single tuning parameter that determines graph width, which can also be eliminated by the way of additional computation. MISTICA outperforms existing alignment methods when applied to microscopy image sequences of mouse arteries.

Leukocyte arrest: Biomechanics and molecular mechanisms of β2 integrin activation

Integrins are a group of heterodimeric transmembrane receptors that play essential roles in cell-cell and cell-matrix interaction. Integrins are important in many physiological processes and diseases. Integrins acquire affinity to their ligand by undergoing molecular conformational changes called activation. Here we review the molecular biomechanics during conformational changes of integrins, integrin functions in leukocyte biorheology (adhesive functions during rolling and arrest) and molecules involved in integrin activation.

Neutrophil recruitment limited by high-affinity bent β2 integrin binding ligand in cis

Neutrophils are essential for innate immunity and inflammation and many neutrophil functions are β2 integrin-dependent. Integrins can extend (E(+)) and acquire a high-affinity conformation with an 'open' headpiece (H(+)). The canonical switchblade model of integrin activation proposes that the E(+) conformation precedes H(+), and the two are believed to be structurally linked. Here we show, using high-resolution quantitative dynamic footprinting (qDF) microscopy combined with a homogenous conformation-reporter binding assay in a microfluidic device, that a substantial fraction of β2 integrins on human neutrophils acquire an unexpected E(-)H(+) conformation. E(-)H(+) β2 integrins bind intercellular adhesion molecules (ICAMs) in cis, which inhibits leukocyte adhesion in vitro and in vivo. This endogenous anti-inflammatory mechanism inhibits neutrophil aggregation, accumulation and inflammation.

Microfluidics-based side view flow chamber reveals tether-to-sling transition in rolling neutrophils

Neutrophils rolling at high shear stress (above 6 dyn/cm²) form tethers in the rear and slings in the front. Here, we developed a novel photo-lithographically fabricated, silicone(PDMS)-based side-view flow chamber to dynamically visualize tether and sling formation. Fluorescently membrane-labeled mouse neutrophils rolled on P-selectin substrate at 10 dyn/cm². Most rolling cells formed 5 tethers that were 2–30 μm long. Breaking of a single tether caused a reproducible forward microjump of the cell, showing that the tether was load-bearing. About 15% of all tether-breaking events resulted in slings. The tether-to-sling transition was fast (<100 ms) with no visible material extending above the rolling cell, suggesting a very low bending modulus of the tether. The sling downstream of the rolling cell aligned according to the streamlines before landing on the flow chamber. These new observations explain how slings form from tethers and provide insight into their biomechanical properties.

Live cell imaging to understand monocyte, macrophage, and dendritic cell function in atherosclerosis

Ley et al. provide a review of the technology and accomplishments of dynamic imaging of myeloid cells in atherosclerosis.

Intravital imaging is an invaluable tool for understanding the function of cells in healthy and diseased tissues. It provides a window into dynamic processes that cannot be studied by other techniques. This review will cover the benefits and limitations of various techniques for labeling and imaging myeloid cells, with a special focus on imaging cells in atherosclerotic arteries. Although intravital imaging is a powerful tool for understanding cell function, it alone does not provide a complete picture of the cell. Other techniques, such as flow cytometry and transcriptomics, must be combined with intravital imaging to fully understand a cell's phenotype, lineage, and function.

How Mouse Macrophages Sense What Is Going On

Macrophages are central to both innate and adaptive immunity. With few exceptions, macrophages are the first cells that sense trouble and respond to disturbances in almost all tissues and organs. They sense their environment, inhibit or kill pathogens, take up apoptotic and necrotic cells, heal tissue damage, and present antigens to T cells. Although the origins (yolk sac versus monocyte-derived) and phenotypes (functions, gene expression profiles, surface markers) of macrophages vary between tissues, they have many receptors in common that are specific to one or a few molecular species. Here, we review the expression and function of almost 200 key macrophage receptors that help the macrophages sense what is going on, including pathogen-derived molecules, the state of the surrounding tissue cells, apoptotic and necrotic cell death, antibodies and immune complexes, altered self molecules, extracellular matrix components, and cytokines, including chemokines.

Abstract 351: MHC-II Tetramer-based Isolation of Atherosclerosis Autoantigen-specific T Cells

Background: Immune cells, including T cells, figure prominently in atherosclerosis. T cells in the atherosclerotic plaque have been proposed to recognize self-antigen, such as peptides derived from Apolipoprotein B-100. However, antigen-specific T cells have not been identified in atherosclerosis. Here, we aimed to establish and optimize a novel MHC-II multimer based assay to detect and isolate T cells specifically recognizing the ApoB-100 amino residues 978-993. We have previously shown that vaccination with this peptide protects mice from atherosclerosis.

Materials and Methods: We screened peptides from mouse ApoB-100 for high affinity binding to the mouse C57BL/6 MHC-II molecule I-Ab and designed a novel MHC-II-tetramer in order to detect CD4 + T cells that specifically recognize one of these peptide epitopes (ApoB-100 978-993) by flow cytometry. This tetramer (P6:I-Ab) consists of four biotinylated MHC-II monomers that are linked by a streptavidin molecule and display the ApoB-100 978-993 peptide in the MHC-II binding grove. It was expressed in a drosophila expression system and subsequently labeled with PE or APC. T cells from mice immunized with the ApoB-100 978-993 peptide were isolated from spleen and lymph nodes, enriched for CD4 + T cells by negative selection, labeled with a mixture of APC- and PE-labeled ApoB-100 978-993 tetramer and positively selected by magnetic beads to PE and APC. P6:I-Ab binding co-localized with CD4 in confocal microscopy. ApoB-100 978-993 P6:I-Ab + T cells showed higher expression of the activation marker CD44 than P6:I-Ab - T cells. When Nur77-GFP transgenic mice were immunized with ApoB-100 978-993, P6:I-Ab + CD4 + T cells showed enhanced GFP signal by flow cytometry, indicating T Cell Receptor signaling. By optimizing these methods, we successfully increased the percentage and absolute number of P6:I-Ab + T cells by 50-100-fold compared to un-enriched samples. The negative control tetramer (2WIS) bound very few T cells.

Conclusion: We have successfully established a method to isolate atherosclerosis autoantigen-specific T cells from mice immunized with ApoB-100 978-993.

SLAT promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion through interaction of its PH domain with Rap1

SLAT (also known as DEF6) promotes T cell activation and differentiation by regulating NFAT-Ca2+ signaling. However, its role in TCR-mediated inside-out signaling, which induces integrin activation and T cell adhesion, a central process in T cell immunity and inflammation, has not been explored. Here, we show that SLAT is crucial for TCR-induced adhesion to ICAM-1 and LFA-1 affinity maturation in CD4+ T cells. Mechanistic studies revealed that SLAT interacts, through its PH domain, with a key component of inside-out signaling, namely the active form of the small GTPase Rap1. This interaction has been further shown to facilitate the interdependent recruitment of Rap1 and SLAT to the T cell immunological synapse upon TCR engagement. Furthermore, a SLAT mutant lacking its PH domain drastically inhibited LFA-1 activation and CD4+ T cell adhesion. Finally, we established that a constitutively active form of Rap1, which is present at the plasma membrane, rescues the defective LFA-1 activation and ICAM-1 adhesion in SLAT-deficient (Def6−/−) T cells. These findings ascribe a new function to SLAT, and identify Rap1 as a target of SLAT function in TCR-mediated inside-out signaling.

Abstract 239: Deficiency of Macrophage Epsins Impedes Atherosclerosis by Inhibiting LRP-1 Internalization and Degradation

Background: Atherosclerosis is caused by the chronic activation of the vascular endothelium and immune and inflammatory cell infiltration of the vascular wall, leading to enhanced inflammation and lipid accumulation. Understanding the molecular mechanisms underlying this disease is critical for the development of new therapies. Epsins are a family of ubiquitin-binding endocytic adaptors. However, their role in vascular inflammation is poorly understood. Our goal is to define the novel role of epsins in regulating atherogenesis.

Methods and Results: We engineered mice with specific deletion of epsins in myeloid cells (MΦ-DKO). Strikingly, MΦ-DKO mice on an ApoE-/- background fed western diet exhibited reduced atherosclerotic lesion and foam cell accumulation, and diminished recruitment of immune or inflammatory cells to aortas by FACS analysis. In primary macrophages, epsin deficiency impaired foam cell formation by Oil Red O staining, and suppressed the pro-inflammatory M1 macrophage phenotype but increased the anti-inflammatory macrophage phenotype by gene profiling. Epsin deficiency did not alter levels of LDL scavenger receptors, or reverse cholesterol transport proteins, but did increase total and surface levels of LRP-1, a protein with anti-inflammatory and anti-atherosclerotic properties. Mechanistically, Epsin interacts with LRP-1 via epsin’s UIM domain. LPS treatment increased LRP-1 ubiquitination and subsequent binding to epsin, suggesting that epsin promotes the ubiquitin-dependent internalization and degradation of LRP-1. Accordingly, macrophages isolated from MΦ-DKO mice on LRP-1 heterozygous background restored the pro-inflammatory phenotype.

Conclusions: Epsins promote atherogenesis by facilitating pro-inflammatory macrophage recruitment and potentiating foam cell formation by downregulating LRP-1 implicating that targeting the epsin-LRP-1 interaction may serve as a novel therapeutic strategy to treat atheromas.