Mononuclear phagocytes egress from an in vitro model of the vascular wall by migrating across endothelium in the basal to apical direction: role of intercellular adhesion molecule 1 and the CD11/CD18 integrins

Dendritic cell development in infection

The immune system protects from infections primarily by detecting and eliminating invading pathogens. This is predominantly mediated by innate immune cells like neutrophils, monocytes and dendritic cells (DCs) expressing specific receptors recognizing pathogen-associated molecular patterns. DC activation by pathogens leads to the initiation of antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. However, various pathogens have evolved immune evasion strategies to ensure their survival. In this review, we highlight recent findings on how various microorganisms or their structural features affect or modulate DC development and whether this has any consequences for a protective immune response.

Angiopoietin-1 promotes atherosclerosis by increasing the proportion of circulating Gr1+ monocytes

Aims

Atherosclerosis is a chronic inflammatory disease occurring within the artery wall. A crucial step in atherogenesis is the infiltration and retention of monocytes into the subendothelial space of large arteries induced by chemokines and growth factors. Angiopoietin-1 (Ang-1) regulates angiogenesis and reduces vascular permeability and has also been reported to promote monocyte migration in vitro. We investigated the role of Ang-1 in atherosclerosis-prone apolipoprotein-E (Apo-E) knockout mouse.

Methods and results

Apo-E knockout (Apo-E^-/-) mice fed a western or normal chow diet received a single iv injection of adenovirus encoding Ang-1 or control vector. Adenovirus-mediated systemic expression of Ang-1 induced a significant increase in early atherosclerotic lesion size and monocyte/macrophage accumulation compared with control animals receiving empty vector. Ang-1 significantly increased plasma MCP-1 and VEGF levels as measured by ELISA. FACS analysis showed that Ang-1 selectively increased inflammatory Gr1⁺monocytes in the circulation, while the cell-surface expression of CD11b, which mediates monocyte emigration, was significantly reduced.

Conclusions

Ang-1 specifically increases circulating Gr1⁺inflammatory monocytes and increases monocyte/macrophage retention in atherosclerotic plaques, thereby contributing to development of atherosclerosis.

Reverse transendothelial cell migration in inflammation: to help or to hinder?

The endothelium provides a strong barrier separating circulating blood from tissue. It also provides a significant challenge for immune cells in the bloodstream to access potential sites of infection. To mount an effective immune response, leukocytes traverse the endothelial layer in a process known as transendothelial migration. Decades of work have allowed dissection of the mechanisms through which immune cells gain access into peripheral tissues, and subsequently to inflammatory foci. However, an often under-appreciated or potentially ignored question is whether transmigrated leukocytes can leave these inflammatory sites, and perhaps even return across the endothelium and re-enter circulation. Although evidence has existed to support "reverse" transendothelial migration for a number of years, it is only recently that mechanisms associated with this process have been described. Here we review the evidence that supports both reverse transendothelial migration and reverse interstitial migration within tissues, with particular emphasis on some of the more recent studies that finally hint at potential mechanisms. Additionally, we postulate the biological significance of retrograde migration, and whether it serves as an additional mechanism to limit pathology, or provides a basis for the dissemination of systemic inflammation.

Reverse Migration of Neutrophils: Where, When, How, and Why?

Neutrophil migration to injured and pathogen-infected tissues is a fundamental component of innate immunity. An array of cellular and molecular events mediate this response to collectively guide neutrophils out of the vasculature and towards the core of the ensuing inflammatory reaction where they exert effector functions. Advances in imaging modalities have revealed that neutrophils can also exhibit motility away from sites of inflammation and injury, although it is unclear under what circumstances this reverse migration is a physiological protective response, and when it has pathophysiological relevance. Here we review different types of neutrophil reverse migration and discuss the current understanding of the associated mechanisms. In this context we propose clarifications to the existing terminology used to describe the many facets of neutrophil reverse migration.

Patients with acute-on-chronic liver failure have increased numbers of regulatory immune cells expressing the receptor tyrosine kinase MERTK

BACKGROUND & AIMS

Characteristics of decompensated cirrhosis and acute-on-chronic liver failure (ACLF) include susceptibility to infection, immuneparesis, and monocyte dysfunction. MER receptor tyrosine kinase (MERTK) is expressed by monocytes and macrophages and contributes to down-regulation of innate immune responses. We investigated whether MERTK expression is altered on monocytes from patients with liver failure.

METHODS

We analyzed blood and liver samples collected from patients admitted to the liver intensive therapy unit at King's College Hospital in London from December 2012 through July 2014. Patients had either ACLF (n = 41), acute decompensation of cirrhosis without ACLF (n = 9), cirrhosis without decompensation (n = 17), or acute liver failure (n = 23). We also analyzed samples from healthy individuals (controls, n = 29). We used flow cytometry to determine the level of innate immune function, and associated the findings with disease severity. We developed an assay to measure recruitment and migration of immune cells from the tissue parenchyma. Immunohistochemistry and confocal microscopy were used to determine levels of MERTK in bone marrow, liver, and lymph node tissues. We performed immunophenotype analyses and measured the production of tumor necrosis factor and interleukin 6 and intracellular killing of Escherichia coli by monocytes and peritoneal macrophages incubated with lipopolysaccharide, with or without an inhibitor of MERTK (UNC569).

RESULTS

The number of monocytes and macrophages that expressed MERTK was greatly increased in the circulation, livers, and lymph nodes of patients with ACLF, compared with patients with stable cirrhosis and controls. MERTK expression (mean fluorescence intensity) correlated with the severity of hepatic and extrahepatic disease and systemic inflammatory responses. Based on immunophenotype, migration, and functional analyses, MERTK-expressing monocytes migrate across the endothelia to localize into tissue sites and regional lymph nodes. Expression of MERTK reduced the response of cultured monocytes to lipopolysaccharide; the addition of UNC569 restored production of inflammatory cytokines in response to lipopolysaccharide.

CONCLUSIONS

Patients with ACLF have increased numbers of immunoregulatory monocytes and macrophages that express MERTK and suppress the innate immune response to microbes. The number of these cells correlates with disease severity and the inflammatory response. MERTK inhibitors restore production of inflammatory cytokines by immune cells from patients with ACLF, and might be developed to increase the innate immune response in these patients.

Leukocyte migration into inflamed tissues

Leukocyte migration through activated venular walls is a fundamental immune response that is prerequisite to the entry of effector cells such as neutrophils, monocytes, and effector T cells to sites of infection, injury, and stress within the interstitium. Stimulation of leukocytes is instrumental in this process with enhanced temporally controlled leukocyte adhesiveness and shape-changes promoting leukocyte attachment to the inner wall of blood vessels under hydrodynamic forces. This initiates polarized motility of leukocytes within and through venular walls and transient barrier disruption facilitated sequentially by stimulated vascular cells, i.e., endothelial cells and their associated pericytes. Perivascular cells such as macrophages and mast cells that act as tissue inflammatory sentinels can also directly and indirectly regulate the exit of leukocytes from the vascular lumen. In this review, we discuss current knowledge and open questions regarding the mechanisms involved in the interactions of different effector leukocytes with peripheral vessels in extralymphoid organs.

Insights how monocytes and dendritic cells contribute and regulate immune defense against microbial pathogens

The immune system protects from infections primarily by detecting and eliminating invading pathogens. Beside neutrophils, monocytes and dendritic cells (DCs) have been recently identified as important sentinels and effectors in combating microbial pathogens. In the steady state mononuclear phagocytes like monocytes and DCs patrol the blood and the tissues. Mammalian monocytes contribute to antimicrobial defense by supplying tissues with macrophage and DC precursors. DCs recognize pathogens and are essential in presenting antigens to initiate antigen-specific adaptive immune responses, thereby bridging the innate and adaptive immune systems. Both, monocytes and DCs play distinct roles in the shaping of immune response. In this review we will focus on the contributions of monocytes and lymphoid organ DCs to immune defense against microbial pathogens in the mouse and their dynamic regulation from steady state to infection.

Neutrophil heterogeneity in health and disease: a revitalized avenue in inflammation and immunity

Leucocytes form the principal cellular components of immunity and inflammation, existing as multiple subsets defined by distinct phenotypic and functional profiles. To date, this has most notably been documented for lymphocytes and monocytes. In contrast, as neutrophils are traditionally considered, to be short-lived, terminally differentiated cells that do not re-circulate, the potential existence of distinct neutrophil subsets with functional and phenotypic heterogeneity has not been widely considered or explored. A growing body of evidence is now challenging this scenario, and there is significant evidence for the existence of different neutrophil subsets under both physiological and pathological conditions. This review will summarize the key findings that have triggered a renewed interest in neutrophil phenotypic changes, both in terms of functional implications and consequences within disease models. Special emphasis will be placed on the potential pro- and anti-inflammatory roles of neutrophil subsets, as indicated by the recent works in models of ischaemia–reperfusion injury, trauma, cancer and sepsis.

Neutrophil reverse migration becomes transparent with zebrafish

The precise control of neutrophil-mediated inflammation is critical for both host defense and the prevention of immunopathology. In vivo imaging studies in zebrafish, and more recently in mice, have made the novel observation that neutrophils leave a site of inflammation through a process called neutrophil reverse migration. The application of advanced imaging techniques to the genetically tractable, optically transparent zebrafish larvae was critical for these advances. Still, the mechanisms underlying neutrophil reverse migration and its effects on the resolution or priming of immune responses remain unclear. Here, we review the current knowledge of neutrophil reverse migration, its potential roles in host immunity, and the live imaging tools that make zebrafish a valuable model for increasing our knowledge of neutrophil behavior in vivo.

Antigen-specific blocking of CD4-specific immunological synapse formation using BPI and current therapies for autoimmune diseases

In this review, we discuss T-cell activation, etiology, and the current therapies of autoimmune diseases (i.e., MS, T1D, and RA). T-cells are activated upon interaction with antigen-presenting cells (APC) followed by a "bull's eye"-like formation of the immunological synapse (IS) at the T-cell-APC interface. Although the various disease-modifying therapies developed so far have been shown to modulate the IS and thus help in the management of these diseases, they are also known to present some undesirable side effects. In this study, we describe a novel and selective way to suppress autoimmunity by using a bifunctional peptide inhibitor (BPI). BPI uses an intercellular adhesion molecule-1 (ICAM-1)-binding peptide to target antigenic peptides (e.g., proteolipid peptide, glutamic acid decarboxylase, and type II collagen) to the APC and therefore modulate the immune response. The central hypothesis is that BPI blocks the IS formation by simultaneously binding to major histocompatibility complex-II and ICAM-1 on the APC and selectively alters the activation of T cells from T(H)1 to T(reg) and/or T(H)2 phenotypes, leading to tolerance.

The junctional adhesion molecule JAM-C regulates polarized transendothelial migration of neutrophils in vivo

The migration of neutrophils into inflamed tissues is a fundamental component of innate immunity. A decisive step in this process is the polarized migration of blood neutrophils through endothelial cells (ECs) lining the venular lumen (transendothelial migration (TEM)) in a luminal-to-abluminal direction. By real-time confocal imaging, we found that neutrophils had disrupted polarized TEM ('hesitant' and 'reverse') in vivo. We noted these events in inflammation after ischemia-reperfusion injury, characterized by lower expression of junctional adhesion molecule C (JAM-C) at EC junctions, and they were enhanced by blockade or genetic deletion of JAM-C in ECs. Our results identify JAM-C as a key regulator of polarized neutrophil TEM in vivo and suggest that reverse TEM of neutrophils can contribute to the dissemination of systemic inflammation.

TLR8 agonists stimulate newly recruited monocyte-derived cells into potent APCs that enhance HBsAg immunogenicity

We previously reported that synthetic or natural Toll-like receptor (TLR) 7/8 agonists present within dead cells enhanced cell-associated antigen presentation both in vitro and in vivo. Here, we investigated the immunopotency of different chemically synthesized TLR7/8 agonists, Resiquimod, Gardiquimod, CL075, and CL097, on HBsAg immunogenicity. These agonists stimulated inflammatory monocyte-derived cells to become potent antigen-presenting dendritic cells (DCs), which augmented HBsAg specific T cell proliferation after they were conditioned with HBsAg. The TLR8 agonist CL075 and the TLR7/8 dual agonist CL097 showed more potent effects than the TLR7 agonist. Compared with alum adjuvant, when HBsAg mixed with CL075 was injected intramuscularly into mice, more monocyte-derived DCs carried antigens into draining lymph nodes and spleens. Specific Abs, particularly IgG2a, were significantly increased, and more IL-5 and IFN-gamma were produced by splenocytes and intrahepatic immunocytes in mice that received HBsAg mixed with CL075 and CL097. These results suggest that TLR8 agonists are good candidates to enhance recombinant HBsAg immunogenicity to induce specific humoral and cellular immune responses.

New observations on the trafficking and diapedesis of monocytes

PURPOSE OF REVIEW

Monocytes play multiple roles in immune system functions and inflammatory diseases such as atherosclerosis. These roles are coupled to diverse trafficking and cellular migration behaviors. Here, we review recent advances in our understanding of such behaviors with emphasis on broad scale trafficking patterns and the cellular and molecular mechanisms regulating diapedesis, a central aspect of trafficking.

RECENT FINDINGS

Monocytes consist of 'inflammatory' and 'resident' subsets, which exhibit differential functions and trafficking properties. Notably, the spleen has recently been identified as a reservoir of inflammatory monocytes, which are readily recruited to injured myocardium and possibly other tissues. Resident monocytes have been shown to undergo long-range crawling within the lumen of the microvasculature, which facilitates immune surveillance and rapid response to infection. Monocyte diapedesis has been demonstrated to utilize both para and transcellular migration routes facilitated by endothelial 'transmigratory cups'. A significant number of new adhesion molecules and signaling pathways have recently been uncovered as functional mediators and modulators of these processes.

SUMMARY

Our improving understanding of monocyte trafficking and migration mechanisms has begun to shed light on the functions of these often enigmatic cells. Continued progress in this area will be critical for elucidating roles of monocytes in disease and for developing therapeutics that target monocytes.

Diabetic conditions promote binding of monocytes to vascular smooth muscle cells and their subsequent differentiation

Diabetes is associated with significantly accelerated rates of atherosclerosis, key features of which include the presence of excessive macrophage-derived foam cells in the subendothelial space. We examined the hypothesis that enhanced monocyte-vascular smooth muscle cell (VSMC) interactions leading to subendothelial monocyte retention and differentiation to macrophages under diabetic conditions may be underlying mechanisms. Human aortic VSMCs (HVSMCs) treated with diabetic stimuli high glucose (HG) or S100B, a ligand of the receptor for advanced glycation end products, exhibited significantly increased binding of THP-1 monocytic cells. Diabetic stimuli increased the expression of the adhesive chemokine fractalkine (FKN) in HVSMCs. Pretreatment of HVSMCs with FKN or monocyte chemoattractant protein-1 (MCP-1) neutralizing antibodies significantly inhibited monocyte-VSMC binding, whereas monocytes treated with FKN showed enhanced binding to VSMC. Mouse aortic VSMCs (MVSMCs) derived from type 2 diabetic db/db mice exhibited significantly increased FKN levels and binding to mouse WEHI78/24 monocytic cells relative to nondiabetic control db/+ cells. The enhanced monocyte binding in db/db cells was abolished by both FKN and MCP-1 antibodies. Endothelium-denuded aortas from db/db mice and streptozotocin-induced diabetic mice also exhibited enhanced FKN expression and monocyte binding, relative to respective controls. Coculture with HVSMCs increased CD36 expression in THP-1 cells, and this was significantly augmented by treatment of HVSMCs with S100B or HG. CD36 mRNA and protein levels were also significantly increased in WEHI78/24 cells after coincubation with db/db MVSMCs relative to control MVSMCs. These results demonstrate that diabetic conditions may accelerate atherosclerosis by inducing key chemokines in the vasculature that promote VSMC-monocyte interactions, subendothelial monocyte retention, and differentiation.

Inflamed lymphatic endothelium suppresses dendritic cell maturation and function via Mac-1/ICAM-1-dependent mechanism

The lymphatic system is essential for the generation of immune responses by facilitating immune cell trafficking to lymph nodes. Dendritic cells (DCs), the most potent APCs, exit tissues via lymphatic vessels, but the mechanisms of interaction between DCs and the lymphatic endothelium and the potential implications of these interactions for immune responses are poorly understood. In this study, we demonstrate that lymphatic endothelial cells (LECs) modulate the maturation and function of DCs. Direct contact of human monocyte-derived DCs with an inflamed, TNF-alpha-stimulated lymphatic endothelium reduced expression of the costimulatory molecule CD86 by DCs and suppressed the ability of DCs to induce T cell proliferation. These effects were dependent on adhesive interactions between DCs and LECs that were mediated by the binding of Mac-1 on DCs to ICAM-1 on LECs. Importantly, the suppressive effects of the lymphatic endothelium on DCs were observed only in the absence of pathogen-derived signals. In vivo, DCs that migrated to the draining lymph nodes upon inflammatory stimuli, but in the absence of a pathogen, showed increased levels of CD86 expression in ICAM-1-deficient mice. Together, these data demonstrate a direct role of LECs in the modulation of immune response and suggest a function of the lymphatic endothelium in preventing undesired immune reactions in inflammatory conditions.

Direct observations of the kinetics of migrating T cells suggest active retention by endothelial cells with continual bidirectional migration

The kinetics and regulatory mechanisms of T cell migration through the endothelium have not been fully defined. In experimental, filter-based assays in vitro, transmigration of lymphocytes takes hours, compared with minutes, in vivo. We cultured endothelial cell (EC) monolayers on filters, solid substrates, or collagen gels and treated them with TNF-α, IFN-γ, or both prior to analysis of lymphocyte migration in the presence or absence of flow. PBL, CD4+ cells, or CD8+ cells took many hours to migrate through EC-filter constructs for all cytokine treatments. However, direct microscopic observations of EC filters, which had been mounted in a flow chamber, showed that PBL crossed the endothelial monolayer in minutes and were highly motile in the subendothelial space. Migration through EC was also observed on clear plastic, with or without flow. After a brief settling without flow, PBL and isolated CD3+ or CD4+ cells crossed EC in minutes, but the numbers of migrated cells varied little with time. Close observation revealed that lymphocytes migrated back and forth continuously across endothelium. Under flow, migration kinetics and the proportions migrating back and forth were altered little. On collagen gels, PBL again crossed EC in minutes and migrated back and forth but showed little penetration of the gel over hours. In contrast, neutrophils migrated efficiently through EC and into gels. These observations suggest a novel model for lymphoid migration in which EC support migration but retain lymphocytes (as opposed to neutrophils), and additional signal(s) are required for onward migration.

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.

In vivo transmigrated monocytes from patients with stable coronary artery disease have a reduced expression of CD11b

Coronary artery disease (CAD) is characterized by infiltration of monocyte derived cells in the intima of the vessel wall. We hypothesized that accumulation of these cells is caused partly by an altered monocyte transmigration process in CAD. To gain insight into this issue we applied the skin blister method that allows collection of in vivo transmigrated cells at sites of local inflammation. Nineteen patients with stable CAD and 19 matched controls were enrolled. Markers of inflammation and gradients of chemokines, as well as adhesion molecule expression and up-regulation capacity, were studied. The expression of inflammatory markers, such as C-reactive protein, interleukin (IL)-6, tumour necrosis factor-alpha and IL-10, was similar in patients and controls, indicating that patients were in a stable phase of the disease. Expression of adhesion molecules, CD11b and very late activation antigen-4, on peripheral monocytes did not differ between patients and controls. However, following in vivo transmigration, monocytes in patients with CAD had a significantly reduced expression and mobilization of CD11b. The effect on CD11b could not be reproduced by in vitro stimulation with blister fluid, representing a local inflammatory milieu, or in an in vitro system of transmigration. These findings point towards differences in monocyte CD11b expression and availability at an inflammatory site between patients with CAD and healthy controls.

Newly recruited human monocytes have a preserved responsiveness towards bacterial peptides in terms of CD11b up-regulation and intracellular hydrogen peroxide production

The transmigration of peripheral human monocytes to the interstitium is a fundamental step in the host-defence mechanism against infections. Little is known about the state of function of in vivo transmigrated interstitial monocytes prior to differentiation into macrophages and dendritic cells. We hypothesized that newly recruited interstitial monocytes have a preserved responsiveness against bacterial-related peptides, giving them a specific role in the immediate defence against invading pathogens. In order to test this hypothesis, we explored the responsiveness of in vivo transmigrated as well as peripheral monocytes, in terms of CD11b expression and H(2)O(2) production towards the bacterial-related peptide formylmethionylleucylphenylalanine (fMLP) by the use of a skin chamber technique. In addition, we analysed the concentration of interleukin (IL)-8, monocyte chemotactic protein-1 (MCP-1) and tumour necrosis factor (TNF)-alpha in the skin blister exudates and in the circulation. We demonstrate that in vivo-transmigrated monocytes had a fivefold higher CD11b expression compared to monocytes obtained from the peripheral circulation. fMLP exposure induced a significantly higher CD11b expression on transmigrated cells compared to peripheral monocytes. In addition, newly recruited monocytes had a preserved H(2)O(2) production. The interstitial concentration of IL-8, MCP-1 and TNF-alpha was significantly higher in blister exudates compared to that in the peripheral circulation. Thus, in vivo transmigrated human monocytes preserve their capacity to respond towards bacterial peptides in terms of CD11b up-regulation and H(2)O(2) generation. These data strengthen a role for newly recruited interstitial human monocytes in the immediate defence against invading pathogens.

Osteopontin prevents monocyte recirculation and apoptosis

Cells of the monocyte/macrophage lineage have been shown to be the principal targets for productive HIV-1 replication within the CNS. In addition, HIV-1-associated dementia (HAD) has been shown to correlate with macrophage abundance in the brain. Although increased entry of monocytes into the brain is thought to initiate this process, mechanisms that prevent macrophage egress from the brain and means that prevent macrophage death may also contribute to cell accumulation. We hypothesized that osteopontin (OPN) was involved in the accumulation of macrophages in the brain in neuroAIDS. Using in vitro model systems, we have demonstrated the role of OPN in two distinct aspects of macrophage accumulation: prevention from recirculation and protection from apoptosis. In these unique mechanisms, OPN would aid in macrophage survival and accumulation in the brain, the pathological substrate of HAD.

Altered phenotype and gene transcription in endothelial cells, induced by Plasmodium falciparum-infected red blood cells: pathogenic or protective?

Severe malaria is associated with sequestration of Plasmodium falciparum-infected red blood cells (PRBC) in the microvasculature and elevation of intercellular adhesion molecule-1 (ICAM-1) and TNF. In vitro co-culture of human umbilical vein endothelial cells (HUVEC), with either PRBC or uninfected RBC, required the presence of low level TNF (5 pg/ml) for significant up-regulation of ICAM-1, which may contribute to increased cytoadhesion in vivo. These effects were independent of P. falciparum erythrocyte membrane protein-1 (PfEMP-1)-mediated adhesion but critically dependent on cell–cell contact. Further changes included increases in IL8 release and soluble TNF receptor shedding. Microarray analysis of HUVEC transcriptome following co-culture, using a human Affymetrix microarray chip, showed significant differential regulation of genes which defined gene ontologies such as cell communication, cell adhesion, signal transduction and immune response. Our data demonstrate that endothelial cells have the ability to mobilise immune and pro-adhesive responses when exposed to both PRBC and TNF. In addition, there is also a previously un-described positive regulation by RBC and TNF and a concurrent negative regulation of a range of genes involved in inflammation and cell-death, by PRBC and TNF. We propose that the balance between positive and negative regulation demonstrated in our study will determine endothelial pathology during a malaria infection.

Posterolateral approach for open reduction and internal fixation of trimalleolar ankle fractures

Cell-cell and cell-matrix interactions are of critical importance in immunobiology. Leukocytes make extensive use of a specialized repertoire of receptors to mediate such processes. Among these receptors, integrins are known to be of crucial importance. This review deals with the central role of integrins and their counterreceptors during the establishment of leukocyte-endothelium contacts, interstitial migration, and final encounter with antigen-presenting cells to develop an appropriate immune response. Particularly, we have addressed the molecular events occurring during these sequential processes, leading to the dynamic subcellular redistribution of adhesion receptors and the reorganization of the actin cytoskeleton, which is reflected in changes in cytoarchitecture, including leukocyte polarization, endothelial docking structure formation, or immune synapse organization. The roles of signaling and structural actin cytoskeleton-associated proteins and organized membrane microdomains in the regulation of receptor adhesiveness are also discussed.

Disparate ability of murine CD8alpha- and CD8alpha+ dendritic cell subsets to traverse endothelium is not determined by differential CD11b expression

Upon Ag uptake and response to maturation stimuli, dendritic cells (DC) are directed through lymphatic or blood vessel endothelium to T cell areas of secondary lymphoid tissues by the constitutively expressed CC chemokines CCL19 and CCL21. We have shown that mature (m) murine CD8alpha+ DC exhibit poorer migratory ability to these chemokines than classic CD8alpha- DC by quantifying their in vitro chemotaxis through unmodified Transwell filters. We hypothesized that lower surface expression (compared to CD8alpha- mDC) of the adhesion molecule CD11b on CD8alpha+ DC might limit their ability to adhere to filter pores in vitro and/or endothelium in vitro/in vivo. To test the role of this and/or other adhesion molecules (CD11a, CD31, CD54 and CD62L) in regulating murine DC subset migration, we used specific mAbs to block their function and quantified their migration through resting or tumour necrosis factor (TNF)-alpha-activated endothelial cell (EC) layered-Transwell filters. Both CD8alpha+ and CD8alpha- subsets migrated through resting EC (albeit less than in the absence of EC) in response to CCL19 and CCL21, and migration through TNF-alpha-activated EC was enhanced. In contrast to reports concerning human DC, transendothelial migration of the murine DC subsets was not dependent on CD11b, CD31, or CD62L expression by these cells. CD54 and CD11a, however, were at least partly involved in DC/EC interactions. This is the first report to examine adhesion molecules involved in transendothelial migration of murine DC subsets.

LFA-1 on CD4+ T Cells Is Required for Optimal Antigen-Dependent Activation In Vivo

The leukocyte-specific integrin, LFA-1, plays a critical role in trafficking of T cells to both lymphoid and nonlymphoid tissues. However, the role of LFA-1 in T cell activation in vivo has been less well understood. Although there have been reports describing LFA-1-deficient T cell response defects in vivo, due to impaired migration to lymphoid structures and to sites of effector function in the absence of LFA-1, it has been difficult to assess whether T cells also have a specific activation defect in vivo. We examined the role of LFA-1 in CD4(+) T cell activation in vivo by using a system that allows for segregation of the migration and activation defects through the adoptive transfer of LFA-1-deficient (CD18(-/-)) CD4(+) T cells from DO11.10 Ag-specific TCR transgenic mice into wild-type BALB/c mice. We find that in addition to its role in trafficking to peripheral lymph nodes, LFA-1 is required for optimal CD4(+) T cell priming in vivo upon s.c. immunization. CD18(-/-) DO11.10 CD4(+) T cells primed in the lymph nodes demonstrate defects in IL-2 and IFN-gamma production. In addition, recipient mice adoptively transferred with CD18(-/-) DO11.10 CD4(+) T cells demonstrate a defect in OVA-specific IgG2a production after s.c. immunization. The defect in priming of CD18(-/-) CD4(+) T cells persists even in the presence of proliferating CD18(+/-) CD4(+) T cells and in lymphoid structures to which there is no migration defect. Taken together, these results demonstrate that LFA-1 is required for optimal CD4(+) T cell priming in vivo.

High glucose-induced alterations in subendothelial matrix perlecan leads to increased monocyte binding

OBJECTIVE

Hyperglycemia is an independent risk factor for cardiovascular disease in diabetic patients, although the link between the two is unknown. These studies were designed to model effects of high glucose on an early event in atherogenesis: the binding of monocytes to subendothelial matrix (SEM).

METHODS AND RESULTS

SEM was prepared from human aortic endothelial cells (HAECs) and bovine aortic endothelial cells (BAECs) cultured in the presence of low (5 mmol/L) or high (30 mmol/L) glucose for 3 to 5 days. Monocyte binding was significantly higher (P<0.05) to the SEM of both HAEC and BAEC exposed to high glucose. This increase was a result of changes in SEM heparan sulfate proteoglycans (HSPGs). Metabolic radiolabeling of BAEC demonstrated a 24% decrease in [35S]sulfate incorporation into SEM HSPG produced by cells incubated in 30 mmol/L versus 5 mmol/L glucose, whereas no glucose-associated differences were measured in [35S]methionine incorporation into proteoglycans (PGs) or non-PG proteins. Autoradiography revealed 2 high-molecular weight SEM HSPGs. One was a hybrid PG that contained both heparan sulfate and chondroitin sulfate/dermatan sulfate chains. Both PGs were identified by Western blotting as perlecan.

CONCLUSIONS

These results illustrate that hyperglycemia-induced structural changes in perlecan may result in a SEM that is more favorable to retention of monocytes.

The contribution of monocyte infection and trafficking to viral persistence, and maintenance of the viral reservoir in HIV infection

Cellular viral reservoirs and anatomic sanctuary sites allow continuing HIV-1 replication in patients with suppressed plasma viremia who are receiving highly active antiretroviral therapy and prevent eradication of HIV-1 by these regimens. Cells of macrophage lineage, including monocytes subsets within the blood, play a role in HIV-1 persistence. Evidence of sequence evolution in blood monocytes, in comparison to resting CD4+ T cells, demonstrates their distinct contribution to plasma viremia. There is evidence to suggest that a specific monocyte subset, of CD14loCD16hi phenotype, is more susceptible to HIV-1 infection than the majority of blood monocytes. Trafficking of monocytes through various tissues following their emigration from the bloodstream allows these cells to differentiate into tissue macrophages, or potentially to egress from the tissues as migratory dendritic cells. This review provides an evaluation of the contribution of monocytes to HIV-1 persistence and the HIV-1 reservoir, essential for the effective design of therapeutic eradication strategies.

Travellers in many guises: the origins and destinations of dendritic cells

The migratory behaviour of dendritic cells (DC) is tightly linked to their differentiation state. Precursor DC constitutively repopulate normal tissues from the bloodstream, and are recruited in elevated numbers to sites of inflammation. Whilst maturing in response to antigenic stimulation, DC acquire the capability to enter lymph nodes via afferent lymphatic vessels, thus facilitating their presentation of antigen to naïve T cells. Peripheral blood monocytes constitute a second DC precursor population, which during an inflammatory response are recruited to the affected site where some differentiate into functional DC. The availability of separate DC precursor populations is thought to be significant for the character, amplification and perpetuation of the resultant immune response. In addition, the balance between steady-state trafficking of incompletely activated DC bearing self-antigens from the periphery, and the migration of fully mature DC from inflammatory sites into lymph nodes might have profound effects upon tolerance induction and activation of T cells, respectively.

The CD16(+) (FcgammaRIII(+)) subset of human monocytes preferentially becomes migratory dendritic cells in a model tissue setting

Much remains to be learned about the physiologic events that promote monocytes to become lymph-homing dendritic cells (DCs). In a model of transendothelial trafficking, some monocytes become DCs in response to endogenous signals. These DCs migrate across endothelium in the ablumenal-to-lumenal direction (reverse transmigration), reminiscent of the migration into lymphatic vessels. Here we show that the subpopulation of monocytes that expresses CD16 (Fcgamma receptor III) is predisposed to become migratory DCs. The vast majority of cells derived from CD16(+) monocytes reverse transmigrated, and their presence was associated with migratory cells expressing high levels of CD86 and human histocompatibility leukocyte antigen (HLA)-DR, and robust capacity to induce allogeneic T cell proliferation. A minority of CD16(-) monocytes reverse transmigrated, and these cells stimulated T cell proliferation less efficiently. CD16 was not functionally required for reverse transmigration, but promoted cell survival when yeast particles (zymosan) were present as a maturation stimulus in the subendothelial matrix. The cell surface phenotype and migratory characteristics of CD16(+) monocytes were inducible in CD16(-) monocytes by preincubation with TGFbeta1. We propose that CD16(+) monocytes may contribute significantly to precursors for DCs that transiently survey tissues and migrate to lymph nodes via afferent lymphatic vessels.

A novel immunostimulating aspect of Lactobacillus gasseri: induction of "Gasserokine" as chemoattractants for macrophages

The chemotactic activity of the culture supernatants from 14 strains of Lactobacillus acidophilus and L. gasseri was examined for murine macrophages. Significant macrophage chemotactic activity was observed in three strains of L. acidophilus and all strains of L. gasseri. The highest activity was observed in the supernatant (1131-sup) from 24-h cultures of L. gasseri JCM1131T. The chemotactic factor from 1131-sup, designated as "Gasserokine", was purified by the C18 reverse phase and ion-exchange chromatography. The purity of Gasserokine was checked by HPLC with the reverse-phase mode. The chemotactic activity of Gasserokine was also observed for human monocytes. The macrophage chemotaxis induced by L. gasseri JCM1131T culture supernatants was discovered to be a new biological function exerted by probiotic lactic acid bacteria. Therefore, the activity is expected to be used for one of the functional parameters in the immunomodulating properties of probiotic lactic acid bacteria.

Dynamic interaction of VCAM-1 and ICAM-1 with moesin and ezrin in a novel endothelial docking structure for adherent leukocytes

Ezrin, radixin, and moesin (ERM) regulate cortical morphogenesis and cell adhesion by connecting membrane adhesion receptors to the actin-based cytoskeleton. We have studied the interaction of moesin and ezrin with the vascular cell adhesion molecule (VCAM)-1 during leukocyte adhesion and transendothelial migration (TEM). VCAM-1 interacted directly with moesin and ezrin in vitro, and all of these molecules colocalized at the apical surface of endothelium. Dynamic assessment of this interaction in living cells showed that both VCAM-1 and moesin were involved in lymphoblast adhesion and spreading on the endothelium, whereas only moesin participated in TEM, following the same distribution pattern as ICAM-1. During leukocyte adhesion in static or under flow conditions, VCAM-1, ICAM-1, and activated moesin and ezrin clustered in an endothelial actin-rich docking structure that anchored and partially embraced the leukocyte containing other cytoskeletal components such as alpha-actinin, vinculin, and VASP. Phosphoinositides and the Rho/p160 ROCK pathway, which participate in the activation of ERM proteins, were involved in the generation and maintenance of the anchoring structure. These results provide the first characterization of an endothelial docking structure that plays a key role in the firm adhesion of leukocytes to the endothelium during inflammation.

Molecular mechanisms of IL-2 gene regulation following costimulation through LFA-1

The integrin LFA-1 serves as an accessory molecule in T cell activation. In addition to its well-known role as an adhesion molecule, LFA-1 can contribute to T cell activation and up-regulation of IL-2 gene expression. However, the specific mechanisms by which LFA-1 influences T cell activation have not been elucidated. Therefore, we examined the impact of LFA-1:ICAM-1 interactions on transcriptional and posttranscriptional IL-2 gene regulation, using a costimulation-negative cell line transfected with MHC class II alone, or in combination with ICAM-1 or B7-1. IL-2 transcription was assessed utilizing transgenic mice expressing an IL-2 promoter luciferase reporter construct crossed to DO11.10 TCR-transgenic mice, and IL-2 mRNA stability was evaluated by real-time RT-PCR. Comparison of naive and previously activated T cells demonstrates a dramatic increase in IL-2-luciferase transcription in activated T cells that can, in part, be attributed to downstream signaling events. Costimulation through LFA-1 enhances transcription of the transgenic reporter construct across a wide Ag dose range, but does not affect IL-2 mRNA stability. In contrast, CD28 costimulation is clearly mediated through up-regulation of IL-2 transcription and through enhancement of mRNA stability. These results indicate that the primary pathway whereby engagement of LFA-1 through its ligand ICAM-1 up-regulates IL-2 gene expression is through enhanced IL-2 transcription, in the absence of any effect on IL-2 mRNA stabilization.

Human leukocyte subpopulations from inflamed gut bind to joint vasculature using distinct sets of adhesion molecules

Reactive arthritis can be triggered by inflammatory bowel diseases. We hypothesized that migration of mucosal immune cells from inflamed gut to joints could contribute to the development of reactive arthritis. Here we isolated gut-derived leukocytes from patients with Crohn's disease and ulcerative colitis. Using function-blocking mAbs and in vitro frozen section adhesion assays we studied whether these cells bind to synovial vessels and which molecules mediate the interaction. The results showed that mucosal leukocytes from inflammatory bowel diseased gut bind well to venules in synovial membrane. Small intestinal lymphocytes adhered to synovial vessels using multiple homing receptors and their corresponding endothelial ligands (CD18-ICAM-1, alpha(4)beta(7)/alpha(4)beta(1)-integrin-VCAM-1, L-selectin-peripheral lymph node addressins, and CD44). Of these, only ICAM-1 significantly supported binding of immunoblasts. In contrast, P-selectin glycoprotein ligand-1-P-selectin interaction accounted for practically all synovial adherence of mucosal macrophages. In addition, blocking of vascular adhesion protein-1 significantly inhibited binding of all these leukocyte subsets to joint vessels. We conclude that different leukocyte populations derived from inflamed gut bind avidly to synovial vessels using distinct repertoire of adhesion molecules, suggesting that their recirculation may contribute to the development of reactive arthritis in inflammatory bowel diseases.

Monocytic cells synthesize, adhere to, and migrate on laminin-8 (alpha 4 beta 1 gamma 1)

Laminins, a growing family of large heterotrimeric proteins with cell adhesive and signaling properties, are major components of vascular and other basement membranes. Expression, recognition, and use of laminin isoforms by leukocytes are poorly understood. In monoblastic THP-1 cells, transcripts for laminin gamma(1)-, beta(1)-, and alpha(4)-chains were detected by RT-PCR. Following immunoaffinity purification on a laminin beta(1) Ab-Sepharose column, laminin beta(1)- (220 kDa), gamma(1)- (200 kDa), and alpha(4)- (180/200 kDa) chains were detected by Western blotting in THP-1 cells and in two other monoblastic cell lines, U-937 and Mono Mac 6. After cell permeabilization, a mAb to laminin gamma(1)-chain reacted with practically all blood monocytes by immunofluorescence flow cytometry, and laminin-8 (alpha(4)beta(1)gamma(1)) could be isolated also from these cells. Monoblastic JOSK-I cells adhered constitutively to immobilized recombinant laminin-8, less than to laminin-10/11 (alpha(5)beta(1)gamma(1)/alpha(5)beta(2)gamma(1)) but to a higher level than to laminin-1 (alpha(1)beta(1)gamma(1)). Compared with the other laminin isoforms, adhesion to laminin-8 was preferentially mediated by alpha(6)beta(1) and beta(2) integrins. Laminin-8 and, to a lower extent, laminin-1 promoted spontaneous and chemokine-induced migration of blood monocytes, whereas laminin-10/11 was inhibitory. Altogether, the results indicate that leukocytes, as other cell types, are able to synthesize complete laminin molecules. Expression, recognition, and use of laminin-8 by leukocytes suggest a major role of this laminin isoform in leukocyte physiology.

The role of the graft endothelium in transplant rejection: evidence that endothelial activation may serve as a clinical marker for the development of chronic rejection

In this review, we discuss the role of the allograft endothelium in the recruitment and activation of leukocytes during acute and chronic rejection. We discuss associations among endothelial activation responses, the expression of adhesion molecules, chemokines and chemokine receptors, and rejection; and we propose that endothelial vascular cellular adhesion molecule-1 (VCAM-1) may be used as a surrogate marker of acute rejection and allograft vasculopathy. In addition, we describe potential mechanistic interpretations of persistent endothelial cell (EC) expression of major histocompatibility complex (MHC) class II molecules in allorecognition. The graft endothelium may provide an antigen-specific signal to transmigrating, previously activated, T cells and may induce B7 expression on locally transmigrating leukocytes to promote costimulation. Taken together, these functions of the EC provide it with a potent regulatory role in rejection and in the maintenance of T-cell activation via the direct and/or the indirect pathways of allorecognition.

Differentiation of phagocytic monocytes into lymph node dendritic cells in vivo

We investigated the differentiation and trafficking of inflammatory monocytes that phagocytosed subcutaneously injected fluorescent microspheres. As expected, most of the monocytes became microsphere+ macrophages, which remained in subcutaneous tissue. However, about 25% of latex+ cells migrated to the T cell area of draining lymph nodes, where they expressed dendritic cell (DC)-restricted markers and high levels of costimulatory molecules. Microsphere-transporting cells were distinct from resident skin DCs, and this transport was reduced by more than 85% in monocyte-deficient osteopetrotic mice. Thus, a substantial minority of inflammatory monocytes carry phagocytosed particles to lymph nodes and differentiate into DCs.

Downregulation by tumor necrosis factor-alpha of monocyte CCR2 expression and monocyte chemotactic protein-1-induced transendothelial migration is antagonized by oxidized low-density lipoprotein: a potential mechanism of monocyte retention in atherosclerotic lesions

The subintimal infiltration with monocytes is crucially involved in the development of complex atherosclerotic plaques. Monocyte chemotactic protein-1 (MCP-1) and its receptor CCR2 are important for monocyte extravasation and formation of atherosclerotic lesions. However, mechanisms of monocyte persistence in atherosclerotic plaques remain to be elucidated. Flow cytometric analysis revealed that monocytoid Mono Mac 6 cells that had transmigrated endothelium towards a MCP-1 gradient expressed higher levels of CCR2 than the non-migratory fraction, while input cells were intermediate, suggesting that high CCR2 levels are essential for transendothelial chemotaxis. Pretreatment of Mono Mac 6 cells or isolated human blood monocytes with the inflammatory cytokine tumor necrosis factor (TNF)-alpha dose- and time-dependently reduced MCP-1-induced transendothelial chemotaxis, which was inhibited by the CCR2 receptor antagonist 9-76 analog. This was paralleled by a decrease in CCR2 surface protein and mRNA expression. as assessed by flow cytometry and reverse transcription-polymerase chain reaction, inferring that inhibition of monocyte transmigration was due to downregulation of CCR2 to levels insufficient for chemotaxis. In contrast, treatment of monocytes with oxidized low-density protein (oxLDL) containing oxidized lipids, such as cholesteryl linoleate 13-hydroxide. but not with LDL, increased CCR2 protein and mRNA expression. Notably, oxLDL counteracted the TNF-alpha-mediated downregulation of CCR2 and CCR2-dependent transendothelial chemotaxis. Macrophage-colony-stimulating factor hardly affected CCR2 expression and function, suggesting that differentiation was not responsible for effects on CCR2. In conclusion, TNF-alpha impairs MCP-1-induced transendothelial migration of monocytes by downregulating CCR2 which appears critical for migration. Exposure to oxLDL antagonized the effects of TNF-alpha, and may thus contribute to monocyte retention and perpetuation of a chronic inflammatory reaction in unstable atherosclerotic lesions.

The Dependence for Leukocyte Function-Associated Antigen-1/ICAM-1 Interactions in T Cell Activation Cannot Be Overcome by Expression of High Density TCR Ligand

The leukocyte-specific integrin, LFA-1, can enhance T cell activation. However, it is unclear whether the binding of LFA-1 to its ligand, ICAM-1, functions through intercellular adhesion alone, resulting in an augmentation of the TCR signal, or involves an additional LFA-1-mediated cellular signal transduction pathway. We have previously shown that naive CD4+ lymph node T cells, isolated from DO11.10 TCR transgenic mice, are activated by increasing doses of exogenous OVA peptide presented by transfectants expressing both class II and ICAM-1, but not by cells expressing class II alone. To determine whether LFA-1/ICAM-1 interactions were simply enhancing the presentation of low concentrations of specific MHC/peptide complexes generated from exogenously added peptide, we transfected cells with class II that is covalently coupled to peptide, alone or in combination with ICAM-1. These cells express 100-fold more specific class II/peptide complexes than can be loaded onto class II-positive cells at maximum concentrations of exogenous peptide. Despite this high density of TCR ligand, activation of naive CD4+ T cells still requires the coexpression of ICAM-1. LFA-1/ICAM-1 interactions are not required for effective conjugate formation and TCR engagement because presentation of class II/peptide complexes in the absence of ICAM-1 does induce up-regulation of CD25 and CD69. Thus, high numbers of engaged TCR cannot compensate for the lack of LFA-1/ICAM-1 interactions in the activation of naive CD4+ T cells.

Monocytes that have ingested Yersinia enterocolitica serotype O:3 acquire enhanced capacity to bind to nonstimulated vascular endothelial cells via P-selectin

Reactive arthritis is usually a self-limiting polyarthritis which develops after certain gastrointestinal or urogenital infections. Microbial antigens found in the inflamed joints are thought to play a key role in the development of this disease. It is not known how antigens of the pathogenic organisms migrate from the mucosal tissues into the joints. The data presented here show that mononuclear phagocytes which mediate the dissemination of several intracellular pathogens acquire an enhanced capacity to bind to nonstimulated vascular endothelial cells after phagocytosis of Yersinia enterocolitica O:3, one of the causative organisms of reactive arthritis. The increased binding to previously nonstimulated endothelial cells was mediated by P-selectin, whose translocation to the endothelial cell surface was induced by monocytes with intracellular Yersinia bacteria. These results suggest that mononuclear phagocytes may be responsible for the dissemination of bacterial antigens and the initiation of the joint inflammation in reactive arthritis.

Role of Tissue Factor in Adhesion of Mononuclear Phagocytes to and Trafficking Through Endothelium In Vitro

An in vitro model consisting of endothelium grown on collagen was used to investigate how mononuclear phagocytes traverse endothelium in the basal-to-apical direction (reverse transmigration), a process that mimics their migration across vascular and/or lymphatic endothelium during atherosclerosis and resolution of inflammation, respectively. Monoclonal antibody (MoAb) VIC7 against tissue factor (TF) inhibited reverse transmigration by 77%. Recombinant tissue factor fragments containing at least six amino acids C-terminal to residue 202 also strongly inhibited reverse transmigration. TF was absent on resting monocytes but was induced on these cells after initial apical-to-basal transendothelial migration. Two additional observations suggest that TF is involved in adhesion between mononuclear phagocytes and endothelium: (1) when monocytes were incubated with lipopolysaccharide (LPS) to stimulate expression of TF before they were added to endothelium, VIC7 or soluble TF modestly inhibited their adhesion to the apical endothelial surface, each by about 35%; and (2) endothelial cells specifically bound to surfaces coated with TF fragments containing amino acids 202-219. This binding was blocked by anti-TF MoAb, suggesting that endothelial cells bear a receptor for TF. These data suggest that mononuclear phagocytes use TF, perhaps as an adhesive protein, to exit sites of inflammation.

Role of Tissue Factor in Adhesion of Mononuclear Phagocytes to and Trafficking Through Endothelium In Vitro

An in vitro model consisting of endothelium grown on collagen was used to investigate how mononuclear phagocytes traverse endothelium in the basal-to-apical direction (reverse transmigration), a process that mimics their migration across vascular and/or lymphatic endothelium during atherosclerosis and resolution of inflammation, respectively. Monoclonal antibody (MoAb) VIC7 against tissue factor (TF) inhibited reverse transmigration by 77%. Recombinant tissue factor fragments containing at least six amino acids C-terminal to residue 202 also strongly inhibited reverse transmigration. TF was absent on resting monocytes but was induced on these cells after initial apical-to-basal transendothelial migration. Two additional observations suggest that TF is involved in adhesion between mononuclear phagocytes and endothelium: (1) when monocytes were incubated with lipopolysaccharide (LPS) to stimulate expression of TF before they were added to endothelium, VIC7 or soluble TF modestly inhibited their adhesion to the apical endothelial surface, each by about 35%; and (2) endothelial cells specifically bound to surfaces coated with TF fragments containing amino acids 202-219. This binding was blocked by anti-TF MoAb, suggesting that endothelial cells bear a receptor for TF. These data suggest that mononuclear phagocytes use TF, perhaps as an adhesive protein, to exit sites of inflammation.

Differentiation of monocytes into dendritic cells in a model of transendothelial trafficking

Essential to the dendritic cell system of antigen-presenting cells are the veiled dendritic cells that traverse afferent lymph to enter lymph nodes, where they initiate immune responses. The origin of veiled cells, which were discovered 20 years ago, is unclear. Monocytes cultured with endothelium differentiated into dendritic cells within 2 days, particularly after phagocytosing particles in subendothelial collagen. These nascent dendritic cells migrated across the endothelium in the ablumenal-to-lumenal direction, as would occur during entry into lymphatics. Monocytes that remained in the subendothelial matrix became macrophages. Therefore, monocytes have two potential fates associated with distinct patterns of migration.

Infectious agents as triggers of reactive arthritis

Reactive arthritis was originally defined as a sterile joint inflammation after infection elsewhere in the body, but this view has been challenged in the past decade since different antigens and DNA and RNA of various triggering microbes have been shown to exist at the sites of inflammation in the joints. It has been suggested that microbial antigens, or intact pathogens, are important for the pathogenesis of reactive arthritis, at least in the early phase of the disease, but the exact mechanism of how the pathogens contribute to the development of this usually self-limiting polyarthritis has not been discovered. This article reviews the theories on the role of infectious agents as triggers of reactive arthritis.

Borrelia burgdorferi and interleukin-1 promote the transendothelial migration of monocytes in vitro by different mechanisms

A prominent feature of Lyme disease is the perivascular accumulation of mononuclear leukocytes. Incubation of human umbilical vein endothelial cells (HUVEC) cultured on amniotic tissue with either interleukin-1 (IL-1) or Borrelia burgdorferi, the spirochetal agent of Lyme disease, increased the rate at which human monocytes migrated across the endothelial monolayers. Very late antigen 4 (VLA-4) and CD11/CD18 integrins mediated migration of monocytes across HUVEC exposed to either B. burgdorferi or IL-1 in similar manners. Neutralizing antibodies to the chemokine monocyte chemoattractant protein 1 (MCP-1) inhibited the migration of monocytes across unstimulated, IL-1-treated, or B. burgdorferi-stimulated HUVEC by 91% +/- 3%, 65% +/- 2%, or 25% +/- 22%, respectively. Stimulation of HUVEC with B. burgdorferi also promoted a 6-fold +/- 2-fold increase in the migration of human CD4(+) T lymphocytes. Although MCP-1 played only a limited role in the migration of monocytes across B. burgdorferi-treated HUVEC, migration of CD4(+) T lymphocytes across HUVEC exposed to spirochetes was highly dependent on this chemokine. The anti-inflammatory cytokine IL-10 reduced both migration of monocytes and endothelial production of MCP-1 in response to B. burgdorferi by approximately 50%, yet IL-10 inhibited neither migration nor secretion of MCP-1 when HUVEC were stimulated with IL-1. Our results suggest that activation of endothelium by B. burgdorferi may contribute to formation of the chronic inflammatory infiltrates associated with Lyme disease. The transendothelial migration of monocytes that is induced by B. burgdorferi is significantly less dependent on MCP-1 than is migration induced by IL-1. Selective inhibition by IL-10 further indicates that B. burgdorferi and IL-1 employ distinct mechanisms to activate endothelial cells.

Patterns for RANTES secretion and intercellular adhesion molecule 1 expression mediate transepithelial T cell traffic based on analyses in vitro and in vivo

Immune cell migration into and through mucosal barrier sites in general and airway sites in particular is a critical feature of immune and inflammatory responses, but the determinants of transepithelial (unlike transendothelial) immune cell traffic are poorly defined. Accordingly, we used primary culture airway epithelial cells and peripheral blood mononuclear cells to develop a cell monolayer system that allows for apical-to-basal and basal-to-apical T cell transmigration that can be monitored with quantitative immunofluorescence flow cytometry. In this system, T cell adhesion and subsequent transmigration were blocked in both directions by monoclonal antibodies (mAbs) against lymphocyte function-associated antigen 1 (LFA-1) or intercellular adhesion molecule 1 (ICAM-1) (induced by interferon gamma [IFN-gamma] treatment of epithelial cells). The total number of adherent plus transmigrated T cells was also similar in both directions, and this pattern fit with uniform presentation of ICAM-1 along the apical and basolateral cell surfaces. However, the relative number of transmigrated to adherent T cells (i.e., the efficiency of transmigration) was increased in the basal-to-apical relative to the apical-to-basal direction, so an additional mechanism was needed to mediate directional movement towards the apical surface. Screening for epithelial-derived beta-chemokines indicated that IFN-gamma treatment caused selective expression of RANTES (regulated upon activation, normal T cell expressed and secreted), and the functional significance of this finding was demonstrated by inhibition of epithelial-T cell adhesion and transepithelial migration by anti-RANTES mAbs. In addition, we found that epithelial (but not endothelial) cells preferentially secreted RANTES through the apical cell surface thereby establishing a chemical gradient for chemotaxis across the epithelium to a site where they may be retained by high levels of RANTES and apical ICAM-1. These patterns for epithelial presentation of ICAM-1 and secretion of RANTES appear preserved in airway epithelial tissue studied either ex vivo with expression induced by IFN-gamma treatment or in vivo with endogenous expression induced by inflammatory disease (i.e., asthma). Taken together, the results define how the patterns for uniform presentation of ICAM-1 along the cell surface and specific apical sorting of RANTES may serve to mediate the level and directionality of T cell traffic through epithelium (distinct from endothelium) and provide a basis for how this process is precisely coordinated to route immune cells to the mucosal surface and maintain them there under normal and stimulated conditions.

A physiologic function for p-glycoprotein (MDR-1) during the migration of dendritic cells from skin via afferent lymphatic vessels

P-glycoprotein (MDR-1) is a well-known transporter that mediates efflux of chemotherapeutic agents from the intracellular milieu and thereby contributes to drug resistance. MDR-1 also is expressed by nonmalignant cells, including leukocytes, but physiologic functions for MDR-1 are poorly defined. Using an initial screening assay that included >100 mAbs, we observed that neutralizing mAbs MRK16, UIC2, and 4E3 against MDR-1 specifically and potently blocked basal-to-apical transendothelial migration of mononuclear phagocytes, a process that may mimic their migration into lymphatic vessels. Antagonists of MDR-1 then were used in a model of authentic lymphatic clearance. In this model, antigen-presenting dendritic cells (DC) migrate out of explants of cultured human skin and into the culture medium via dermal lymphatic vessels. DC and T cells derived from skin expressed MDR-1 on their surfaces. Addition of anti-MDR-1 mAbs MRK16, UIC2, or the MDR-1 antagonist verapamil to skin explants at the onset of culture inhibited the appearance of DC, and accompanying T cells, in the culture medium by approximately 70%. Isotype-matched control mAbs against other DC molecules including CD18, CD31, and major histocompatibility complex I did not block. In the presence of MDR-1 antagonists, epidermal DC were retained in the epidermis, in contrast to control conditions. In summary, this work identifies a physiologic function for MDR-1 during the mobilization of DC and begins to elucidate how these critical antigen-presenting cells migrate from the periphery to lymph nodes to initiate T lymphocyte-mediated immunity.