Venular basement membranes ubiquitously express matrix protein low-expression regions: characterization in multiple tissues and remodeling during inflammation

The fibrotic scar in neurological disorders

Tissue fibrosis, or scar formation, is a common response to damage in most organs of the body. The central nervous system (CNS) is special in that fibrogenic cells are restricted to vascular and meningeal niches. However, disruption of the blood-brain barrier and inflammation can unleash stromal cells and trigger scar formation. Astroglia segregate from the inflammatory lesion core, and the so-called "glial scar" composed of hypertrophic astrocytes seals off the intact neural tissue from damage. In the lesion core, a second type of "fibrotic scar" develops, which is sensitive to inflammatory mediators. Genetic fate mapping studies suggest that pericytes and perivascular fibroblasts are activated, but other precursor cells may also be involved in generating a transient fibrous extracellular matrix in the CNS. The stromal cells sense inflammation and attract immune cells, which in turn drive myofibroblast transdifferentiation. We believe that the fibrotic scar represents a major barrier to CNS regeneration. Targeting of fibrosis may therefore prove to be a valuable therapeutic strategy for neurological disorders such as stroke, spinal cord injury and multiple sclerosis.

Assembly, heterogeneity, and breaching of the basement membranes

Basement membranes are thin sheets of self-assembled extracellular matrices that are essential for embryonic development and for the homeostasis of adult tissues. They play a role in structuring, protecting, polarizing, and compartmentalizing cells, as well as in supplying them with growth factors. All basement membranes are built from laminin and collagen IV networks stabilized by nidogen/perlecan bridges. The precise composition of basement membranes, however, varies between different tissues. Even though basement membranes represent physical barriers that delimit different tissues, they are breached in many physiological or pathological processes, including development, the immune response, and tumor invasion. Here, we provide a brief overview of the molecular composition of basement membranes and the process of their assembly. We will then illustrate the heterogeneity of basement membranes using two examples, the epithelial basement membrane in the gut and the vascular basement membrane. Finally, we examine the different strategies cells use to breach the basement membrane.

The extracellular matrix protein laminin α2 regulates the maturation and function of the blood-brain barrier

Laminins are major constituents of the gliovascular basal lamina of the blood-brain barrier (BBB); however, the role of laminins in BBB development remains unclear. Here we report that Lama2(-/-) mice, lacking expression of the laminin α2 subunit of the laminin-211 heterotrimer expressed by astrocytes and pericytes, have a defective BBB in which systemically circulated tracer leaks into the brain parenchyma. The Lama2(-/-) vascular endothelium had significant abnormalities, including altered integrity and composition of the endothelial basal lamina, inappropriate expression of embryonic vascular endothelial protein MECA32, substantially reduced pericyte coverage, and tight junction abnormalities. Additionally, astrocytic endfeet were hypertrophic and lacked appropriately polarized aquaporin4 channels. Laminin-211 appears to mediate these effects at least in part by dystroglycan receptor interactions, as preventing dystroglycan expression in neural cells led to a similar set of BBB abnormalities and gliovascular disturbances, which additionally included perturbed vascular endothelial glucose transporter-1 localization. These findings provide insight into the cell and molecular changes that occur in congenital muscular dystrophies caused by Lama2 mutations or inappropriate dystroglycan post-translational modifications, which have accompanying brain abnormalities, including seizures. Our results indicate a novel role for laminin-dystroglycan interactions in the cooperative integration of astrocytes, endothelial cells, and pericytes in regulating the BBB.

Intravital microscopic methods to evaluate anti-inflammatory effects and signaling mechanisms evoked by hydrogen sulfide

Hydrogen sulfide (H2S) is an endogenous gaseous signaling molecule with potent anti-inflammatory properties. Exogenous application of H2S donors, administered either acutely during an inflammatory response or as an antecedent preconditioning intervention that invokes the activation of anti-inflammatory cell survival programs, effectively limits leukocyte rolling, adhesion and emigration, generation of reactive oxygen species, chemokine and cell adhesion molecule expression, endothelial barrier disruption, capillary perfusion deficits, and parenchymal cell dysfunction and injury. This chapter focuses on intravital microscopic methods that can be used to assess the anti-inflammatory effects exerted by H2S, as well as to explore the cellular signaling mechanisms by which this gaseous molecule limits the aforementioned inflammatory responses. Recent advances include use of intravital multiphoton microscopy and optical biosensor technology to explore signaling mechanisms in vivo.

Neutrophil migration under normal and sepsis conditions

Neutrophil migration is critical for pathogen clearance and host survival during severe sepsis. Interaction of neutrophil adhesion receptors with ligands on endothelial cells results in firm adhesion of the circulating neutrophils, followed by neutrophil activation and directed migration to sites of infection through the basement membrane and interstitial extracellular matrix. Proteolytic enzymes and reactive oxygen species are produced and released by neutrophils in response to a variety of inflammatory stimuli. Although these mediators are important for host defense, they also promote tissue damage. Excessive neutrophil migration during the early stages of sepsis may lead to an exaggerated inflammatory response with associated tissue damage and subsequent organ dysfunction. On the other hand, dysregulation of migration and insufficient migratory response that occurs during the latter stages of severe sepsis contributes to neutrophils' inability to contain and control infection and impaired wound healing. This review discusses the major steps and associated molecules involved in the balance of neutrophil trafficking, the precise regulation of which during sepsis spells life or death for the host.

Vaccines to prevent tuberculosis infection rather than disease: Physiological and immunological aspects

There is increasing enthusiasm and optimism that a vaccine could be developed that prevents infection rather than disease. In this article I discuss the fact that despite this optimism nothing has been produced so far that seems to have this capability, and moreover even the borderline between when infection ends and disease begins has not even been defined. To be effective such a vaccine, or at least the immunity it would generate, would have to work within the confines of the pulmonary physiological systems, which are complex. To date much of the emphasis here has turned away from T cell mediated immunity and towards establishing specific antibodies in the lungs. Here, I argue that with the exception of a possible exclusionary function, most claims of a protective role of antibody are completely over-blown. Finally, even if we had a potential "anti-infection" vaccine, how would we test and validate it?

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.

Basement membrane zone collagens XV and XVIII/proteoglycans mediate leukocyte influx in renal ischemia/reperfusion

Collagen type XV and XVIII are proteoglycans found in the basement membrane zones of endothelial and epithelial cells, and known for their cryptic anti-angiogenic domains named restin and endostatin, respectively. Mutations or deletions of these collagens are associated with eye, muscle and microvessel phenotypes. We now describe a novel role for these collagens, namely a supportive role in leukocyte recruitment. We subjected mice deficient in collagen XV or collagen XVIII, and their compound mutant, as well as the wild-type control mice to bilateral renal ischemia/reperfusion, and evaluated renal function, tubular injury, and neutrophil and macrophage influx at different time points after ischemia/reperfusion. Five days after ischemia/reperfusion, the collagen XV, collagen XVIII and the compound mutant mice showed diminished serum urea levels compared to wild-type mice (all p<0.05). Histology showed reduced tubular damage, and decreased inflammatory cell influx in all mutant mice, which were more pronounced in the compound mutant despite increased expression of MCP-1 and TNF-α in double mutant mice compared to wildtype mice. Both type XV and type XVIII collagen bear glycosaminoglycan side chains and an in vitro approach with recombinant collagen XVIII fragments with variable glycanation indicated a role for these side chains in leukocyte migration. Thus, basement membrane zone collagen/proteoglycan hybrids facilitate leukocyte influx and tubular damage after renal ischemia/reperfusion and might be potential intervention targets for the reduction of inflammation in this condition.

T and B cells participate in bone repair by infiltrating the fracture callus in a two-wave fashion

Fracture healing is a regenerative process in which bone is restored without scar tissue formation. The healing cascade initiates with a cycle of inflammation, cell migration, proliferation and differentiation. Immune cells invade the fracture site immediately upon bone damage and contribute to the initial phase of the healing process by recruiting accessory cells to the injury site. However, little is known about the role of the immune system in the later stages of fracture repair, in particular, whether lymphocytes participate in soft and hard callus formation. In order to answer this question, we analyzed femoral fracture healing in mice by confocal microscopy. Surprisingly, after the initial inflammatory phase, when soft callus developed, T and B cells withdrew from the fracture site and were detectable predominantly at the femoral neck and knee. Thereafter lymphocytes massively infiltrated the callus region (around day 14 after injury), during callus mineralization. Interestingly, lymphocytes were not found within cartilaginous areas of the callus but only nearby the newly forming bone. During healing B cell numbers seemed to exceed those of T cells and B cells progressively underwent effector maturation. Both, osteoblasts and osteoclasts were found to have direct cell-cell contact with lymphocytes, strongly suggesting a regulatory role of the immune cells specifically in the later stages of fracture healing.

Neutrophils recruited by chemoattractants in vivo induce microvascular plasma protein leakage through secretion of TNF

Adherent neutrophils responding to chemoattractants release TNF in proximity of endothelial cell junctions to mediate microvascular leakage.

Microvascular plasma protein leakage is an essential component of the inflammatory response and serves an important function in local host defense and tissue repair. Mediators such as histamine and bradykinin act directly on venules to increase the permeability of endothelial cell (EC) junctions. Neutrophil chemoattractants also induce leakage, a response that is dependent on neutrophil adhesion to ECs, but the underlying mechanism has proved elusive. Through application of confocal intravital microscopy to the mouse cremaster muscle, we show that neutrophils responding to chemoattractants release TNF when in close proximity of EC junctions. In vitro, neutrophils adherent to ICAM-1 or ICAM-2 rapidly released TNF in response to LTB₄, C5a, and KC. Further, in TNFR^−/− mice, neutrophils accumulated normally in response to chemoattractants administered to the cremaster muscle or dorsal skin, but neutrophil-dependent plasma protein leakage was abolished. Similar results were obtained in chimeric mice deficient in leukocyte TNF. A locally injected TNF blocking antibody was also able to inhibit neutrophil-dependent plasma leakage, but had no effect on the response induced by bradykinin. The results suggest that TNF mediates neutrophil-dependent microvascular leakage. This mechanism may contribute to the effects of TNF inhibitors in inflammatory diseases and indicates possible applications in life-threatening acute edema.

Crosstalk between sentinel and helper macrophages permits neutrophil migration into infected uroepithelium

The phagocytes of the innate immune system, macrophages and neutrophils, contribute to antibacterial defense, but their functional specialization and cooperation is unclear. Here, we report that three distinct phagocyte subsets play highly coordinated roles in bacterial urinary tract infection. Ly6C(-) macrophages acted as tissue-resident sentinels that attracted circulating neutrophils and Ly6C(+) macrophages. Such Ly6C(+) macrophages played a previously undescribed helper role: once recruited to the site of infection, they produced the cytokine TNF, which caused Ly6C(-) macrophages to secrete CXCL2. This chemokine activated matrix metalloproteinase-9 in neutrophils, allowing their entry into the uroepithelium to combat the bacteria. In summary, the sentinel macrophages elicit the powerful antibacterial functions of neutrophils only after confirmation by the helper macrophages, reminiscent of the licensing role of helper T cells in antiviral adaptive immunity. These findings identify helper macrophages and TNF as critical regulators in innate immunity against bacterial infections in epithelia.

Traversing the basement membrane in vivo: a diversity of strategies

The basement membrane is a dense, highly cross-linked, sheet-like extracellular matrix that underlies all epithelia and endothelia in multicellular animals. During development, leukocyte trafficking, and metastatic disease, cells cross the basement membrane to disperse and enter new tissues. Based largely on in vitro studies, cells have been thought to use proteases to dissolve and traverse this formidable obstacle. Surprisingly, recent in vivo studies have uncovered a remarkably diverse range of cellular- and tissue-level strategies beyond proteolysis that cells use to navigate through the basement membrane. These fascinating and unexpected mechanisms have increased our understanding of how cells cross this matrix barrier in physiological and disease settings.

Laminins affect T cell trafficking and allograft fate

Lymph nodes (LNs) are integral sites for the generation of immune tolerance, migration of CD4⁺ T cells, and induction of Tregs. Despite the importance of LNs in regulation of inflammatory responses, the LN-specific factors that regulate T cell migration and the precise LN structural domains in which differentiation occurs remain undefined. Using intravital and fluorescent microscopy, we found that alloreactive T cells traffic distinctly into the tolerant LN and colocalize in exclusive regions with alloantigen-presenting cells, a process required for Treg induction. Extracellular matrix proteins, including those of the laminin family, formed regions within the LN that were permissive for colocalization of alloantigen-presenting cells, alloreactive T cells, and Tregs. We identified unique expression patterns of laminin proteins in high endothelial venule basement membranes and the cortical ridge that correlated with alloantigen-specific immunity or immune tolerance. The ratio of laminin α4 to laminin α5 was greater in domains within tolerant LNs, compared with immune LNs, and blocking laminin α4 function or inducing laminin α5 overexpression disrupted T cell and DC localization and transmigration through tolerant LNs. Furthermore, reducing α4 laminin circumvented tolerance induction and induced cardiac allograft inflammation and rejection in murine models. This work identifies laminins as potential targets for immune modulation.

Leukocyte migration in the interstitial space of non-lymphoid organs

Leukocyte migration through interstitial tissues is essential for mounting a successful immune response. Interstitial motility is governed by a vast array of cell-intrinsic and cell-extrinsic factors that together ensure the proper positioning of immune cells in the context of specific microenvironments. Recent advances in imaging modalities, in particular intravital confocal and multi-photon microscopy, have helped to expand our understanding of the cellular and molecular mechanisms that underlie leukocyte navigation in the extravascular space. In this Review, we discuss the key factors that regulate leukocyte motility within three-dimensional environments, with a focus on neutrophils and T cells in non-lymphoid organs.

Transendothelial migration of effector T cells across inflamed endothelial barriers does not require heparan sulfate proteoglycans

Leukocyte diapedesis is a chemotactic multistep process that requires optimal chemoattractant presentation by the endothelial barrier. Recent studies have described a critical role for heparan sulfate glycosaminoglycans (HSGAGs) in the presentation and functions of chemokines essential for lymphocyte interactions with the lymph node vasculature. We wished to test whether HS expression by a prototypic endothelial cell type, i.e. human umbilical vein endothelial cells (HUVECs), is critical for their ability to support neutrophil and lymphocyte adhesion and transendothelial migration (TEM) under shear flow. We found that HUVECs deposit HS GAGs mainly at their basolateral compartments in both their resting and inflamed states. We next inactivated the key enzyme involved in HS biosynthesis, exostosin-1 (Ext1). Silencing Ext1 resulted in a complete loss of HS biosynthesis; nonetheless, TNF-α and IL-1β stimulation of key adhesion molecules and inflammatory chemokines necessary for neutrophil or lymphocyte adhesion and TEM remained intact. Ext1 silencing reduced neutrophil arrest and markedly impaired TEM, consistent with a role of basolateral HS GAGs in directing neutrophil crossing of inflamed endothelial barriers. Strikingly, however, the TEM of effector T cells across identically Ext1-silenced HUVECs remained normal. Importantly, the biosynthesis of the main promigratory chemokines for effector T cells and neutrophils, respectively, CCL2 and CXCL1, and their vesicle distributions were also Ext1 independent. These results suggest that transmigrating neutrophils must respond to chemokines transiently presented by apical and basolateral endothelial HS GAGs. In contrast, effector T cells can integrate chemotactic TEM signals directly from intra-endothelial chemokine stores rather than from externally deposited chemokines.

A composite model of the human postcapillary venule for investigation of microvascular leukocyte recruitment

Neutrophil extravasation occurs across postcapillary venules, structures composed of endothelial cells (ECs), pericytes (PCs), and basement membrane (BM). We constructed composite models of the human postcapillary venule, combining ECs with PCs or PC-deposited BM, to better study this process. Quiescent and tumor necrosis factor α (TNF-α)-activated composites demonstrated in situ-like expression of cadherins, E-selectin, intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), platelet-endothelial cell adhesion molecule 1 (PECAM-1), CD99, and interleukin 8 (IL-8). After TNF-α activation, the ECs supported greater neutrophil adhesion (66.1 vs. 23.7% of input cells) and transmigration (35.1 vs. 7.20% of input cells) than did the PCs, but the composites behaved comparably (no significant difference) to ECs in both assays. TNF-α-activated EC-conditioned medium (CM) increased transmigration across the PCs, whereas TNF-α-activated PC-CM decreased transmigration across the ECs, and culturing on PC-derived BM decreased both adhesion to and transmigration across the ECs. Anti-very late antigen 4 (VLA-4; on neutrophils) inhibited adhesion to TNF-α-activated composites, but not to ECs alone. Anti-CD99 (expressed on all 3 cell types) inhibited transmigration across the composites (14.5% of control) more than across the ECs (39.0% of control), and venular shear stress reduced transmigration across the ECs (17.3% of static) more than across the composites (36.7% of static). These results provide proof of concept that our composite human EC/PC/BM venular construct can reveal new interactions in the inflammatory cascade.

Crossing the endothelial barrier during metastasis

During metastasis, cancer cells disseminate to other parts of the body by entering the bloodstream in a process that is called intravasation. They then extravasate at metastatic sites by attaching to endothelial cells that line blood vessels and crossing the vessel walls of tissues or organs. This Review describes how cancer cells cross the endothelial barrier during extravasation and how different receptors, signalling pathways and circulating cells such as leukocytes and platelets contribute to this process. Identification of the mechanisms that underlie cancer cell extravasation could lead to the development of new therapies to reduce metastasis.

Endothelial cell junctional adhesion molecule C plays a key role in the development of tumors in a murine model of ovarian cancer

Junctional adhesion molecule C (JAM-C) is a transmembrane protein with significant roles in regulation of endothelial cell (EC) functions, including immune cell recruitment and angiogenesis. As these responses are important in promoting tumor growth, the role of EC JAM-C in tumor development was investigated using the ID8 syngeneic model of ovarian cancer. Within 10-15 wk, intraperitoneally injected ID8 cells form multiple tumor deposits and ascites that resemble human high-grade serous ovarian cancer. Compared to wild-type mice, survival in this model was increased in EC JAM-C knockouts (KOs; 88 vs. 96 d, P=0.04) and reduced in EC JAM-C transgenics (88 vs. 78.5 d, P=0.03), mice deficient in or overexpressing EC JAM-C, respectively. While tumor growth was significantly reduced in EC JAM-C KOs (87% inhibition at 10 wk, P<0.0005), this was not associated with alterations in tumor vessel density or immune cell infiltration. However, tumor microvessels from EC JAM-C-deficient mice exhibited reduced pericyte coverage and increased vascular leakage, suggesting a role for EC JAM-C in the development of functional tumor vessels. These findings provide evidence for a role for EC JAM-C in tumor growth and aggressiveness as well as recruitment of pericytes to newly formed blood vessels in a model of ovarian cancer.

Transendothelial migration enables subsequent transmigration of neutrophils through underlying pericytes

During acute inflammation, neutrophil recruitment into extravascular tissue requires neutrophil tethering and rolling on cytokine-activated endothelial cells (ECs), tight adhesion, crawling towards EC junctions and transendothelial migration (TEM). Following TEM, neutrophils must still traverse the subendothelial basement membrane and network of pericytes (PCs). Until recently, the contribution of the PC layer to neutrophil recruitment was largely ignored. Here we analyze human neutrophil interactions with interleukin (IL)-1β-activated human EC monolayers, PC monolayers and EC/PC bilayers in vitro. Compared to EC, PC support much lower levels of neutrophil binding (54.6% vs. 7.1%, respectively) and transmigration (63.7 vs. 8.8%, respectively) despite comparable levels of IL-8 (CXCL8) synthesis and display. Remarkably, EC/PC bilayers support intermediate levels of transmigration (37.7%). Neutrophil adhesion to both cell types is Mac-1-dependent and while ICAM-1 transduction of PCs increases neutrophil adhesion to (41.4%), it does not increase transmigration through PC monolayers. TEM, which increases neutrophil Mac-1 surface expression, concomitantly increases the ability of neutrophils to traverse PCs (19.2%). These data indicate that contributions from both PCs and ECs must be considered in evaluation of microvasculature function in acute inflammation.

Neutrophil transmigration: emergence of an adhesive cascade within venular walls

Recruitment of neutrophils from the blood circulation to sites of infection or injury is a key innate immune response against invading pathogens and tissue injury. However, if inappropriately triggered, excessive and/or prolonged, this host defence response can also lead to severe pathological disorders. The migration of all leucocytes out of the vasculature is classically described by the leucocyte adhesion cascade that depicts a well-characterised sequence of cellular and molecular events within the vascular lumen. Recent findings have now illustrated that beyond the vascular lumen, the breaching of the venular wall can also involve an analogous cascade of adhesive events. For neutrophils this involves a tightly regulated and sequential series of responses within venular walls, initiating with adhesive steps that guide neutrophils through endothelial cells lining the venular wall, followed by responses that mediate and regulate their migration through the pericyte sheath and the venular basement membrane. The present review aims to provide a brief summary of novel additions to the classical adhesion cascade within the vascular lumen and then to discuss the emergence of a second adhesion cascade for neutrophils within venular walls, the latter illustrating the intricacies and complexities of neutrophil transmigration.

Pericytes regulate vascular basement membrane remodeling and govern neutrophil extravasation during inflammation

During inflammation polymorphonuclear neutrophils (PMNs) traverse venular walls, composed of the endothelium, pericyte sheath and vascular basement membrane. Compared to PMN transendothelial migration, little is known about how PMNs penetrate the latter barriers. Using mouse models and intravital microscopy, we show that migrating PMNs expand and use the low expression regions (LERs) of matrix proteins in the vascular basement membrane (BM) for their transmigration. Importantly, we demonstrate that this remodeling of LERs is accompanied by the opening of gaps between pericytes, a response that depends on PMN engagement with pericytes. Exploring how PMNs modulate pericyte behavior, we discovered that direct PMN-pericyte contacts induce relaxation rather than contraction of pericyte cytoskeletons, an unexpected response that is mediated by inhibition of the RhoA/ROCK signaling pathway in pericytes. Taking our in vitro results back into mouse models, we present evidence that pericyte relaxation contributes to the opening of the gaps between pericytes and to the enlargement of the LERs in the vascular BM, facilitating PMN extravasation. Our study demonstrates that pericytes can regulate PMN extravasation by controlling the size of pericyte gaps and thickness of LERs in venular walls. This raises the possibility that pericytes may be targeted in therapies aimed at regulating inflammation.

Microvascular remodeling and wound healing: a role for pericytes

Physiologic wound healing is highly dependent on the coordinated functions of vascular and non-vascular cells. Resolution of tissue injury involves coagulation, inflammation, formation of granulation tissue, remodeling and scarring. Angiogenesis, the growth of microvessels the size of capillaries, is crucial for these processes, delivering blood-borne cells, nutrients and oxygen to actively remodeling areas. Central to angiogenic induction and regulation is microvascular remodeling, which is dependent upon capillary endothelial cell and pericyte interactions. Despite our growing knowledge of pericyte-endothelial cell crosstalk, it is unclear how the interplay among pericytes, inflammatory cells, glia and connective tissue elements shape microvascular injury response. Here, we consider the relationships that pericytes form with the cellular effectors of healing in normal and diabetic environments, including repair following injury and vascular complications of diabetes, such as diabetic macular edema and proliferative diabetic retinopathy. In addition, pericytes and stem cells possessing "pericyte-like" characteristics are gaining considerable attention in experimental and clinical efforts aimed at promoting healing or eradicating ocular vascular proliferative disorders. As the origin, identification and characterization of microvascular pericyte progenitor populations remains somewhat ambiguous, the molecular markers, structural and functional characteristics of pericytes will be briefly reviewed.

Imaging of cell adhesion events in 3D matrix environments

Cell adhesion plays an essential role in development and homeostasis, but is also a key regulator of many diseases such as cancer and immune dysfunction. Numerous studies over the past three decades have revealed a wealth of information detailing signalling molecules required for cell adhesion to two-dimensional surfaces. However, in vivo many cells are completely surrounded by matrix and this will very likely influence the size, composition and dynamics of adhesive structures. The study of adhesion in cells within three-dimensional environments is still in its infancy, thus the role and regulation of adhesions in these complex environments remains unclear. The recent development of new experimental models coupled with significant advances in cell imaging approaches have provided platforms for researchers to begin to dissect adhesion signalling in cells in 3D matrices. Here we summarise the recent insights in cell adhesion formation and regulation in 3D model systems and the imaging approaches used to analyse these events.

Pericytes support neutrophil subendothelial cell crawling and breaching of venular walls in vivo

Neutrophil transmigration through venular walls that are composed of endothelial cells (ECs), pericytes, and the venular basement membrane is a key component of innate immunity. Through direct analysis of leukocyte-pericyte interactions in inflamed tissues using confocal intravital microscopy, we show how pericytes facilitate transmigration in vivo. After EC migration, neutrophils crawl along pericyte processes to gaps between adjacent pericytes in an ICAM-1-, Mac-1-, and LFA-1-dependent manner. These gaps were enlarged in inflamed tissues through pericyte shape change and were used as exit points by neutrophils in breaching the venular wall. The findings identify previously unknown roles for pericytes in neutrophil transmigration in vivo and add additional steps to the leukocyte adhesion cascade that supports leukocyte trafficking into sites of inflammation.

The effects of extracellular matrix proteins on neutrophil-endothelial interaction--a roadway to multiple therapeutic opportunities

Polymorphoneuclear leukocytes or neutrophils, a major component of white blood cells, contribute to the innate immune response in humans. Upon sensing changes in the microenvironment, neutrophils adhere to the vascular wall, migrate through the endothelial cell (EC)-pericyte bilayer, and subsequently through the extracellular matrix to reach the site of inflammation. These cells are capable of destroying microbes, cell debris, and foreign proteins by oxidative and non-oxidative processes. While primarily mediators of tissue homeostasis, there are an increasing number of studies indicating that neutrophil recruitment and transmigration can also lead to host-tissue injury and subsequently inflammation-related diseases. Neutrophil-induced tissue injury is highly regulated by the microenvironment of the infiltrated tissue, which includes cytokines, chemokines, and the provisional extracellular matrix, remodeled through increased vascular permeability and other cellular infiltrates. Thus, investigation of the effects of matrix proteins on neutrophil-EC interaction and neutrophil transmigration may help identify the proteins that induce pro- or anti-inflammatory responses. This area of research presents an opportunity to identify therapeutic targets in inflammation-related diseases. This review will summarize recent literature on the role of neutrophils and the effects of matrix proteins on neutrophil-EC interactions, with focus on three different disease models: 1) atherosclerosis, 2) COPD, and 3) tumor growth and progression. For each disease model, inflammatory molecules released by neutrophils, important regulatory matrix proteins, current anti-inflammatory treatments, and the scope for further research will be summarized.

Pericytes on the tumor vasculature: jekyll or hyde?

The induction of tumor vasculature, known as the 'angiogenic switch', is a rate-limiting step in tumor progression. Normal blood vessels are composed of two distinct cell types: endothelial cells which form the channel through which blood flows, and mural cells, the pericytes and smooth muscle cells which serve to support and stabilize the endothelium. Most functional studies have focused on the responses of endothelial cells to pro-angiogenic stimuli; however, there is mounting evidence that the supporting mural cells, particularly pericytes, may play key regulatory roles in both promoting vessel growth as well as terminating vessel growth to generate a mature, quiescent vasculature. Tumor vessels are characterized by numerous structural and functional abnormalities, including altered association between endothelial cells and pericytes. These dysfunctional, unstable vessels contribute to hypoxia, interstitial fluid pressure, and enhanced susceptibility to metastatic invasion. Increasing evidence points to the pericyte as a critical regulator of endothelial activation and subsequent vessel development, stability, and function. Here we discuss both the stimulatory and inhibitory effects of pericytes on the vasculature and the possible utilization of vessel normalization as a therapeutic strategy to combat cancer.

Modeling inflammation and microvascular dysfunction

We are using confocal intravital microscopy to understand the mechanisms behind leukocyte trafficking in the brain, thus providing potential therapeutic targets for neurovascular diseases, for example, stroke and multiple sclerosis.

Regulation of coronary venular barrier function by blood borne inflammatory mediators and pharmacological tools: insights from novel microvascular wall models

We hypothesized that postcapillary venules play a central role in the control of the tightness of the coronary system as a whole, particularly under inflammatory conditions. Sandwich cultures of endothelial cells and pericytes of precapillary arteriolar or postcapillary venular origin from human myocardium as models of the respective vascular walls (sandwich cultures of precapillary arteriolar or postcapillary venular origin) were exposed to thrombin and components of the acutely activatable inflammatory system, and their hydraulic conductivity ( LP) was registered. LP of SC-PAO remained low under all conditions (3.24 ± 0.52·10−8cm·s−1·cmH2O−1). In contrast, in the venular wall model, PGE2, platelet-activating factor (PAF), leukotriene B4 (LTB4), IL-6, and IL-8 induced a prompt, concentration-dependent, up to 10-fold increase in LP with synergistic support when combined. PAF and LTB4 released by metabolically cooperating platelets, and polymorphonuclear leucocytes (PMNs) caused selectively venular endothelial cells to contract and to open their clefts widely. This breakdown of the barrier function was preventable and even reversible within 6–8 h by the presence of 50 μM quercetin glucuronide (QG). LTB4 synthesis was facilitated by biochemical involvement of erythrocytes. Platelets segregated in the arterioles and PMNs in the venules of blood-perfused human myocardium (histological studies on donor hearts refused for heart transplantation). Extrapolating these findings to the coronary microcirculation in vivo would imply that the latter's complex functionality after accumulation of blood borne inflammatory mediators can change rapidly due to selective breakdown of the postcapillary venular barrier. The resulting inflammatory edema and venulo-thrombosis will severely impair myocardial performance. The protection afforded by QG could be of particular relevance in the context of cardiosurgical intervention.

Extracellular matrix determinants of proteolytic and non-proteolytic cell migration

Cell invasion into the 3D extracellular matrix (ECM) is a multistep biophysical process involved in inflammation, tissue repair, and metastatic cancer invasion. Migrating cells navigate through tissue structures of complex and often varying physicochemical properties, including molecular composition, porosity, alignment and stiffness, by adopting strategies that involve deformation of the cell and engagement of matrix-degrading proteases. We review how the ECM determines whether or not pericellular proteolysis is required for cell migration, ranging from protease-driven invasion and secondary tissue destruction, to non-proteolytic, non-destructive movement that solely depends on cell deformability and available tissue space. These concepts call for therapeutic targeting of proteases to prevent invasion-associated tissue destruction rather than the migration process per se.

Pericytes: developmental, physiological, and pathological perspectives, problems, and promises

Pericytes, the mural cells of blood microvessels, have recently come into focus as regulators of vascular morphogenesis and function during development, cardiovascular homeostasis, and disease. Pericytes are implicated in the development of diabetic retinopathy and tissue fibrosis, and they are potential stromal targets for cancer therapy. Some pericytes are probably mesenchymal stem or progenitor cells, which give rise to adipocytes, cartilage, bone, and muscle. However, there is still confusion about the identity, ontogeny, and progeny of pericytes. Here, we review the history of these investigations, indicate emerging concepts, and point out problems and promise in the field of pericyte biology.

Wall structures of myocardial precapillary arterioles and postcapillary venules reexamined and reconstructed in vitro for studies on barrier functions

The barrier functions of myocardial precapillary arteriolar and postcapillary venular walls (PCA or PCV, respectively) are of considerable scientific and clinical interest (regulation of blood flow and recruitment of immune defense). Using enzyme histochemistry combined with confocal microscopy, we reexamined the cell architecture of human PCA and PVC and reconstructed appropriate in vitro models for studies of their barrier functions. Contrary to current opinion, the PCA endothelial tube is encompassed not by smooth muscle cells but rather by a concentric layer of pericytes cocooned in a thick, microparticle-containing extracellular matrix (ECM) that contributes substantially to the tightness of the arteriolar wall. This core tube extends upstream into the larger arterioles, there additionally enwrapped by smooth muscle. PCV consist of an inner layer of large, contractile endothelial cells encompassed by a fragile, wide-meshed pericyte network with a weakly developed ECM. Pure pericyte and endothelial cell preparations were isolated from PCA and PCV and grown in sandwich cultures. These in vitro models of the PCA and PCV walls exhibited typical histological and functional features. In both plasma-like (PLM) and serum-containing (SCM) media, the PCA model (including ECM) maintained its low hydraulic conductivity (L(P) = 3.24 ± 0.52·10(-8)cm·s(-1)·cmH(2)O(-1)) and a high selectivity index for transmural passage of albumin (SI(Alb) = 0.95 ± 0.02). In contrast, L(P) and SI(Alb) in the PCV model (almost no ECM) were 2.55 ± 0.32·10(-7)cm·s(-1)·cmH(2)O(-1) and 0.88 ± 0.03, respectively, in PLM, and 1.39 ± 0.10·10(-6)cm·s(-1)·cmH(2)O(-1) and 0.49 ± 0.04 in SCM. With the use of these models, systematic, detailed studies on the regulation of microvascular barrier properties now appear to be feasible.

A novel role for calpain in the endothelial dysfunction induced by activation of angiotensin II type 1 receptor signaling

RATIONALE

The cytosolic protease calpain has been recently implicated in the vascular remodeling of angiotensin II (Ang II) type 1 receptor (AT(1)R) signaling. The role of Ang II/AT(1)R/calpain signaling on endothelial function, an important and early determinant of vascular pathology, remains though totally unknown. Accordingly, we investigated the role of calpain in the endothelial dysfunction of Ang II.

OBJECTIVE

To demonstrate a mechanistic role for calpain in the endothelial dysfunction induced by Ang II/AT(1)R signaling. To establish endothelial-expressed calpains as an important target of AT(1)R signaling.

METHODS AND RESULTS

Subchronic administration of nonpressor doses of Ang II to rats and mice significantly increased vascular calpain activity via AT(1)R signaling. Intravital microscopy studies revealed that activation of vascular expressed calpains causes endothelial dysfunction with increased leukocyte-endothelium interactions and albumin permeability in the microcirculation. Western blot and immunohistochemistry studies confirmed that Ang II/AT(1)R signaling preferentially activates the constitutively expressed μ-calpain isoform and demonstrated a calpain-dependent degradation of IκBα, along with upregulation of nuclear factor κB-regulated endothelial cell adhesion molecules. These physiological and biochemical parameters were nearly normalized following inhibition of AT(1)R or calpain in vivo. RNA silencing studies in microvascular endothelial cells, along with knockout and transgenic mouse studies, further confirmed the role of μ-calpain in the endothelial adhesiveness induced by Ang II.

CONCLUSIONS

This study uncovers a novel role for calpain in the endothelial dysfunction of Ang II/AT(1)R signaling and establishes the calpain system as a novel molecular target of the vascular protective action of renin-angiotensin system inhibition. Our results may have significant clinical implications in vascular disease.

LFA-1 and Mac-1 define characteristically different intralumenal crawling and emigration patterns for monocytes and neutrophils in situ

To exit blood vessels, most (∼80%) of the lumenally adhered monocytes and neutrophils crawl toward locations that support transmigration. Using intravital confocal microscopy of anesthetized mouse cremaster muscle, we separately examined the crawling and emigration patterns of monocytes and neutrophils in blood-perfused unstimulated or TNF-α-activated venules. Most of the interacting cells in microvessels are neutrophils; however, in unstimulated venules, a greater percentage of the total monocyte population is adherent compared with neutrophils (58.2 ± 6.1% versus 13.6 ± 0.9%, adhered/total interacting), and they crawl for significantly longer distances (147.3 ± 13.4 versus 61.8 ± 5.4 μm). Intriguingly, after TNF-α activation, monocytes crawled for significantly shorter distances (67.4 ± 9.6 μm), resembling neutrophil crawling. Using function-blocking Abs, we show that these different crawling patterns were due to CD11a/CD18 (LFA-1)- versus CD11b/CD18 (Mac-1)-mediated crawling. Blockade of either Mac-1 or LFA-1 revealed that both LFA-1 and Mac-1 contribute to monocyte crawling; however, the LFA-1-dependent crawling in unstimulated venules becomes Mac-1 dependent upon inflammation, likely due to increased expression of Mac-1. Mac-1 alone was responsible for neutrophil crawling in both unstimulated and TNF-α-activated venules. Consistent with the role of Mac-1 in crawling, Mac-1 block (compared with LFA-1) was also significantly more efficient in blocking TNF-α-induced extravasation of both monocytes and neutrophils in cremaster tissue and the peritoneal cavity. Thus, mechanisms underlying leukocyte crawling are important in regulating the inflammatory responses by regulating the numbers of leukocytes that transmigrate.

Cell-matrix interactions in mammary gland development and breast cancer

The mammary gland is an organ that at once gives life to the young, but at the same time poses one of the greatest threats to the mother. Understanding how the tissue develops and functions is of pressing importance in determining how its control mechanisms break down in breast cancer. Here we argue that the interactions between mammary epithelial cells and their extracellular matrix (ECM) are crucial in the development and function of the tissue. Current strategies for treating breast cancer take advantage of our knowledge of the endocrine regulation of breast development, and the emerging role of stromal-epithelial interactions (Fig. 1). Focusing, in addition, on the microenvironmental influences that arise from cell-matrix interactions will open new opportunities for therapeutic intervention. We suggest that ultimately a three-pronged approach targeting endocrine, growth factor, and cell-matrix interactions will provide the best chance of curing the disease.

Breaching multiple barriers: leukocyte motility through venular walls and the interstitium

The shuttling of leukocytes between the bloodstream and interstitial tissues involves different locomotion strategies that are governed by locally presented soluble and cell-bound signals. Recent studies have furthered our understanding of the rapidly advancing field of leukocyte migration, particularly regarding cellular and subcellular events at the level of the venular wall. Furthermore, emerging cellular models are now addressing the transition from an adherent mode to a non-adherent state, incorporating mechanisms that support an efficient migratory profile of leukocytes in the interstitial tissue beyond the venular wall.

The melanocortin MC(1) receptor agonist BMS-470539 inhibits leucocyte trafficking in the inflamed vasculature

BACKGROUND AND PURPOSE

Over three decades of research evaluating the biology of melanocortin (MC) hormones and synthetic peptides, activation of the MC type 1 (MC(1)) receptor has been identified as a viable target for the development of novel anti-inflammatory therapeutic agents. Here, we have tested a recently described selective agonist of MC(1) receptors, BMS-470539, on leucocyte/post-capillary venule interactions in murine microvascular beds.

EXPERIMENTAL APPROACH

Intravital microscopy of two murine microcirculations were utilized, applying two distinct modes of promoting inflammation. The specificity of the effects of BMS-470539 was assessed using mice bearing mutant inactive MC(1) receptors (the recessive yellow e/e colony).

KEY RESULTS

BMS-470539, given before an ischaemia-reperfusion protocol, inhibited cell adhesion and emigration with no effect on cell rolling, as assessed 90 min into the reperfusion phase. These properties were paralleled by inhibition of tissue expression of both CXCL1 and CCL2. Confocal investigations of inflamed post-capillary venules revealed immunostaining for MC(1) receptors on adherent and emigrated leucocytes. Congruently, the anti-inflammatory properties of BMS-470539 were lost in mesenteries of mice bearing the inactive mutant MC(1) receptors. Therapeutic administration of BMS-470539 stopped cell emigration, but did not affect cell adhesion in the cremasteric microcirculation inflamed by superfusion with platelet-activating factor.

CONCLUSIONS AND IMPLICATIONS

Activation of MC(1) receptors inhibited leucocyte adhesion and emigration. Development of new chemical entities directed at MC(1) receptors could be a viable approach in the development of novel anti-inflammatory therapeutic agents with potential application to post-ischaemic conditions.

Recent developments and complexities in neutrophil transmigration

PURPOSE OF REVIEW

As the migration of neutrophils from blood to inflamed tissues is an essential component of innate immunity and a key contributing factor to the pathogenesis of inflammatory disorders, this aspect of leukocyte biology continues to be a highly dynamic field of research. This review summarizes recent findings in this area, focusing on the mechanisms that mediate neutrophil transmigration, an area where significant progress has been made.

RECENT FINDINGS

The topics to be covered will include responses that are prerequisite to neutrophil migration through venular walls, such as leukocyte luminal crawling and cellular and molecular changes in leukocytes and endothelial cells (e.g. formation of protrusions) that collectively support leukocyte transendothelial cell migration. Advances in both paracellular and transcellular neutrophil migration through endothelial cells will be discussed, addressing the associated roles and regulation of expression of endothelial cell luminal and junctional adhesion molecules. Beyond the endothelium, migration through the vascular pericyte coverage and basement membrane will be reviewed.

SUMMARY

The unquestionable role of neutrophils in the development and progression of inflammatory conditions suggests that a better understanding of the tissue-specific and stimulus-specific mechanisms that mediate this response may identify novel pathways that could be exploited for the development of more specific anti-inflammatory interventions.