The A-domain of beta 2 integrin CR3 (CD11b/CD18) is a receptor for the hookworm-derived neutrophil adhesion inhibitor NIF

Phagocytosis via complement receptor 3 enables microbes to evade killing by neutrophils

CR3 (CD11b/CD18; αmβ2 integrin) is a conserved phagocytic receptor. The active conformation of CR3 binds the iC3b fragment of complement C3 as well as many host and microbial ligands, leading to actin-dependent phagocytosis. There are conflicting reports about how CR3 engagement affects the fate of phagocytosed substrates. Using imaging flow cytometry, we confirmed that binding and internalization of iC3b-opsonized polystyrene beads by primary human neutrophils was CR3-dependent. iC3b-opsonized beads did not stimulate neutrophil reactive oxygen species, and most beads were found in primary granule-negative phagosomes. Similarly, Neisseria gonorrhoeae that does not express phase-variable Opa proteins suppresses neutrophil reactive oxygen species and delays phagolysosome formation. Here, binding and internalization of Opa-deleted (Δopa) N. gonorrhoeae by adherent human neutrophils was inhibited using blocking antibodies against CR3 and by adding neutrophil inhibitory factor, which targets the CD11b I-domain. No detectable C3 was deposited on N. gonorrhoeae in the presence of neutrophils alone. Conversely, overexpressing CD11b in HL-60 promyelocytes enhanced Δopa N. gonorrhoeae phagocytosis, which required the CD11b I-domain. Phagocytosis of N. gonorrhoeae was also inhibited in mouse neutrophils that were CD11b-deficient or treated with anti-CD11b. Phorbol ester treatment upregulated surface CR3 on neutrophils in suspension, enabling CR3-dependent phagocytosis of Δopa N. gonorrhoeae. Neutrophils exposed to Δopa N. gonorrhoeae had limited phosphorylation of Erk1/2, p38, and JNK. Neutrophil phagocytosis of unopsonized Mycobacterium smegmatis, which also resides in immature phagosomes, was CR3-dependent and did not elicit reactive oxygen species. We suggest that CR3-mediated phagocytosis is a silent mode of entry into neutrophils, which is appropriated by diverse pathogens to subvert phagocytic killing.

CAP superfamily proteins from venomous animals: Who we are and what to do?

Cysteine-rich secretory proteins (CRISPs), antigen 5 (Ag5), and pathogenesis-related (PR-1) superfamily proteins (CAP superfamily proteins) are found in diverse species across the bacterial, fungal, plant, mammalian, and venomous animal kingdoms. Notably, CAP proteins are found in a remarkable range of species across the venomous animal kingdom and are present almost ubiquitously in venoms, even when venoms are produced in very small quantities. Meanwhile, in comparison to mammals, venomous animals are underappreciated and easy to ignore. Overwhelming evidence suggests that CAP proteins derived from venomous animals exhibit diverse activities, including ion channel, inflammatory, proteolysis, and immune regulatory activities. To understand the potential biological functions of CAP proteins in venom more effectively, we need to examine the significance of the evolution of venomous animals in the animal kingdom, for their survival. In this article, we will review the current status of research on CAP proteins in venomous animals, including their isolation, characterization, known biological activities, and sequence alignments. We will also discuss the rapid evolution of CAP proteins with varied subtypes in venomous animals. A treasure trove of information can be obtained by studying the CAP proteins in venomous animals; hence, it is necessary to explore these proteins further.

Prenatal environmental stressors impair postnatal microglia function and adult behavior in males

Gestational exposure to environmental toxins and socioeconomic stressors is epidemiologically linked to neurodevelopmental disorders with strong male bias, such as autism. We model these prenatal risk factors in mice by co-exposing pregnant dams to an environmental pollutant and limited-resource stress, which robustly activates the maternal immune system. Only male offspring display long-lasting behavioral abnormalities and alterations in the activity of brain networks encoding social interactions. Cellularly, prenatal stressors diminish microglial function within the anterior cingulate cortex, a central node of the social coding network, in males during early postnatal development. Precise inhibition of microglial phagocytosis within the anterior cingulate cortex (ACC) of wild-type (WT) mice during the same critical period mimics the impact of prenatal stressors on a male-specific behavior, indicating that environmental stressors alter neural circuit formation in males via impairing microglia function during development.

Block et al. show that combined exposure to air pollution and maternal stress during pregnancy activates the maternal immune system and induces male-specific impairments in social behavior and circuit connectivity in offspring. Cellularly, prenatal stressors diminish microglia phagocytic function, and inhibition of microglia phagocytosis phenocopies behavioral deficits from prenatal stressors.

Tissue-specific immunity in helminth infections

A characteristic feature of host responses to helminth infections is the development of profound systemic and tissue-localised Type 2 immune responses that play critical roles in immunity, tissue repair and tolerance of the parasite at tissue sites. These same Type 2 responses are also seen in the tissue-associated immune-pathologies seen in asthma, atopic dermatitis and many forms of allergies. The recent identification of new subtypes of immune cells and cytokine pathways that influence both immune and non-immune cells and tissues creates the opportunity for reviewing helminth parasite-host responses in the context of tissue specific immunity. This review focuses on the new discoveries of the cells and cytokines involved in tissue specific immune responses to helminths and how these contribute to host immunity against helminth infection and allow the host to accommodate the presence of parasites when they cannot be eliminated.

Amygdala microglia modify neuronal plasticity via complement C1q/C3-CR3 signaling and contribute to visceral pain in a rat model

Stress can trigger symptoms in patients with irritable bowel syndrome (IBS). Previously we demonstrated that chronic psychological stress induced microglial remodeling in the central nucleus of amygdala (CeA) and contributed to the development of visceral hypersensitivity via synaptic engulfment. However, the specific signaling mechanisms that microglia depend upon to recognize target neurons to facilitate visceral pain remain unknown. Here, we test the hypothesis that the microglia in the CeA contribute to chronic stress-induced visceral hypersensitivity via complement C1q/C3-CR3 signaling-mediated synaptic remodeling. In male and female Fischer-344 rats, micropellets of corticosterone (CORT) or cholesterol (control) were stereotaxically implanted bilaterally onto the CeA. After 7 days, microglial C1q, complement receptor 3 (CR3) expression, and microglia-mediated synaptic engulfment were assessed via RNAscope, quantitative PCR, and immunofluorescence. The microglial inhibitor minocycline, CR3 antagonist neutrophil inhibitory factor (NIF), or vehicle were daily infused into the CeA following CORT implantations. Visceral sensitivity was assessed via a visceromotor response (VMR) to graded pressures of isobaric colorectal distension (CRD). Our results suggest that chronic exposure to elevated CORT in the CeA induced visceral hypersensitivity and amygdala microglial morphological remodeling. CORT increased microglial C1q and CR3 expression and increased microglia-mediated synaptic engulfment. Both groups of animals with minocycline or NIF infusions reversed microglia-mediated synaptic remodeling and attenuated CORT-induced visceral hypersensitivity. Our findings demonstrate that C1q/C3-CR3 signaling is critical for microglia-mediated synaptic remodeling in the CeA and contributes to CORT-induced visceral hypersensitivity.NEW & NOTEWORTHY Patients with irritable bowel syndrome (IBS) show altered amygdala activity. We showed previously that stress induces visceral hypersensitivity partially through microglia-modulated synaptic plasticity in the central nucleus of the amygdala (CeA). Our current data suggest that the C1q/C3-CR3 cascade initiates microglia-mediated synaptic remodeling in the CeA. Blocking C3-CR3 interaction attenuates stress-induced visceral hypersensitivity. These findings uncover a role of microglia-synapse signaling in the brain-gut regulation and support a future therapeutic target to treat visceral pain.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Structural Characterization of the Interaction between the αMI-Domain of the Integrin Mac-1 (αMβ2) and the Cytokine Pleiotrophin

Integrin Mac-1 (α_Mβ₂) is an adhesion receptor vital to many functions of myeloid leukocytes. It is also the most promiscuous member of the integrin family capable of recognizing a broad range of ligands. In particular, its ligand-binding α_MI-domain is known to bind cationic proteins/peptides depleted in acidic residues. This contradicts the canonical ligand-binding mechanism of αI-domains, which requires an acidic amino acid in the ligand to coordinate the divalent cation within the metal ion-dependent adhesion site (MIDAS) of αI-domains. The lack of acidic amino acids in the α_MI-domain-binding sequences suggests the existence of an as-yet uncharacterized interaction mechanism. In the present study, we analyzed interactions of the α_MI-domain with a representative Mac-1 ligand, the cationic cytokine pleiotrophin (PTN). Through NMR chemical shift perturbation analysis, cross saturation, NOESY, and mutagenesis studies, we found the interaction between the α_MI-domain and PTN is divalent cation-independent and mediated mostly by hydrophobic contacts between the N-terminal domain of PTN and residues in the α5-β5 loop of α_MI-domain. The observation that increased ionic strength weakens the interaction between the proteins indicates electrostatic forces may also play a significant role in the binding. On the basis of the results from these experiments, we formulated a model of the interaction between the α_MI-domain and PTN.

Twenty-five-year research progress in hookworm excretory/secretory products

Hookworm infection is a major public health problem that threatens about 500 million people throughout tropical areas of the world. Adult hookworms survive for many years in the host intestine, where they suck blood, causing iron deficiency anemia and malnutrition. Numerous molecules, named excretory/secretory (ES) products, are secreted by hookworm adults and/or larvae to aid in parasite survival and pathobiology. Although the molecular cloning and characterization of hookworm ES products began 25 years ago, the biological role and molecular nature of many of them are still unclear. Hookworm ES products, with distinct structures and functions, have been linked to many essential events in the disease pathogenesis. These events include host invasion and tissue migration, parasite nourishment and reproduction, and immune modulation. Several of these products represent promising vaccine targets for controlling hookworm disease and therapeutic targets for many inflammatory diseases. This review aims to summarize our present knowledge about hookworm ES products, including their role in parasite biology, host-parasite interactions, and as vaccine and pharmaceutical targets and to identify research gaps and future research directions in this field.

Structural Immunology of Complement Receptors 3 and 4

Complement receptors (CR) 3 and 4 belong to the family of beta-2 (CD18) integrins. CR3 and CR4 are often co-expressed in the myeloid subsets of leukocytes, but they are also found in NK cells and activated T and B lymphocytes. The heterodimeric ectodomain undergoes considerable conformational change in order to switch the receptor from a structurally bent, ligand-binding in-active state into an extended, ligand-binding active state. CR3 binds the C3d fragment of C3 in a way permitting CR2 also to bind concomitantly. This enables a hand-over of complement-opsonized antigens from the cell surface of CR3-expressing macrophages to the CR2-expressing B lymphocytes, in consequence acting as an antigen presentation mechanism. As a more enigmatic part of their functions, both CR3 and CR4 bind several structurally unrelated proteins, engineered peptides, and glycosaminoglycans. No consensus motif in the proteinaceous ligands has been established. Yet, the experimental evidence clearly suggest that the ligands are primarily, if not entirely, recognized by a single site within the receptors, namely the metal-ion dependent adhesion site (MIDAS). Comparison of some recent identified ligands points to CR3 as inclined to bind positively charged species, while CR4, by contrast, binds strongly negative-charged species, in both cases with the critical involvement of deprotonated, acidic groups as ligands for the Mg²⁺ ion in the MIDAS. These properties place CR3 and CR4 firmly within the realm of modern molecular medicine in several ways. The expression of CR3 and CR4 in NK cells was recently demonstrated to enable complement-dependent cell cytotoxicity toward antibody-coated cancer cells as part of biological therapy, constituting a significant part of the efficacy of such treatment. With the flexible principles of ligand recognition, it is also possible to propose a response of CR3 and CR4 to existing medicines thereby opening a possibility of drug repurposing to influence the function of these receptors. Here, from advances in the structural and cellular immunology of CR3 and CR4, we review insights on their biochemistry and functions in the immune system.

Helminth Immunomodulation in Autoimmune Disease

Helminths have evolved to become experts at subverting immune surveillance. Through potent and persistent immune tempering, helminths can remain undetected in human tissues for decades. Redirecting the immunomodulating "talents" of helminths to treat inflammatory human diseases is receiving intensive interest. Here, we review therapies using live parasitic worms, worm secretions, and worm-derived synthetic molecules to treat autoimmune disease. We review helminth therapy in both mouse models and clinical trials and discuss what is known on mechanisms of action. We also highlight current progress in characterizing promising new immunomodulatory molecules found in excretory/secretory products of helminths and their potential use as immunotherapies for acute and chronic inflammatory diseases.

Structural and functional characterization of the CAP domain of pathogen-related yeast 1 (Pry1) protein

The production, crystal structure, and functional characterization of the C-terminal cysteine-rich secretory protein/antigen 5/pathogenesis related-1 (CAP) domain of pathogen-related yeast protein-1 (Pry1) from Saccharomyces cerevisiae is presented. The CAP domain of Pry1 (Pry1CAP) is functional in vivo as its expression restores cholesterol export to yeast mutants lacking endogenous Pry1 and Pry2. Recombinant Pry1CAP forms dimers in solution, is sufficient for in vitro cholesterol binding, and has comparable binding properties as full-length Pry1. Two crystal structures of Pry1CAP are reported, one with Mg²⁺ coordinated to the conserved CAP tetrad (His208, Glu215, Glu233 and His250) in spacegroup I4₁ and the other without divalent cations in spacegroup P6₁22. The latter structure contains four 1,4-dioxane molecules from the crystallization solution, one of which sits in the cholesterol binding site. Both structures reveal that the divalent cation and cholesterol binding sites are connected upon dimerization, providing a structural basis for the observed Mg²⁺-dependent sterol binding by Pry1.

A recombinant fusion protein derived from dog hookworm inhibits autoantibody-induced dermal-epidermal separation ex vivo

The proteins secreted by parasitic nematodes are evolutionarily optimized molecules with unique capabilities of suppressing the immune response of the host organism. Neutrophil inhibitory factor (NIF), which is secreted by the dog hookworm Ancylostoma caninum, binds to the β2 integrin CD11b/CD18, which is expressed on human neutrophils, eosinophils, monocytes and macrophages and inhibits neutrophil-dependent lung injury and neutrophil invasion of ischaemic brain tissue. Neutrophils are key players in the pathogenesis of subepidermal autoimmune blistering diseases (sAIBDs), and their pathogenic activities are crucially dependent on β2 integrin functionality. Based on the template of single-stranded, dimerizing antibody derivatives, which are already used in cancer treatment, we designed a novel biologic, NIF-IGHE-CH4, comprising NIF and the dimerizing but otherwise inert constant heavy subdomain 4 (CH4) of human IgE (IGHE). This molecule was evaluated in a variety of in vitro assays, demonstrating its ability to inhibit pathogenically relevant neutrophil functions such as migration, adhesion and spreading, and release of reactive oxygen species. Finally, we confirmed that NIF-IGHE-CH4 inhibits blister formation in an ex vivo assay of sAIBD. These results suggest that NIF-IGHE-CH4 is a novel potential anti-inflammatory drug for the treatment of neutrophil-mediated diseases such as sAIBDs. This study promotes the drugs from bugs concept and encourages further research and development focused on turning parasite proteins into useful anti-inflammatory biologics.

Evidence for a lectin specific for sulfated glycans in the salivary gland of the malaria vector, Anopheles gambiae

Salivary gland homogenate (SGH) from the female mosquitoes Anopheles gambiae, An. stephensi, An. freeborni, An. dirus and An. albimanus were found to exhibit hemagglutinating (lectin) activity. Lectin activity was not found for male An. gambiae, or female Ae aegypti, Culex quinquefasciatus, Phlebotomus duboscqi, and Lutzomyia longipalpis. With respect to species-specificity, An. gambiae SGH agglutinates red blood cells (RBC) from humans, horse, sheep, goat, pig, and cow; it is less active for rats RBC, and not detectable for guinea-pigs or chicken RBC. Notably, lectin activity was inhibited by low concentrations of dextran sulfate 50–500 K, fucoidan, heparin, laminin, heparin sulfate proteoglycan, sialyl-containing glycans (e.g. 3′-sialyl Lewis X, and 6′-sialyl lactose), and gangliosides (e.g. GM3, GD1, GD1b, GTB1, GM1, GQ1B), but not by simple sugars. These results imply that molecule(s) in the salivary gland target sulfated glycans. SGH from An. gambiae was also found to promote agglutination of HL-60 cells which are rich in sialyl Lewis X, a glycan that decorates PSGL-1, the neutrophils receptor that interacts with endothelial cell P-selectin. Accordingly, SGH interferes with HL-60 cells adhesion to immobilized P-selectin. Because An. gambiae SGH expresses galectins, one member of this family (herein named Agalectin) was expressed in E. coli. Recombinant Agalectin behaves as a non-covalent homodimer. It does not display lectin activity, and does not interact with 500 candidates tested in a Glycan microarray. Gel-filtration chromatography of the SGH of An. gambiae identified a fraction with hemagglutinating activity, which was analyzed by 1D PAGE followed by in-gel tryptic digestion, and nano-LC MS/MS. This approach identified several genes which emerge as candidates for a lectin targeting sulfated glycans, the first with this selectivity to be reported in the SGH of a blood-sucking arthropod. The role of salivary molecules (sialogenins) with lectin activity is discussed in the context of inflammation, and parasite-vector-host interactions.

Probing of a human proteome microarray with a recombinant pathogen protein reveals a novel mechanism by which hookworms suppress B-cell receptor signaling

Na-ASP-2 is an efficacious hookworm vaccine antigen. However, despite elucidation of its crystal structure and studies addressing its immunobiology, the function of Na-ASP-2 has remained elusive. We probed a 9000-protein human proteome microarray with Na-ASP-2 and showed binding to CD79A, a component of the B-cell antigen receptor complex. Na-ASP-2 bound to human B lymphocytes ex vivo and downregulated the transcription of approximately 1000 B-cell messenger RNAs (mRNAs), while only approximately 100 mRNAs were upregulated, compared with control-treated cells. The expression of a range of molecules was affected by Na-ASP-2, including factors involved in leukocyte transendothelial migration pathways and the B-cell signaling receptor pathway. Of note was the downregulated transcription of lyn and pi3k, molecules that are known to interact with CD79A and control B-cell receptor signaling processes. Together, these results highlight a previously unknown interaction between a hookworm-secreted protein and B cells, which has implications for helminth-driven immunomodulation and vaccine development. Further, the novel use of human protein microarrays to identify host-pathogen interactions, coupled with ex vivo binding studies and subsequent analyses of global gene expression in human host cells, demonstrates a new pipeline by which to explore the molecular basis of infectious diseases.

Antagonism of CD11b with neutrophil inhibitory factor (NIF) inhibits vascular lesions in diabetic retinopathy

Leukocytes and proteins that govern leukocyte adhesion to endothelial cells play a causal role in retinal abnormalities characteristic of the early stages of diabetic retinopathy, including diabetes-induced degeneration of retinal capillaries. Leukocyte integrin αmβ2 (CD11b/CD18, MAC1), a protein mediating adhesion, has been shown to mediate damage to endothelial cells by activated leukocytes in vitro. We hypothesized that Neutrophil Inhibitory Factor (NIF), a selective antagonist of integrin αmβ2, would inhibit the diabetes-induced degeneration of retinal capillaries by inhibiting the excessive interaction between leukocytes and retinal endothelial cells in diabetes. Wild type animals and transgenic animals expressing NIF were made diabetic with streptozotocin and assessed for diabetes-induced retinal vascular abnormalities and leukocyte activation. To assess if the leukocyte blocking therapy compromised the immune system, animals were challenged with bacteria. Retinal superoxide production, leukostasis and leukocyte superoxide production were increased in wild type mice diabetic for 10 weeks, as was the ability of leukocytes isolated from diabetic animals to kill retinal endothelial cells in vitro. Retinal capillary degeneration was significantly increased in wild type mice diabetic 40 weeks. In contrast, mice expressing NIF did not develop any of these abnormalities, with the exception that non-diabetic and diabetic mice expressing NIF generated greater amounts of superoxide than did similar mice not expressing NIF. Importantly, NIF did not significantly impair the ability of mice to clear an opportunistic bacterial challenge, suggesting that NIF did not compromise immune surveillance. We conclude that antagonism of CD11b (integrin αmβ2) by NIF is sufficient to inhibit early stages of diabetic retinopathy, while not compromising the basic immune response.

Neutrophil Inhibitory Factor Selectively Inhibits the Endothelium-Driven Transmigration of Eosinophils In Vitro and Airway Eosinophilia in OVA-Induced Allergic Lung Inflammation

Leukocyte adhesion molecules are involved in cell recruitment in an allergic airway response and therefore provide a target for pharmaceutical intervention. Neutrophil inhibitory factor (NIF), derived from canine hookworm (Ancylostoma caninum), binds selectively and competes with the A-domain of CD11b for binding to ICAM-1. The effect of recombinant NIF was investigated. Intranasal administration of rNIF reduced pulmonary eosinophilic infiltration, goblet cell hyperplasia, and Th₂ cytokine production in OVA-sensitized mice. In vitro, transendothelial migration of human blood eosinophils across IL-4-activated umbilical vein endothelial cell (HUVEC) monolayers was inhibited by rNIF (IC₅₀: 4.6 ± 2.6 nM; mean ± SEM), but not across TNF or IL-1-activated HUVEC monolayers. Treatment of eosinophils with rNIF together with mAb 60.1 directed against CD11b or mAb 107 directed against the metal ion-dependent adhesion site (MIDAS) of the CD11b A-domain resulted in no further inhibition of transendothelial migration suggesting shared functional epitopes. In contrast, rNIF increased the inhibitory effect of blocking mAbs against CD18, CD11a, and VLA-4. Together, we show that rNIF, a selective antagonist of the A-domain of CD11b, has a prominent inhibitory effect on eosinophil transendothelial migration in vitro, which is congruent to the in vivo inhibition of OVA-induced allergic lung inflammation.

Molecular mechanisms of hookworm disease: stealth, virulence, and vaccines

Hookworms produce a vast repertoire of structurally and functionally diverse molecules that mediate their long-term survival and pathogenesis within a human host. Many of these molecules are secreted by the parasite, after which they interact with critical components of host biology, including processes that are key to host survival. The most important of these interactions is the hookworm's interruption of nutrient acquisition by the host through its ingestion and digestion of host blood. This results in iron deficiency and eventually the microcytic hypochromic anemia or iron deficiency anemia that is the clinical hallmark of hookworm infection. Other molecular mechanisms of hookworm infection cause a systematic suppression of the host immune response to both the parasite and to bystander antigens (eg, vaccines or allergens). This is achieved by a series of molecules that assist the parasite in the stealthy evasion of the host immune response. This review will summarize the current knowledge of the molecular mechanisms used by hookworms to survive for extended periods in the human host (up to 7 years or longer) and examine the pivotal contributions of these molecular mechanisms to chronic hookworm parasitism and host clinical outcomes.

The buccal gland of Lampetra japonica is a source of diverse bioactive proteins

The parasitic phase lampreys (Lampetra japonica) are bloodsuckers in the marine, and their buccal gland secretion (lamphredin) contains various regulators such as anticoagulants, ion channel blockers, and immune suppressors like those from leeches, insects, ticks, vampire bats, and snakes. This review focuses on the functions and characteristics of the active proteins from the buccal gland of L. japonica for the first time, and provides new insights into the parasitic mechanisms of lampreys and the possibilities of developing drugs such as novel anticoagulants, thrombolytic agents, local anesthetics, and immunosuppressants.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Disintegrins from hematophagous sources

Bloodsucking arthropods are a rich source of salivary molecules (sialogenins) which inhibit platelet aggregation, neutrophil function and angiogenesis. Here we review the literature on salivary disintegrins and their targets. Disintegrins were first discovered in snake venoms, and were instrumental in our understanding of integrin function and also for the development of anti-thrombotic drugs. In hematophagous animals, most disintegrins described so far have been discovered in the salivary gland of ticks and leeches. A limited number have also been found in hookworms and horseflies, and none identified in mosquitoes or sand flies. The vast majority of salivary disintegrins reported display a RGD motif and were described as platelet aggregation inhibitors, and few others as negative modulator of neutrophil or endothelial cell functions. This notably low number of reported disintegrins is certainly an underestimation of the actual complexity of this family of proteins in hematophagous secretions. Therefore an algorithm was created in order to identify the tripeptide motifs RGD, KGD, VGD, MLD, KTS, RTS, WGD, or RED (flanked by cysteines) in sialogenins deposited in GenBank database. The search included sequences from various blood-sucking animals such as ticks (e.g., Ixodes sp., Argas sp., Rhipicephalus sp., Amblyommasp.), tabanids (e.g., Tabanus sp.), bugs (e.g., Triatoma sp., Rhodnius prolixus), mosquitoes (e.g., Anopheles sp., Aedes sp., Culex sp.), sand flies (e.g., Lutzomyia sp., Phlebotomus sp.), leeches (e.g., Macrobdella sp., Placobdella sp.) and worms (e.g., Ancylostoma sp.). This approach allowed the identification of a remarkably high number of novel putative sialogenins with tripeptide motifs typical of disintegrins (>450 sequences) whose biological activity remains to be verified. This database is accessible online as a hyperlinked worksheet and displays biochemical, taxonomic, and gene ontology aspects for each putative disintegrin. It is also freely available for download (right click with the mouse) at links http://exon.niaid.nih.gov/transcriptome/RGD/RGD-Peps-WEB.xlsx (web version) and http://exon.niaid.nih.gov/transcriptome/RGD/RGD-sialogenins.zip (stand alone version).

Stable coordination of the inhibitory Ca2+ ion at the metal ion-dependent adhesion site in integrin CD11b/CD18 by an antibody-derived ligand aspartate: implications for integrin regulation and structure-based drug design

A central feature of integrin interaction with physiologic ligands is the monodentate binding of a ligand carboxylate to a Mg(2+) ion hexacoordinated at the metal ion-dependent adhesion site (MIDAS) in the integrin A domain. This interaction stabilizes the A domain in the high-affinity state, which is distinguished from the default low-affinity state by tertiary changes in the domain that culminate in cell adhesion. Small molecule ligand-mimetic integrin antagonists act as partial agonists, eliciting similar activating conformational changes in the A domain, which has contributed to paradoxical adhesion and increased patient mortality in large clinical trials. As with other ligand-mimetic integrin antagonists, the function-blocking mAb 107 binds MIDAS of integrin CD11b/CD18 A domain (CD11bA), but in contrast, it favors the inhibitory Ca(2+) ion over the Mg(2+) ion at MIDAS. We determined the crystal structures of the Fab fragment of mAb 107 complexed to the low- and high-affinity states of CD11bA. Favored binding of the Ca(2+) ion at MIDAS is caused by the unusual symmetric bidentate ligation of a Fab-derived ligand Asp to a heptacoordinated MIDAS Ca(2+) ion. Binding of the Fab fragment of mAb 107 to CD11bA did not trigger the activating tertiary changes in the domain or in the full-length integrin. These data show that the denticity of the ligand Asp/Glu can modify the divalent cation selectivity at MIDAS and hence integrin function. Stabilizing the Ca(2+) ion at MIDAS by bidentate ligation to a ligand Asp/Glu may provide one approach for designing pure integrin antagonists.

Novel neutrophil inhibitory factor homologue in the buccal gland secretion of Lampetra japonica

Abstract To identify the functional gene fragment, a neutrophil inhibitory factor (NIF) like protein was found in the buccal gland of Lampetra japonica, suggesting that this related lamprey protein represents a novel class of integrin receptor antagonists. The recombinant Lampetra japonica-NIF like (rLj-NIF) was identified by SDS-PAGE and purified by using His·Bind affinity chromatography. Effect of rLj-NIF on neutrophil migration suggested that rLj-NIF can act as a neutrophil inhibitory factor. Besides that, oxidative burst activity of neutriphil was tested by flow cytometry using dihydrorhodamine (DHR123) as a fluorogenic substrate, and the data suggested that the mean fluorescence intensity significantly decreased compared with positive controls (p<0.01). All above results indicated that rLj-NIF could also prevent the binding of β2 integrins to the surface of PMN and its FITC-labeled monoclonal antibodies (p<0.05). These data suggest that Lampetra japonica-NIF like protein is secreted by the stage of the parasite at the site of attachment. rLj-NIF plays an essential role in physiological reaction of neutrophil by a novel class of β2 integrin receptor antagonists. The activity of immunosuppressant of L. japonica-NIF could have potential medicinal value in anti-inflammation and therapy of autoimmune diseases.

Mast cells as early responders in the regulation of acute blood-brain barrier changes after cerebral ischemia and hemorrhage

The inflammatory response triggered by stroke has been viewed as harmful, focusing on the influx and migration of blood-borne leukocytes, neutrophils, and macrophages. This review hypothesizes that the brain and meninges have their own resident cells that are capable of fast host response, which are well known to mediate immediate reactions such as anaphylaxis, known as mast cells (MCs). We discuss novel research suggesting that by acting rapidly on the cerebral vessels, this cell type has a potentially deleterious role in the very early phase of acute cerebral ischemia and hemorrhage. Mast cells should be recognized as a potent inflammatory cell that, already at the outset of ischemia, is resident within the cerebral microvasculature. By releasing their cytoplasmic granules, which contain a host of vasoactive mediators such as tumor necrosis factor-alpha, histamine, heparin, and proteases, MCs act on the basal membrane, thus promoting blood-brain barrier (BBB) damage, brain edema, prolonged extravasation, and hemorrhage. This makes them a candidate for a new pharmacological target in attempts to even out the inflammatory responses of the neurovascular unit, and to stabilize the BBB after acute stroke.

Modulation of cutaneous inflammation induced by ticks in contrasting phenotypes of infestation in bovines

Tick saliva contains molecules that are inoculated at the site of attachment on their hosts in order to modulate local immune responses and facilitate a successful blood meal. Bovines express heritable, contrasting phenotypes of infestations with the cattle tick, Rhipicephalus (Boophilus) microplus: breeds of Bos taurus indicus are significantly more resistant than those of Bos taurus taurus. Tick saliva may contain molecules that interfere with adhesion of leukocytes to endothelium and resistant hosts may mount an inflammatory profile that is more efficient to hamper the tick's blood meal. We show in vitro that adhesion of peripheral blood mononuclear cells to monolayers of cytokine-activated bovine umbilical endothelial cells was significantly inhibited by tick saliva. The inflammatory response to bites of adults of R. microplus mounted by genetically resistant and susceptible bovine hosts managed in the same pasture was investigated in vivo. The inflammatory infiltrates and levels of message coding for adhesion molecules were measured in biopsies of tick-bitten and control skin taken when animals of both breeds were exposed to low and high tick infestations. Histological studies reveal that cutaneous reactions of resistant hosts to bites of adult ticks contained significantly more basophils and eosinophils compared with reactions of the susceptible breed. Expression of the adhesion molecules - intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1) and P-selectin - was higher in adult-infested skin of susceptible hosts undergoing low infestations compared to resistant hosts; when host was exposed to high infestations expression of these adhesion molecules was down-regulated in both phenotypes of infestations. Expression of leukocyte adhesion glycoprotein-1 (LFA-1) was higher in skin from susceptible hosts undergoing low or high infestations compared to resistant hosts. Conversely, higher levels of E-selectin, which promotes adhesion of memory T cells, were expressed in skin of resistant animals. This finding may explain the resistant host's ability to mount more rapid and efficient secondary responses that limit hematophagy and infestations. The expression profiles observed for adhesion molecules indicate that there are differences in the kinetics of the inflammatory reactions mounted by resistant and susceptible hosts and the balance between tick and host is affected by the number of tick bites a host receives. We show that the contrasting phenotypes of infestations seen in bovines infested with R. microplus are correlated with differences in the cellular and molecular composition of inflammatory infiltrates elicited by bites with adult ticks.

Active MAC-1 (CD11b/CD18) on DCs inhibits full T-cell activation

The β2 integrins are important for transendothelial migration of leukocytes as well as for T-cell activation during antigen presentation. Despite abundant expression of β2 integrins on antigen-presenting cells (APCs), their functional relevance for antigen presentation is completely unclear. We show here that dendritic cells (DCs) from CD18-deficient mice, which lack all functional β2 integrins, have no defect in antigen presentation. Moreover, DCs from normal mice express inactive β2 integrins that do not become activated on contact with T cells, at least in vitro. Pharmacologic activation of β2 integrins on DCs results in a significant reduction of their T cell–activating capacity. This effect is mediated by Mac-1 (CD11b/CD18) on DCs because it could be reversed via blocking antibodies against CD18 and CD11b. Furthermore, the antigen-presenting capacity of macrophages, which express constitutively active β2 integrins, is significantly enhanced on Mac-1 blockade. We therefore conclude that active CD11b/CD18 (Mac-1) on APCs directly inhibits T-cell activation.

Altered localization of CXCL13 expressing cells in mice deficient in Pactolus following an inflammatory stimulus

The mouse Pactolus gene is an evolutionary paralogue to the CD18/beta2 integrin subunit and is preferentially expressed by neutrophils. When first identified, it was assumed Pactolus would function as an adhesion receptor similar to other beta integrin subunits. The analysis of mice genetically deficient in Pactolus, however, did not define any lesion in neutrophil migration, adhesion or phagocytosis. Therefore a wider analysis of the Pactolus deficiency was initiated using transcriptional profiling during an inflammatory insult. This screen identified a single transcript, CXCL13, that was elevated in cells from a peritoneal lavage of the wild type animal compared to the Pactolus-deficient animal. Our analyses confirmed resident macrophages as being responsible for the chemokine using intracellular CXCL13 staining and additional cell markers to phenotypically characterize such cells. The resident CXCL13-expressing cells (which do not express Pactolus) are functionally distinct from the macrophages recruited into the peritoneal cavity following the inflammatory stimulation since the recruited macrophages do not express detectable levels of the chemokine. The numbers and expression patterns of these resident CXCL13-expressing cells do not vary in naïve animals of wild type or Pactolus-deficient origin. Additionally, Pactolus-deficient neutrophils do not preferentially kill (compared to wild type) CXC13-expressing macrophages. These data suggest that during an inflammatory response, Pactolus may help retain CXCL13-expressing cells within the peritoneal environment.

Mechanism and effect of thrombospondin-4 polymorphisms on neutrophil function

High-throughput genomic technology identified an association between a single nucleotide polymorphism (SNP), a proline (P387) rather than the predominant alanine (A387) at position 387 in thrombospondin-4 (TSP-4) and premature myocardial infarction. The inflammatory hypothesis of atherosclerosis invokes a prominent role of leukocytes and cytokines in pathogenesis. As the expression of TSP-4 by vascular cells permits its exposure to circulating leukocytes, the interactions of human neutrophils (polymorphonuclear leukocytes [PMNs]) with both TSP-4 variants were investigated. Phorbol 12-myristate 13-acetate (PMA)-stimulated PMNs adhered and migrated well and equally on the TSP-4 variants. Integrin alpha(M)beta2 was identified as the TSP-4 receptor mediating these responses, and the 3 epidermal growth factor (EGF)-like domains of TSP-4 harboring the SNPs interacted with the alpha(M)I-domain. Despite the similarity in these responses, the P387 variant induced more robust tyrosine phosphorylation of the stress-related mitogen-activated protein kinases (MAPKs): p38MAPK and c-Jun NH2-terminal kinase (JNK), as well as signal transducer and activator of transcription-1 (STAT1) and heat shock protein 27 (HSP27) than the A387 variant. Additionally, cells adherent to P387 TSP-4 variant released 4-fold more H2O2 and secreted 2-fold more interleukin 8 (IL-8) as compared with the A387. H2O2 release and p38MAPK activation were totally inhibited by blockade of alpha(M)beta2. Thus, alpha(M)beta2 plays a central role in proinflammatory activities of TSP-4 (P387) and may contribute to the prothrombotic phenotype associated with this variant.

Identity of the amino acid residues involved in C3bi binding to the I-domain supports a mosaic model to explain the broad ligand repertoire of integrin alpha M beta 2

Interactions between the complement degradation product C3bi and leukocyte integrin alpha(M)beta(2) are critical for host defense against foreign pathogens and in tumor cell surveillance. To gain insight into the mechanism by which the alpha(M)I-domain of the integrin interacts with C3bi, detailed mapping of the C3bi binding site was undertaken. Previous mutagenesis studies had implicated five small structural segments within the alpha(M)I-domain in recognition of this ligand. Sets of three amino acids within the five implicated segments were mutated to the corresponding alpha(L)I-domain residues. Then, within the affected mutants, single point mutations were introduced to precisely define the requisite residues. Ultimately, H148, F150, Q204, L205, R208, T211, T213, I256, P257 were identified as being critical for C3bi binding. A synthetic peptide approach confirmed the involvement of the specified residues with the complex midsegment, Q204-I215, in C3bi recognition. Furthermore, the alpha(D)I-domain, which has a low intrinsic affinity for C3bi, acquired high affinity for the ligand when the implicated residues were inserted. The residues necessary to engage C3bi reside on or adjacent to the cation binding MIDAS site of the alpha(M)I-domain. The amino acids involved in C3bi binding are distinct from those involved in interaction of previously mapped ligands with the alpha(M)I-domain. This divergence supports a mosaic model, in which different ligands engage different amino acids to bind to alpha(M)I-domain, accounting for the broad recognition capacity of integrin alpha(M)beta(2).

Distinct roles for the alpha and beta subunits in the functions of integrin alphaMbeta2

Integrin alphaMbeta2 (Mac-1, CD11b/CD18) is a noncovalently linked heterodimer of alphaM and beta2 subunits on the surface of leukocytes, where it plays a pivotal role in the adhesion and migration of these cells. Using HEK293 cells expressing alphaMbeta2 or the individual constituent chains on their surface, we analyzed the contributions of the alphaM or beta2 subunits to functional responses mediated by the integrin. In cells expressing only alphaM or beta2, the individual subunits were not associated with the endogenous integrins of the cells, and other partners for the subunits were not detected by surface labeling and immunoprecipitation under a variety of conditions. The alphaM cells mediated adhesion and spreading on a series of alphaMbeta2 ligands (fibrinogen, Factor X, iC3b, ICAM-1 (intercellular adhesion molecule-1), and denatured ovalbumin) but could not support cell migration to any of these. The spreading of the alphaM cells suggested an unanticipated linkage of this subunit to the cytoskeleton. The beta2 cells supported migration and attachment but not spreading on a subset of the alphaMbeta2 ligands. The heterodimeric receptor and its individual subunits were purified from the cells by affinity chromatography and recapitulated the ligand binding properties of the corresponding cell lines. These data indicate that each subunit of alphaMbeta2 contributes distinct properties to alphaMbeta2 and that, in most but not all cases, the response of the integrin is a composite of the functions of its individual subunits.

Binding Affinity of Metal Ions to the CD11b A-domain Is Regulated by Integrin Activation and Ligands

The divalent cations Mg(2+) and Ca(2+) regulate the interaction of integrins with their cognate ligands, with Mg(2+) uniformly facilitating and Ca(2+) generally inhibiting such interactions in vitro. Because both cations are present in mm concentrations in vivo, the physiologic relevance of the in vitro observations is unclear. We measured the affinity of both cations to the inactive and active states of the ligand- and cation-binding A-domain (CD11bA) from integrin CD11b/CD18 in the absence and presence of the single-chain 107 antibody (scFv107), an activation-insensitive ligand-mimetic antibody. Using titration calorimetry, we found that Mg(2+) and Ca(2+) display equivalent (mm) affinities to inactive CD11bA. Activation induced a approximately 10-fold increase in the binding affinity of Mg(2+) to CD11bA with no change in that of Ca(2+) (106 microm +/- 16 and 2.1 mm +/- 0.19, respectively, n = 4). This increase is largely driven by favorable enthalpy. scFv107 induced a 50-80-fold increase in the binding affinity of Ca(2+) (but not Mg(2+) or Mn(2+)) to either form of CD11bA. Thus the affinity of metal ions to integrins is itself regulated by the activation state of these receptors and by certain ligands. These findings, which we expect will be applicable in vivo, elucidate a new level of regulation of the integrin-metal-ligand ternary complex and help explain some of the discrepant effects of Ca(2+) on integrin-ligand interactions.

Binding Stoichiometry and Kinetics of the Interaction of a Human Anthrax Toxin Receptor, CMG2, with Protective Antigen

The protective antigen (PA) moiety of anthrax toxin binds to cellular receptors and mediates entry of the two enzymatic moieties of the toxin into the cytosol. Two PA receptors, anthrax toxin receptor (ATR)/tumor endothelial marker 8 (TEM8) and capillary morphogenesis protein 2 (CMG2), have been identified. We expressed and purified the von Willebrand A (VWA) domain of CMG2 and examined its interactions with monomeric and heptameric forms of PA. Monomeric PA bound a stoichiometric equivalent of CMG2, whereas the heptameric prepore form bound 7 eq. The Kd of the VWA domain-PA interaction is 170 pm when liganded by Mg2+, reflecting a 1000-fold tighter interaction than most VWA domains with their endogenous ligands. The dissociation rate constant is extremely slow, indicating a 30-h lifetime for the CMG2.PA monomer complex. CMG2 metal ion-dependent adhesion site (MIDAS) was studied kinetically and thermodynamically. The association rate constant (approximately 10(5) m(-1) s(-1)) is virtually identical in the presence or absence of Mg2+ or Ca2+ , but the dissociation rate of metal ion liganded complex is up to 4 orders of magnitude slower than metal ion free complex. Residual affinity (Kd approximately 960 nm) in the absence of divalent metal ions allowed the free energy for the contribution of the metal ion to be calculated as 5 kcal mol(-1), demonstrating that the metal ion-dependent adhesion site is directly coordinated by CMG2 and PA in the binding interface. The high affinity of the VWA domain for PA supports its potency in neutralizing anthrax toxin, demonstrating its potential utility as a novel therapeutic for anthrax.

Crystal structure of the von Willebrand factor A domain of human capillary morphogenesis protein 2: an anthrax toxin receptor

Anthrax toxin is released from Bacillus anthracis as three monomeric proteins, which assemble into toxic complexes at the surface of receptor-bearing host cells. One of the proteins, protective antigen (PA), binds to receptors and orchestrates the delivery of the other two (the lethal and edema factors) into the cytosol. PA has been shown to bind to two cellular receptors: anthrax toxin receptor/tumor endothelial marker 8 and capillary morphogenesis protein 2 (CMG2). Both are type 1 membrane proteins that include an approximately 200-aa extracellular von Willebrand factor A (VWA) domain with a metal ion-dependent adhesion site (MIDAS) motif. The anthrax toxin receptor/tumor endothelial marker 8 and CMG2 VWA domains share approximately 60% amino acid identity and bind PA directly in a metal-dependent manner. Here, we report the crystal structure of the CMG2 VWA domain, with and without its intramolecular disulfide bond, to 1.5 and 1.8 A, respectively. Both structures contain a carboxylate ligand-mimetic bound at the MIDAS and appear as open conformations when compared with the VWA domains from alpha-integrins. The CMG2 structures provide a template to begin probing the high-affinity CMG2-PA interaction (200 pM) and may facilitate understanding of toxin assembly/internalization and the development of new anthrax treatments. The structural data also allow molecular interpretation of known CMG2 VWA domain mutations linked to the genetic disorders, juvenile hyaline fibromatosis, and infantile systemic hyalinosis.

Integrin αMβ2 Orchestrates and Accelerates Plasminogen Activation and Fibrinolysis by Neutrophils

Plasmin, the pivotal thrombolytic enzyme, is generated on the surface of many cell types, where urokinase receptor (uPAR)-bound urokinase (uPA) activates cell-bound plasminogen (Plg). It has been reported that neutrophils mediate endogenous thrombolysis involving a uPA-dependent mechanism, and we previously demonstrated that both uPAR and integrin alpha(M)beta(2) recognize uPA to control cell migration and adhesion. In the present study, we report that the alpha(M)beta(2) regulates neutrophil-dependent fibrinolysis. Phorbol 12-myristate 13-acetate (PMA)-stimulated but not resting neutrophils dissolved fibrin clots, and this activity was not only uPA- and Plg-dependent but also alpha(M)beta(2)-dependent. Purified alpha(M)beta(2) directly bound uPA (K(d) = 40 nm) and Plg (K(d) = 1 microm) in a dose-dependent and saturable manner. In Plg activation assays, addition of purified alpha(M)beta(2), but not a control protein, to a single chain uPA (sc-uPA)/Plg mixture, decreased the K(m) from 2 to 0.1 microm, thereby augmenting the overall reaction efficiency by 50-fold. The binding of sc-uPA to alpha(M)beta(2) was critical for the alpha(M)beta(2)-mediated enhancement of plasmin (Plm) generation, because this effect was lost when WT-sc-uPA was replaced with a kringle-less mutant (DeltaK-sc-uPA), which does not bind to alpha(M)beta(2). Plm inactivation by alpha(2)-antiplasmin was significantly delayed when Plm was preincubated with purified, soluble alpha(M)beta(2). When Plg was added to PMA-stimulated neutrophils, both uPA and Plg were co-immunoprecipitated with alpha(M)beta(2.) Thus, assembly of Plg and uPA on integrin alpha(M)beta(2) regulates Plm activity and, thereby, plays a crucial role in neutrophil-mediated thrombolysis.

Surface translocation of pactolus is induced by cell activation and death, but is not required for neutrophil migration and function

Pactolus is a cell surface protein expressed by murine neutrophils. Pactolus is similar to the beta integrins, except it lacks a functional metal ion-dependent adhesion site domain and is expressed without an alpha-chain partner. The majority of the Pactolus protein is held within the cell in dense granules in a highly glycosylated form. This intracellular form of Pactolus can be released to the cell surface following inflammatory activation or ligation of Pactolus on the cell surface. In addition, intracellular Pactolus translocates to the neutrophil surface following induction of apoptosis. Neutrophil activation studies suggest that Pactolus does not serve as an activating or phagocytic receptor for the neutrophil. To further define the function of Pactolus, a Pactolus-null mouse was generated. Pactolus-deficient animals mature appropriately and possess normal numbers of neutrophils, display appropriate migration into sites of inflammation, and combat introduced infections efficiently. These data suggest that Pactolus does not function as a neutrophil phagocytic or adhesion receptor, but may instead serve as a sugar-bearing ligand for lectin recognition by other cells.

Targeting platelet-leukocyte interactions: identification of the integrin Mac-1 binding site for the platelet counter receptor glycoprotein Ibalpha

The firm adhesion and transplatelet migration of leukocytes on vascular thrombus are dependent on the interaction of the leukocyte integrin Mac-1 (alphaMbeta2, CD11b/CD18) and the platelet counter receptor glycoprotein (GP) Ibalpha. Previous studies have established a central role for the I domain, a stretch of approximately 200 amino acids within the alphaM subunit, in the binding of GP Ibalpha. This study was undertaken to establish the molecular basis of GP Ibalpha recognition by alphaMbeta2. The P201-K217 sequence, which spans an exposed loop and amphipathic alpha4 helix in the three-dimensional structure of the alphaMI domain, was identified as the binding site for GP Ibalpha. Mutant cell lines in which the alphaMI domain segments P201-G207 and R208-K217 were switched to the homologous, but non-GP Ibalpha binding, alphaL domain segments failed to support adhesion to GP Ibalpha. Mutation of amino acid residues within P201-K217, H210-A212, T213-I215, and R216-K217 resulted in the loss of the binding function of the recombinant alphaMI domains to GP Ibalpha. Synthetic peptides duplicating the P201-K217, but not scrambled versions, directly bound GP Ibalpha and inhibited alphaMbeta2-dependent adhesion to GP Ibalpha and adherent platelets. Finally, grafting critical amino acids within the P201-K217 sequence onto alphaL, converted alphaLbeta2 into a GP Ibalpha binding integrin. Thus, the P201-K217 sequence within the alphaMI domain is necessary and sufficient for GP Ibalpha binding. These observations provide a molecular target for disrupting leukocyte-platelet complexes that promote vascular inflammation in thrombosis, atherosclerosis, and angioplasty-related restenosis.

Leukocyte–epithelial interactions

As a 'double-edged sword', neutrophil (polymorphonuclear leukocyte) migration across epithelial-lined organs is an important component of host defense, but it also results in epithelial pathophysiology and disease symptoms. There have been significant advances in better understanding the mechanisms of how leukocytes cross the vascular endothelium to exit the bloodstream; however, many of the mechanisms that govern polymorphonuclear leukocyte transepithelial migration are different and we are only just beginning to understand them. Recent findings include new junctional adhesion molecules and carbohydrate moieties as receptors for migrating neutrophils. In addition, new insights into leukocyte-epithelial signaling events have emerged that are beginning to shed light on the role of SIRP-CD47 interactions in regulating the rate of neutrophil transepithelial migration and how neutrophils modulate epithelial barrier function.

Isolation and molecular cloning of a secreted hookworm platelet inhibitor from adult Ancylostoma caninum

Hookworms, bloodfeeding intestinal nematodes, are a leading cause of iron deficiency anemia in the developing world. These parasites have evolved potent mechanisms of interfering with mammalian hemostasis, presumably for the purpose of facilitating bloodfeeding. Adult Ancylostoma caninum worm extracts contain an activity that inhibits platelet aggregation and adhesion by blocking the function of two cell surface integrin receptors, Glycoprotein IIb/IIIa and GPIa/IIa. Using rpHPLC, the hookworm platelet inhibitor activities have been purified from protein extracts of A. caninum. Because the two inhibitory activities co-purified through multiple chromatographic steps, have similar molecular masses and share identical N-terminal as well as internal amino acid sequence homology, it is likely that they represent a single gene product. A cDNA corresponding to the purified hookworm platelet inhibitor (HPI) protein has been cloned from adult A. caninum RNA, and the translated amino acid sequence shows significant homology to Neutrophil Inhibitory Factor and Ancylostoma Secreted Proteins, suggesting that these related hookworm proteins represent a novel class of integrin receptor antagonists. Polyclonal antibodies raised against the recombinant HPI protein recognize corresponding native proteins in A. caninum extracts and excretory/secretory products, and immunohistochemistry data have identified the cephalic glands as the major source of the inhibitor within the adult hookworm. These data suggest that HPI is secreted by the adult stage of the parasite at the site of intestinal attachment. As such, it may represent a viable target for a vaccine-based strategy aimed at interfering with hookworm-induced gastrointestinal hemorrhage and iron deficiency anemia.

Divalent cations modulate human colon cancer cell adhesion

BACKGROUND

Iatrogenic tumor implantation within surgical sites can compromise curative cancer surgery. Cancer cell adhesion to extracellular matrix proteins is mediated by diverse matrix receptors, most notably integrins. Divalent cations may modulate integrin-ligand interactions in some cells.

MATERIALS AND METHODS

We studied adhesion of SW620 and Caco-2 human colon cancer cells to collagen I, the dominant collagen of the interstitial matrix, and confirmed our results in primary human colon cancer cells from surgical specimens. Single cell suspensions in either HEPES/NaCl buffer or media supplemented with 0-1 mM Mn2+ or Mg2+, and 0-10 mM Zn2+ or Ca2+ were plated onto collagen-I-precoated dishes for 30 min.

RESULTS

Supplementation of the HEPES/NaCl/BSA buffer with 1 mM Mn2+, Mg2+, Zn2+, or Ca2+ affected adhesion differently. Mn2+ (1 mM) markedly promoted SW620 adhesion vs control (21.17 +/- 0.08-fold). Mg2+ (1 mM) had a similar but lesser effect (14.71 +/- 0.02-fold). However, 1-10 mM Ca2+ inhibited basal cell adhesion by 22.0 +/- 3.1 to 88.0 +/- 7.3 % inhibition. Ca2+ (2.5-10 mM) also inhibited Mn2+-induced adhesion. Zn2+ stimulated basal adhesion slightly at lower concentrations but inhibited Mn2+-stimulated adhesion similarly to Ca2+ at higher concentrations. Results were duplicated in conventional serum containing culture medium supplemented with these cations. Caco-2 cells and primary cancer cells yielded similar results. All results are significant to P < 0.01.

DISCUSSION

Integrin-mediated colon cancer cell adhesion is affected by extracellular divalent cation concentrations. Washing the surgical site with dilute calcium or zinc solutions might diminish perioperative tumor implantation.

Convergence of the adhesive and fibrinolytic systems: recognition of urokinase by integrin alpha Mbeta 2 as well as by the urokinase receptor regulates cell adhesion and migration

Previous studies demonstrated that integrin alpha(M)beta(2) (CD11b/18, Mac-1) forms a physical complex with the urokinase-type plasminogen activator receptor (uPAR/CD87) on leukocytes. In this study, we used human peripheral blood neutrophils and transfected cells expressing alpha(M)beta(2), uPAR, or both receptors to show that the integrin can directly interact with urokinase (uPA). We demonstrate that alpha(M)beta(2) supported adhesion and migration of these cells to uPA, and, in each case, blockade of alpha(M)beta(2) suppressed the response. Within uPA, both the kringle and proteolytic domains are recognized by alpha(M)beta(2), which are distinct from the growth factor domain that binds to uPAR. Within the alpha(M) subunit of the integrin, the I domain interacts with uPA, which is distinct from the region that interacts with uPAR. On cells expressing uPAR and alpha(M)beta(2), both receptors mediated adhesion and migration. This cooperation was particularly apparent in the responses of neutrophils to uPA, where blockade of alpha(M)beta(2) reduced uPAR-mediated responses and engagement of uPAR enhanced recognition of uPA by alpha(M)beta(2). Thus, recognition of uPA by alpha(M)beta(2) allows for formation of a multicontact trimolecular complex, in which a single uPA ligand may bind simultaneously to both uPAR and alpha(M)beta(2). This complex may play an important role in the control of inflammatory cell migration and vascular homeostasis.

CD11b/CD18-dependent interactions of neutrophils with intestinal epithelium are mediated by fucosylated proteoglycans

CD11b/CD18-mediated adhesive interactions play a key role in regulating polymorphonuclear leukocytes (PMN)) migration across intestinal epithelium. However, the identity of epithelial ligands for migrating PMN remains obscure. In this study we investigated the role of carbohydrates in mediating adhesive interactions between T84 intestinal epithelial cells and CD11b/CD18 purified from PMN. Fucoidin, heparin/heparin sulfate, N-acetyl-D-glucosamine, mannose-6-phosphate, and laminarin were found to inhibit adhesion of T84 cells to CD11b/CD18. The most potent inhibitory effects were observed with fucoidin (50% inhibition at 1-5 x 10(-8) M). Binding assays demonstrated that fucoidin directly bound to CD11b/CD18 in a divalent cation- and sulfation-dependent fashion that was blocked by anti-CD11b mAbs. Experiments employing CD11b/CD18 as a probe to blot T84 cell fucosylated proteins purified via fucose-specific lectin column revealed several candidate CD11b/CD18 binding proteins with molecular masses of 95, 50, 30, 25, and 20 kDa. Fucosidase treatment of T84 cells resulted in significantly reduced cell adhesion to CD11b/CD18, while no inhibition was observed after neuraminidase treatment. Finally, significant inhibition of T84 cell adhesion to CD11b/CD18 was observed after blocking cell proteoglycan synthesis with p-nitrophenyl-beta-D-xylopyranoside. These findings implicate epithelial cell surface proteoglycans decorated with sulfated fucose moieties as ligands for CD11b/CD18 during PMN migration across mucosal surfaces.

Coming to grips with integrin binding to ligands

Integrins are alphabeta heterodimeric cell-surface receptors that are vital to the survival and function of nucleated cells. They recognize aspartic-acid- or a glutamic-acid-based sequence motifs in structurally diverse ligands. Integrin recognition of most ligands is divalent cation dependent and conformationally sensitive. In addition to this common property, there is an underlying binding specificity between integrins and ligands for which there has been no structural basis. The recently reported crystal structures of the extracellular segment of an integrin in its unliganded state and in complex with a prototypical Arg-Gly-Asp (RGD) ligand have provided an atomic basis for cation-mediated binding of aspartic-acid-based ligands to integrins. They also serve as a basis for modelling other integrins in complex with larger physiologic ligands. These models provide new insights into the molecular basis for ligand binding specificity in integrins and its regulation by activation-driven tertiary and quaternary changes.

Mycobacterium tuberculosis-infected human macrophages exhibit enhanced cellular adhesion with increased expression of LFA-1 and ICAM-1 and reduced expression and/or function of complement receptors, FcgammaRII and the mannose receptor

The entry of Mycobacterium tuberculosis (Mtb) into the host macrophage and its survival in this environment are key components of tuberculosis pathogenesis. Following intracellular replication of the bacterium within alveolar macrophages, there is spread of bacilli to regional lymph nodes in the lungs and subsequent presentation of antigens to the host immune system. How this process occurs remains poorly understood, but one mechanism may involve the migration of macrophages containing Mtb across the alveoli to lymph nodes, where there is development of a protective host response with formation of granulomas composed in part of aggregated and fused, apoptotic, infected macrophages. Leukocyte integrins, including lymphocyte function-associated antigen-1 (LFA-1) and complement receptors CR3 and CR4, and their counter receptors play a major role in macrophage adhesion processes and phagocytosis. In this study, the appearance of Mtb-infected macrophages over time was examined, using inverted-phase microscopy and an in vitro culture model of human monocyte-derived macrophages (MDMs). Prior to and immediately following infection of the MDMs with Mtb, the macrophages appeared as individual cells in monolayer culture; however, within 24 h of infection with Mtb, the MDMs began to migrate and adhere to each other. The kinetics of this response were dependent on both the m.o.i. and the length of infection. Quantitative transmission electron microscopy studies revealed that macrophage adhesion was accompanied by increases in levels of LFA-1 and its counter receptor (ICAM-1), decreases in surface levels of the phagocytic receptors CR3, CR4 and FcgammaRII, and an increase in major histocompatibility complex Class II (MHC-II) molecules at 72 h post-infection. Decreases in surface levels of CR3 and CR4 had a functional correlate, with macrophages containing live bacilli showing a diminished phagocytic capacity for complement-opsonized sheep erythrocytes; macrophages containing heat-killed bacilli did not show this diminished capacity. The modulation of macrophage adhesion and phagocytic proteins may influence the trafficking of Mtb-infected macrophages within the host, with increases in levels of LFA-1 and ICAM-1 enhancing the adhesive properties of the macrophage and decreases in phagocytic receptors diminishing the phagocytic capacity of an already-infected cell, potentially allowing for maintenance of the intracellular niche of Mtb.

Delineation of the key amino acids involved in neutrophil inhibitory factor binding to the I-domain supports a mosaic model for the capacity of integrin alphaMbeta 2 to recognize multiple ligands

To gain insight into the mechanism by which the alpha(M)I-domain of integrin alpha(M)beta(2) interacts with multiple and unrelated ligands, the identity of the neutrophil inhibitory factor (NIF) recognition site was sought. A systematic strategy in which individual amino acid residues within three previously implicated segments were changed to those in the alpha(L)I-domain, which is structurally very similar but does not bind NIF, was implemented. The capacity of the resulting mutants, expressed as glutathione S-transferase fusion proteins, to recognize NIF was assessed. These analyses ultimately identified Asp(149), Arg(151), Gly(207), Tyr(252), and Glu(258) as critical for NIF binding. Cation binding, a function of the metal ion-dependent adhesion site (MIDAS) motif, was assessed by terbium luminescence to evaluate conformational perturbations induced by the mutations. All five mutants bound terbium with unaltered affinities. When the five residues were inserted into the alpha(L)I-domain, the chimera bound NIF with high affinity. Another ligand of alpha(M)beta(2), C3bi, which is known to use the same segments of the alpha(M)I-domain in engaging the receptor, failed to bind to the chimeric alpha(L)I-domain. Thus, the alpha(M)I-domain appears to present a mosaic of exposed amino acids within surface loops on its MIDAS face, and different ligands interact with different residues to attain high affinity binding.

Characterization of a conformationally sensitive murine monoclonal antibody directed to the metal ion-dependent adhesion site face of integrin CD11b

Integrin binding to physiologic ligands requires divalent cations and an inside-out-driven switch of the integrin to a high-affinity state. Divalent cations at the metal ion-dependent adhesion site (MIDAS) face of the alpha subunit-derived A domain provide a direct bridge between ligands and the integrin, and it has been proposed that activation dependency is caused by reorientation of the surrounding residues relative to the metal ion, forming an optimal binding interface. To gain more insight into the functional significance of the protein movements on the MIDAS face, we raised and characterized a murine mAb 107 directed against the MIDAS face of the A domain from integrin CD11b. We find that mAb 107 behaves as a ligand mimic. It binds in a divalent-cation-dependent manner to solvent-exposed residues on the MIDAS face of CD11b, blocks interaction of 11bA or the holoreceptor with ligands, and inhibits spreading and phagocytosis by human neutrophils. However, in contrast to physiologic ligands, mAb 107 preferentially binds to the inactive low-affinity form of the integrin, suggesting that its antagonistic effects are exerted in part by stabilizing the receptor in the low-affinity state. These data support a functional relevance of the protein movements on the MIDAS face and suggest that stabilizing the A domain in the low-affinity state may have therapeutic benefit.

The alternatively spliced alpha(E)C domain of human fibrinogen-420 is a novel ligand for leukocyte integrins alpha(M)beta(2) and alpha(X)beta(2)

The interaction of human plasma fibrinogen with leukocyte integrins alpha(M)beta(2) (CD11b/CD18, Mac-1) and alpha(X)beta(2) (CD11c/CD18, p150,95) is an important component of the inflammatory response. Previously, it was demonstrated that binding of fibrinogen to these integrins is mediated by gammaC, the globular C-terminal domain of the gamma chain. In this study, evidence was found of another fibrinogen domain that can serve as a ligand for the 2 leukocyte integrins: alpha(E)C, a homologous domain that extends the alpha chains in a recently discovered subclass of fibrinogen known as fibrinogen-420. Recombinant alpha(E)C supported strong adhesion and migration of cells expressing alpha(M)beta(2) and alpha(X)beta(2), including nonactivated and activated U937 and THP-1 monocytoid cells, and neutrophils. Cells transfected with complementary DNA for these integrins also bound alpha(E)C. The specificity of interaction was substantiated by inhibition of cell adhesion with antibodies against alpha(M), alpha(X), and beta(2) subunits. Also, neutrophil inhibitory factor, a specific inhibitor of alpha(M)beta(2) and alpha(X)beta(2) function, efficiently blocked cell adhesion to alpha(E)C. In alpha(M)beta(2) and alpha(X)beta(2), the I domain is the binding site for alpha(E)C, since alpha(E)C bound to recombinant alpha(M) I and alpha(X)I domains in a dose-dependent and saturable manner. Synthetic peptides that duplicated sequences gamma190 to 202 and gamma377 to 395, previously considered putative binding sites in gammaC, effectively inhibited alpha(M)beta(2)- and alpha(X)beta(2)-mediated adhesion to alpha(E)C, suggesting that recognition of alpha(E)C by the I domain involves structural features in common with those of gammaC. These findings identify alpha(E)C as a second domain in fibrinogen-420 that binds alpha(M)beta(2) and alpha(X)beta(2) and can mediate leukocyte adhesion and migration.

Functional characterization of Pactolus, a beta-integrin-like protein preferentially expressed by neutrophils

Murine Pactolus is a beta-integrin-like molecule expressed exclusively on the surface of granulocytes. Cell surface expression of Pactolus is dramatically increased following activation of bone marrow neutrophils with known agonists, and cross-linking of cell surface Pactolus, suggesting the bulk of the protein is in intracellular stores. The mature protein is found in two forms depending upon the extent of N-linked glycosylation. There is no evidence to suggest that Pactolus requires an associated alpha chain for expression. In some mouse strains, a truncated form of the protein is predicted based upon alternative splicing: this form, however, is unstable and rapidly degraded after synthesis. Differences in the quantities of these Pactolus mRNA isoforms have defined two alleles. BALB/c and C3H/HeJ mice possess allele B and preferentially express the truncated, unstable product, whereas C57BL/6 mice possess allele A and only produce the membrane-bound form. Sequence analysis has shown the difference between these two alleles is due to a single base pair difference at the splice acceptor site for the truncated product. The increased expression of the membrane form of Pactolus by granulocytes of C57BL/6 mice suggests a compensatory adhesion function that is reduced in cells from the low producing strains.

Recognition of fibrinogen by leukocyte integrins

Numerous studies have provided evidence that fibrinogen plays a multifaceted role in the immune and inflammatory response. The ability of fibrinogen to participate in the inflammatory response depends on its specific interaction with leukocyte cell surface adhesion receptors, integrins. Two leukocyte integrins, alpha M beta 2 (CD11b/CD18, Mac-1) and alpha X beta 2 (CD11c/CD18, p150,95), are the main fibrinogen receptors expressed on neutrophils, monocytes, macrophages and several subsets of lymphocytes. The recognition site for alpha M beta 2 has been previously mapped to the carboxyl-terminal globular gamma C domains (gamma 143-411) and two sequences, gamma 190-202 (P1) and gamma 377-395 (P2), were implicated as the putative binding sites. We now demonstrate that a second leukocyte integrin, alpha X beta 2, which is highly homologous to alpha M beta 2, mediates adhesion of the alpha X beta 2-bearing cells to the D fragment and to the recombinant gamma-module, gamma 143-411. Within the gamma C domain, alpha X beta 2 may recognize P1 and P2 sequences since synthetic peptides duplicating these sequences effectively inhibits adhesion of the alpha X beta 2-expressing cells to the D fragment. In addition, neutrophil inhibitory factor, NIF, a potent inhibitor of alpha X beta 2, also inhibited alpha X beta 2-mediated cell adhesion. These data suggest that recognition of the gamma C domain of fibrinogen by alpha M beta 2 and alpha X beta 2 may have common structural requirements.

Vaccination with neutrophil inhibitory factor reduces the fecundity of the hookworm Ancylostoma ceylanicum

Neutrophil inhibitory factor (NIF), a protein isolated from hookworms of the genus Ancylostoma, inhibits CD11b/18-dependent leucocyte function, binding to the I domain of CD11b. Historically, NIF was serendipitously isolated from whole worm extracts during a search for novel antihaemostatic agents, and little is known of its source or biological significance to the parasite. NIF has also been identified as a possible hookworm vaccine candidate. Ancylostoma ceylanicum recombinant NIF, expressed in its active form in Pichia pastoris, was purified and its functional activity confirmed using neutrophil adhesion assays and confirmatory immunoassay. Recombinant NIF was subsequently used in vaccination trials in the A. ceylanicum-hamster model system for human hookworm infection. Vaccinated and challenged animals were not protected in terms of worm burden or haematocrit values, despite the presence of high levels of specific antibody against NIF. However, adult worms resident in vaccinated animals showed a significant reduction in fecundity (85.8% by day 21 postinfection), indicating a degree of protection against subsequent transmission by vaccination. These data indicate that targeted vaccination with recombinant subunit material, derived from a known and effective immune suppressant secreted by the parasite, may offer partial protection against the transmission of hookworm infection. Furthermore, we can also report that a biological activity characteristic of NIF is detectable in the secretions of A. ceylanicum using two complementary bioassays. Complete neutralization of this secreted activity by vaccination in combination with other vaccine candidates may result in improved protection against A. ceylanicum infection.

Neutrophil migration across intestinal epithelium

Transmigration of neutrophils across epithelial surfaces is the hallmark of inflammatory mucosal diseases of diverse organs. In disorders such as Crohn's disease, ulcerative colitis, pyelonephritis, and bronchitis, for example, neutrophil transmigration correlates with clinical disease activity, is associated morphologically with injury to the epithelium, and is central to disease pathophysiology. The mechanisms by which neutrophils transmigrate across epithelia are, therefore, of considerable significance for numerous pathologic states. In this paper, we discuss current evidence that defines these mechanisms in intestinal epithelium, emphasizing the structural constituents determining adhesive interactions and a subset of the complex regulatory signals between neutrophils and epithelium.