Integrin Biology Research Articles

Heterotropic roles of divalent cations in the establishment of allostery and affinity maturation of integrin αXβ2.

SUMMARYAllosteric activation and silencing of leukocyte β2-integrins transpire through cation-dependent structural changes, which mediate integrin biosynthesis and recycling, and are essential to designing leukocyte-specific drugs. Stepwise addition of Mg2+ reveals two mutually coupled events for the αXβ2 ligand-binding domain—the αX I-domain—corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the Mg2+-binding site establish long-range couplings, leading to both pH− and Mg2+-occupancy-dependent biphasic stability change in the αX I-domain fold. The ligand-binding sensorgrams show composite affinity events for the αX I-domain accounting for the multiplicity of the αX I-domain conformational states existing in the solution. On cell surfaces, increasing Mg2+ concentration enhanced adhesiveness of αXβ2. This work highlights how intrinsically flexible pH− and cation-sensitive architecture endows a unique dynamic continuum to the αI-domain structure on the intact integrin, thereby revealing the importance of allostery establishment and affinity maturation in both extracellular and intracellular integrin events.

RGD-Binding Integrins Revisited: How Recently Discovered Functions and Novel Synthetic Ligands (Re-)Shape an Ever-Evolving Field.

Simple SummaryIntegrins, a superfamily of cell adhesion receptors, were extensively investigated as therapeutic targets over the last decades, motivated by their multiple functions, e.g., in cancer (progression, metastasis, angiogenesis), sepsis, fibrosis, and viral infections. Although integrin-targeting clinical trials, especially in cancer, did not meet the high expectations yet, integrins remain highly interesting therapeutic targets. In this article, we analyze the state-of-the-art knowledge on the roles of a subfamily of integrins, which require binding of the tripeptide motif Arg-Gly-Asp (RGD) for cell adhesion and signal transduction, in cancer, in tumor-associated exosomes, in fibrosis and SARS-CoV-2 infection. Furthermore, we outline the latest achievements in the design and development of synthetic ligands, which are highly selective and affine to single integrin subtypes, i.e., αvβ3, αvβ5, α5β1, αvβ6, αvβ8, and αvβ1. Lastly, we present the substantial progress in the field of nuclear and optical molecular imaging of integrins, including first-in-human and clinical studies.Integrins have been extensively investigated as therapeutic targets over the last decades, which has been inspired by their multiple functions in cancer progression, metastasis, and angiogenesis as well as a continuously expanding number of other diseases, e.g., sepsis, fibrosis, and viral infections, possibly also Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2). Although integrin-targeted (cancer) therapy trials did not meet the high expectations yet, integrins are still valid and promising targets due to their elevated expression and surface accessibility on diseased cells. Thus, for the future successful clinical translation of integrin-targeted compounds, revisited and innovative treatment strategies have to be explored based on accumulated knowledge of integrin biology. For this, refined approaches are demanded aiming at alternative and improved preclinical models, optimized selectivity and pharmacological properties of integrin ligands, as well as more sophisticated treatment protocols considering dose fine-tuning of compounds. Moreover, integrin ligands exert high accuracy in disease monitoring as diagnostic molecular imaging tools, enabling patient selection for individualized integrin-targeted therapy. The present review comprehensively analyzes the state-of-the-art knowledge on the roles of RGD-binding integrin subtypes in cancer and non-cancerous diseases and outlines the latest achievements in the design and development of synthetic ligands and their application in biomedical, translational, and molecular imaging approaches. Indeed, substantial progress has already been made, including advanced ligand designs, numerous elaborated pre-clinical and first-in-human studies, while the discovery of novel applications for integrin ligands remains to be explored.

Crystal structures of Rea1-MIDAS bound to its ribosome assembly factor ligands resembling integrin\u2013ligand-type complexes

The Rea1 AAA+-ATPase dislodges assembly factors from pre-60S ribosomes upon ATP hydrolysis, thereby driving ribosome biogenesis. Here, we present crystal structures of Rea1-MIDAS, the conserved domain at the tip of the flexible Rea1 tail, alone and in complex with its substrate ligands, the UBL domains of Rsa4 or Ytm1. These complexes have structural similarity to integrin α-subunit domains when bound to extracellular matrix ligands, which for integrin biology is a key determinant for force-bearing cell–cell adhesion. However, the presence of additional motifs equips Rea1-MIDAS for its tasks in ribosome maturation. One loop insert cofunctions as an NLS and to activate the mechanochemical Rea1 cycle, whereas an additional β-hairpin provides an anchor to hold the ligand UBL domains in place. Our data show the versatility of the MIDAS fold for mechanical force transmission in processes as varied as integrin-mediated cell adhesion and mechanochemical removal of assembly factors from pre-ribosomes.

De novo designed transmembrane peptides activating the α5β1 integrin.

Computationally designed transmembrane α-helical peptides (CHAMP) have been used to compete for helix-helix interactions within the membrane, enabling the ability to probe the activation of the integrins αIIbβ3 and αvβ3. Here, this method is extended towards the design of CHAMP peptides that inhibit the association of the α5β1 transmembrane (TM) domains, targeting the Ala-X3-Gly motif within α5. Our previous design algorithm was performed alongside a new workflow implemented within the widely used Rosetta molecular modeling suite. Peptides from each computational approach activated integrin α5β1 but not αVβ3 in human endothelial cells. Two CHAMP peptides were shown to directly associate with an α5 TM domain peptide in detergent micelles to a similar degree as a β1 TM peptide does. By solution-state nuclear magnetic resonance, one of these CHAMP peptides was shown to bind primarily the integrin β1 TM domain, which itself has a Gly-X3-Gly motif. The second peptide associated modestly with both α5 and β1 constructs, with slight preference for α5. Although the design goal was not fully realized, this work characterizes novel CHAMP peptides activating α5β1 that can serve as useful reagents for probing integrin biology.

Integrins promote axonal regeneration after injury of the nervous system.

ABSTRACTIntegrins are cell surface receptors that form the link between extracellular matrix molecules of the cell environment and internal cell signalling and the cytoskeleton. They are involved in several processes, e.g. adhesion and migration during development and repair. This review focuses on the role of integrins in axonal regeneration. Integrins participate in spontaneous axonal regeneration in the peripheral nervous system through binding to various ligands that either inhibit or enhance their activation and signalling. Integrin biology is more complex in the central nervous system. Integrins receptors are transported into growing axons during development, but selective polarised transport of integrins limits the regenerative response in adult neurons. Manipulation of integrins and related molecules to control their activation state and localisation within axons is a promising route towards stimulating effective regeneration in the central nervous system.

Integrins in wound healing, fibrosis and tumor stroma: High potential targets for therapeutics and drug delivery.

Wound healing is a complex process, which ultimately leads to fibrosis if not repaired well. Pathologically very similar to fibrosis is the tumor stroma, found in several solid tumors which are regarded as wounds that do not heal. Integrins are heterodimeric surface receptors which control various physiological cellular functions. Additionally, integrins also sense ECM-induced extracellular changes during pathological events, leading to cellular responses, which influence ECM remodeling. The purpose and scope of this review is to introduce integrins as key targets for therapeutics and drug delivery within the scope of wound healing, fibrosis and the tumor stroma. This review provides a general introduction to the biology of integrins including their types, ligands, means of signaling and interaction with growth factor receptors. Furthermore, we highlight integrins as key targets for therapeutics and drug delivery, based on their biological role, expression pattern within human tissues and at cellular level. Next, therapeutic approaches targeting integrins, with a focus on clinical studies, and targeted drug delivery strategies based on ligands are described.

Coronin 1A, a novel player in integrin biology, controls neutrophil trafficking in innate immunity

AbstractTrafficking of polymorphonuclear neutrophils (PMNs) during inflammation critically depends on the β2 integrins lymphocyte function–associated antigen 1 (LFA-1) (CD11a/CD18) and macrophage-1 antigen (CD11b/CD18). Here, we identify coronin 1A (Coro1A) as a novel regulator of β2 integrins that interacts with the cytoplasmic tail of CD18 and is crucial for induction of PMN adhesion and postadhesion events, including adhesion strengthening, spreading, and migration under flow conditions. Transition of PMN rolling to firm adhesion critically depends on Coro1A by regulating the accumulation of high-affinity LFA-1 in focal zones of adherent cells. Defective integrin affinity regulation in the genetic absence of Coro1A impairs leukocyte adhesion and extravasation in inflamed cremaster muscle venules in comparison with control animals. In a Helicobacter pylori mouse infection model, PMN infiltration into the gastric mucosa is dramatically reduced in Coro1A−/− mice, resulting in an attenuated gastric inflammation. Thus, Coro1A represents an important novel player in integrin biology, with key functions in PMN trafficking during innate immunity.

Potential of Integrin Inhibitors for Treating Ovarian Cancer: A Literature Review.

Epithelial ovarian cancer is a fatal disease, with a cure rate of only 30%. Several recent studies have targeted integrins for cancer treatment. Preclinical studies have shown the effectiveness of several integrin inhibitors for blocking cancer progression, especially by blocking angiogenesis. Because the initial critical step in ovarian cancer metastasis is the attachment of cancer cells to the peritoneum or omentum and because clinical trials have provided positive results for anti-angiogenic therapy, therapies targeting integrins may be the most feasible approach for treating cancer. This review summarizes the current understanding of integrin biology in ovarian cancer metastasis and various therapeutic approaches involving integrin inhibitors. However, no integrin inhibitor has shown favorable results thus far. However, conjugates of cytotoxic agents with the triplet sequence arginine-glycine-aspartate (RGD) peptides targeting α5β1-, αvβ3-, and αvβ6-integrins may be promising integrin-targeting therapies for further clinical investigation.

Nature Biotechnology's academic spinouts of 2016.

Our annual survey highlights several academic startups developing immunotherapies as well as ventures focusing on microbiomes, proteostasis, integrin biology, nucleic acid delivery and subcellular imaging.

Convergence of eicosanoid and integrin biology: Role of Src in 12-LOX activation

12-Lipoxygenase (12-LOX) metabolizes arachidonic acid to 12(S)-hydroxyeicosatetraenoic acid, or 12(S)-HETE, a proinflammatory bioactive lipid implicated in tumor angiogenesis, growth, and metastasis. The mechanisms underlying 12-LOX-mediated signaling in cancer progression are still ill-defined. In the present study we demonstrate that 12-LOX phosphorylation and subsequent enzymatic activity occurs after integrin β4 stimulation and Src kinase recruitment to the integrin subunit. Inhibition of Src activity by PP2 or Src dominant-negative mutants reduced 12-LOX tyrosine phosphorylation and 12(S)-HETE production in response to integrin β4 stimulation in A431 cells. The pertinent Src-targeted residues for 12-LOX activity were mapped to Y19 and Y614, where 12-LOX mutants Y19F and Y614F showed 70% less enzymatic activity. Furthermore, we have shown that the 12-LOX activity modulated by these residues impacts migration. To our knowledge, this is the first report that c-Src kinase activity is required for β4-integrin-mediated phosphorylation of 12-LOX.

Integrins and Integrin-Associated Proteins in the Cardiac Myocyte

Integrins are heterodimeric, transmembrane receptors that are expressed in all cells, including those in the heart. They participate in multiple critical cellular processes including adhesion, extracellular matrix organization, signaling, survival, and proliferation. Particularly relevant for a contracting muscle cell, integrins are mechanotransducers, translating mechanical to biochemical information. Although it is likely that cardiovascular clinicians and scientists have the highest recognition of integrins in the cardiovascular system from drugs used to inhibit platelet aggregation, the focus of this article will be on the role of integrins specifically in the cardiac myocyte. After a general introduction to integrin biology, the article will discuss important work on integrin signaling, mechanotransduction, and lessons learned about integrin function from a range of model organisms. Then we will detail work on integrin-related proteins in the myocyte, how integrins may interact with ion channels and mediate viral uptake into cells, and also play a role in stem cell biology. Finally, we will discuss directions for future study.

Matched rabbit monoclonal antibodies against αv-series integrins reveal a novel αvβ3-LIBS epitope, and permit routine staining of archival paraffin samples of human tumors

SummaryThe relationship between integrin expression and function in pathologies is often contentious as comparisons between human pathological expression and expression in cell lines is difficult. In addition, the expression of even integrins αvβ6 and αvβ8 in tumor cell lines is not comprehensively documented. Here, we describe rabbit monoclonal antibodies (RabMabs) against the extracellular domains of αv integrins that react with both native integrins and formalin fixed, paraffin embedded (FFPE) human tissues. These RabMabs, against αvβ3 (EM22703), αvβ5 (EM09902), αvβ6 (EM05201), αvβ8 (EM13309), and pan-αv (EM01309), recognize individual integrin chains in Western blots and in flow cytometry. EM22703 detected a ligand-induced binding site (LIBS), reporting an epitope enhanced by the binding of an RGD-peptide to αvβ3. αvβ8 was rarely expressed in human tumor specimens, and weakly expressed in non-small-cell lung carcinoma (NSCLC). However, ovarian carcinoma cell lines expressed αvβ8, as did some melanoma cells, whereas U87MG glioma lacked αvβ8 expression. We observed an unexpected strong expression of αvβ6 in tumor samples of invasive ductal breast adenoma, colorectal carcinoma (CRC), and NSCLC. αvβ3 was strongly expressed in some invasive NSCLC cohorts. Interestingly, PC3 prostate cell and human prostate tumors did not express αvβ3. The RabMabs stained plasma membranes in FFPE-immunohistochemistry (IHC) samples of tumor cell lines from lung, ovary, colon, prostate, squamous cell carcinoma of head and neck (SCCHN), breast, and pancreas carcinomas. The RabMabs are unique tools for probing αv integrin biology, and suggest that especially αvβ6 and αvβ8 biologies still have much to reveal.

Mechanistic Insights into αIIbβ3 Integrin Signaling from Long-Timescale Molecular Dynamics Simulations

Integrin αIIbβ3 is a heterodimeric cell adhesion receptor composed of α and β subunits, each of which features an extracellular domain (ECD), a single transmembrane (TM) α-helix, and an intracellular cytoplasmic tail (CT). Like other integrins, αIIbβ3 can signal bi-directionally through interaction of its ECD with extracellular ligands (“outside-in” signaling) or its CT with intracellular proteins (“inside-out” signaling). Notwithstanding the recent developments in structural biology of integrins, questions remain about the molecular determinants that are responsible for protein activation and signaling. To obtain rigorous mechanistic insights into αIIbβ3 “inside-out” signaling at an unprecedented level of molecular detail, we carried out microsecond-scale all-atom molecular dynamics (MD) simulations of various experimentally-based three-dimensional models of the TM and CT regions of αIIbβ3 in an explicit 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocoline bilayer, and in the presence or absence of the F2-F3 subdomains of the intracellular activator talin-1.These simulations broaden our current understanding of the mechanism of αIIbβ3 activation by talin-1. Specifically, we observe: a) different interactions between the αIIb and β3 CTs with or without the F2-F3 subdomains of talin-1; b) a modulatory effect of F2-F3 on the αIIbβ3 TM and CT structures, c) specific electrostatic interactions between talin-1 F2-F3 and the phospholipid polar groups; and d) flexibility of the linker region between the F3 and F2 resulting in their re-orientation with respect to the corresponding crystal structure. Notably, these simulations reveal novel stable interactions between residues of the αIIb and β3 CTs (E1005-K725, E1006-K729, and E1008-R736, respectively), the αIIb CT and the talin-1 F3 (E1001-K316 and D1004-K364, respectively), as well as the β3 CT and the talin-1 F2-F3 (E726-K316, E726-Q381, T753-N355, F754-Y373, and R760-E293, respectively), the mutation of which may be worthy of experimental testing based on their expected interference with αIIbβ3 activation.

Integrin Inhibitors as a Therapeutic Agent for Ovarian Cancer

Ovarian cancer is a deadly disease, with a cure rate of only 30%. Despite aggressive treatments, relapse remains almost inevitable in patients with advanced-stage disease. In recent years, great progress has been made towards targeting integrins in cancer treatment, and clinical studies with various integrin inhibitors have demonstrated their effectiveness in blocking cancer progression. Given that the initial critical step of ovarian cancer metastasis is the attachment of cancer cells onto the peritoneum or omentum, in addition to the proven positive clinical results of anti-angiogenic therapy, targeting integrins is likely to be one of the most feasible approaches. This paper summarizes the current understanding of the integrin biology in ovarian cancer metastasis and the various therapeutic approaches attempted with integrin inhibitors. Although no integrin inhibitors have shown favorable results so far, integrin-targeted therapies continue to be a promising approach to be explored for further clinical investigation.

Opening the Field of Integrin Biology to “Biased Agonism”

See related article, pages 1269–1279 Integrins are α- and β-subunit heterodimeric receptors that mediate divalent cation-dependent cell-cell and cell-matrix adhesion through tightly regulated interactions with ligands central to inflammation, immunity, hemostasis, and tumor metastasis.1 Integrins have been viewed to function within a 2-state model. In response to cell activation signals, integrins switch from a low-affinity to a high-affinity ligand-binding state. These inactive and active integrin conformations have been corroborated structurally.2 In this issue of Circulation Research , Wolf and coworkers3 have mapped a novel interaction site within the ligand-binding I-domain of the αM-subunit. Taken with the synthesis herein, these data provide tantalizing evidence that the 2-state model of integrin activation and deactivation may be oversimplified. Recruitment of circulating leukocytes in inflammation requires multistep adhesive and signaling events mediated, in part, by members of the β2-integrin family, LFA-1 (αLβ2, CD11a/CD18), Mac-1 (αMβ2, CD11b/CD18), p150,95 (αMβ2, CD11c/CD18), and αDβ2 (CD11d/CD18).4 Mac-1 is the most abundant β2-integrin on neutrophils and monocytes and is a highly promiscuous receptor, capable of binding a broad repertoire of ligands (Figure, A) and facilitating key leukocyte functions including migration, coagulation, proteolysis, phagocytosis, oxidative burst, and signaling.5–9 Elegant structural studies by several groups have begun to elucidate the molecular basis for such broad ligand recognition by Mac-1. A subset of integrin α-subunits, including αM of Mac-1, contains an inserted domain (I domain) of ≈200 amino acids that is implicated in ligand binding. The αMI-domain contributes broadly to the recognition of ligands by αMβ …

Abstract 2890: A novel interaction between 12-lipoxygenase and integrin subunit Beta4 plays a role in tumor migration, invasion and metastasis

Abstract This study has the potential to establish a new paradigm in adhesion-mediated control of eicosanoid production and could be relevant to tumor migration, invasion, and metastasis. 12-Lipoxygenase metabolizes arachidonic acid to form 12(S)-HETE, a metabolite known to play a significant role in the process of tumor-induced angiogenesis and metastasis. The role of 12-LOX in proliferation, survival, and motility of tumor cells as well as in angiogenesis has been described for a variety of cancers. Similarly Alpha6Beta4 integrin also plays a role in both angiogenesis and tumorigenesis in addition to having a role in the formation of hemidesmosomes (HD). Evidence suggests a correlation between 12-LOX and integrin Beta4 since they play a similar role in cancer progression. The physical interaction between 12-LOX and Beta4 was first identified by a yeast two-hybrid screen using 12-LOX as bait against a library made from A431 cells that are known to have endogenous expression of both proteins. We verified this interaction by co-immunoprecipitation from A431 cells, and recapitulated it in CHO and PC-3 cells ectopically expressing 12-LOX and the cytoplasmic tail of Beta4 integrin. Additionally our data shows that ligation or clustering of Alpha6Beta4 integrin by the natural ligand laminin or an antibody mimic of laminin, 3E1, caused an increase in translocation of 12-LOX from the cytosol to membrane and an increase in 12-LOX enzymatic activities as assayed by immunofluorescence and 12(S)-HETE measurements respectively. 3E1-stimulated A431 cells showed increased cell migration but there was 40% decrease in presence of 12-LOX inhibitor BMD122 which confirms the role of 12-LOX in cell invasion. Here we describe for the first time the interaction domains that unite Beta4 integrin and 12-LOX. We mapped the domains in the cytoplasmic tail of Beta4 critical for interacting with 12-LOX and found that the 1126-1157 domain in the cytoplasmic tail of Beta4 is important for binding of 12-LOX. Also we have shown the mutant fragment (1126-1315) interacts with 12-lipoxygenase in the cytosol and prevent its binding to native, full-length membrane-associated Beta4 and thereby prevents 12-LOX activation. This physical interaction has functional implications with regard to decreased cell survival and migration. Determination of the structural basis for protein-protein interaction may provide potential means to manipulate or block such interaction, which may yield therapeutic benefits. We propose that the results obtained from these studies will provide a novel, and important dimension to both integrin and eicosanoid biology. Acknowledgements: This project is supported by NIH: NCI, 2 R01 CA029997-17A1 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2890. doi:10.1158/1538-7445.AM2011-2890

Influence of stress on extracellular matrix and integrin biology

Dynamic interactions between cells and extracellular matrix (ECM) through integrins influence most cellular functions. Normal cells, but even more, tumor cells are subjected to different forms of stress, including ischemia, radical oxygen species production, starvation, mechanical stress or genotoxic insults due to anti-cancer drugs or irradiation. In these situations, an adaptative cellular response occurs, integrating a complex network of intracellular signaling modules, which, depending on stress intensity, may result to either damage repair followed by complete restitution of cellular functions, or programmed cell death. Because of its implication in oncogenesis and anti-cancer therapy, cellular stress response has been thoroughly investigated. However, most of these studies have been performed in the context of isolated cells without taking into consideration that most cells are part of the tissue within which they interact with ECM through integrin. Few studies have described the influence of stress on cell-to-ECM interaction. However, one can speculate that, in these conditions, cells could functionally interact with protein microenvironment either to create positive interactions to survive (for example by facilitating protective pathways) or negative interaction to die (for example by facilitating detachment). In this review, we summarize the knowledge relative to the influence of different stress modalities on ECM remodeling, integrin expression and/or function modifications, and possible functional consequences, independently from the cellular model as these findings came from a large variety of cells (mesenchymal, endothelial, muscular, epithelial and glandular) and fields of application (cancer, vascular biology and tissue engineering). Most studies support the general notion that non-lethal stress favors ECM stiffness, integrin activation and enhanced survival. This field opens large perspectives not only in tumor biology but also in anti-cancer therapy by targeting one or several steps of the integrin-mediated signaling pathway, including integrin ligation, or activation of integrin-linked enzymes or integrin adaptors.

Integrin Targeted Imaging and Therapy

This theme issue provides an overview on the biology and pathology of various integrins as well as in-depth discussion on the use of integrin as targeting molecules for molecular imaging and molecular therapy.

Three-dimensional Model of Human Platelet Integrin αIIbβ3 in Solution Obtained by Small Angle Neutron Scattering

Integrin alphaIIbbeta3 is the major membrane protein and adhesion receptor at the surface of blood platelets, which after activation plays a key role in platelet plug formation in hemostasis and thrombosis. Small angle neutron scattering (SANS) and shape reconstruction algorithms allowed formation of a low resolution three-dimensional model of whole alphaIIb beta3 in Ca(2+)/detergent solutions. Model projections after 90 degrees rotation along its long axis show an elongated and "arched" form (135 degrees) not observed before and a "handgun" form. This 20-nm-long structure is well defined, despite alphaIIb beta3 multidomain nature and expected segmental flexibility, with the largest region at the top, followed by two narrower and smaller regions at the bottom. Docking of this SANS envelope into the high resolution structure of alphaIIb beta3, reconstructed from crystallographic and NMR data, shows that the solution structure is less constrained, allows tentative assignment of the disposition of the alphaIIb and beta3 subunits and their domains within the model, and points out the structural analogies and differences of the SANS model with the crystallographic models of the recombinant ectodomains of alphaIIb beta3 and alphaV beta3 and with the cryo-electron microscopy model of whole alphaIIb beta3. The ectodomain is in the bent configuration at the top of the model, where alphaIIb and beta3 occupy the concave and convex sides, respectively, at the arched projection, with their bent knees at its apex. It follows the narrower transmembrane region and the cytoplasmic domains at the bottom end. AlphaIIb beta3 aggregated in Mn(2+)/detergent solutions, which impeded to get its SANS model.

A fundamental role of mAbp1 in neutrophils: impact on β2 integrin–mediated phagocytosis and adhesion in vivo

AbstractThe mammalian actin-binding protein 1 (mAbp1, Hip-55, SH3P7) is phosphorylated by the nonreceptor tyrosine kinase Syk that has a fundamental effect for several β2 integrin (CD11/CD18)–mediated neutrophil functions. Live cell imaging showed a dynamic enrichment of enhanced green fluorescence protein–tagged mAbp1 at the phagocytic cup of neutrophil-like differentiated HL-60 cells during β2 integrin–mediated phagocytosis of serum-opsonized Escherichia coli. The genetic absence of Syk or its pharmacologic inhibition using piceatannol abrogated the proper localization of mAbp1 at the phagocytic cup. The genetic absence or down-regulation of mAbp1 using the RNA interference technique significantly compromised β2 integrin–mediated phagocytosis of serum-opsonized E coli or Salmonella typhimurium in vitro as well as clearance of S typhimurium infection in vivo. Moreover, the genetic absence of mAbp1 almost completely abrogated firm neutrophil adhesion under physiologic shear stress conditions in vitro as well as leukocyte adhesion and extravasation in inflamed cremaster muscle venules of mice treated with tumor-necrosis factor α. Functional analysis showed that the down-regulation of mAbp1 diminished the number of β2 integrin clusters in the high-affinity conformation under flow conditions. These unanticipated results define mAbp1 as a novel molecular player in integrin biology that is critical for phagocytosis and firm neutrophil adhesion under flow conditions.