The beta 2 integrin Mac-1 but not p150,95 associates with Fc gamma RIIA

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.

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.

Cytohesin-1 regulates fMLF-mediated activation and functions of the β2 integrin Mac-1 in human neutrophils

The nucleotide exchange factor cytohesin-1 was previously reported to interact with the cytoplasmic domains of the integrin β-chain common to all β(2) integrins such as LFA-1 and Mac-1. We show here that cytohesin-1, which contributes to fMLF-induced functional responses in PMNs through activation of Arf6, restrains the activation of the β(2) integrin Mac-1 (αMβ(2)) in PMNs or dcAMP-differentiated PLB-985 cells. We found that the cytohesin-1 inhibitor SecinH3 or siRNA increased cell adhesion to immobilized fibrinogen and fMLF-mediated conformational changes of Mac-1, monitored using mAb CBRM1/5, specific for the activation epitope of the αM subunit. In contrast, PLB-985 cells overexpressing cytohesin-1 showed little adhesion to fibrinogen. The use of SecinH3 and siRNA also revealed that interference with cytohesin-1 signaling also enhanced phagocytosis of zymosan particles and chemotaxis toward fMLF in transwell migration assays. These increments of phagocytosis and chemotaxis in cells treated with SecinH3 and cytohesin-1 siRNA were reversed by a blocking mAb to the integrin-αM subunit. We provide evidence for increased polymerized cortical actin in cells treated with SecinH3 and that altered signaling through cytohesin-1 increased cell surface expression of FPRL-1 and impairs the late calcium mobilization response elicited by fMLF. The data provide evidence that stimulation with fMLF initiates a signaling cascade that restrains Mac-1 activation in PMNs. Such crosstalk between FPRL-1 and Mac-1 involves cytohesin-1. We suggest that cytohesin-1 may coordinate activation of the β(2) integrins to regulate PMN adhesion, phagocytosis, and chemotaxis.

The role of integrins in the recognition and response of dendritic cells to biomaterials

Biomaterials have the potential to be utilized as immunostimulatory or immunosuppressive delivery agents for biologics. It is hypothesized that this is directed by the ability of a biomaterial to drive dendritic cells (DC) in situ toward an immunostimulatory or an immunosuppressive phenotype, respectively. However, the specific pattern recognition receptors (PRRs) that DCs use to recognize and respond to biomaterials are unknown. From among the many receptors that DCs use to recognize and respond to foreign entities, herein the focus is on integrins. A biomaterial that induces DC maturation, namely poly(lactic-co-glycolic) acid (PLGA), supported increased human monocyte-derived DC adhesion and up-regulation of integrin receptor gene expression, measured via RT-PCR, as compared to culture on tissue culture polystyrene (TCPS). This was not observed for a biomaterial that does not support DC maturation. Through antibody-blocking techniques, the adhesion to both TCPS and PLGA was found to be β(2) integrin dependent and β(1) independent. Significantly, inhibiting β(2)-mediated adhesion to biomaterials via blocking antibodies also lowered the level of maturation of DCs (CD86 expression). β(2) integrins (but not β(1)) were found localized in biomaterial-adherent DC podosomes and also were found in direct contact with the PLGA surface. Therefore, it appeared that β(2) integrin-mediated adhesion is involved in determining the state of DC maturation on the PLGA surface. DC adhesion to biomaterials may be engaged or avoided to manipulate an immune response to biological component delivered with a biomaterial carrier.

Urokinase-type plasminogen activator receptor induces conformational changes in the integrin alphaMbeta2 headpiece and reorientation of its transmembrane domains

The glycosylphosphatidylinositol-linked urokinase-type plasminogen activator receptor (uPAR) interacts with the heterodimer cell adhesion molecules integrins to modulate cell adhesion and migration. Devoid of a cytoplasmic domain, uPAR triggers intracellular signaling via its associated molecules that contain cytoplasmic domains. Interestingly, uPAR changes the ectodomain conformation of one of its partner molecules, integrin alpha(5)beta(1), and elicits cytoplasmic signaling. The separation or reorientation of integrin transmembrane domains and cytoplasmic tails are required for integrin outside-in signaling. However, there is a lack of direct evidence showing these conformational changes of an integrin that interacts with uPAR. In this investigation we used reporter monoclonal antibodies and fluorescence resonance energy transfer analyses to show conformational changes in the alpha(M)beta(2) headpiece and reorientation of its transmembrane domains when alpha(M)beta(2) interacts with uPAR.

A simple, no-wash cell adhesion-based high-throughput assay for the discovery of small-molecule regulators of the integrin CD11b/CD18

The leukocyte-specific integrin CD11b/CD18 plays a key role in the biological function of these cells and represents a validated therapeutic target for inflammatory diseases. Currently, the low affinity interaction between CD11b/CD18 integrin and its respective ligand poses a challenge in the development of cell-based adhesion assays for the high-throughput screening (HTS) environment. Here the authors describe a simple cell-based adhesion assay that can be readily used for HTS for the discovery of functional regulators of CD11b/CD18. The assay consistently produces acceptable Z' values (> 0.5) for HTS. After testing the assay using 2 established blocking antibodies as reference biologicals, the authors performed a proof-of-concept primary screen using a library of 6612 compounds and identified both agonist and antagonist hits.

Both Fcgamma and complement receptors mediate transfer of immune complexes from erythrocytes to human macrophages under physiological flow conditions in vitro

Abnormal clearance by the mononuclear phagocytic system of immune complexes (IC) is important in the pathogenesis of systemic lupus erythematosus (SLE). We have developed an in vitro model to investigate the cellular mechanisms involved in the transfer of soluble IC from erythrocytes to human macrophages under physiological flow conditions. In this assay, erythrocytes bearing fluorescently labelled IC are perfused over monolayers of human monocytes or monocyte-derived macrophages in a parallel-plate flow chamber, and transfer quantified using confocal microscopy and flow cytometry. Using aggregated human IgG as a model IC, we have been able to demonstrate transfer of IC from erythrocytes to macrophages. Blocking studies with specific neutralizing antibodies have shown that both complement and Fcgamma receptors are required for IC transfer. Blockade of CR4 (alpha(x)beta(2) integrin), FcgammaRIIa or FcgammaRIII reduced transfer, while anti-CR3 (alpha(m)beta(2) integrin) had no effect. Blockade of CR3, FcgammaRIIa or FcgammaRIII also reduced the number of adhesive interactions between fluorescently labelled IC-bearing erythrocytes and macrophage monolayers. Taken together with the transfer data, this suggests differing roles for these receptors in the human IC transfer reaction that includes an adhesive function which facilitates IC processing by mononuclear phagocytes. Finally, a functional effect of the FcgammaRIIa R131/H131 polymorphism, important in susceptibility to SLE, has also been demonstrated using this model. Uptake of IgG(2) but not IgG(1)-containing soluble IC was reduced by macrophages from individuals homozygous for the R131 allelic variant of the receptor.

The beta-tail domain (betaTD) regulates physiologic ligand binding to integrin CD11b/CD18

Crystallographic and electron microscopy studies revealed genuflexed (bent) integrins in both unliganded (inactive) and physiologic ligandbound (active) states, suggesting that local conformational changes are sufficient for activation. Herein we have explored the role of local changes in the contact region between the membrane-proximal beta-tail domain (betaTD) and the ligand-binding betaA domain of the bent conformation in regulating interaction of integrin CD11b/CD18 (alphaMbeta2) with its physiologic ligand iC3b. We replaced the betaTD CD loop residues D658GMD of the CD18 (beta2) subunit with the equivalent D672SSG of the beta3 subunit, with AGAA or with NGTD, expressed the respective heterodimeric receptors either transiently in epithelial HEK293T cells or stably in leukocytes (K562), and measured their ability to bind iC3b and to conformation-sensitive mAbs. In the presence of the physiologic divalent cations Ca(2+) plus Mg(2+) (at 1 mM each), the modified integrins showed increased (in HEK293) or constitutive (in K562) binding to iC3b compared with wild-type receptors. K562 expressing the betaTD-modified integrins bound in Ca(2+)Mg(2+) to the betaA-directed high-affinity reporter mAb 24 but not to mAb KIM127, a reporter of the genu-straightened state. These data identify a role for the membrane proximal betaTD as an allosteric modulator of integrin activation.

Antileukoproteinase: Modulation of neutrophil function and therapeutic effects on anti-type II collagen antibody-induced arthritis

OBJECTIVE

Antileukoproteinase (ALP) is a physiologic inhibitor of granulocytic serine proteases. The present study was undertaken to investigate its therapeutic benefit in an antibody-transfer model of erosive polyarthritis and to elucidate its potential to interfere with immune complex-dependent inflammatory pathways.

METHODS

Arthritis development was induced in male (BALB/c x B10.Q)F(1) mice by intravenous injection of two monoclonal antibodies specific for type II collagen and was quantified by clinical scoring and histopathology. Arthritis severity was assessed in a cohort of mice under systemic treatment with recombinant human ALP (daily doses of 0.1 mg for 5 days starting immediately after disease induction) in comparison with untreated controls. Concomitantly, functional assays (phagocytosis, oxidative burst, fluorescence-activated cell sorting analysis of integrin expression) were performed on neutrophils upon in vitro stimulation by IgG-coated latex beads.

RESULTS

ALP treatment reduced arthritis incidence and severity and had a protective effect against cartilage and bone erosion. ALP inhibited the conversion of the leukocyte beta2 integrins into an active conformation upon Fc receptor stimulation of granulocytes. ALP bound to the actin-bundling protein L-plastin and down-modulated filamentous actin assembly in response to stimulation with IgG-coated latex beads in granulocytes. ALP exerted additional inhibitory effects on neutrophil functions associated with cytoskeletal reorganization, such as phagocytosis and oxidative burst.

CONCLUSION

In addition to its antiprotease activity, ALP exerts a variety of blocking effects on neutrophil functions, probably due to modulation of cytoskeletal changes, that may contribute to this inhibitor's antiarthritis potential and qualify it as a multifunctional regulator of inflammatory responses.

Involvement of Fcgamma receptors and beta2 integrins in neutrophil activation by anti-proteinase-3 or anti-myeloperoxidase antibodies

We previously described the requirement of tumour necrosis factor-alpha (TNF-alpha) and the role of beta2 integrins in the Fc-gamma receptor IIa (FcgammaRIIa)-mediated mechanism of neutrophil activation by antiproteinase-3 (anti-PR3) or anti-myeloperoxidase (anti-MPO) antibodies. In the present study, we assessed the involvement of FcgammaRIIIb by studying the respiratory burst activation of completely FcgammaRIIIb-deficient neutrophils primed by TNF-alpha and exposed to anti-PR3 or anti-MPO. Activation of the NADPH oxidase occurred normally in these neutrophils, which indicates that engagement of FcgammaRIIIb is not essential in our model. Experiments performed with neutrophils from severe leucocyte adhesion deficiency (LAD) patients confirmed that beta2 integrins play a pivotal role in this activation. We next studied whether adhesion per se, beta2-integrin-mediated adhesion, or beta2-integrin ligation without adhesion is necessary or sufficient for this activation. Anti-PR3 or anti-MPO induced an FcgammaRIIa-dependent burst in TNF-primed neutrophils incubated in wells coated with poly-L-lysine, known to induce beta2-integrin-independent adhesion, but this reaction was still inhibited by blocking CD18 antibodies. In a system with granulocyte-macrophage colony-stimulating factor (GM-CSF)-primed neutrophils, which did not enhance adhesion, we measured a similar activation by anti-PR3 or anti-MPO and inhibition by CD18. We also noticed that treatment with the beta2-integrin-activating CD18 MoAb KIM185 per se is insufficient for neutrophil activation by anti-PR3 or anti-MPO. We therefore conclude that ligation of beta2 integrins rather than adherence per se is essential for this activation, and that TNF-alpha or GM-CSF is needed for priming but not for adherence.

Neutrophil-activating potential of antineutrophil cytoplasm autoantibodies

Accumulating in vivo and in vitro evidence supports the hypothesis that antineutrophil cytoplasm autoantibodies (ANCA) with specificity for proteinase 3 (PR3) and myeloperoxidase (MPO) are involved in the pathophysiology of small-vessel vasculitis. The best-described effector function of these autoantibodies is stimulation of neutrophils to produce reactive oxygen species and to release proteolytic enzymes. Neutrophil activation requires interaction of monomeric ANCA with PR3/MPO and Fcgamma receptors, but also other mechanisms--for instance, stimulation by ANCA-containing immune complexes--cannot be excluded. This review focuses on the mechanisms of neutrophil activation by ANCA. We discuss the molecules involved in ANCA binding to the neutrophil surface and in triggering the functional responses. We summarize current knowledge on the signal-transduction pathways initiated by ANCA and on the factors determining susceptibility of neutrophils to activation by these autoantibodies.

Downregulation of mac-1 expression in monocytes by surface-bound IgG

Physical and functional association between the beta2-integrin Mac-1 (CD11b/CD18) and receptors of immunoglobulin G (IgG) (FcgammaRs) has been previously reported. In this study, we examined the modulation of Mac-1 expression by IgG in different leucocyte populations. Our data show that human monocytes, but not neutrophils, macrophages, dendritic or natural killer cells, downregulate the expression of Mac-1 after overnight exposure to surface-bound IgG. This effect, which requires at least 6 h of incubation, is not associated with a general downmodulation of membrane antigens, and is selectively induced by immobilized IgG (iIgG), as the stimulation of monocytes with N-formyl-methionyl-leucyl-phenylalanine, lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha) or soluble IgG did not modify the Mac-1 expression after 18 h in culture. The loss of Mac-1 was completely prevented by blocking antibodies (Abs) directed to FcgammaRII (CD32) or CD18. On the other hand, the serine protease inhibitor, phenyl methyl sulphonyl fluoride, but not inhibitors of cysteine proteases or neutral endopeptidases, partially prevented the downregulation of Mac-1 by iIgG. Monocytes cultured overnight on iIgG exhibited a dramatic decrease in their capacity to ingest zymosan particles that could be attributed to the reduced expression of Mac-1. However, there was no inhibition of TNF-alpha production induced by zymosan, suggesting that Mac-1-dependent responses require different levels of its expression to be fully activated.

Interactions of integrins with their partner proteins in leukocyte membranes

Integrins participate in many aspects of immunologic and inflammatory responses, especially those involving cell migration, adherence, and activation. Although leukocyte integrins such as complement receptor type 3 (CR3) are known to carry out certain functions without the intervention of other plasma membrane receptors, many plasma membrane proteins are now known to physically interact and functionally cooperate with integrins. Several of these interactions are highly dynamic within cell membranes; thus integrin-partner protein interactions change during certain physiological processes. This allows an extraordinary adaptability of the system to prime and promote proinflammatory signaling. Since our discovery of the CR3-FcgammaRIIIB interaction, the plasma membrane protein repertoire of beta1, beta2, and beta3 integrins has grown to include: FcgammaRIIA (CD32), uPAR (urokinase-type plasminogen activator receptor; CD87), CD14, voltage-gated K+channels (Kv1.3), integrin-associated protein (IAP), CD98, tetraspans (TM4SF), insulin receptors, and PDGFbeta receptors. In this article we will highlight certain features of this growing field of research, especially with regard to their relevance in immunology and inflammation.

Mac-1 (CD11b/CD18) is essential for Fc receptor-mediated neutrophil cytotoxicity and immunologic synapse formation

Receptors for human immunoglobulin (Ig)G and IgA initiate potent cytolysis of antibody (Ab)-coated targets by polymorphonuclear leukocytes (PMNs). Mac-1 (complement receptor type 3, CD11b/CD18) has previously been implicated in receptor cooperation with Fc receptors (FcRs). The role of Mac-1 in FcR-mediated lysis of tumor cells was characterized by studying normal human PMNs, Mac-1-deficient mouse PMNs, and mouse PMNs transgenic for human FcR. All PMNs efficiently phagocytosed Ab-coated particles. However, antibody-dependent cellular cytotoxicity (ADCC) was abrogated in Mac-1(-/-) PMNs and in human PMNs blocked with anti-Mac-1 monoclonal Ab (mAb). Mac-1(-/-) PMNs were unable to spread on Ab-opsonized target cells and other Ab-coated surfaces. Confocal laser scanning and electron microscopy revealed a striking difference in immunologic synapse formation between Mac-1(-/-) and wild-type PMNs. Also, respiratory burst activity could be measured outside membrane-enclosed compartments by using Mac-1(-/-) PMNs bound to Ab-coated tumor cells, in contrast to wild-type PMNs. In summary, these data document an absolute requirement of Mac-1 for FcR-mediated PMN cytotoxicity toward tumor targets. Mac-1(-/-) PMNs exhibit defective spreading on Ab-coated targets, impaired formation of immunologic synapses, and absent tumor cytolysis.

Immune complex size and complement regulate cytokine production by peripheral blood mononuclear cells

We have previously shown that immune complexes isolated from children with juvenile rheumatoid arthritis are heterogeneous in their size, composition, and proinflammatory capacities. The experiments described here were undertaken to clarify further the roles of size and composition in determining the proinflammatory effects of immune complexes. We incubated peripheral blood mononuclear cells (PBMCs) with different soluble immune complex preparations: opsonized complexes, which were formed in the presence of serum, unopsonized complexes, which were formed in the absence of serum, and immune precipitates solubilized by complement after their formation. ELISA assays showed that immune complexes formed in the presence of complement were less efficient than unopsonized complexes in inducing IL-1beta and IL-8 secretion from leukocytes. Solubilized immune precipitates showed intermediate capacity to stimulate the release of both cytokines. Complexes formed in heat-inactivated serum were as efficient as unopsonized complexes in eliciting cytokine secretion from the cells. The capacity of complement to regulate cytokine secretion from leukocytes was related, at least in part, to immune complex size. Sucrose density gradients showed unopsonized complexes and solubilized immune precipitates were larger than opsonized immune complexes. In contrast, fluid-phase binding of C4 to immune complexes, which did not appreciably change immune complex size, substantially increased IL-1beta secretion from PBMC.

Lipopolysaccharide enhances FcgammaR-dependent functions in vivo through CD11b/CD18 up-regulation

Fc receptors for immunoglobulin G (IgG) (FcgammaR) mediate several defence mechanisms in the course of inflammatory and infectious diseases. In Gram-negative infections, cellular wall lipopolysaccharides (LPS) modulate different immune responses. We have recently demonstrated that murine LPS in vivo treatment significantly increases FcgammaR-dependent clearance of immune complexes (IC). In addition, we and others have reported the induction of adhesion molecules on macrophages and neutrophils by LPS in vivo and by tumour necrosis factor-alpha (TNF-alpha) in vitro. The aim of this paper was to investigate CD11b/CD18 participation in LPS enhancing effects on Fcgamma-dependent functionality of tissue macrophages. Our results have demonstrated that LPS can enhance antibody-dependent cellular cytotoxicity (ADCC) and IC-triggered cytotoxicity (IC-Ctx), two reactions which involve the Fcgamma-receptor but different lytic mechanisms. In vitro incubation of splenocytes from LPS-treated mice with anti-CD11b/CD18 abrogated ADCC and IC-Ctx enhancement, without affecting FcgammaR expression. Similar results were obtained with physiological concentrations of fibrinogen. In this way cytotoxic values of LPS-splenocytes decreased to the basal levels of control mice. Time and temperature requirements for such inhibition strongly suggested that anti-CD11b/CD18 could modulate intracellular signals leading to downregulation of FcgammaR functionality. Data presented herein support the hypothesis that functional and/or physical associations between integrins and FcgammaR could be critical for the modulation of effector functions during an inflammatory response.

Complement deposition on immune complexes reduces the frequencies of metabolic, proteolytic, and superoxide oscillations of migrating neutrophils

Neutrophils exhibit intrinsic sinusoidal metabolite concentration oscillations of 3 min in resting cells and an additional approximately 10- or 20-s oscillation in migrating/adhering cells. To better understand immune complex (IC)-mediated leukocyte activation, we have studied neutrophil metabolic oscillations in the presence of ICs either with or without fixed complement. Using a microscope photometer we quantitated NAD(P)H autofluorescence oscillations. Cells exposed to ICs exhibited metabolic oscillation periods of approximately 12 s in the absence of complement and approximately 22 s in the presence of complement opsonization. To determine if the effects could be associated with C3 deposition, we used ICs opsonized with only C3 or only C1 and C4. Untreated ICs, heat-inactivated complement-treated ICs, and C1,C4-treated ICs trigger rapid metabolic oscillations, as do fMLP and yeast; in contrast, ICs treated with full complement or C3 alone did not affect NAD(P)H oscillations in comparison to controls. The induction of higher frequency (approximately 10 s) NAD(P)H oscillations by ICs could be blocked by addition of anti-FcgammaRII, but not FcgammaRIII mAb fragments, suggesting the participation of FcgammaRII in cellular metabolic responses to ICs. Parallel changes in the frequencies of oxidant release and pericellular proteolysis were found for all of these stimuli. Thus, immune complex composition affects both intracellular metabolic signals and extracellular functional oscillations. We suggest that complement attenuates the phlogistic potential of ICs by reducing the frequency of cytoplasmic NAD(P)H oscillations.

Aberrant integrin (CR4; alpha(x)beta2; CD11c/CD18) oscillations on neutrophils in a mild form of pyoderma gangrenosum

We have previously shown that the beta2 integrins CR3 and CR4 physically and functionally interact with urokinase receptors (uPAR) on neutrophil plasma membranes in an oscillatory fashion. In this study we have analyzed neutrophils from patient SC, a 34 y old African American female, with aberrant skin window results and recurrent perianal abscesses and pretibial lesions diagnosed as pyoderma gangrenosum. Although untreated migrating normal neutrophils exhibited 20 s sinusoidal oscillations in CR4-uPAR proximity, neutrophils from SC demonstrated a faster oscillation (10 s) in the form of a flyback sawtooth wave. This waveform mimicked that observed for normal neutrophils treated with subsaturating doses of the kinase inhibitors staurosporine, genistein, and erbstatin. As beta2 integrins are regulated by phosphorylation, we tested the hypothesis that the aberrant CR4-uPAR proximity oscillations seen in SC's neutrophils are due to defective kinase activity that might be balanced by a decrease in phosphatase activity. When SC's cells are exposed to subsaturating concentrations of the phosphatase inhibitor pervanadate, this caused the CR4-uPAR oscillations to become sinusoidal in shape with a 20 s period, as seen in normal migrating neutrophils. Although SC's neutrophils were deficient in spontaneous and N-formyl-methionyl-leucyl-phenylalanine-induced polarization, 0.5 microM pervanadate returned cell polarization to nearly normal levels, thus paralleling the acquisition of normal receptor interactions. Inasmuch as SC's cellular phenotype is mimicked by kinase inhibitors and corrected by phosphatase inhibitors, we suggest that a mutation(s) affecting the kinetics of intracellular signaling enzymes, but not blocking the pathway per se, may be responsible for this clinical state.

The Human S100 Protein MRP-14 Is a Novel Activator of the β2 Integrin Mac-1 on Neutrophils

The 14-kDa myeloid-related protein (MRP-14) and its heterodimeric partner, MRP-8, are members of the S100 family of calcium-binding proteins (S100A9 and S100A8, respectively). Their importance in neutrophil function is implied by their unusual abundance in neutrophil cytosol (∼40% of cytosolic protein). Previous work from our laboratory has demonstrated the extracellular association of these proteins with vascular endothelium adjacent to transmigrating leukocytes. We report here a function for MRP-14 as a stimulator of neutrophil adhesion mediated by the β2 integrin, Mac-1. MRP-14 is an affinity regulator of Mac-1 because it promotes binding of soluble ligand and expression of an “activation reporter” epitope of high affinity β2 integrins recognized by mAb24. The activity of MRP-14 is confined to regulating integrin function because, unlike other inflammatory agonists, there was no release of L-selectin, up-regulation of cytosolic Mac-1, or induction of neutrophil respiratory burst or calcium flux. Furthermore, MRP-14 does not act as a chemoattractant or cause alterations in cell shape or cytoskeleton. MRP-8 has a regulatory role in MRP-14 activity, inhibiting the adhesion induced by MRP-14 through the formation of the heterodimer. In terms of mechanism of action, MRP-14 does not increase Mac-1 function by direct binding to this integrin but recognizes a distinct receptor on neutrophils. This receptor interaction is pertussis toxin sensitive, indicating that MRP-14-generated signals leading to a Mac-1 affinity increase are heterotrimeric G protein dependent. We postulate that MRP-14 and MRP-8 are important in vivo candidates for the regulated adhesion of neutrophils through control of Mac-1 activity.

Ectodomain interactions of leukocyte integrins and pro-inflammatory GPI-linked membrane proteins

Although glycosylphosphatidyl-inositol (GPI) linked membrane proteins do not possess transmembrane or cytosolic sequences they elicit transmembrane signals. Using microscopic fluorescence imaging and resonance energy transfer (RET) techniques we have shown that certain pro-inflammatory GPI-linked membrane proteins can interact with leukocyte beta 2 integrins (complement receptor type 3 (CR3) and 4 (CR4) and the leukocyte function-associated antigen-1 (LFA-1)). For example, physical associations between CR3 and Fc gamma RIIIB, CR3 and urokinase receptors, and CR3 and CD14 (lipopolysaccharide receptor) have been found. Although Fc gamma RIIIB appears to be constitutively associated with CR3, urokinase receptors and CD14 associations with CR3 are influenced by their ligation status and cell function (e.g. adherence and locomotion). CR3-to-urokinase receptor interactions have been confirmed by immunoprecipitation techniques. Immunoprecipitation of CR3 from Brij-58 lysates after biotinylation of neutrophil membranes revealed proteins of M(r) = 40,000, 50,000, 74,000 and 120,000, in addition to bands corresponding to the integrin alpha and beta chains. Cell functions such as transmembrane signaling and superoxide release/priming have been linked to these interactions. Importantly, reagents that affect the lectin-like site of CR3, such as N-acetyl-D-glucosamine, alpha-methyl-D-mannoside and beta-glucan alter these interactions and, in parallel, leukocyte functions. Thus, the interactions of GPI-linked proteins and integrins can be highly dynamic events linked to cell activities. Our studies suggest that it may be possible to develop new drugs directed at the lectin-like site of beta 2 integrins that block GPI-linked protein-to-integrin coupling thereby controlling inflammatory cell processes including cell adherence, locomotion and activation. Such drugs may be useful in clinical conditions such as ischemia-reperfusion injury, sepsis, arthritis and others.

Beta 2 (CD11/CD18) integrins can serve as signaling partners for other leukocyte receptors

Fig. 1 depicts our current thinking about the ways in which Mo1 and p150,95 form cis interactions with other leukocyte receptors. With respect to the associations of Mo1 with Fc gamma RIIIB and uPAR, the inhibitory effect of saccharides such as NADG suggests a lectin-carbohydrate interaction that may involve the recognition of Mo1's beta-glucan site for N-linked carbohydrates4 that are expressed by both Fc gamma RIIIB and uPAR. This hypothesis is supported by the results of Stockl et al., who showed that the binding of C-terminal-specific mAb VIM12 to Mo1, which enhances the phospholipase C-mediated release of Fc gamma RIIIB, was inhibited by NADG. However, unlike the sample lectin-carbohydrate interaction that appears to govern the association between Mo1 and Fc gamma RIIIB, effective Mo1-dependent uPAR signaling also depends on the binding of intact uPA to uPAR (the receptor-binding ATF of uPA proving insufficient to prime neutrophils for an enhanced burst response to FMLP). We speculate that ATF (residues 6-135) binds to uPAR while the carboxyl terminal fragment (residues 136-411), which includes a glycosylation site at residue 144, binds to the lectinlike site of Mo1, thus fostering the linkage between the two receptors. In support of this model is the fact that exposure of neutrophils to ATF reduced the degree of molecular proximity between Mo1 and uPAR (the latter probably occupied by endogenous intact uPA) and increased the molecular association between Mo1 and Fc gamma RIIIB (both as detected by quantitative RET). This hypothesis is analogous to the concept proposed by Nykjaer et al in which plasminogen activator inhibitor-1 initially binds to uPA to form a complex that secondarily binds to the alpha 2 macroglobulin receptor, leading to internalization of the complex. Whereas the contribution of intact uPA to the interaction between Mo1 and uPAR remains speculative (based on the indirect data available), no such ambiguity exists for the role of the LPS/LBP ligand in regulating the association between Mo1 and CD14. In this circumstance, no physical linkage exists between the two receptors without the ligand complex. This observation is consistent with the previously described affinity of the beta 2 integrins for LPS, leading to the notion that the LPS portion of the LPS/LPB complex binds to Mo1, serving to link it with LPS/LBP bound to CD14. The observed reversibility of the interactions between the integrin glycoproteins and uPAR or CD14 illustrates the fact that these associations can be highly dynamic and tied to cellular processes that include directed motility (Mo1-uPAR), adherence to substrates (Mo1-CD14), and energy metabolism (p150,95-uPAR). We speculate that the GPI-anchored receptor proteins serve as rapidly diffusible, expendable "scouts" for the beta 2 integrins, which serve to expand their ligand binding repertoire in a cis-acting fashion.

Integrins as promiscuous signal transduction devices

Recent studies suggest physical and functional interactions of glycosylphosphatidylinositol (GPI)-linked proteins (CD14, CD16b and CD87) with leukocyte beta 2 integrins. As discussed in this article, it now appears that beta 2 integrins relay proinflammatory information from GPI-anchored membrane receptors to the cytoplasm via exodomain interactions.