An immobile subset of plasma membrane CD11b/CD18 (Mac-1) is involved in phagocytosis of targets recognized by multiple receptors

CD11b/CD18 is a heterodimeric leukocyte surface receptor which functions in both C3bi-ligand binding and homotypic and heterotypic cell adherence. We have examined the effect of several anti-CD11b/18 mAb on phagocytosis of IgG (EIgG) or complement (EC4b) opsonized erythrocytes by polymorphonuclear leukocytes (PMN) and monocytes. F(ab')2 of two mAb (IB4, an anti-beta-chain mAb and Mo-1 an anti-alpha-chain mAb), inhibited both phagocytosis of EIgG and phorbol ester-stimulated phagocytosis of EC4b by PMN and monocytes. These F(ab')2 inhibited the binding of EIgG to monocytes, but they had no effect on binding of EIgG to PMN, or EC4b to either phagocyte. In addition, IB4 inhibited phorbol-ester stimulated phagocytosis of sheep E opsonized with C component 3bi (EC3bi) without inhibiting rosetting of these same targets. These data separate the anti-phagocytic effect of these mAb from effects on phagocyte-target adherence. When PMN were adherent to an anti-CD11b/CD18 F(ab')2-coated surface, EC3bi binding was abolished, but phagocytosis of EIgG or EC4b was unaffected. Subsequent addition of fluid- phase IB4 or Mo-1 F(ab')2 inhibited phagocytosis of EIgG or EC4b by the adherent cells. This suggested that the CD11b/CD18 involved in C3bi rosetting were mobile in the membrane, whereas those involved in phagocytosis of EIgG or EC4b were not. Cytochalasin treatment of PMN during adherence to F(ab')2-coated plates decreased both apical expression of CD11b/18 and subsequent ingestion of EIgG by 70%, suggesting that microfilaments are important in maintaining immobile CD11b/18 on the apical PMN surface. We conclude that there are functionally distinct populations of CD11b/CD18 on monocytes and PMN: one involved in C3bi rosetting and another involved in the process of phagocytosis mediated via several different receptors. CD11b/18 is not required for optimal target binding in all cases, but is always required for ingestion. As with several other integrins, the CD11b/18 molecules involved in phagocytosis have a functional association with the cell cytoskeleton.     

Neutrophil-activating peptide 1/interleukin 8 mRNA expression and protein secretion by human monocytes: effect of cyclosporin A

Neutrophil-activating peptide 1/interleukin 8 (NAP-1/IL-8) is a recently described cytokine with potent chemotactic activity for human neutrophil granulocytes (PMN) and T cells. In psoriasis, a chronic hyperproliferative and inflammatory skin disorder, PMN and T cells are found as prominent cells in the inflammatory infiltrate of the lesions; however, monocytes were shown to be the first cells invading a newly formed plaque. NAP-1/IL-8 was found to be present in high amounts in the skin and in scale material of psoriatic patients. Psoriasis responds well to systemic treatment with cyclosporin A (CsA), an immunosuppressive peptide. Therefore, we addressed the question of whether the clinical improvement of psoriatic patients during CsA therapy may be due to an inhibition of NAP-1/IL-8 production and secretion from monocytes. Purified human monocytes were stimulated by lipopolysaccharide in the presence or absence of various concentrations of CsA. Production of NAP-1/IL-8 was determined as expression of specific mRNA by fluorescent in situ hybridization. Secreted peptide was measured by bioassay (PMN chemotaxis) and enzyme-linked immunosorbent assay (ELISA) using specific monoclonal antibodies. The results show that CsA neither inhibited mRNA expression for NAP-1/IL-8 nor secretion of the peptide. These findings support the hypothesis that the pharmacological effect of CsA may be restricted to the inhibition of T-cell activation and proliferation.     

Interleukin-10 inhibits neutrophil phagocytic and bactericidal activity

Abstract Effective host defense against bacterial invasion is characterized by the vigorous recruitment and activation of inflammatory cells, which is dependent upon the coordinated expression of both pro- and anti-inflammatory cytokines. Interleukin-10 (IL-10) is a recently described cytokine with potent anti-inflammatory properties in vivo and in vitro. In this study we investigated whether IL-10 could directly regulate the ability of neutrophils (PMN) to phagocytose and kill bacteria. Initial studies demonstrated that human recombinant IL-10 (hrIL-10) inhibited the ability of PMN to phagocytose Escherichia coli in vitro. Inhibition of phagocytosis occurred in the absence of changes in CR1 (C3b) or Fc receptor expression, as treatment of PMN with IL-10 failed to induce significant changes in FcγIIR, FcγIIIR or CR1 cell surface expression. However, incubation of PMN with IL-10 resulted in a dose-dependent decrease in CD11b (Mac-1) expression. In addition to effects on PMN phagocytosis, hrIL-10 significantly attenuated PMN microbicidal activity, as bactericidal assays revealed that co-incubation of PMN with hrIL-10 resulted in a marked decrease in killing of phagocytosed bacteria. Furthermore, IL-10 inhibited the production of superoxide from PMA-stimulated PMN, suggesting that the detrimental effects of IL-10 on PMN microbicidal activity were due, in part, to suppression of respiratory burst. In summary, our studies indicate that IL-10 inhibits PMN-dependent phagocytosis and killing of E. coli in vitro, and suggest that this cytokine may impair effective antibacterial host defense in vivo.     

Monokines released during short-term Fc gamma receptor phagocytosis up-regulate polymorphonuclear leukocytes and monocyte-phagocytic function.

Freshly explanted monocytes phagocytosing IgG antibody-coated erythrocyte targets (EIgG) release a factor(s) that stimulates phagocytosis by neighboring monocytes and polymorphonuclear leukocytes (PMN). Culture supernatants obtained after 30-min incubation of adherent monocytes with EIgG, but not unopsonized sheep erythrocytes, markedly up-regulated the extent of PMN phagocytosis and enhanced the rate at which monocytes ingested EIgG. The presence of this factor(s) was first evident in phagocytic studies in which monocytes were prepared by a colloidal silica-based continuous gradient technique (Sepracell-Mn). After introduction of erythrocyte targets, there was a 20- to 30-min delay before initiation of phagocytosis that was not observed with monocytes prepared by the standard Percoll-gradient technique. Experiments suggest that, when compared with monocytes prepared by the Percoll-gradient method, Sepracell-Mn monocytes are closer to a base line state of activation with regard to the expression of Fc gamma RI and the ability to ingest EIgG. The mechanism of PMN upregulation by the monocyte factor(s) was explored. Monocyte supernatants did not induce an increase in the surface expression of PMN Fc gamma RI, II, or III. Neither anti-TNF, anti-IL-2, nor anti-GM-CSF had any significant effect on monocyte supernatant activity. Neutrophil activating protein-1 was not detected by ELISA. In contrast, anti-IL-1 completely blocked the effect of the supernatant on subsequent monocyte phagocytosis, and partially inhibited its effect on PMN phagocytosis. Furthermore, it was shown that RIL-1 as well as TNF markedly enhanced monocyte and PMN ingestion of EIgG. These results suggest that monocytes, after Fc gamma R-mediated phagocytosis, release monokines, including at least IL-1, which enhance the phagocytic function of neighboring PMN and monocytes to augment the host defense process.     

Neutrophil-activating peptide 2 and gro/melanoma growth-stimulatory activity interact with neutrophil-activating peptide 1/interleukin 8 receptors on human neutrophils

Neutrophil-activating peptide 1/interleukin 8 (NAP-1/IL-8), neutrophil-activating peptide 2 (NAP-2), and gro/melanoma growth-stimulatory activity (gro/MGSA) are potent inflammatory cytokines with homologous structure and similar neutrophil-activating properties. Receptors on human neutrophils that interact with these peptides were studied. Analysis of 125I-NAP-1/IL-8 binding at 0-4 degrees C revealed 64,500 +/- 14,000 receptors/cell with an apparent Kd of 0.18 +/- 0.07 nM (mean +/- S.D. of six independent experiments). Unlabeled NAP-1/IL-8, NAP-2, and gro/MGSA competed with 125I-NAP-1/IL-8 for binding to human neutrophils. Competition with increasing concentrations of unlabeled NAP-2 and gro/MGSA resolved two classes of NAP-1/IL-8 binding sites: about 70% of them bound NAP-2 and gro/MGSA with high affinity (Kd: 0.34 +/- 0.2 and 0.14 +/- 0.02), while 30% were of low affinity (Kd: 100 +/- 20 and 130 +/- 10 nM). Different binding sites, however, were not apparent upon competition with unlabeled NAP-1/IL-8, suggesting that both classes of receptors have similar affinities for NAP-1/IL-8. The existence of two receptors was also suggested by ligand cross-linking and cross-desensitization experiments. Two neutrophil membrane proteins with apparent Mr of 66,000-74,000 and 42,000-46,000 became cross-linked to 125I-NAP-1/IL-8, and the labeling was decreased when excess NAP-1/IL-8, NAP-2, or gro/MGSA was present. Stimulation of neutrophils with NAP-1/IL-8 resulted in desensitization toward a subsequent challenge with NAP-2 or gro/MGSA as shown by the rise in cytosolic free calcium. By contrast, following primary stimulation with NAP-2 or gro/MGSA, responses to NAP-1/IL-8 were only moderately attenuated, supporting the existence of NAP-1/IL-8 receptors which bind NAP-2 or gro/MGSA with low affinity. In conclusion, our results demonstrate that NAP-2 and gro/MGSA act upon human neutrophils by directly interacting with two classes of receptors for NAP-1/IL-8.     

NAP-1/IL-8 induces up-regulation of CR1 receptors in human neutrophil leukocytes

The effect of the neutrophil-activating peptide NAP-1/IL-8 on the expression of complement receptor type 1 (CR1) in human neutrophils was studied. NAP-1/IL-8 enhanced CR1 expression at concentrations between 10(-10) and 10(-8) M. The maximum increase with respect to unstimulated control cells was on average 2.3 fold. The effect was rapid: Half-maximum enhancement was obtained in 4 min and the plateau was reached in 15 min. The chemotactic peptide fMLP, tested for comparison, was effective between 10(-9) and 10(-7) M, showed a similar time course and a somewhat higher maximum effect (2.8 fold increase). The effect of NAP-1/IL-8 was prevented by pretreatment of the cells with B.pertussis toxin and desensitization was observed following restimulation. Stimulus combination experiments suggested that NAP-1/IL-8 mobilizes the same or a similar intracellular pool of CR1 receptors as fMLP or C5a.     

Unusual expression of IgG Fc receptors on peripheral granulocytes from patients with leukocyte adhesion deficiency (CD11/CD18 deficiency)

Leukocyte adhesion deficiency (LAD) is a hereditary disease characterized by defective expression of leukocyte adhesion glycoproteins; lymphocyte function-associated Ag-1 (CD11a/CD18), CR3 (CD11b/CD18) and p150,95 (CD11c/CD18). Granulocytes, monocytes, and lymphocytes of patients with LAD show profoundly defective in vivo and in vitro adherence-dependent immune functions. We investigated the expression of FcR for IgG on polymorphonuclear cells (PMN) and monocytes from patients with LAD, and their luminol- and lucigenin-enhanced chemiluminescence production in response to SRBC sensitized with murine (m) IgG2a and IgG2b. Unstimulated patient PMN showed an enhanced chemiluminescence in response to mIgG2a-SRBC and an increased phagocytosis of mIgG2a-SRBC. The up-regulated functions were inhibited by monomeric human IgG in a dose-dependent manner, which was attributed to an increase in expression of FcRI on patient PMN, as shown by flow cytometry using monoclonal antibody, 32.2, specific for human FcRI. In contrast, neither the expression of FcR on the monocytes of LAD patients nor their FcR-mediated functions were different from those of controls.     

Exudation induces clustering of CR1 receptors at the surface of human polymorphonuclear leukocytes

The complement receptor type 1 (CR1) surface distribution, density and immune adherence efficiency were determined in circulating PMN activated by fMLP, NAP-1/IL-8, TNF, GM-CSF and C5a, or exudate PMN harvested from skin-blisters. These observations were compared with those observed on resting peri-pheral blood PMN. PMN activators known to upregulate CR1 expression did not induce a significant increase in CR1 clustering, or immune adherence efficiency towards opsonized immune complexes. By contrast, increase in CR1 density at the surface of exudated PMN was accompanied by an increased clustering. This clustering was however insufficient to increase the binding efficiency for immune complexes. Eventually, CR1 expression of exudated neutrophil could not be increased further by stimulation with fMLP or PMA. These results indicated that clustering of CR1 on PMN may occur in vivo. Such reaction might determine the phagocytic potential of the cell for opsonized micro-organisms or debris. This clustering could not be attributed to one of the PMN activators tested.     

Stimulation of human neutrophil Fc receptor-mediated phagocytosis by a low molecular weight cytokine

Human neutrophil Fc receptor-mediated phagocytosis can be markedly enhanced by a low m.w. (less than 10,000) heat-labile cytokine(s) derived from specifically stimulated human mononuclear cells and from a human T cell line, MO(t). PMN incubated with supernatant from control mononuclear leukocyte (MNL) culture bound EIgG (percentage of rosettes = 73.7% +/- 7.1) but did not ingest the attached targets (phagocytic index, PI = 40.7 +/- 9.5) as efficiently as PMN incubated with supernatant from adherent MNL, which had ingested EIgG and were then cocultured with nonadherent MNL (PI = 264.3 +/- 46.3). Cytokine-containing supernatants were fractionated on YM-10 Centricon microconcentrators, and the effluent (YM-10E) was found to contain the phagocytosis-enhancing activity. Optimal Fc receptor-mediated ingestion by YM-10E-stimulated PMN required a critical level of target-bound IgG; stimulation was dose dependent and detectable after 5 min at 37 degrees C with a maximal response by 15 min. Monoclonal antibody 3G8 (anti-PMN Fc receptor) inhibited in a dose-dependent fashion both Fc receptor-mediated rosette formation and ingestion by nonstimulated and YM-10E-stimulated PMN. Solid-phase 3G8 Fab had the same effect. A previously undescribed monoclonal antibody, 1C2, exhibited a different pattern of inhibition. It had no effect on rosetting or ingestion of EIgG by nonstimulated PMN; however, it inhibited EIgG phagocytosis by YM-10E-stimulated PMN down to the level of nonstimulated ingestion without affecting rosette formation. Solid-phase 1C2 had the same effect. These data indicate that phagocytosis mediated by 3G8-positive Fc receptors may be enhanced by cytokine(s) stimulation in a manner requiring the molecule recognized by 1C2. Monoclonal antibodies to the alpha-chain of CR3 had only minimal effects on YM-10E-stimulated ingestion. Fluorescence flow cytometry of YM-10E-stimulated PMN, indirectly stained with 3G8 or 1C2, indicated that cytokine enhancement of EIgG ingestion occurred without an increase in either 3G8 or 1C2 binding sites. These data show that the low avidity Fc receptor, which binds immune complexes, may be functionally modulated at sites of inflammation where PMN and macrophages mediate clearance and destruction of immune complexes and opsonized particles.     

Differential requirements for cellular cytoskeleton in human macrophage complement receptor- and Fc receptor-mediated phagocytosis.

We investigated the requirement for cellular cytoskeleton in CR- and FcR-mediated phagocytosis by human monocyte-derived macrophages (M phi). Inhibition of actin microfilament (MF) assembly and stability by cytochalasins B and D completely inhibited M phi phagocytosis of sheep E coated with C3b (EC3b), iC3b (EC3bi), and IgG (EIgG) via CR1, CR3, and FcR, respectively. Ligand-binding to either CR or FcR was not effected by cytochalasins. Nocodazole (NOC), which prevents microtubule (MT) polymerization, and taxol, which causes random polymerization of MT inhibited M phi phagocytosis of EC3b(i) but not EIgG. However, the combination of taxol (5 x 10(-4) M) and NOC (2 x 10(-6) M) augmented M phi CR-mediated phagocytosis. In addition, agents known to increase intracellular cGMP augmented phagocytosis of EC3b(i). Conversely, agents that increase intracellular cAMP inhibited CR-mediated phagocytosis. These agents had no effect on FcR-mediated phagocytosis, and did not effect ligand-binding to CR or FcR. PMA markedly enhanced CR- but not FcR-mediated phagocytosis, and augmentation of CR-mediated phagocytosis by PMA was inhibited by both CD and NOC. In contrast, the synthetic diacylglycerol, 1-oleoyl-2-acetoyl-sn-3-glycerol augmented, and inhibitors of protein kinase C inhibited M phi phagocytosis via CR and FcR. These data indicate that for adherently cultured human M phi: 1) binding of ligand-coated E to CR or FcR does not require an intact cytoskeleton; 2) intact actin microfilament are required for phagocytosis via CR and FcR; 3) phagocytosis via CR1 and CR3 but not FcR is dependent on MT assembly; 4) PMA most likely augments CR-mediated phagocytosis through promotion of MT assembly; and 5) PKC activity is involved in the phagocytic signal generated by both CR and FcR.     

Receptor-operated activation of polymorphonuclear leukocytes: different effects of NAP-1/IL-8 and fMet-Leu-Phe or C5a.

We examined the production of PAF and LTB4 by PMN in response to NAP1/IL-8 alone, or after preincubation with GM-CSF, which has been shown to enhance PMN responsiveness and to prime PMN for production of those bioactive lipids. NAP-1/IL-8 does not induce the synthesis of PAF and LTB4 from endogenous phospholipid precursors, even after preincubation with GM-CSF. In addition and again in contrast to fMLP and C5a, NAP-1/IL-8 fails to induce an enhanced oxidative burst in GM-CSF primed PMN. Exogenously added PAF or LTB4 can mimic the priming effect of GM-CSF for an enhanced oxidative burst in response to all examined chemotactic peptides including NAP1/IL-8. Our data reveal a possible autocrine role of PAF and LTB4 in the enhanced responsiveness of GM-CSF primed PMN towards fMLP or C5a, but not NAP-1/IL-8.     

Endothelial-leukocyte adhesion molecule-1-dependent and leukocyte (CD11/CD18)-dependent mechanisms contribute to polymorphonuclear leukocyte adhesion to cytokine-activated human vascular endothelium

We have examined the contributions of endothelial-leukocyte adhesion molecule-1 (ELAM-1) and the complex of leukocyte surface adhesion molecules designated CD11/CD18 to the adhesion of human polymorphonuclear leukocytes (PMN) to cultured human endothelial cells (HEC), activated by rIL-1 beta for 4 or 24 h. Inhibition of PMN attachment to IL-1-activated HEC was measured in a quantitative in vitro monolayer adhesion assay, after treatment with mAb directed to ELAM-1 (mAb H18/17), and to CD11a (mAb L11), CD11b (mAb 44), CD11c (mAb L29), and CD18 (mAb 10F12), alone or in combination. Pretreatment of activated HEC with mAb H18/7 inhibited PMN adhesion by 47 +/- 8% whereas control mAb had no effect. CD11/CD18-directed mAb significantly blocked PMN adhesion to activated HEC (anti-CD11a, 40 +/- 3%; anti-CD11b, 34 +/- 4%; anti-CD18, 78+/- 6% inhibition). The combination of mAb H18/7 and each of the various anti-CD11/CD18 mAb resulted in greater inhibition of PMN adhesion than any Mab alone. After 24 h of rIL-1 beta treatment, when ELAM-1 was markedly decreased but elevated PMN adhesion was still observed, mAb H18/7 had no effect on PMN adhesion. At this time, CD11/CD18-dependent adhesive mechanisms predominated and a CD11c-dependent mechanism became apparent (anti-CD11a, 67 +/- 4% inhibition; anti-CD11b, 45 +/- 9%; anti-CD11c, 26 +/- 6%; anti-CD18, 97 +/- 1%). In summary, PMN adhesion to IL-1-activated HEC involves both CD11/CD18-dependent mechanisms and an ELAM-1-dependent mechanism, and the relative contribution of these varies at different times of IL-1-induced HEC activation. The additive blocking observed at 4 h with mAb H18/7 in combination with CD11/CD18-directed Mab implies that members of the CD11/CD18 complex do not function as an obligate ligand(s) for ELAM-1.     

Leukocyte adhesion molecule-1 (LAM-1, L-selectin) interacts with an inducible endothelial cell ligand to support leukocyte adhesion.

The human lymphocyte homing receptor, LAM-1, mediates the adhesion of lymphocytes to specialized high endothelial venules (HEV) of peripheral lymph nodes. We now report that LAM-1 is also a major mediator of leukocyte attachment to activated human endothelium. In a novel adhesion assay, LAM-1 was shown to mediate approximately 50% of the adhesion of both lymphocytes and neutrophils to TNF-activated human umbilical vein endothelial cells at 4 degrees C. The contribution of LAM-1 to leukocyte adhesion was only detectable when the assays were carried out under rotating (nonstatic) conditions, suggesting that LAM-1 is involved in the initial attachment of leukocytes to endothelium. In this assay at 37 degrees C, essentially all lymphocyte attachment to endothelium was mediated by LAM-1, VLA-4/VCAM-1, and the CD11/CD18 complex, whereas neutrophil attachment was mediated by LAM-1, endothelial-leukocyte adhesion molecule-1, and CD11/CD18. Thus, multiple receptors are necessary to promote optimal leukocyte adhesion to endothelium. LAM-1 also appeared to be involved in optimal neutrophil transendothelial migration using a videomicroscopic in vitro transmigration model system. LAM-1-dependent leukocyte adhesion required the induction and surface expression of a neuraminidase-sensitive molecule that was expressed for at least 24 h on activated endothelium. Expression of the LAM-1 ligand by endothelium was optimally induced by LPS and the proinflammatory cytokines TNF-alpha and IL-1 beta, whereas IFN-gamma and IL-4 induced lower levels of expression. The LAM-1 ligand on HEV and cytokine treated endothelium may be similar carbohydrate-containing molecules, because phosphomannan monoester core complex from yeast Hansenula hostii cell wall blocked binding of lymphocytes to both cell types, and identical epitopes on LAM-1-mediated lymphocyte attachment to HEV and activated endothelium. Thus, LAM-1 and its inducible endothelial ligand constitute a new pair of adhesion molecules that may regulate initial leukocyte/endothelial interactions at sites of inflammation.     

Granulocyte-macrophage colony-stimulating factor increases synthesis and expression of CR1 and CR3 by human peripheral blood neutrophils

Polymorphonuclear leukocytes (PMN) constitutively synthesize various plasma membrane proteins including CR1(3) (CD35), CR3 (or Mac-1) alpha-chain (CD11b) and MHC class I. PMN are also able to up-regulate rapidly the expression of CR1 and CR3 to the plasma membrane in response to agonists such as FMLP. To determine whether constitutive PMN translation was static or up-regulatable, PMN were cultured in the presence or absence of the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) for 8 h. CR1, CR3 and class I proteins immunoprecipitated from lysates of 35S-methionine pulse-labeled PMN were resolved by SDS-PAGE, fluorographed and quantified by densitometry. GM-CSF-treated PMN synthesized 4.5-fold more class I protein, 3.7-fold more CR1, 2.4-fold more CD11b and 3.4-fold more CR3 beta-chain (CD18), compared with untreated control cells. Actinomycin D treatment of replicate samples of PMN decreased the amount of these proteins synthesized by each group of PMN from 30 to 90%, implying that continued translation was required for the increases in protein synthesis. Nascent CR and class I proteins were inserted into the plasma membrane of PMN, thereby supplementing the molecules already expressed on the cell surface. In addition to these longer term effects of GM-CSF, we observed its acute up-regulatory effects on PMN. GM-CSF induced a five- to 12-fold increase in the expression of CR1 and CR3 on the PMN cell surface within 30 min. These increases were both dose- and time-dependent with maximum up-regulation occurring at 25 pM and at 30 min. In contrast to the long term biosynthetic events, this rapid up-regulation was not dependent on protein synthesis but was due instead to mobilization of CR from intracellular compartments similar to those up-regulated by FMLP. These results demonstrate that PMN can respond to microenvironmental stimuli such as GM-CSF both by rapidly up-regulating and increasing translation and expression of functionally important plasma membrane proteins.     

Impairment of HIV polymorphonuclear leukocyte transmigration across T84 cell monolayers: an alternative mechanisms for increased intestinal bacterial infections in AIDS?

Our objective was to study the influence of HIV infection of polymorphonuclear leukocytes (PMN) on transepithelial migration. To date, reports of functional PMN chemotaxis in AIDS are contradictory. This is the first attempt to assess this function via an in vitro model allowing transmigration of neutrophils through an intestinal epithelial barrier. PMN were isolated from 45 HIV-infected patients and 45 healthy volunteers. PMN transmigration across T84 epithelial cells was initiated by applying either various concentrations of formyl-met-leu-phe peptide (f-MLP) or interleukin-8 and assayed by quantification of myeloperoxidase activity. CD11b, CD18, and CD47 expression on PMN was compared before and after transepithelial migration by flow cytometry analysis. CD11b expression was studied by electron microscopy. Apoptosis of transmigrated HIV PMN and control PMN was investigated by morphology and DNA fragmentation characterization. Compared to control PMN, HIV PMN exhibited a decrease in transepithelial migration that directly correlated with CD4+ counts. Basal and transepithelial migration-mediated expression of CD11b, CD18, and CD47 were unmodified in HIV PMN compared to control PMN. Electron microscopy labeling confirmed no difference in CD11b expression on HIV and control PMN. The index of apoptosis in transmigrated HIV PMN and control PMN was identical. These data provide evidence of a defect in the f-MLP-induced chemotaxis of PMN from HIV-infected patients across an intestinal epithelial barrier. This defective migration is not due to a quantitative modification of CD11b, CD18 and CD47 on HIV PMN suggesting a more subtle alteration. The impairment in the transmigration function may contribute in vivo to an increased susceptibility to intestinal bacterial infection in HIV-infected patients.     

Two leukocyte receptors (CD11a/CD18 and CD11b/CD18) mediate transient adhesion to endothelium by binding to different ligands

Upon stimulation with C5a, TNF, or phorbol dibutyrate (PDB), polymorphonuclear leukocytes (PMN) exhibit first an increase then a decrease in adhesion to unstimulated endothelial cells (EC). Essentially all of this adhesion is mediated by the CD18 family of leukocyte integrins on PMN. To determine the individual roles of CD11a/CD18 (LFA-1), CD11b/CD18 (CR3, Mac-1) and CD11c/CD18 (p150,95) in adhesion of PDB-stimulated PMN to unstimulated EC, mAb against the CD11 chains were used. mAb against CD11a or CD11b each blocked adhesion of PMN to EC by approximately 50%, but mAb against CD11c had no effect. Inasmuch as a combination of anti-CD11a and CD11b mAb completely blocked adhesion, it appears that CD11a/CD18 and CD11b/CD18 make approximately equal contributions to binding, and CD11c does not participate. Anti-CD11a or CD11b each blocked adhesion by about 50% throughout the transient time course of PDB-stimulated adhesion, indicating that the capacity of each of these receptors to bind EC is transiently activated by PDB. We next examined the role of ICAM-1 on EC as a ligand for CD18. Two anti-ICAM-1 mAb (LB-2 and 84H10) each inhibited PMN adhesion in a dose-dependent fashion, reaching a maximal inhibition of approximately 50%. Anti-ICAM-1 mAb blocked the CD11a/CD18-dependent portion of adhesion because concomitant use of anti-CD11a and anti-ICAM-1 did not cause additive inhibition. In contrast, anti-CD11b plus anti-ICAM-1 resulted in complete blockade of adhesion. This result suggests that CD11a/CD18 recognizes ICAM-1 on EC, but CD11b/CD18 recognizes a different ligand(s). To determine if CD11b CD18 has the ability to recognize ICAM-1, human macrophages were plated on culture surfaces coated with purified ICAM-1. Interaction of CD11a/CD18 with the surface-bound ICAM-1 resulted in selective down-modulation of CD11a/CD18 from the apical portion of the macrophages. In contrast, ICAM-1-coated surfaces did not down-modulate CD11b/CD18. The data suggest that CD11b/CD18 does not recognize ICAM-1, and that this receptor functions in adhesion of PMN to EC by recognizing novel ligand(s) on EC.     

The role of beta2 integrins and lipopolysaccharide-binding protein in the phagocytosis of dead Neisseria meningitidis

Phagocytosis of microbial pathogens is essential for the host immune response to infection. Our previous work has shown that lipooligosaccharide (LOS) expression on the surface of Neisseria meningitidis (Nm) is essential for phagocytosis, but the receptor involved remained unclear. In this study, we show that human CR3 (CD11b/CD18) and CR4 (CD11c/CD18) are phagocytic receptors for Nm as illustrated by the capacity of CR3- and CR4-transfected Chinese hamster ovary (CHO) cells to facilitate Nm uptake. A CR3-signalling mutant failed to internalize Nm, showing that the ability of CR3 to signal is essential for phagocytosis. Internalization of Nm by CR3-transfected CHO cells could be inhibited by the presence of CR3-specific antibodies. Furthermore, dendritic cells from leukocyte adhesion deficiency-1 patients, who have diminished expression of β2 integrins, showed markedly reduced phagocytosis of Nm. The CR3-mediated phagocytosis required the presence of lipopolysaccharide-binding protein (LBP). Furthermore, the expression of LOS by Nm was essential for LBP binding and phagocytosis via CR3. These results reveal a critical role of CR3 and LBP in the phagocytosis of Nm and provide important insights into the initial interaction meningococci have with the immune system.     

The effects of IL-1, IL-2, and tumor necrosis factor on polymorphonuclear leukocyte Fc gamma receptor-mediated phagocytosis. IL-2 down-regulates the effect of tumor necrosis factor

It has been reported that the Fc gamma R-mediated phagocytic activity of polymorphonuclear leukocytes (PMN) from patients with acute bacterial infections is markedly enhanced when compared with healthy controls. Inasmuch as several potent cytokines are known to be involved in inflammatory and infectious processes, we studied the effects of three such cytokines (IL-1 beta, IL-2, and TNF-alpha) on normal PMN Fc gamma R-mediated phagocytosis. IL-1 beta and TNF alpha both caused a significant increase in the ingestion of EIgG by adherent PMN. In combination, IL-1 beta and TNF-alpha had an additive effect, even when each was used at its optimal concentration. In contrast to the enhancing effects mediated by IL-1 beta and TNF-alpha, IL-2 alone had no significant effect on PMN phagocytosis. Notably, however, IL-2 at a concentration of 10(4) U/ml partially inhibited TNF-alpha-mediated enhancement of phagocytosis by decreasing TNF binding to the PMN cell surface. This inhibitory effect of IL-2 on TNF was reversed by anti-IL-2 antibody and mAb directed against the low affinity IL-2R (anti-Tac), whereas mAb directed against the intermediate affinity receptor (mik-beta 1) had no such effect. These findings may have important physiologic implications, because patients receiving IL-2 therapy have been shown to have increased susceptibility to infection.     

The CD11/CD18 (beta2) integrins modulate neutrophil caspase activation and survival following TNF-alpha or endotoxin induced transendothelial migration

Neutrophils (PMN) are short-lived cells but their survival is often prolonged in inflammation. The beta2 (CD11/CD18) integrins are involved in PMN migration into inflammation but their role in PMN survival is not well understood. We investigated the role of beta2 integrins in PMN caspase activation, a key enzyme cascade in apoptosis. After 20 h, caspase activation (Western blotting) was markedly decreased in PMN cultured on fibrinogen, a ligand for Mac-1 (CD11b/CD18), but not on fibronectin or albumin. In the presence of TNF-alpha or endotoxin (LPS), blockade of CD18 (beta2 chain) with mAb markedly increased caspase activation in PMN on fibrinogen. PMN which migrated through endothelium in vitro in response to TNF-alpha, LPS, IL-1alpha, IL-8 or C5a contained 58% fewer active caspase positive PMN after 20 h than non-migrated PMN remaining on the endothelium. When beta2 (CD18) integrin or lymphocyte function antigen (LFA)-1 (CD11a) plus Mac1 (CD11b) were blocked by mAb (intact or Fab'), the proportion of migrated PMN (but not of non-migrated PMN) with active caspases was significantly increased (2-4-fold) and this was associated with accelerated PMN apoptosis and death. Thus, engagement of ligands on extracellular matrix and endothelium by the beta2 integrins Mac-1 and LFA-1 plays a role in delaying apoptosis in PMN recruited in response to LPS and TNF-alpha. Inhibition of beta2 integrin function may not only inhibit PMN infiltration, but also accelerate PMN clearance from inflamed tissue.     

Melanoma cell extravasation under flow conditions is modulated by leukocytes and endogenously produced interleukin 8

Attachment of tumor cells to the endothelium (EC) under flow conditions is critical for the migration of tumor cells out of the vascular system to establish metastases. Innate immune system processes can potentially promote tumor progression through inflammation dependant mechanisms. White blood cells, neutrophils (PMN) in particular, are being studied to better understand how the host immune system affects cancer cell adhesion and subsequent migration and metastasis. Melanoma cell interaction with the EC is distinct from PMN-EC adhesion in the circulation. We found PMN increased melanoma cell extravasation, which involved initial PMN tethering on the EC, subsequent PMN capture of melanoma cells and maintaining close proximity to the EC. LFA-1 (CD11a/CD18 integrin) influenced the capture phase of PMN binding to both melanoma cells and the endothelium, while Mac-1 (CD11b/CD18 integrin) affected prolonged PMN-melanoma aggregation. Blocking E-selectin or ICAM-1 (intercellular adhesion molecule) on the endothelium or ICAM-1 on the melanoma surface reduced PMN-facilitated melanoma extravasation. Results indicated a novel finding that PMN-facilitated melanoma cell arrest on the EC could be modulated by endogenously produced interleukin-8 (IL-8). Functional blocking of the IL-8 receptors (CXCR1 and CXCR2) on PMN, or neutralizing soluble IL-8 in cell suspensions, significantly decreased the level of Mac-1 up-regulation on PMN while communicating with melanoma cells and reduced melanoma extravasation. These results provide new evidence for the complex role of hemodynamic forces, secreted chemokines, and PMN-melanoma adhesion in the recruitment of metastatic cancer cells to the endothelium in the microcirculation, which are significant in fostering new approaches to cancer treatment through anti-inflammatory therapeutics.