Opsonin-independent ligation of Fc gamma receptors. The 3G8-bearing receptors on neutrophils mediate the phagocytosis of concanavalin A-treated erythrocytes and nonopsonized Escherichia coli

Potential Association of Maker Expression of Low-Density Neutrophils and Their Phenotypes in Patients with Periodontitis: Control Study

Background

Neutrophils play an important role in maintaining periodontal status in conditions of healthy homeostasis. They achieve their surveillance function by continuously migrating to the gingival sulcus and eradicating periodontal pathogens. In addition, neutrophils are considered an integral element in the pathogenesis of periodontal diseases. Among several neutrophil subsets, low-density neutrophils (LDN) have recently received attention and are linked with cancer, immunological, inflammatory, and infectious diseases. However, the presence, phenotypes, and potential role of LDN in the pathogenesis of periodontitis have not yet been investigated.

Objectives

To investigate the presence, subsets (normal, band, suppressive, and active), and phenotypes via marker expression surface protein known as the cluster of differentiation (CD) (CD16b, CD14, CD15, and CD62L) of LDN in patients with periodontitis.

Materials and Methods

The observational case-control study was conducted to estimate the potential role of LDNs in periodontitis. Venous blood and periodontal indices were obtained from 40 healthy control individuals and 60 periodontitis patients. Subsequently, CD16b, CD62L, CD14, and CD15 expression on the surface of LDN was examined by multicolor flow cytometry, and their subsets were classified as "normal" (CD16brightCD62Lbright), "bands" (CD16dimCD62Lbright), "suppressive" (CD16brightCD62Ldim), and "active" (CD16brightCD62Lnegative).

Results

There was a significant difference in the expression of LDN markers for active and suppressive phenotypes, respectively, favoring periodontitis over the control group. In contrast, there were significantly higher levels of CD16b, CD62L, and CD15 ("normal") in the control group when compared with the periodontitis group.

Conclusion

LDN was associated with periodontitis as it was significantly increased in the periodontitis group in comparison with the control group and was positively correlated with all periodontal parameters. Cells from both groups of patients (periodontitis and control) expressed a normal mature phenotype (CD16b + High, CD62L + High, CD15+, and CD14-). Regarding subsets, the normal LDN (CD16brightCD62Lbright) was the most predominant phenotype in both periodontitis and control groups. However, the active subset increased in periodontitis compared to normal, indicating their destructive role in periodontitis.

The impact of nanoparticle protein corona on cytotoxicity, immunotoxicity and target drug delivery

In a perfect sequence of events, nanoparticles (NPs) are injected into the bloodstream where they circulate until they reach the target tissue. The ligand on the NP surface recognizes its specific receptor expressed on the target tissue and the drug is released in a controlled manner. However, once injected in a physiological environment, NPs interact with biological components and are surrounded by a protein corona (PC). This can trigger an immune response and affect NP toxicity and targeting capabilities. In this review, we provide a survey of recent findings on the NP-PC interactions and discuss how the PC can be used to modulate both cytotoxicity and the immune response as well as to improve the efficacy of targeted delivery of nanocarriers.

Effect of the protein corona on nanoparticles for modulating cytotoxicity and immunotoxicity

Although the cytotoxicity of nanoparticles (NPs) is greatly influenced by their interactions with blood proteins, toxic effects resulting from blood interactions are often ignored in the development and use of nanostructured biomaterials for in vivo applications. Protein coronas created during the initial reaction with NPs can determine the subsequent immunological cascade, and protein coronas formed on NPs can either stimulate or mitigate the immune response. Along these lines, the understanding of NP-protein corona formation in terms of physiochemical surface properties of the NPs and NP interactions with the immune system components in blood is an essential step for evaluating NP toxicity for in vivo therapeutics. This article reviews the most recent developments in NP-based protein coronas through the modification of NP surface properties and discusses the associated immune responses.

CD16 inhibition increases host survival in a murine model of severe sepsis

BACKGROUND

To investigate the therapeutic effect of monoclonal antibody (mAb)-induced CD16 (FcγRIII) inhibition in a murine model of high-grade (severe) sepsis.

MATERIALS AND METHODS

In a prospective controlled animal study, 2 μg of CD16/32 (FcγRIII/FcγRII) or the same volume of normal saline was administered intraperitoneally to BALB/c FcγRII(-/-) mice at the time of cecal ligation and puncture (CLP) in a murine model of high-grade sepsis. Subcutaneous administration of CD16/32 (0.5 μg/24 h) or normal saline continued for 7 d. Survival was evaluated, and the underlying therapeutic mechanism of mAb-induced CD16 inhibition was investigated.

RESULTS

CD16 expression was significantly increased on peripheral blood CD14(+) monocytes from mice with high-grade sepsis compared with non-septic control mice (1579.40 ± 217.75 versus 461.10 ± 36.13; P < 0.05). CD16/32 mAb treatment increased the survival of mice with high-grade sepsis (P < 0.05) and significantly decreased their elevated levels of serum tumor necrosis factor α (36.70 ± 9.97 versus 52.60 ± 10.69; P < 0.05) and interleukin 1β (1149.40 ± 244.09 versus 2605.60 ± 353.74; P < 0.05) at 6 and 24 h after CLP, respectively. Moreover, CD16/32 mAb-treated mice with high-grade sepsis had fewer bacteria in their blood and peritoneal lavage than mice just treated with normal saline at 24 h after CLP (P < 0.05).

CONCLUSIONS

CD16/32 mAb-induced CD16 inhibition increased the survival of mice with high-grade sepsis, which may have been because of the concomitant suppression of tumor necrosis factor α and interleukin 1β as well as the enhancement of monocyte phagocytosis. Thus, targeted inhibition of CD16 can potentially improve the outcome of selected patients with severe sepsis.

Inhibitory ITAMs: a matter of life and death

The balance between activating and inhibitory signals is essential to control immune responses to microorganisms. Innate and adaptive immune responses are regulated by receptors that signal through either an immunoreceptor tyrosine-based activation motif (ITAM) or an immunoreceptor tyrosine-based inhibitory motif (ITIM). When clustered, these motifs are, respectively, responsible for activating and inhibitory signals. Recently, the concept of inhibitory ITAM (ITAM(i)) has emerged as a new means to negatively control the immune response. In this Opinion, we will discuss the ability of Escherichia coli to evade the immune system by eliciting ITAM(i) function through FcgammaRIII (CD16) on phagocytes leading to uncontrolled systemic infection and sepsis. Elucidating such mechanisms will open opportunities for specific therapeutic manipulation of ITAM(i)-based signaling pathways.

CD16b associates with high-density, detergent-resistant membranes in human neutrophils

CD16b is unique in that it is the only Fc receptor linked to the plasma membrane by a GPI (glycosylphosphatidylinositol) anchor. GPI-anchored proteins often preferentially localize to DRMs (detergent-resistant membranes) that are rich in sphingolipids and cholesterol and play an important role in signal transduction. Even though the responses to CD16b engagement have been intensively investigated, the importance of DRM integrity for CD16b signalling has not been characterized in human neutrophils. We provide direct evidence that CD16b constitutively partitions with both low- and high-density DRMs. Moreover, upon CD16b engagement, a significant increase in the amount of the receptor is observed in high-density DRMs. Similarly to CD16b, CD11b also resides in low- and high-density DRMs. In contrast with CD16b, the partitioning of CD11b in DRMs does not change in response to CD16b engagement. We also provide evidence for the implication of Syk in CD16b signalling and its partitioning to DRMs in resting and activated PMNs (polymorphonuclear neutrophils). Additionally, DRM-disrupting agents, such as nystatin and methyl-beta-cyclodextrin, alter cellular responses to CD16b receptor ligation. Notably, a significant increase in the mobilization of intracellular Ca2+ and in tyrosine phosphorylation of intracellular substrates after CD16b engagement is observed. Altogether, the results of this study provide evidence that high-density DRMs play a role in CD16b signalling in human neutrophils.

Native and modified C-reactive protein bind different receptors on human neutrophils

Native C-reactive protein (CRP) is a planar pentamer of identical subunits expressed at high serum levels during the acute phase of inflammation. At inflammatory sites, an isomeric form termed modified CRP (mCRP) is expressed and reveals neoantigenic epitopes associated with modified monomeric CRP subunits. mCRP cannot assume the native pentameric conformation but rather forms a nonpentameric aggregate of monomers. While native CRP inhibits neutrophil movement in vitro and in vivo, the effect of mCRP on neutrophil movement has not been reported. To model the biological function and biochemical interaction of mCRP on neutrophils, in vitro chemotaxis and binding experiments were performed using mCRP. Reported here, mCRP effectively inhibited fMLP-induced chemotaxis similar to native CRP. Additionally, mCRP increased binding of labeled native CRP to neutrophils. This increased binding occurred by direct protein-protein interaction of the C-terminus thereby implicating the CRP(199-206) sequence. Binding of mCRP to neutrophils was blocked by anti-CD16 monoclonal antibody whereas native CRP was not. These results suggest that modified forms of CRP inhibit chemotaxis, a function similar to native CRP, but that mCRP and native molecule bind different receptors on human neutrophils.

Lectinocytochemical detection of apoptotic murine leukemia L1210 cells

BACKGROUND

Various cytomorphologic and biochemical markers of apoptosis are found in different compartments (plasma membrane, cytoplasm, nucleus, and mitochondria) of target cells. Although the plasma membrane is an easily accessible cellular compartment, relatively little is known about its changes during apoptosis. We investigated whether specific changes in the expression of plasma membrane glycoproteins take place during apoptosis and whether these changes could be used for a quantitative estimation of apoptosis.

METHODS

Lectin cytochemical study of normal and apoptotic murine leukemia cells of the L1210 line was done. Horseradish peroxidase-labeled Laburnum anagyroides bark agglutinin, Phaseolus vulgaris agglutinin, Pisum sativum lectin (PSL), Ricinus communis agglutinin (RCA-120; 120 kDa), Solanum tuberosum agglutinin, Triticum vulgaris lectin (wheat germ agglutinin), Viscum album agglutinin, Canavalia ensiformis lectin (concanavalin A), and Helix pomatia lectin were used to compare specific glycoprotein expressions in normal and apoptotic murine leukemia cells of the L1210 line sensitive (L1210) and resistant (L1210R) to apoptosis induction by cisplatin.

RESULTS

The data demonstrated significantly increased binding of alpha-D-mannose-specific PSL lectin (P<0.01) and beta-D-galactose-specific RCA-120 lectin (P<0.001) by the apoptotic cells of the L1210 and L1210R lines in comparison with the intact cells. That binding was shown to be specific because it was blocked by the corresponding inhibitory sugars.

CONCLUSIONS

Lectins specific to alpha-D-mannose (PSL) and beta-D-galactose (RCA-120) can be used to distinguish between native and apoptotic murine leukemia L1210 cells and to quantitatively estimate apoptosis in a population of these cells.

Comparison of IgG preparations by a turbidimetric assay of opsonizing capacity

A convenient turbidimetric phagocytosis assay was applied for the functional comparison of various intravenous IgG preparations. Staphylococcus aureus (Oxford) was opsonized by the immunoglobulin samples in the presence of an IgG deficient serum as a source of complement. The opsonized bacteria were subjected to phagocytosis by neutrophil granulocytes isolated from healthy adults. The time course of phagocytosis was monitored by the decrease of light absorbance at 400 nm. Changes in light absorbance during a 15 min period of opsonophagocytosis (delta E(400)) were expressed as a percentage of delta E(400) obtained by a reference IgG preparation. The opsonizing effect of five commercially available i.v. IgG preparations was compared. Three different preparations containing whole, non-modified IgG molecules had a comparable opsonizing effect while a further one prepared by propiolacton modification displayed a reduced activity (52%) of the reference preparation, taken as 100%. A preparation consisting of IgG molecules without an Fc-region proved to be practically ineffective (8.7%).

Adherence of Salmonella typhimurium to murine peritoneal macrophages is mediated by lipopolysaccharide and complement receptors

Adherence of Salmonella typhimurium to mouse peritoneal macrophages (Mø) was monitored using a direct microscopic assay and flow cytometry. Competitive binding studies using wild-type lipopolysaccharide and derivatives confirmed a role for this moiety in bacterial adherence. Mø pretreated with 2-deoxy-D-glucose exhibited lower binding activity than did untreated controls, suggesting involvement of either Fc or complement receptors. Pre-exposing Mø to Fc fragments, however, failed to reduce bacterial binding, thus eliminating a role for Fc receptors in this process. Mø pretreated with neutrophil elastase exhibited a diminished ability to bind S. typhimurium, suggesting involvement of complement receptor 1. Monoclonal antibodies M1/70 and M18/2, specific for epitopes on the alpha and beta chains, respectively, of complement receptor 3, also blocked this adherence. In each case we were unable to eliminate completely bacterial adhesion to Mø. Monoclonal antibodies to two additional Mø receptors, Mac-2 and Mac-3, did not block bacterial attachment. These data indicate that multiple mechanisms are involved in the initial adhesion of S. typhimurium to mouse Mø.

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.

Polymorphonuclear leukocyte opsonic receptor expression after hypoxia/reoxygenation

We investigated the effects of hypoxia/reoxygenation (H/R) and subsequent stimulation of polymorphonuclear leukocytes (PMNs) with either formyl-methionyl-leucyl-phenylalanine (FMLP) or phorbol myristate acetate (PMA) on CD32, CD16, CD35, and CD11b/CD18 expression and on degranulation and superoxide anion production. H/R primed both adherent and fluid-phase PMNs for subsequent up-regulation of CD32 and CD16 (Fcgamma receptors) when stimulated with FMLP and primed both Fcgamma and complement (CD35, CD11b/CD18) receptors when stimulated with PMA. Kinetics assays demonstrated maximal up-regulation of CD32 and CD16 induced by H/R plus FMLP after 30 minutes of reoxygenation, whereas maximal receptor stimulation by H/R plus PMA occurred within 15 minutes of reoxygenation. Neither actinomycin D nor cycloheximide abrogated the effect of H/R with subsequent stimulation of PMNs on receptor expression; however, 10(-5) to 10(-8) mol/L concentrations of either taxol or phalloidin completely abrogated the effect of H/R plus FMLP or PMA on opsonic receptor expression. The effect of H/R plus FMLP on CD32 and CD16 expression was blocked by pertussis toxin, whereas staurosporine, H-7, H-9, and genistein had no effect. Conversely, the effect of H/R plus PMA on CD32, CD16, CD35, and CD11b/CD18 expression was blocked by staurosporine and H-7 but not by H-9, pertussis toxin, or genistein. The up-regulation of CD32, CD16, CD35, and CD11b/CD18 induced by H/R plus FMLP or PMA in the presence or absence of matrix proteins resulted in the increased rosetting of E-anti-CD32, E anti-CD16, E-Con A, EC3b, and EC3bi, respectively. Reduced nicotinamide adenine dinucleotide phosphate oxidase inhibition with diphenyleneiodonium blocked the effect of H/R on receptor expression, degranulation, and superoxide anion production. These results demonstrate that H/R primes PMNs for subsequent receptor up-regulation by divergent intracellular signal transduction pathways and that the receptors induced to the cell surface are biologically active.

Ligand binding and phagocytosis by CD16 (Fc gamma receptor III) isoforms. Phagocytic signaling by associated zeta and gamma subunits in Chinese hamster ovary cells

CD16, the low affinity Fc gamma receptor III for IgG (Fc gamma RIII), exists as a polypeptide-anchored form (Fc gamma RIIIA or CD16A) in human natural killer cells and macrophages and as a glycosylphosphatidylinositol-anchored form (Fc gamma RIIIB or CD16B) in neutrophils. CD16A requires association of the gamma subunit of Fc epsilon RI or the zeta subunit of the TCR-CD3 complex for cell surface expression. The CD16B is polymorphic and the two alleles are termed NA1 and NA2. In this study, CD16A and the two alleles of CD16B have been expressed in Chinese hamster ovary (CHO) cells and their ligand binding and phagocytic properties analyzed. The two allelic forms of CD16B showed a similar affinity toward human IgG1. However, the NA1 allele showed approximately 2-fold higher affinity for the IgG3 than the NA2 allele. Although all three forms of CD16 efficiently bound rabbit IgG-coated erythrocytes (EA), only CD16A coexpressed with the gamma subunit phagocytosed EA. The phagocytosis mediated by CD16A expressed on CHO cells was independent of divalent cations but dependent on intact microfilaments. CHO cells expressing CD16A-gamma and CD16A-zeta chimeras also phagocytosed EA. The phagocytosis was specifically inhibited by tyrphostin-23, a tyrosine kinase inhibitor. In summary, our results demonstrate that glycosylphosphatidylinositol-anchored CD16B alleles differ from CD16A in their ability to mediate phagocytosis. Furthermore, since studies with other Fc gamma Rs have shown that CHO cells lack the phagocytic pathway mediated by the cytoplasmic domain of Fc gamma Rs, the phagocytosis of EA by CHO cells stably transfected with CD16A and CD16A-subunit chimera provides an ideal system to dissect the phagocytic signaling pathways mediated by these Fc gamma R-associated subunits.

Putative glycoprotein and glycolipid polymorphonuclear leukocyte receptors for the Actinomyces naeslundii WVU45 fimbrial lectin

Recognition of receptors on sialidase-treated polymorphonuclear leukocytes (PMNs) by the Gal/GalNAc lectin associated with the type 2 fimbriae of certain strains of actinomyces results in activation of the PMNs, phagocytosis, and destruction of the bacteria. In the present study, plant lectins were utilized as probes to identify putative PMN receptors for the actinomyces lectin. The Gal-reactive lectin from Ricinus communis (RCAI), the Gal/GalNAc-reactive lectins from R. communis (RCAII) and Bauhinia purpurea (BPA), as well as the Gal beta 1-3GalNAc-specific lectins from Arachis hypogaea (PNA) and Agaricus bisporus (ABA) inhibited killing of Actinomyces naeslundii WVU45 by sialidase-treated PMNs. These five lectins detected a 130-kDa surface-labeled glycoprotein on nitrocellulose transfers of PMN extracts separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This glycoprotein was revealed only after treatment of the transfers with sialidase, a condition analogous to the sialidase dependence of the lectin-mediated biological responses of the PMNs to the actinomyces. The mannose-reactive lectin concanavalin A did not inhibit killing of the actinomyces and failed to detect the 130-kDa glycoprotein but did block PMN-dependent killing of Escherichia coli B, a bacterium that possesses mannose-sensitive fimbriae. Therefore, the PMN glycoprotein receptor for A. naeslundii is clearly distinct from those recognized by E. coli. Two major putative glycolipid receptors were also identified by actinomyces and RCAI overlays on sialidase-treated thin-layer chromatograms of PMN gangliosides. Thus, both a 130-kDa glycoprotein and certain gangliosides are implicated in the attachment of the actinomyces to PMNs.

Heterotypic Fc gamma R clusters evoke a synergistic Ca2+ response in human neutrophils

Both Fc gamma receptors on human neutrophils (Fc gamma RIIa and Fc gamma RIIIb) are capable of initiating signal transduction after multivalent cross-linking. However, immune complexes most likely activate neutrophils by a combined homotypic and heterotypic cross-linking of Fc gamma Rs. We have investigated the effect of homotypic and heterotypic Fc gamma R cluster formation on changes in the intracellular free Ca2+ concentration. Combined heterotypic and homotypic cluster formation resulted in a Ca2+ response that was strongly enhanced as compared to the sum of both individual Fc gamma R responses. This synergistic response was caused by the formation of heterotypic clusters of Fc gamma Rs and not by the simultaneous formation of homotypic clusters. This conclusion was supported by experiments with a bispecific antibody binding to both Fc gamma RIIa and Fc gamma RIIIb. The heterotypic Fc gamma R cross-linking results in efficient activation of Ca2+ influx, probably caused by a more pronounced depletion of intracellular Ca2+ stores. Stimulation with immune complexes also induced Ca2+ influx in normal neutrophils, but not in Fc gamma RIIIb-deficient neutrophils. The synergism between both Fc gamma Rs was also apparent in other responses of neutrophils, such as the activation of the respiratory burst. This study shows that the two different Fc gamma Rs on neutrophils complement each other in mediating an important cellular response.

Human immunoglobulin G (IgG) Fc receptor IIA (CD32) polymorphism and IgG2-mediated bacterial phagocytosis by neutrophils

Human immunoglobulin G (IgG) Fc receptor IIa (Fc gamma RIIa; CD32) is expressed on phagocytes, triggers phagocytosis, and represents the sole Fc receptor for IgG (Fc gamma R) capable of interaction with IgG2, the main IgG subclass induced in response to bacterial capsular polysaccharides. The two genetically determined structurally different allotypes of human Fc gamma RIIa, the products of the Fc gamma RIIa-R131 and IIa-H131 alleles, have functionally different reactivities with human IgG2. In humans, the Fc gamma RIIa-H131 allotype is known to interact efficiently with complexed human IgG2, whereas the IIa-H131 allotype does so only poorly. This polymorphism may therefore have implications for IgG2-mediated phagocytosis of encapsulated bacteria and susceptibility to bacterial infections. Phagocytosis of IgG2-opsonized bacteria by homozygous Fc gamma RIIa-R/R131, heterozygous IIa-H/R131, and homozygous IIa-H/H131 polymorphonuclear cells (PMN) was compared. A higher phagocytic capacity of IgG2-opsonized group B type III streptococci by PMN of homozygous H/H131 individuals compared with PMN from homozygous R/R131 individuals was observed (P = 0.001), while heterozygous IIa-H/R131 PMN showed intermediate phagocytosis. In this model system, IgG2-mediated phagocytosis was independent of the Fc gamma RIIIb-NA1/NA2 allelic polymorphism.

Neutrophil Fc receptor participation in phagocytosis of type III group B streptococci

Human peripheral blood neutrophils bear receptors for immunoglobulin G, FcRII, and FcRIII that differ structurally and functionally. We investigated the role of FcRII and FcRIII in the phagocytosis of group B streptococci (GBS) by measuring neutrophil uptake of radiolabeled type III GBS. The mean uptake of GBS opsonized with human serum containing complement and specific antibody was 76%, but when this serum was heated, the mean uptake was only 22%. A monoclonal antibody to FcRIII, Leu-11b, inhibited in a dose-dependent manner uptake of GBS opsonized with heated or intact serum to maxima of 40 and 30%, respectively. Conversely, a monoclonal antibody to FcRII, IV.3, inhibited by 77% the uptake of GBS opsonized with heated serum but had no effect when GBS was opsonized with intact serum. Leu-11b and IV.3 had an additive inhibitory effect with heated but not with intact serum. Neither monoclonal antibody inhibited the uptake of GBS opsonized with hypogammaglobulinemic serum. Therefore, FcRII is the primary mediator of the phagocytosis of GBS opsonized by antibody alone, whereas FcRIII plays a lesser role. Surprisingly, FcRII is not necessary for phagocytosis when complement is also present. FcRIII participates, to a limited extent, in phagocytosis of GBS opsonized with antibody whether or not complement is present. These findings suggest that the function of FcRII in triggering phagocytosis may be particularly important in host defense against type III GBS in the setting of complement deficiency of young infants.

Functional capacity of Fc gamma receptor III (CD16) on human neutrophils

Receptors for the Fc region of immunoglobulin G (IgG) are a structurally diverse group of molecules. Within the three Fc gamma R families (Fc gamma RI, Fc gamma RII and Fc gamma RIII), the presence of distinct genes and alternative splicing variants leads to a variety of receptor isoforms that are most strikingly different in the transmembrane and intracellular regions. An obvious example of structural variation in the transmembrane and cytoplasmic domains is observed in the Fc gamma RIII family. Fc gamma RIIIB, which is nearly identical to Fc gamma RIIIA in the extracellular domains, lacks both transmembrane and cytoplasmic protein domains and is anchored to the cell through a glycosyl phosphatidylinositol anchor. Analysis of Fc gamma RIII function presents a considerable challenge in understanding the role of different Fc gamma R receptors in polymorphonuclear neutrophil (PMN) function. While one hypothesis for the role of Fc gamma RIII in Fc gamma R-dependent PMN effector functions is that Fc gamma RIII serves as a binding molecule which focuses the IgG ligand for more efficient recognition and intracellular signaling by Fc gamma RII, recent observations from a number of laboratories suggest that Fc gamma RIII on PMN can transduce signals across the membrane independent of ligand-dependent engagement of Fc gamma RII. We will review these data and present recent data which suggest that the role of Fc gamma RIII extends beyond direct initiation of functions to a more complex role of synergistic receptor interactions. These findings will be reviewed in the context of the experimental approaches that have been used to examine the roles of Fc gamma RII and Fc gamma RIII on PMN function.

Surface expression of Fc gamma receptor III (CD16) on chemoattractant-stimulated neutrophils is determined by both surface shedding and translocation from intracellular storage compartments

Stimulation of neutrophils (PMN) with chemoattractants markedly increases surface expression of several membrane proteins, including the complement receptors, CR1 and CR3, by translocation from intracellular storage compartments to the cell surface. When we stimulated freshly-isolated PMN with FMLP, we observed little net change in surface Fc gamma receptor (R) III expression. However, if elastase was first used to cleave most (85-90%) of the Fc gamma R III from the PMN surface, subsequent treatment with FMLP induced a rapid renewal of surface Fc gamma R III, achieving levels of approximately 70% of that originally present on the cell surface after 15 min, suggesting translocation of intracellular receptors. This was confirmed by demonstrating concomitant depletion of greater than 80% of the intracellular Fc gamma R III. Studies of density gradient fractions of N2-cavitated PMN indicated at least two distinct intracellular membrane fractions that contain Fc gamma R III. Shedding of Fc gamma R III induced by FMLP was about half-maximal by 15 min and nearly complete by 60 min. Stoichiometric assessment of FMLP-induced changes in PMN surface and intracellular Fc gamma R III showed a marked depletion in intracellular Fc gamma R III, little net change in surface Fc gamma R III, and a large overall loss of total cell Fc gamma R III that could be attributed to shedding. We conclude that stimulation by chemoattractants causes a rapid translocation of intracellular Fc gamma R III to the PMN surface that is roughly balanced by the concomitant FMLP-induced shedding of this receptor.

Signal transduction by Fc gamma RIII (CD16) is mediated through the gamma chain

To determine the functional role of the two isoforms of Fc gamma RIII (CD16) (IIIA, IIIB), the signal transduction capabilities of wild-type and mutant forms of these receptors were analyzed in transfected lymphoid, myeloid, and fibroblastic cell lines. Functional reconstitution of receptor signalling was observed in hematopoietic T and mast cells, and was absent in nonhematopoietic (CHO) cells. Fc gamma RIIIA, a hetero-oligomeric receptor composed of a ligand-binding subunit alpha and dimeric gamma chains, generated both proximal and distal responses in Jurkat and P815 cells, typical of what is seen in natural killer cells and macrophages upon receptor activation. In contrast, Fc gamma RIIIB, which is normally attached to the cell surface via a glycosyl-phosphatidylinositol anchor, was incapable of transducing signals. After crosslinking, Fc gamma RIIIA signalling was dependent only upon the gamma chain. Fc gamma RIIIA chimeras in which the alpha subunit transmembrane and cytoplasmic domains were substituted with the corresponding gamma chain sequences functioned as well as wild-type hetero-oligomeric receptors. These data indicate that the ability of the Fc gamma RIIIA complex to activate the appropriate pathways for cell activation is cell-type restricted and independent of the transmembrane and cytoplasmic domains of the alpha subunit. The presence of the gamma chain is responsible for the assembly of, as well as the signal transduction by, the functional cell surface complex.

Allelic polymorphisms of human Fc gamma receptor IIA and Fc gamma receptor IIIB. Independent mechanisms for differences in human phagocyte function

Two different allelic polymorphisms among the isoforms of human Fc gamma receptors have been defined: the low-responder (LR)-high-responder (HR) polymorphism of huFc gamma RIIA expressed on both PMN and monocytes and the NA1-NA2 polymorphism of the neutrophil Fc gamma RIII (huFc gamma RIIIB). To address the issues of whether the LR-HR polymorphism has a significant impact on Fc gamma R-mediated functions in human blood cells and whether any differences in LR-HR might be related to higher Fc gamma R-mediated phagocytosis in NA1 donors, we examined Fc gamma R-specific binding and internalization by donors homozygous for the two huFc gamma RIIA alleles. PMN from LR homozygotes showed consistently higher levels of internalization of erythrocytes opsonized with pooled human IgG (E-hIgG). The absence of an LR-HR phagocytic difference with erythrocytes opsonized with either anti-Fc gamma RIIA MAb IV.3 or rabbit IgG, as opposed to E-hIgG, suggested that the Fc piece of the opsonin might be important for this LR-HR difference. Accordingly, we studied HR and LR homozygotes with human IgG subclass-specific probes. Both PMN (independent of huFc gamma RIIIB phenotype) and monocytes from LR donors bound and internalized erythrocytes coated with human IgG2 (E-hIgG2) efficiently, whereas phagocytes from HR donors did so poorly. E-hIgG2 internalization was completely abrogated by blockade of the ligand binding site of huFc gamma RIIA with IV.3 Fab, indicating that huFc gamma RIIA is essential for the handling of hIgG2 and that the mechanism of the LR-HR phagocytic difference is at the level of ligand binding to huFc gamma RIIA. In contrast, the difference in internalization of E-hIgG between NA1 and NA2 homozygous donors was independent of the huFc gamma RIIA phenotype and did not manifest differences in ligand binding. Thus, the two known allelic polymorphisms of human Fc gamma R have distinct and independent mechanisms for altering receptor function, which may influence host defense and immune complex handling.

Granulocyte Fc gamma receptor recognition of cell bound and aggregated IgG: effect of gamma-interferon

Granulocyte Fc gamma receptors are important components in the recognition of IgG-coated cells and immune complexes. Two proteins have been identified on resting human granulocytes which function as Fc gamma receptors, Fc gamma RII (CD32) and Fc gamma RIII (CD16). A third protein, Fc gamma RI (CD64), is not constitutively expressed on resting granulocytes, but can be induced by activation with gamma-interferon. We examined the role of these three Fc gamma receptors on human granulocytes in the binding of both IgG-sensitized erythrocytes and soluble oligomeric IgG. In these studies we employed anti-Fc gamma receptor antibodies which complete for the Fc gamma RII and Fc gamma RIII ligand binding sites. Preincubation of granulocytes with saturating concentrations of high-affinity anti-Fc gamma RII monoclonal antibody did not alter the recognition of IgG sensitized human cells by granulocytes. Furthermore, ligand binding studies demonstrated that anti-Fc gamma RII antibody altered neither the number nor the affinity of granulocyte binding sites for human trimeric IgG. In contrast, Fab anti-Fc gamma RIII inhibited the binding of both IgG (anti-D) sensitized human RBCs and IgG sensitized sheep RBCs. Similarly, a reduction in the expression of Fc gamma RIII by treatment with phosphatidyl-inositol specific phospholipase C reduced PMN recognition of IgG-sensitized cells. Also, anti-Fc gamma RIII decreased the number of granulocyte binding sites for human IgG trimer without a change in receptor affinity. Fc gamma RI, which was induced by gamma-IFN, increased granulocyte recognition of both IgG sensitized RBCs and IgG trimer. These data suggest that Fc gamma RIII is the primary Fc gamma receptor on granulocytes which recognizes IgG sensitized RBCs and low molecular weight complexes of IgG. With gamma-interferon activated granulocytes, Fc gamma RI appears to enhance this recognition process.

Identification of the leukocyte adhesion molecules CD11 and CD18 as receptors for type 1-fimbriated (mannose-specific) Escherichia coli

Attachment of bacteria to phagocytic cells may be mediated by lectin-carbohydrate interactions, resulting in lectinophagocytosis. The best-studied system is the interaction of type 1-fimbriated (mannose-specific) Escherichia coli with human phagocytic cells. Here we demonstrate that the leukocyte integrins CD11 and CD18 (CD11/CD18) constitute the major receptors for type 1-fimbriated E. coli. Bacteria were bound in a dose-dependent and saturable manner to CD11/CD18, which was immobilized to microwells, whereas nonfimbriated E. coli cells failed to bind. The binding was efficiently inhibited (82 to 85%) by methyl-alpha-mannoside but not by galactose, and it was reduced by treatment of the immobilized CD11/CD18 with sodium metaperiodate, endoglycosidase H, or a mixture of endoglycosidase F and N-glycosidase. The fimbriated bacteria also bound to CD11a,b,c and CD18 separated by polyacrylamide gel electrophoresis with sodium dodecyl sulfate and blotted onto nitrocellulose paper. This binding was inhibited specifically by methyl-alpha-mannoside and was significantly diminished by treatment of the blots with sodium metaperiodate. Only minimal binding to the blotted CD11/CD18 that had been deglycosylated enzymatically prior to electrophoresis was observed. On blots of granulocyte lysates, specific binding to two glycoproteins (Mrs, 90,000 to 100,000 and 165,000) with mobilities similar to that of CD11/CD18 was observed. Monoclonal antibodies to CD11a, CD11b, or CD18 inhibited the binding of the bacteria to intact human granulocytes by 55 to 80%, whereas antibodies against other leukocyte surface antigens were not inhibitory. We conclude that type 1-fimbriated E. coli binds to human granulocytes via the oligomannose and hybrid N-linked units of CD11/CD18. Since CD11b/CD18 and CD11c/CD18 are known to serve as receptors for complement fragment iC3b, this study provides a link between opsonophagocytosis and lectinophagocytosis.

IgM anti-Fc gamma R autoantibodies trigger neutrophil degranulation

Anti-Fc gamma R IgM monoclonal antibodies (mAbs) isolated from lipopolysaccharide-stimulated spleen cells from tightskin (TSK) mice were found to be polyspecific, reacting with a wide variety of molecules, including double-stranded DNA, topoisomerase, RNA polymerase, and different collagen types. Approximately 60% of the polyspecific IgM mAbs have anti-Fc gamma R specificity. These anti-Fc gamma R mAbs induce the release of hydrolases from both azurophil and specific granules of human neutrophils. 25-45% of the total cellular content (determined in Nonidet P-40 lysates) of neutrophil elastase, 10-25% of beta-glucuronidase, and 30-50% of alkaline phosphatase was released after incubation with the mAbs. The degranulation process was accompanied by dramatic morphological changes shown by scanning and transmission electron microscopy. The release of hydrolytic enzymes stimulated by the IgM anti-Fc gamma R mAbs was inhibited by preincubation of neutrophils with Fab fragments of either anti-human Fc gamma RII (IV.3) or anti-human Fc gamma RIII (3G8) mAbs. The binding of the anti-Fc gamma R TSK mAbs to human neutrophils was inhibited by Fab fragments of mAb 3G8. However, we found that the TSK anti-Fc gamma R mAbs do not bind to human Fc gamma RII expressed in either CHO cells or the P388D1 mouse macrophage cell line. Since the enzyme release could be inhibited by Fab fragments of mAb IV.3, we suggest that the signal transduction may require Fc gamma RII activation subsequent to crosslinking of the glycan phosphatidyl inositol-anchored Fc gamma RIII-1. These data demonstrate for the first time that polyspecific autoantibodies with Fc gamma R specificity can trigger neutrophil enzyme release via human Fc gamma RIII-1 in vitro and indicate a possible role for such autoantibodies in autoimmune inflammatory processes.

Preferential expression of human Fc gamma RIIIPMN (CD16) in paroxysmal nocturnal hemoglobinuria. Discordant expression of glycosyl phosphatidylinositol-linked proteins

The isoform of Fc gamma RIII (CD16) expressed on PMN has a GPI membrane anchor, and in paroxysmal nocturnal hemoglobinuria (PNH) there is a deficiency in Fc gamma RIII expression on PMN. Contrary to expectation, however, CD16 expression is preserved (albeit at reduced levels) in all affected PNH PMN that completely lack the GPI-anchored proteins DAF (CD55) and CD59. Fc gamma RIII negative PMN are not observed in any of the six PNH patients examined in this study. Analysis of the molecular weight of both glycosylated and deglycosylated Fc gamma RIII from PMN with reduced Fc gamma RIII expression indicates no variations in size relative to normal donor Fc gamma RIIIPMN. Indeed, the Fc gamma RIII expressed at intermediate levels is phosphatidylinositol-specific phospholipase C (PI-PLC)-sensitive. Thus, there is no evidence suggestive of expression of a transmembrane isoform and all data indicate that Fc gamma RIIIPMN on affected cells in PNH is a GPI-linked isoform. With Fc gamma RIIIPMN expression preserved at reduced levels on affected cells in PNH, PMN from PNH patients retain the capacity to internalize the Fc gamma RIIIPMN-specific probe E-ConA (at reduced levels) as well as IgG-opsonized erythrocytes. Reduced expression of GPI-anchored molecules on PNH PMN is not restricted to Fc gamma RIIIPMN since intermediate levels of CD59 were observed in the PNH PMN that were decay-accelerating factor (DAF)-negative and Fc gamma RIIIPMN intermediate. In addition, discordant expression of GPI-linked molecules in individual cells is not restricted to PMN since DAF+/CD14- monocytes were observed in one PNH patient. These data suggest that, when analyzed on an individual cell level, the GPI anchor defect in PNH is not absolute and must involve either a hierarchy of access of different protein molecules to available GPI anchors, distinct anchor biochemistries for the different proteins, or differential regulation of protein-anchor assembly.

The glycosyl phosphatidylinositol-linked Fc gamma RIIIPMN mediates transmembrane signaling events distinct from Fc gamma RII

To investigate the ability of FcgammaRIII(PMN), the GPI-anchored isoform of FcgammaRIII (CD16) in polymorphonuclear leukocytes (PMN), to mediate transmembrane signaling events, we measured changes in membrane potential with DiOC(5) and in intracellular calcium with indo-1. FcgammaR were ligated by anti-FcgammaRIII mAb 3G8 (IgG and Fab), anti-FcgammaRII mAb IV.3 (IgG and Fab), and human IgG aggregates. Cell bound mAbs were also crosslinked by goat F(ab')(2) anti-mouse IgG. 3G8 IgG elicited a rapid change in [Ca(2+)](i), which was unaffected by EGTA, Vibrio cholerae toxin (CT), or Bordetella pertussis toxin (PT), and was abolished by BAPTA . Univalent receptor binding with 3G8 Fab gave no response but crosslinking with F(aV)2 GAM gave a rapid [Ca2,](i) response. Neither IV.3 Fab, IV.3 IgG, nor crosslinking of IV.3 Fab elicited a calcium signal. PI-PLC-treated PMN with the density of FcgammaRIII(PMN) reduced to that of FcgammaRII showed an unattenuated change in [Ca(2+)](i), with a 3G8 stimulus. The effects of IgG aggregates paralleled those of 3G8 mAb. These data indicate that multivalent ligation of FcgammaRIII(PMN) initiates an increase in [Ca(2+)];, derived from intracellular stores, that is distinct from both the FMLP- and FcgammaRII-induced responses. Ligand-dependent interaction with FcgammaRII is not required. Since FcgammaRIII(PMN) can internalize the FcgammaRIII-specific probe Con A-opsonized E and lyse anti-FcgammaRIII heteroantibody-opsonized chick E, this GPI-anchored molecule mediates both signal transduction and integrated cell responses.

Fc gamma receptor III on human neutrophils. Allelic variants have functionally distinct capacities

As a model system to explore the functional consequences of structural variants of human Fc gamma receptors (Fc gamma R), we have investigated Fc gamma R-mediated phagocytosis in relation to the NA1-NA2 polymorphism of Fc gamma RIII (CD16) on neutrophils (Fc gamma RIIIPMN). The neutrophil-specific NA antigen system is a biallelic polymorphism with codominant expression demonstrating a gene dose effect with the anti-NA1 MAb CLB-gran 11 in a large donor population. To explore the impact of this allelic variation of Fc gamma RIIIPMN on phagocytosis, we used two Fc gamma RIII-dependent probes, IgG-sensitized erythrocytes (EA) and concanavalin. A-treated erythrocytes (E-ConA). Comparison of Fc gamma R-mediated phagocytosis by PMN from NA1 subjects and from NA2 subjects showed lower levels of phagocytosis of both probes by the NA2 individuals. The difference was most pronounced with lightly opsonized EA: at the lowest level of sensitization the phagocytic index was 72% lower for NA2 donors, whereas at the highest level of sensitization it was 21% lower (P less than 0.003). Blockade of Fc gamma RII with MAb IV.3 Fab amplified by threefold the difference between NA1 and NA2 donors. NA1 and NA2 individuals had identical phagocytic capacities for the non-Fc gamma RIII probes, serum-treated and heat-treated zymosan. These individuals did not show differential quantitative cell surface expression of Fc gamma RIIIPMN measured by a panel of anti-CD16 MAb (3G8, CLB FcR-gran 1, VEP13, BW209/2) and by Scatchard analysis of 125I-IgG dimer binding. The difference in Fc gamma R-mediated phagocytosis was not explicable on the basis of differential collaboration of Fc gamma RIIIPMN alleles with Fc gamma RII, since (a) the difference in phagocytic capacity between NA1 and NA2 individuals was readily apparent with the E-ConA probe (which is independent of Fc gamma RII) and (b) the difference in phagocytosis of EA was magnified by Fc gamma RII blockade. The demonstration that allelic polymorphisms in Fc gamma R can have significant consequences for physiological functions implies that within the structural complexity of human Fc gamma Rs, including both allelic forms and cell type-specific isoforms, there will be differences in quantitative, and perhaps qualitative, function with potential importance for disease processes.

Phagocytosis mediated by three distinct Fc gamma receptor classes on human leukocytes

We have evaluated the capacity of the three major classes of human Fc gamma R to mediate phagocytosis by measuring the ability of adherent phagocytes to internalize erythrocytes coated with anti-Fc gamma R mAb. Five different cell types were studied, freshly purified monocytes, cultured monocytes, alveolar macrophages, freshly purified polymorphonuclear neutrophilic leukocytes, and PMNs cultured in IFN-gamma. Fc gamma RI and Fc gamma RII on whichever cells they were expressed were capable of phagocytosing anti-Fc gamma R mAb-coated erythrocytes. Furthermore, Fc gamma RIII on mononuclear phagocytes, which appears to be a conventional integral membrane protein that spans the lipid bilayer, was capable of phagocytosing anti-Fc gamma RIII-coated erythrocytes. However, Fc gamma RIII on neutrophils, a molecule linked to the membrane by a phosphatidylinositol-glycan moiety, although binding anti-Fc gamma RIII-coated erythrocytes vigorously was incapable of mounting a phagocytic response. This deficiency correlates with the limited capacity of Fc gamma RIII on neutrophils to mediate superoxide generation and antibody-dependent cell-mediated cytotoxicity and it may be related to the unique structural features of Fc gamma RIII.

The opsonizing ligand on Salmonella typhimurium influences incorporation of specific, but not azurophil, granule constituents into neutrophil phagosomes

Phagosomes were purified from human neutrophils ingesting Salmonella typhimurium opsonized with adsorbed normal human serum or with rabbit IgG. Constituents within the phagosome were endogenously labeled by supplying the cells with 125INa during phagocytosis. Lactoferrin and vitamin B12 binding protein (TC1 and TC3), markers for specific granules, were present in the phagosomes from neutrophils ingesting S. typhimurium opsonized with IgG but were 3.5- to 5-fold less prominent in phagosomes from cells phagocytosing Salmonella bearing C3 fragments only. In contrast, iodinated azurophilic granule components, most prominently defensins, were the major constituents in phagosomes prepared under both opsonization conditions. Furthermore, labeled complement (CR1 and CR3) and immunoglobulin (Fc gamma RIII) receptors were incorporated in the phagosome regardless of the ligand mediating phagocytosis. These results suggest that the ligand-receptor interactions mediating phagocytosis influence incorporation of neutrophil-specific granule contents into phagosomes.

A rapid turbidimetric assay of phagocytosis and serum opsonizing capacity

An in vitro assay has been developed to measure the opsonizing capacity of serum and the extent of bacterial uptake by phagocytes. Various micro-organisms were preopsonized for 10 min with a serum concentration previously determined to be optimal for the respective types of micro-organism. Subsequently, neutrophils from a healthy donor were added to the preopsonized bacteria in a cuvette of a spectrophotometer. The decrease in turbidity at 400 nm, resulting from the uptake of the micro-organisms by the neutrophils, was measured for 20-30 min and the area under the curves was taken as a measure of the opsonizing capacity of the serum or the phagocytic capacity of the neutrophils. The results correlated well with standard opsonophagocytic assays. By excluding Ca2+ from the buffer of the assay, phagocytosis was distinguished from the combined response of phagocytosis and aggregation. In the presence of Ca2+ ions, both phagocytosis and aggregation contributed to the decrease in turbidity. In the absence of Ca2+, phagocytosis was normal, but aggregation was completely inhibited. Phagocytosis in the absence of Ca2+ was also observed using microscopic and radiometric methods of evaluation. Neutrophils from a patient with a deficiency of leukocyte adhesion molecules, ingested as many bacteria as did normal neutrophils without Ca2+. Experiments with NaF, to inhibit phagocytosis, indicated that the change in turbidity measured in the absence of Ca2+ was mainly caused by phagocytosis, not by attachment of bacteria to the neutrophils. The opsonizing capacity of sera, as determined in our assay, depended both on antibodies and on an intact complement system and the inter-assay variance was less than 5%. We found a close correlation between turbidity changes measured in the presence or absence of Ca2+, suggesting that both phagocytosis and aggregation are opsonin-dependent. This assay is applicable to a variety of opsonizing fluids and micro-organisms, and can be used for assessing the phagocytic capacity of patients' neutrophils as well as for assessing the opsonizing capacity of patients' sera.

The human mannose-binding protein functions as an opsonin

The human mannose-binding protein (MBP) is a multimeric serum protein that is divided into three domains: a cysteine-rich NH2-terminal domain that stabilizes the alpha-helix of the second collagen-like domain, and a third COOH-terminal carbohydrate binding region. The function of MBP is unknown, although a role in host defense is suggested by its ability to bind yeast mannans. In this report we show that native and recombinant human MBP can serve in an opsonic role in serum and thereby enhance clearance of mannose rich pathogens by phagocytes. MBP binds to wild-type virulent Salmonella montevideo that express a mannose-rich O-polysaccharide. Interaction of MBP with these organisms results in attachment, uptake, and killing of the opsonized bacteria by phagocytes. These results demonstrate that MBP plays a role in first line host defense against certain pathogenic organisms.