Staphylococcus aureus infection of human endothelial cells potentiates Fc receptor expression

Vasculitis, a recognized complication of staphylococcal-endovascular infections, may result in part, from the expression of FcR by Staphylococcus aureus-infected endothelial cells. FcR were measured using [51]Cr labeled SRBC preincubated with rabbit anti-SRBC IgG. FcR were not detected on uninfected endothelial cells, but were demonstrated on S. aureus infected cells using IgG, but not IgM labeled SRBC. FcR expression was dependent on the initial bacterial density (greater than or equal to 8 x 10(7) cfu/ml) and on phagocytosis of the staphylococci, but not on new protein synthesis. IgG labeled SRBC binding was blocked by aggregated IgG but not IgM. SRBC coated with the F(ab')2 portion of IgG did not bind, thus confirming that FcR were specifically involved in this interaction. FcR are expressed after S. aureus invasion of human endothelial cells and may contribute to the vasculitis which often accompanies S. aureus-endovascular infections.     

A human lymphoid cell line secreting immunoglobulin G and retaining immunoglobulin M in the plasma membrane

A selected clone, LA 85.2, of a human lymphoid cell line produces μ, γ, and light chains. The cells secrete IgG but not IgM. Assembly of μ chains and light chains produces 8S IgM which is retained in the plasma membrane. IgM is produced at a slow rate and in lesser amounts than IgG. LA 85.2 cells produce a plasma membrane protein which can bind to antibody-antigen precipitates. It is suggested that this protein plays a role in holding the surface IgM in the plasma membrane.     

The carboxyl-terminal domains of IgA and IgM direct isotype-specific polymerization and interaction with the polymeric immunoglobulin receptor

Mucosal surfaces are protected by polymeric immunoglobulins that are transported across the epithelium by the polymeric immunoglobulin receptor (pIgR). Only polymeric IgA and IgM containing a small polypeptide called the "joining" (J) chain can bind to the pIgR. J chain-positive IgA consists of dimers, and some larger polymers, whereas only IgM pentamers incorporate the J chain. We made domain swap chimeras between human IgA1 and IgM and found that the COOH-terminal domains of the heavy chains (Calpha3 and Cmu4, respectively) dictated the size of the polymers formed and also which polymers incorporated the J chain. We also showed that chimeric IgM molecules engineered to contain Calpha3 were able to bind the rabbit pIgR. Since the rabbit pIgR normally does not bind IgM, these results suggest that the COOH-terminal domain of the polymeric immunoglobulins is primarily responsible for interaction with the pIgR. Finally, we made a novel chimeric IgA immunoglobulin, containing the terminal domain from IgM. This recombinant molecule formed J chain-containing pentamers that could, like IgA, efficiently form covalent complexes with the human pIgR ectodomain, known as secretory component.     

Prevention of Bacteriophage Adsorption to Staphylococcus aureus by Immunoglobulin G

Normal human and rabbit sera when incubated with Staphylococcus aureus inhibit the adsorption of bacteriophages. The bacteriophage adsorption was also inhibited by separated normal immunoglobulin M (IgM), F(ab')(2), and Fab-fragments of IgG. No inhibition was obtained with myeloma IgG or Fc-fragments of normal human and rabbit IgG. The results indicate that the serum inhibition of bacteriophage adsorption to S. aureus is not due to a binding of IgG to protein A on the surface of S. aureus.     

Bound class M, A and G immunoglobulins in the thymus of myasthenia gravis patients]

Deposits of granular material containing IgM, IgA, and IgG were revealed in the thymus of patients with myasthenia by direct immunofluorescence. Treatment of the thymus sections by unlabeled preparations against individual classes of human immunoglobulins inhibited the reaction of the granular material with the homologous labeled preparations. Disappearance of fluorescence of these deposits was also seen in treatment of the sections with glycine-HCl-buffer, pH 2.8. These data permitted a suggestion that granular material represented immune complexes where IgM, IgA, and IgG served as antibody, and thymus tissue components--as an antigen. The presence of bound immunoglobulin in the thymus indicated that an autoimmune process directed against the tissues of this organ occurred in myasthenia.     

Identification of the secreted macromolecular immunogens of Staphylococcus aureus by analysis of serum

The ability of sera to recognise secreted macromolecules of Staphylococcus aureus was examined by ELISA and Western immunoblotting. Individual secreted proteins were also studied using both human sera and sera from rabbits immunised with secreted macromolecules. Patients sera showed a wide range of IgG antibody titres to secreted macromolecules and whole bacteria. Controls showed a significantly lower IgG response. Western immunoblotting revealed that a significant number of secreted proteins were recognised by circulating IgG antibodies. Surprisingly, both the sera from controls and from patients recognised similar macromolecules including a number of potential virulence factors. The major difference was in the IgG binding to a 16-kDa component, which was recognised by the majority of the sera from infected individuals, but only by a small number of sera from healthy controls. The higher incidence of antibodies recognising the 16 kDa component may be related to our earlier finding that the major bone resorbing component of S. aureus is a heterodimeric protein containing a 16-kDa subunit, the activity of which could be blocked by sera.     

Heterotrimers formed by tumor necrosis factors of different species or muteins

Incubation of murine tumor necrosis factor (mTNF) at subnanomolar concentrations results in partial dissociation of the trimers, coinciding with a decrease in bioactivity. Using size-exclusion chromatography, we observed that the conversion of labeled mTNF to monomers is not only prevented by coincubation with an excess of unlabeled mTNF but also with unlabeled human TNF (hTNF). Moreover, after coincubation of mTNF and hTNF four different TNF complexes were revealed by native polyacrylamide gel electrophoresis, viz. homotrimeric mTNF and hTNF, as well as two complexes with an intermediate migration pattern. Analytical gel filtration in combination with native polyacrylamide gel electrophoresis and Western blot immunodetection indicated that these new complexes consisted of heterotrimeric TNF molecules. We conclude that an exchange of monomers takes place during coincubation of two different species of TNF, which results in homotrimeric and heterotrimeric TNF. To assess receptor interaction in vitro, TNF heterotrimeric molecules were used as obtained after incubation of mTNF with labeled hTNF (which only binds to mTNF receptor I) or with labeled mutein mTNF75 (specific for mTNF receptor II). These heterotrimers were retained by both mTNF receptors, which means that the mTNF subunits incorporated in heterotrimeric complexes still can bind to both types of TNF receptor. In addition, the gradual decrease in mTNF bioactivity during preincubation at subnanomolar concentrations was prevented by the presence of mutein mTNF75, which is inactive in an L929 cytotoxicity assay, indicating that heterotrimerization can influence the overall bioactivity.     

The interaction of monomeric and complexed mouse monoclonal IgG with rat basophilic leukemia cells: a subset of IgG molecules can bind to RBL cell Fc receptors for IgG

Rat basophilic leukemia (RBL) cells were shown to bind mouse monoclonal (MC) IgE and certain mouse monomeric IgG1 and IgG2b monoclonal antibodies (MAb) by using a haptenated sheep red blood cell (SRBC) rosetting assay. Rosette formation was antibody concentration dependent with all three immunoglobulin isotypes, but at least 100 times more IgG than IgE was required to form a similar number of rosettes. It was shown by FACS analysis and rosette formation that a subset (8/23) of the IgG MC was able to bind to RBL cells as monomers. However, the majority 15/23 did not bind or bound weakly (less than 25% rosettes) unless in the form of antigen-antibody complexes. As complexes, all IgG subclasses except IgG3 could produce rosettes with RBL cells. None of the IgM or IgA MC tested formed rosettes, even in complexed form. By inhibition studies it is demonstrated that mouse IgG1, IgG2a, and IgG2b MC bind to the same Fc receptor. Mouse IgE was only partially able to inhibit IgG-dependent rosettes at high concentrations, and none of the IgG MC were able to inhibit IgE-dependent rosettes. These results suggest that the interaction of mouse IgG is quite specific for the RBL cell FcG receptor. Because deaggregated polyclonal mouse IgG was a weak inhibitor of MC IgG sensitization of RBL cells, the results are discussed in terms of the heterogeneity and possible abnormality of some MAb.     

Adherence of Staphylococcus aureus is enhanced by an endogenous secreted protein with broad binding activity

A novel mechanism for enhancement of adherence of Staphylococcus aureus to host components is described. A secreted protein, Eap (extracellular adherence protein), was purified from the supernatant of S. aureus Newman and found to be able to bind to at least seven plasma proteins, e.g., fibronectin, the alpha-chain of fibrinogen, and prothrombin, and to the surface of S. aureus. Eap bound much less to cells of Staphylococcus epidermidis, Streptococcus mutans, or Escherichia coli. The protein can form oligomeric forms and is able to cause agglutination of S. aureus. Binding of S. aureus to fibroblasts and epithelial cells was significantly enhanced by addition of Eap, presumably due to its affinity both for plasma proteins on the cells and for the bacteria.     

Binding characteristics of dimeric IgG subclass complexes to human neutrophils

Immune complexes were prepared by incubation of human IgG paraproteins with F(ab')2 fragments of the mAb K35 against the kappa-L chain of human IgG. The composition of these complexes was analyzed by centrifugation over sucrose gradients, by gel filtration, by RIA with either IgG Sepharose or K35 Sepharose and by double-labeling studies. The results indicated that the complexes consist of saturated tetramers composed of two IgG molecules cross-linked by two F(ab')2 fragments of the mAb. These complexes were used to study the binding of the different IgG subclasses to human neutrophils at 4 degrees C. Human neutrophils bound IgG3 complexes approximately three times faster than IgG1 complexes. Binding of IgG2 or IgG4 dimers to the neutrophils was undetectable. The same number of IgG1 complexes and IgG3 complexes bound to the neutrophils, but considerable inter-donor variation was found (mean number of Fc gamma R per neutrophil: 190,000, range 120,000 to 400,000). The Ka for the binding of IgG1 complexes to neutrophils (median 11 x 10(7) M-1) was lower than the Ka for the binding of IgG3 complexes (median 47 x 10(7) M-1). Competition studies between labeled IgG1 complexes or IgG3 complexes and unlabeled complexes showed that the Fc gamma R of human neutrophils do not display an IgG subclass specificity. Incubation of neutrophils with a mAb against the FcRIII completely blocked the binding of IgG1 complexes and IgG3 complexes. Incubation with a mAb against the FcRII reduced the affinity of the complexes for the neutrophils but had no effect on the maximum number of complexes bound. This indicates that one complex may bind simultaneously to one FcRIII and to one FcRII.     

Polymerization of IgA and IgM: roles of Cys309/Cys414 and the secretory tailpiece

We have investigated how the secretory tailpiece (tp), Cys414 and the amino acids flanking Cys414 or Cys309 are involved in regulating the different polymerization of IgM and IgA to pentamers and dimers/monomers, respectively. Whereas changing the tp of IgM to that of IgA has little effect on IgM polymerization, introducing the mu tp to IgA leads to the formation of larger than wild-type IgA polymers, including pentamers and hexamer. This shows that the secretory tp can differentially regulate polymerization depending on the heavy chain context. Cys414, which is engaged in intermonomeric disulfide bonds in IgM, is not crucial for the difference in IgM and IgA polymerization; IgM with a C414S mutation forms more large polymers than IgA. Also, IgA with IgM-like mutations in the five amino acids flanking Cys309, which is homologous to Cys414, oligomerize similarly as IgA wild type. Thus, IgA appears to have an inherent tendency to form monomers and dimers that is partially regulated by the tp, while the Cys309 region has only a minor effect. We also show that complement activation by IgM is sensitive to alterations in the polymeric structure, while IgA is inactive in classical complement activation even for polymers such as pentamers and hexamers.     

Identification of the coated vesicle proteins that bind calmodulin

The constituent proteins of coated vesicles responsible for binding calmodulin were identified by photoaffinity labeling with the reagent azido-¹²⁵I-calmodulin. Three protein complexes with apparent molecular weights of 130,000, 93,000 and 52,000 were labeled. Specificity was demonstrated by the dependence of labeling on Ca²⁺, and by its reduction in the presence of unlabeled calmodulin or Stelazine. Urea-soluble components of coated vesicles and material isolated by Sepharose CL4B chromatography formed a 52,000 MW labeled complex. Subtracting an apparent molecular weight of calmodulin of 20,000 from the weights of the covalently labeled complexes, the coated vesicle proteins that bind calmodulin are 110,000, 73,000 and 32,000 MW. The 32,000 MW protein is thought to participate in the coat structure but the other two are most likely associated with the vesicle.     

Ability of various oral bacteria to bind human plasma fibronectin

The present study describes the ability of various oral bacteria to bind human plasma fibronectin (PFN). Avid binding of 125I-PFN was found for Streptococcus mutans (serotypes a to h), Streptococcus sanguis, group A Streptococcus pyogenes and Staphylococcus aureus, while other gram-positive bacterial species tested demonstrated only weak or negligible PFN binding ability. Two gram-negative bacterial species, Bacteroides gingivalis and Escherichia coli, did not significantly bind PFN. 125I-PFN binding to S. mutans 6715 cells was decreased by pretreatment with unlabeled PFN, and the radiolabeled PFN bound to the cell surface was released on addition of unlabeled PFN. Strong inhibition of 125I-PFN binding to S. mutans 6715 cells was obtained by protease pretreatment, while partial inhibition was also observed following treatment with acid, alkali, lipase, and monoclonal anti-polyglycerophosphate. These results suggest that PFN binding to S. mutans cells is reversible and that PFN receptors on the cell surface appear to be heat-stable multiple proteins.     

Secretion of recombinant proteins into the culture medium by Escherichia coli and Staphylococcus aureus

The ability of staphylococcal protein A (SPA) to bind to the Fc part of IgG has been used for the purification of a number of heterologous gene products as fusion proteins. Both the SPA promoter and signal sequence function in Escherichia coli, as well as in a number of Gram-positive bacteria, which facilitates comparisons of the expressed specific products in different hosts. The expression system developed for E. coli yields excretion of the fusion protein to the growth medium, which makes E. coli a competitive alternative to Gram-positive bacteria for the expression of secreted products. The human peptide hormones insulin-like growth factors (IGF) I and II were expressed using the protein A system in E. coli and Staphylococcus aureus. Despite a high degree of structural homology, large differences in the yields were observed in the two hosts. This underlines the importance of investigating different bacterial hosts for a particular protein product.     

S-layer-mediated display of the immunoglobulin G-binding domain of streptococcal protein G on the surface of Caulobacter crescentus: development of an immunoactive reagent

The immunoglobulin G (IgG)-binding streptococcal protein G is often used for immunoprecipitation or immunoadsorption-based assays, as it exhibits binding to a broader spectrum of host species IgG and IgG subclasses than the alternative, Staphylococcus aureus protein A. Caulobacter crescentus produces a hexagonally arranged paracrystalline protein surface layer (S-layer) composed of a single secreted protein, RsaA, that is notably tolerant of heterologous peptide insertions while maintaining the surface-attached crystalline character. Here, a protein G IgG-binding domain, GB1, was expressed as an insertion into full-length RsaA on the cell surface to produce densely packed immunoreactive particles. GB1 insertions at five separate sites were expressed, and all bound rabbit and goat IgG, but expression levels were reduced compared to those of wild-type RsaA and poor binding to mouse IgG was noted. To remedy this, we used the 20-amino-acid Muc1 peptide derived from human mucins as a spacer, since insertions of multiple tandem repeats were well tolerated for RsaA secretion and assembly. This strategy worked remarkably well, and recombinant RsaA proteins, containing up to three GB1 domains, surrounded by Muc1 peptides, not only were secreted and assembled but did so at wild-type levels. The ability to bind IgG (including mouse IgG) increased as GB1 units were added, and those with three GB1 domains bound twice as much rabbit IgG per cell as S. aureus cells (Pansorbin). The ability of recombinant protein G-Caulobacter cells to function as immunoactive reagents was assessed in an immunoprecipitation assay using a FLAG-tagged protein and anti-FLAG mouse monoclonal antibody; their performance was comparable to that of protein G-Sepharose beads. This work demonstrates the potential for using cells expressing recombinant RsaA/GB1 in immunoassays, especially considering that protein G-Caulobacter cells are more cost-effective than protein G beads and exhibit a broader species and IgG isotype binding range than protein A.     

Binding of elastin to Staphylococcus aureus.

Many pathogenic bacteria specifically bind to components of the extracellular matrix. In this study, we report the specific association of Staphylococcus aureus with elastin, a major structural component of elastic tissue. Competition assays in which the binding of radiolabeled tropoelastin was inhibited by excess unlabeled elastin peptides, but not by other proteins, established the specificity of the interaction. Kinetic studies showed that tropoelastin binding to the bacteria was rapid and saturable. Scatchard analysis of the equilibrium binding data indicated the presence of a single class of high affinity binding sites (KD approximately 4-7 nM) with approximately 1000 sites per organism. Protease susceptibility suggested that the elastin binding moiety on S. aureus was a protein, which was confirmed by the isolation of a 25-kDa elastin-binding protein from S. aureus extracts through affinity chromatography. Using a truncated form of tropoelastin, the bacterial binding domain on elastin was mapped to a 30-kDa fragment at the amino end of the molecule. Although the precise amino acid sequence recognized by the staphylococcal elastin receptor has not been characterized, it is clearly different from the region of tropoelastin that specifies binding to mammalian elastin receptors.     

Surfactant protein A binds to IgG and enhances phagocytosis of IgG-opsonized erythrocytes

Surfactant protein (SP)-A and SP-D, immunoglobulins, and complement all modulate inflammation within the lung by regulating pathogen clearance. For example, SP-A binds to and opsonizes a variety of bacteria and viruses, thereby enhancing their phagocytosis by innate immune cells such as alveolar macrophages. Immunoglobulins, which bind to antigen and facilitate Fc receptor-mediated phagocytosis, can also activate complement, a family of soluble proteins with multiple host defense functions. Previous studies showed that SP-A and complement protein C1q interact. Since complement protein C1q binds to IgG and IgM immune complexes, the hypothesis tested in this study was that SP-A, which is structurally homologous to C1q, also binds to IgG and affects its functions. SP-A binds to the Fc, rather than the Fab, region of IgG. Binding is calcium dependent but not inhibited by saccharides known to bind to SP-A's carbohydrate recognition domain. The binding of SP-A does not inhibit the formation of immune complexes or the binding of IgG to C1q. In contrast, SP-A enhances the uptake of IgG-coated erythrocytes, suggesting that SP-A might be influencing Fc receptor-mediated uptake. In summary, this study shows a novel interaction between SP-A and IgG and a functional consequence of the binding.     

Nonimmune binding of Ig to Clostridium perfringens. Preferential binding of IgM and aggregated IgG

In an attempt to find a bacterial IgM receptor, a large number of bacterial strains of different species were screened for the ability to bind human IgM. Certain strains of the anaerobic bacterium Clostridium perfringens were found to bind a major fraction of polyclonal IgM. One bacterial strain showed a particularly high binding capacity and was studied in more detail. This strain is also able to bind a minor fraction of polyclonal IgA and IgG. Inhibition experiments indicate that the different Ig classes bind to one and the same R structure. The ability of the strain to bind polyclonal Ig is correlated to the number of subunits in the Ig. This correlation can most simply be explained by increasing avidity with increasing number of subunits. In agreement with this hypotheses, experiments with aggregated IgG show that binding ability increases with aggregate size. Experiments with Ig fragments indicate that the binding structure in Ig is located in the F(ab')2 region. The ability of this bacterial strain to bind a majority of IgM molecules as well as aggregated IgG is potentially useful in immunologic work and represents a new type of Ig binding to bacteria.     

IgM expressed by leukemic CD5(+) B cells binds mouse immunoglobulin light chain

Mouse immunoglobulin (Ig) molecules have previously been shown to bind to the surface of CD5(+) B cells from patients with B-cell chronic lymphocytic leukemia (B-CLL). The results indicated that surface IgM was involved in the interaction and suggested the phenomenon was an example of the polyreactive binding capacity of the surface Ig (sIg) expressed by these malignant cells. This article describes the further characterization of the interaction between human IgM and mouse Ig molecules and subunits. Mouse Ig molecules of both kappa and lambda light chain classes bound to the B-CLL cell surface. The dissociation constant for the interaction of mouse IgG1 (K121) with the B-CLL cell surface was 3.6 x 10(-7) M. To confirm the involvement of the human IgM expressed by the B-CLL cells in the interaction, the malignant cells were stimulated in vitro to induce secretion of human IgM. Enzyme immunoassay was used to show that secreted human IgM bound to intact mouse Ig, as occurred with the cell surface analysis. The mouse Ig epitope recognized by the purified secreted human IgM was shown by Western blot analysis to be located on the light chain of the mouse Ig molecule and to be conformationally dependent. K121 light chain was cloned and expressed in E. coli and the recombinant light chain bound to the surface of CLL B cells. The results confirm that human IgM is the reactive ligand in the interaction with mouse Ig and indicate that the interaction of polyreactive IgM with mouse IgG occurs via the light chain component of IgG.     

Autoantibody to vasoactive intestinal peptide in human circulation

In a radioassay for Vasoactive Intestinal Peptide (VIP)-binding, eight out of 33 plasma samples from healthy human subjects exhibited specific binding ranging from 2.6% to 46.7% of total [125 I]VIP. This binding was competitively displaced by unlabeled VIP. The structurally homologous peptides, Peptide Histidine Isoleucine (PHI) and secretin, were, respectively, 72-fold and 413-fold less potent than VIP in displacing bound [125 I]VIP, whereas the unrelated peptides, neurotensin, eledoisin, bombesin and metenkephalin, were without effect on the binding. The antibody nature of the VIP-binding factor was suggested by its precipitation with ammonium sulfate, attenuation after absorption with Staphylococcus aureus preparations, precipitation with antisera against human IgG and IgM, and coelution with standard IgG and IgM on anion-exchange and high-performance gel-filtration columns. Pepsin treatment of purified IgG fraction yielded a VIP-binding species with apparent molecular weight of 108 +/- 13 kDa that was precipitated by antiserum against the F(ab)2 fragment of the IgG molecule. These results demonstrate the existence in some human plasmas of an autoantibody that binds VIP.