Evasion of Toll-like receptor 2 activation by staphylococcal superantigen-like protein 3

Toll-like receptors (TLRs) are crucial for our host defense against microbial infections. TLR2 is especially important to fight bacterial infections, as it specifically recognizes bacterial lipoproteins of both Gram-positive and Gram-negative origin. Present on a variety of immune cells, TLR2 is critical for host protection against several bacterial infections, including those caused by Staphylococcus aureus. This major human pathogen causes increasing health care problems due to its increased resistance to antibiotics. S. aureus secretes a wide variety of proteins that inhibit innate immune responses. Recently, several staphylococcal superantigen-like proteins (SSLs) have been described to mediate immune evasive properties. Here, we describe that SSL3 specifically binds and inhibits TLR2 activation on human and murine neutrophils and monocytes. Through binding of the extracellular TLR2 domain, SSL3 inhibits IL-8 production by HEK cells expressing TLR1/2 and TLR2/6 dimers, stimulated with their specific ligands. The SSL3-TLR2 interaction is partially glycan dependent as binding of SSL3 to TLR2 is affected upon removal of sialic acid residues. Moreover, the SSL3(R308A) mutant lacking glycan-binding properties shows lower TLR2 inhibition. An SSL3 mutant, lacking the N-terminal 126 amino acids, still retains full TLR2 inhibiting activity. Of other SSLs tested, only SSL4, which shares the highest homology with SSL3, blocks TLR2 activation. SSL3 is the first-described bacterial protein that blocks TLR2 activation through direct extracellular interaction with the receptor. This unique function of SSL3 adds to the arsenal of immune evasive molecules that S. aureus can employ to subvert both innate and adaptive immunity.     

Structural relationships and cellular tropism of staphylococcal superantigen-like proteins

The staphylococcal superantigen-like proteins (SSLs) are a family of polymorphic paralogs encoded in the Staphylococcus aureus genome whose function is unknown. The crystal structure of SSL7 was determined and compared to that of SSL5 and that of a classical superantigen, streptococcal pyrogenic exotoxin. Although the overall architecture of the superantigen family is retained in both SSL7 and SSL5, there are significant differences in the structures which suggest that the characteristic major histocompatibility complex binding site of superantigens has been lost. To complement these data, the abilities of SSL7 and a closely related paralog, SSL9, to interact with cells of the immune system were investigated. In populations of human white blood cells, both SSLs interacted selectively with monocytes via specific saturable but separate binding sites, which led to rapid uptake of the SSLs. In addition, SSLs were rapidly taken up by dendritic cells, but not by macrophages, into the same endosomal compartment as dextran. The ability of these secreted proteins to target antigen-presenting cells may enhance a misplaced antibody response against the proteins, which may facilitate bacterial colonization rather than contribute to host protection. Like classical superantigens, therefore, SSLs may distract the host's immune system, but they may do so via entirely different molecular mechanisms.     

Interaction of collagen with serum complement: inhibition of complement-mediated hemolysis.

Collagens from various vertebrate tissues were tested for their ability to consume complement (C) activity upon incubation in human serum or with isolated components of complement. 10 of 12 collagens tested had anticomplementary activity. The heat-denatured form of collagen, gelatin, was found weakly anticomplementary, but elastin was found inactive in the interaction with C. Inactivation of C is a reaction which is dependent on the time of incubation and the collagen concentration and partially dependent on the temperature of incubation. Most collagens depleted C from human serum in presence of cation chelators, EDTA and EGTA, whereas the large part of anticomplementary activity of soluble collagens obtained from rat skin was abolished in presence of EDTA. Evidence is presented that two different principles in collagens play a role in inactivation of C. A factor, contained in insoluble collagens and inhibitable by mild oxidation with periodate, inactivates C1 directly even in presence of chelating agents. Another principle, contained in soluble and insoluble collagen and resistant to periodate treatment, depletes C in serum by utilization of C via the alternate pathway (the C3 shunt).     

Therapeutic potential of protein kinase C inhibitors

The serine/threonine protein kinase, protein kinase C (PKC) is a family of closely related isoforms which are physiologically activated by diacylglycerol generated by the binding of a variety of agonists to their cellular receptors. Free fatty ccids may also play a role in activating PKC. The enzyme apparently mediates a wide range of signal transduction processes in cells and, therefore, inhibitors directed selectively against PKC may have wide-ranging therapeutic potential. This review highlights the evidence that inappropriate activation of PKC occurs in a number of disease states. Such evidence, however, is often seriously flawed because it relies on the use of phorbol esters, which are potent and direct PKC activators but may not mimic the physiological triggering of the enzyme in cells, or on the use of non-selective protein kinase inhibitors such as H7 and staurosporine. A new generation of bis-indolylmaleimides, derived from the lead provided by staurosporine, shows a high degree of selectivity for PKC over closely related protein kinases and such agents may provide more appropriate tools to investigate the role of PKC in cellular processes.     

S-D-lactoylglutathione as a potential state marker for hemolysis

Red blood cells represent the most abundant cell type in the blood and their energy production is exclusively dependent on glycolysis. However, about 0.1-0.4% of glucose is metabolized via methylglyoxal, a metabolite which is highly toxic to the cells. S-d-lactoylglutathione is an intermediate of methylglyoxal degradation by glyoxalases and is unable to cross cell membrane. Nevertheless, it is measurable in human plasma. This paper claims the introduction of the evaluation of plasma S-d-lactoylglutathione in hemolytic states and proposes its use as a state marker for such cases. According to this hypothesis, higher the rate of hemolysis in non-diabetic patients higher the level of S-d-lactoylglutathione in their plasma. The measurement of S-d-lactoylglutathione in plasma, parallel with other parameters, can be a useful tool in distinguishing hemolytic states and in monitoring the effectiveness of treatment.     

Therapeutic effects of dengue 2 virus capsid protein and staphylococcal nuclease fusion protein on dengue-infected cell cultures

Summary. Dengue infection poses a serious public health problem in most tropical and subtropical areas. No effective antiviral drugs or vaccines are currently available against dengue infection. To explore the feasibility of using capsid-targeted viral inactivation (CTVI) as an antiviral strategy against dengue infection, we constructed a plasmid expressing a fusion protein consisting of staphylococcal nuclease (SN) fused to dengue 2 virus capsid protein (D2C), and investigated its effects on the production of infectious virions when introduced into BHK cells infected with dengue virus. The results indicated that D2C-SN can be expressed and tolerated in this mammalian cell culture. The enzymatically active SN moiety was incorporated into nascent virions during the process of viral assembly. By comparing the effects of incorporated SN and SN*, an enzymatically inactive missense mutant form of wild-type SN, on the infectivity of progeny virions, we clearly demonstrated that nucleolytic activity was the major antiviral mechanism. Expression of D2C-SN fusion protein as a therapeutic agent resulted in a reduction in infectious titers of 12- to 60-fold. Therefore, dengue virus may be particularly vulnerable to a CTVI therapeutic approach.     

The gene for staphylococcal protein A

Protein A from Staphylococcus aureus has become an important tool in immunology and molecular biology due to its specific binding to the constant region of immunoglobulins (Igs) from most mammalian species¹. Many qualitative and quantitative techniques have been developed which take advantage of this ‘pseudo-immune’ reaction². In addition, solid state protein A has recently been introduced in medical therapy to decrease the amount of circulating immune complexes in sera³. In this article Mathias Uhlén, Martin Lindberg and Lennart Philipson describe the structure of the protein A molecule and its gene. They also discuss the possibilities for fusing the protein A gene to other genes.     

Effect of calcium ions on staphylococcal alpha-toxin-induced hemolysis of rabbit erythrocytes.

Calcium in millimolar concentrations protected rabbit erythrocytes from hemolysis caused by staphylococcal alpha-toxin. This effect was maximal at 30 mM CaCl2 and required the continued presence of calcium. The protection was not absolute and could be overcome by increased concentrations of alpha-toxin. Calcium did not block the binding of alpha-toxin to erythrocytes but inhibited the alpha-toxin-induced release of small ions from the cell as measured by 86Rb release. The transient removal of calcium was sufficient to abrogate its protective effect, suggesting that its action involves a reversible alteration in the state of the membrane. The three steps of the alpha-toxin-induced hemolytic sequence are: (i) binding to specific receptors, (ii) formation of transmembrane pores, and (iii) cell lysis. We concluded that calcium acted at step ii by impeding the lateral movement of alpha-toxin necessary to form the transmembrane hexamer pores.     

Interaction of staphylococcal protein A in enzyme-linked immunosorbent assays for detecting staphylococcal antigens

ELISA procedures for detecting staphylococcal antigens may be subject to interference by reactions between staphylococcal protein A (SPA) and IgG molecules. It was found that rabbit IgG reacted with SPA, both in the native state and after conjugation with peroxidase. Sheep IgG, however, did not react with SPA if conjugated with peroxidase. Peroxidase conjugated SPA reacted with rabbit IgG but not with sheep IgG. These results demonstrate that the source of IgG used in an ELISA system is of major importance to correct quantitation of staphylococcal antigens.     

Reevaluation of ethylene oxide hemolysis and irritation potential

The in vitro hemolytic and in vivo mucosal irritation potential of ethylene oxide (EO) was investigated with standard procedures used to determine the biocompatibility of medical devices. Test solutions containing EO at concentrations of 25, 50, 100, 250, 500, 1,250, 2,500, 5,000, or 10,000 microg/mL were prepared in saline to simulate a worst-case aqueous extraction of standard medical devices containing 125, 250, 500, 1,250, 2,500, 6,250, 12,500, 25,000, or 50,000 microg/g of EO, respectively. Concentrations of EO up to 500 microg/mL were not hemolytic ( < 5% hemolysis after a 4-h exposure), whereas > or =1250 microg/mL of EO resulted in significant hemolysis. Hamster cheek pouches exposed to cotton pellets saturated with EO at concentrations of up to 2500 microg/mL for 4 h with a recovery period of 14 days were without effects attributable to EO. However, at > or =5000 microg/mL of EO, significant histomorphological alterations of the buccal mucosa were observed and attributed to EO exposure. It was concluded that solutions of EO of up to 500 microg/mL representing an aqueous extract of a general medical device containing at least 2500 microg/g of EO residue do not result in significant hemolysis and irritation.     

Transfer of prostasomal CD59 to CD59-deficient red blood cells results in protection against complement-mediated hemolysis

PROBLEM: Prostasomes isolated from human seminal plasma have complement regulatory properties because of their content of CD59, a glycosylphosphatidylinositol (GPI)-anchored protein. We investigated a functional role of prostasomes by the possibility of transferring CD59 from prostasomes to rabbit erythrocytes (RE) and human erythrocytes obtained from patients with paroxysmal nocturnal hemoglobinuria (PNH), both types of cells lacking CD59. METHOD OF STUDY: We used the assay of hemolytic activity of the alternative pathway of the complement system to compare the liability of the erythrocytes to hemolysis by the complement system with and without pre-incubation with prostasomes. CD59 gained by the RE and PNH erythrocytes was established by flow cytometry. The effect of phosphatidylinositol phospholipase C (PIPLC) on the GPI anchor of prostasomal CD59 and the effect of heat treatment on the prostasomes were also studied. Anti-CD59 antibodies were used to block the protective effect of prostasomes on erythrocytes. RESULTS: Both RE and PNH erythrocytes showed diminished complement-mediated hemolysis after incubation with prostasomes. This was because of the transfer of CD59 from prostasomes to the red blood cells during pre-incubation as evidenced by the hemolytic assay and flow-cytometry. The efficacy of the prostasomes was affected by heat treatment and was totally lost at 100 degrees C. Phosphatidylinositol phospholipase C broke the GPI anchor and released CD59 from prostasomes and the RE surface (after pre-incubation with prostasomes) but not from the human PNH erythrocytes. CONCLUSIONS: A transfer mechanism of CD59 takes place during pre-incubation from prostasomes to erythrocytes lacking CD59 which supports the idea that transfer of prostasomal CD59 can protect cells from lysis elicited by C5b-9. This might be a mechanism by which autologous and allogeneic cells are protected against complement attack in the genital tracts.     

Inhibitory effect of flavin mononucleotide on the hemolysis of rabbit erythrocytes by staphylococcal alpha-toxin.

Flavin mononucleotide diminished the hemolytic action of staphylococcal alpha-toxin on rabbit erythrocytes by competitive inhibition, probably by its interaction with the alpha-toxin binding sites on the cell membrane.     

Therapeutic potential of anti-IgE antibodies

Anti-IgE antibodies directed against the FcRI-binding region on IgE inhibit binding of IgE to IgE receptors without inducing mediator release from IgE sensitized cells. In mice these antibodies selectively reduce serum IgE, inhibit antigen induced skin reactions, cytokine production by lung Th2 cells, and pulmonary eosinophil infiltration. Clinical trials in humans reveal that such antibodies are well tolerated and reduce rhinitis symptoms and early and late phase bronchoconstriction responses. Thus interruption of the allergic cascade at the IgE antibody level with non-anaphylactogenic anti-IgE antibodies is effective and represents an attractive intervention for the treatment of allergic diseases.