Thomas Areschoug

Surface Proteins of Streptococcus agalactiae and Related Proteins in Other Bacterial Pathogens

SUMMARY Streptococcus agalactiae (group B Streptococcus) is the major cause of invasive bacterial disease, including meningitis, in the neonatal period. Although prophylactic measures have contributed to a substantial reduction in the number of infections, development of a vaccine remains an important goal. While much work in this field has focused on the S. agalactiae polysaccharide capsule, which is an important virulence factor that elicits protective immunity, surface proteins have received increasing attention as potential virulence factors and vaccine components. Here, we summarize current knowledge about S. agalactiae surface proteins, with emphasis on proteins that have been characterized immunochemically and/or elicit protective immunity in animal models. These surface proteins have been implicated in interactions with human epithelial cells, binding to extracellular matrix components, and/or evasion of host immunity. Of note, several S. agalactiae surface proteins are related to surface proteins identified in other bacterial pathogens, emphasizing the general interest of the S. agalactiae proteins. Because some S. agalactiae surface proteins elicit protective immunity, they hold promise as components in a vaccine based only on proteins or as carriers in polysaccharide conjugate vaccines.

Scavenger receptors: role in innate immunity and microbial pathogenesis

Accumulating evidence shows that many scavenger receptors (SR), including SR‐A, MARCO and CD36, represent an important part of the innate immune defence by acting as pattern‐recognition receptors, in particular against bacterial pathogens. Several SR are expressed on macrophages and dendritic cells, where they act as phagocytic receptors mediating non‐opsonic phagocytosis of pathogenic microbes. Another important function of some SR is to act as co‐receptors to Toll‐like receptors (TLR), modulating the inflammatory response to TLR agonists. On bacteria, the SR ligands have commonly been reported to be lipopolysaccharide and lipoteichoic acid, but recent advances in the field indicate that bacterial surface proteins play a more important role as target molecules for SR than previously thought. Interestingly, recent data show that major pathogens, including Streptococcus pyogenes and the group B streptococcus, have evolved mechanisms to evade SR‐mediated recognition. Moreover, intracellular pathogens, such as hepatitis C virus and Plasmodium falciparum, utilize the SR to gain entry into host cells, focusing interest on the importance of SR also in the molecular pathogenesis of infectious diseases. This review highlights the complex interactions between SR and pathogenic microbes, and discusses the role of these interactions in host defence and microbial pathogenesis.

The R28 protein of Streptococcus pyogenes is related to several group B streptococcal surface proteins, confers protective immunity and promotes binding to human epithelial cells.

The R28 protein is a surface molecule expressed by some strains of Streptococcus pyogenes (group A streptococcus). Here, we present evidence that R28 may play an important role in virulence. Sequence analysis demonstrated that R28 has an extremely repetitive sequence and can be viewed as a chimera derived from the three surface proteins Rib, α and β of the group B streptococcus (GBS). Thus, the gene encoding R28 may have originated in GBS. The R28 protein promotes adhesion to human epithelial cells, as shown by experiments with an R28‐negative mutant and by the demonstration that antibodies to highly purified R28 inhibited adhesion. In a mouse model of lethal intraperitoneal S. pyogenes infection, antibodies to R28 conferred protective immunity. However, the virulence of an R28‐negative mutant was similar to that of the parental strain in the intraperitoneal infection model. Together, these data indicate that R28 represents a novel type of adhesin expressed by S. pyogenes and that R28 may also act as a target for protective antibodies at later stages of an infection. We consider the hypothesis that R28 played a pathogenetic role in the well‐known epidemics of childbed fever (puerperal fever), which were caused by S. pyogenes. A role for R28 in these epidemics is suggested by epidemiological data.

Group B Streptococcus suppression of phagocyte functions by protein-mediated engagement of human Siglec-5.

Group B Streptococcus (GBS) is a leading cause of invasive bacterial infections in human newborns. A key GBS virulence factor is its capsular polysaccharide (CPS), displaying terminal sialic acid (Sia) residues which block deposition and activation of complement on the bacterial surface. We recently demonstrated that GBS Sia can bind human CD33-related Sia-recognizing immunoglobulin (Ig) superfamily lectins (hCD33rSiglecs), a family of inhibitory receptors expressed on the surface of leukocytes. We report the unexpected discovery that certain GBS strains may bind one such receptor, hSiglec-5, in a Sia-independent manner, via the cell wall–anchored β protein, resulting in recruitment of SHP protein tyrosine phosphatases. Using a panel of WT and mutant GBS strains together with Siglec-expressing cells and soluble Siglec-Fc chimeras, we show that GBS β protein binding to Siglec-5 functions to impair human leukocyte phagocytosis, oxidative burst, and extracellular trap production, promoting bacterial survival. We conclude that protein-mediated functional engagement of an inhibitory host lectin receptor promotes bacterial innate immune evasion.

Pattern Recognition Receptors and Their Role in Innate Immunity: Focus on Microbial Protein Ligands

Antigen-presenting cells, such as macrophages and dendritic cells, represent a central and important part of the immune defence against invading microorganisms, as they participate in initial capture and processing of microbial antigens (innate immunity) and then activation of specific T and B cell effector mechanisms (acquired immunity). Recognition of microbial molecules by antigen-presenting cells occurs through so called pattern recognition receptors (PRRs), which recognize conserved structures, or pathogen-associated molecular patterns, in pathogenic microbes. The Toll-like receptors are the most extensively studied of these receptors, but accumulating evidence shows that other PRRs, such as scavenger receptors, C-type lectin receptors and NOD-like receptors, also play important roles in the innate immune defence. Here, we summarize current knowledge of the role of various PRRs in the defence against pathogenic microorganisms and we report recent advances in studies of different receptor-ligand interactions. In particular, we focus on the importance of microbial proteins as ligands for PRRs.

Streptococcal beta protein has separate binding sites for human factor H and IgA-Fc.

The group B streptococcus (GBS) is the most important cause of life-threatening bacterial infections in newborn infants. Protective immunity to GBS infection is elicited by several surface proteins, one of which, the β protein, is known to bind human IgA-Fc. Here, we show that the β protein also binds human factor H (FH), a negative regulator of complement activation. Absorption experiments with whole human plasma demonstrated binding of FH to a GBS strain expressing β protein but not to an isogenic β-negative mutant. This binding was due to a direct interaction between β and FH, as shown by experiments with purified proteins. Inhibition tests and studies with β fragments demonstrated that FH and IgA-Fc bind to separate and nonoverlapping regions in β. Heparin, a known ligand for FH, specifically inhibited the binding between β and FH, suggesting that FH has overlapping binding sites for β and heparin. Bacteria-bound FH retained its complement regulatory activity, implying that β-expressing GBS may use bound FH to evade complement attack. The finding that β protein binds FH adds to a growing list of interactions between human pathogens and complement regulatory proteins, supporting the notion that these interactions are of general importance in bacterial pathogenesis.

Isolation and Detection of Human IgA Using a Streptococcal IgA-Binding Peptide

Bacterial proteins that bind to the Fc part of IgG have found widespread use in immunology. A similar protein suitable for the isolation and detection of human IgA has not been described. Here, we show that a 50-residue synthetic peptide, designated streptococcal IgA-binding peptide (Sap) and derived from a streptococcal M protein, can be used for single-step affinity purification of human IgA. High affinity binding of IgA required the presence in Sap of a C-terminal cysteine residue, not present in the intact M protein. Passage of human serum through a Sap column caused depletion of >99% of the IgA, and elution of the column allowed quantitative recovery of highly purified IgA, for which the proportions of the IgA1 and IgA2 subclasses were the same as in whole serum. Moreover, immobilized Sap could be used for single-step purification of secretory IgA of both subclasses from human saliva, with a recovery of ∼45%. The Sap peptide could also be used to specifically detect IgA bound to Ag. Together, these data indicate that Sap is a versatile Fc-binding reagent that may open new possibilities for the characterization of human IgA.

Siglec-5 and Siglec-14 are polymorphic paired receptors that modulate neutrophil and amnion signaling responses to group B Streptococcus

Siglec-5 and Siglec-14 are shown to be paired inhibitory/activating receptors expressed on neutrophils and amniotic epithelium and modulating immune responses to group B Streptococcus.

Structural requirements for the interaction of human IgM and IgA with the human Fcα/μ receptor

Here we unravel the structural features of human IgM and IgA that govern their interaction with the human Fcα/μ receptor (hFcα/μR). Ligand polymerization status was crucial for the interaction, because hFcα/μR binding did not occur with monomeric Ab of either class. hFcα/μR bound IgM with an affinity in the nanomolar range, whereas the affinity for dimeric IgA (dIgA) was tenfold lower. Panels of mutant IgM and dIgA were used to identify regions critical for hFcα/μR binding. IgM binding required contributions from both Cμ3 and Cμ4 Fc domains, whereas for dIgA, an exposed loop in the Cα3 domain was crucial. This loop, comprising residues Pro440–Phe443, lies at the Fc domain interface and has been implicated in the binding of host receptors FcαRI and polymeric Ig receptor (pIgR), as well as IgA‐binding proteins produced by certain pathogenic bacteria. Substitutions within the Pro440–Phe443 loop resulted in loss of hFcα/μR binding. Furthermore, secretory component (SC, the extracellular portion of pIgR) and bacterial IgA‐binding proteins were shown to inhibit the dIgA–hFcα/μR interaction. Therefore, we have identified a motif in the IgA–Fc inter‐domain region critical for hFcα/μR interaction, and highlighted the multi‐functional nature of a key site for protein–protein interaction at the IgA Fc domain interface.

Cross-Protection between Group A and Group B Streptococci Due to Cross-Reacting Surface Proteins

The R28 protein of group A streptococcus (GAS) and the Rib protein of group B streptococcus (GBS) are surface molecules that elicit protective immunity to experimental infection. These proteins are members of the same family and cross-react immunologically. In spite of extensive amino acid residue identity, the cross-reactivity between R28 and Rib was found to be limited, as shown by analysis with highly purified proteins and specific antisera. Nevertheless, immunization of mice with purified R28 conferred protection against lethal infection with Rib-expressing GBS strains, and immunization with Rib conferred protection against R28-expressing GAS. Thus, R28 and Rib elicited cross-protective immunity. Characterization of many clinical GAS and GBS isolates expressing R28 or Rib, respectively, indicated that most of them expressed proteins similar to those of the reference strains. Analysis of these data suggests that cross-protection may influence the outcome of natural infections with R28-expressing GAS and Rib-expressing GBS.