Characterization of an IgA receptor from group B streptococci: specificity for serum IgA

Emerging infection: streptococcal toxic shock-like syndrome caused by group B Streptococcus (GBS), Streptococcus agalactiae

Streptococcus agalactiae or Group B Streptococcus (GBS) infections are commonly associated with infections in neonates and pregnant women. However, there has been a rising incidence in nonpregnant adults. The risk of GBS infection in nonpregnant adults is increased for patients of advanced age and those with underlying medical conditions such as diabetes mellitus and cancer. We present a 77-year-old female with type-2 diabetes mellitus, hypertension, and bilateral foot ulcers that presented in probable septic shock with necrotic foot ulcers and necrotizing fasciitis and underwent bilateral lower limb amputations. The patient fulfilled the Streptococcal Toxic Shock Syndrome (STSS) criteria as defined by The Working Group on Severe Streptococcal Infections. These criteria were created for group A Streptococcus (Streptococcus pyogenes). Our patient fulfilled the Working Group's criteria, except that the blood culture was positive for group B Streptococcus (Streptococcus agalactiae). Numerous studies demonstrate the importance of early detection and antibiotic treatment for GBS infections in general and early surgical management for necrotizing soft tissue infections (NSTIs) such as necrotizing fasciitis.

Immunomodulating Enzymes from Streptococcus pyogenes-In Pathogenesis, as Biotechnological Tools, and as Biological Drugs

Streptococcus pyogenes, or Group A Streptococcus, is an exclusively human pathogen that causes a wide variety of diseases ranging from mild throat and skin infections to severe invasive disease. The pathogenesis of S. pyogenes infection has been extensively studied, but the pathophysiology, especially of the more severe infections, is still somewhat elusive. One key feature of S. pyogenes is the expression of secreted, surface-associated, and intracellular enzymes that directly or indirectly affect both the innate and adaptive host immune systems. Undoubtedly, S. pyogenes is one of the major bacterial sources for immunomodulating enzymes. Major targets for these enzymes are immunoglobulins that are destroyed or modified through proteolysis or glycan hydrolysis. Furthermore, several enzymes degrade components of the complement system and a group of DNAses degrade host DNA in neutrophil extracellular traps. Additional types of enzymes interfere with cellular inflammatory and innate immunity responses. In this review, we attempt to give a broad overview of the functions of these enzymes and their roles in pathogenesis. For those enzymes where experimentally determined structures exist, the structural aspects of the enzymatic activity are further discussed. Lastly, we also discuss the emerging use of some of the enzymes as biotechnological tools as well as biological drugs and vaccines.

Human CEACAM1 is targeted by a Streptococcus pyogenes adhesin implicated in puerperal sepsis pathogenesis

Life-threatening bacterial infections in women after childbirth, known as puerperal sepsis, resulted in classical epidemics and remain a global health problem. While outbreaks of puerperal sepsis have been ascribed to Streptococcus pyogenes, little is known about disease mechanisms. Here, we show that the bacterial R28 protein, which is epidemiologically associated with outbreaks of puerperal sepsis, specifically targets the human receptor CEACAM1. This interaction triggers events that would favor the development of puerperal sepsis, including adhesion to cervical cells, suppression of epithelial wound repair and subversion of innate immune responses. High-resolution structural analysis showed that an R28 domain with IgI3-like fold binds to the N-terminal domain of CEACAM1. Together, these findings demonstrate that a single adhesin-receptor interaction can drive the pathogenesis of bacterial sepsis and provide molecular insights into the pathogenesis of one of the most important infectious diseases in medical history.

Group B Streptococcus Surface Protein β: Structural Characterization of a Complement Factor H-Binding Motif and Its Contribution to Immune Evasion

The β protein from group B Streptococcus (GBS) is a ∼132-kDa, cell-surface exposed molecule that binds to multiple host-derived ligands, including complement factor H (FH). Many details regarding this interaction and its significance to immune evasion by GBS remain unclear. In this study, we identified a three-helix bundle domain within the C-terminal half of the B75KN region of β as the major FH-binding determinant and determined its crystal structure at 2.5 Å resolution. Analysis of this structure suggested a role in FH binding for a loop region connecting helices α1 and α2, which we confirmed by mutagenesis and direct binding studies. Using a combination of protein cross-linking and mass spectrometry, we observed that B75KN bound to complement control protein (CCP)3 and CCP4 domains of FH. Although this binding site lies within a complement regulatory region of FH, we determined that FH bound by β retained its decay acceleration and cofactor activities. Heterologous expression of β by Lactococcus lactis resulted in recruitment of FH to the bacterial surface and a significant reduction of C3b deposition following exposure to human serum. Surprisingly, we found that FH binding by β was not required for bacterial resistance to phagocytosis by neutrophils or killing of bacteria by whole human blood. However, loss of the B75KN region significantly diminished bacterial survival in both assays. Although our results show that FH recruited to the bacterial surface through a high-affinity interaction maintains key complement-regulatory functions, they raise questions about the importance of FH binding to immune evasion by GBS as a whole.

Bacterial protein domains with a novel Ig-like fold target human CEACAM receptors

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is the major cause of neonatal sepsis in humans. A critical step to infection is adhesion of bacteria to epithelial surfaces. GBS adhesins have been identified to bind extracellular matrix components and cellular receptors. However, several putative adhesins have no host binding partner characterised. We report here that surface-expressed β protein of GBS binds to human CEACAM1 and CEACAM5 receptors. A crystal structure of the complex showed that an IgSF domain in β represents a novel Ig-fold subtype called IgI3, in which unique features allow binding to CEACAM1. Bioinformatic assessment revealed that this newly identified IgI3 fold is not exclusively present in GBS but is predicted to be present in adhesins from other clinically important human pathogens. In agreement with this prediction, we found that CEACAM1 binds to an IgI3 domain found in an adhesin from a different streptococcal species. Overall, our results indicate that the IgI3 fold could provide a broadly applied mechanism for bacteria to target CEACAMs.

Interactions of head-kidney leucocytes from giant grouper, Epinephelus lanceolatus, with pathogenic Streptococcus agalactiae strains from marine and terrestrial origins

Streptococcus agalactiae (Group B Streptococcus, GBS) is emerging as a genetically diverse species infecting farmed and wild fish, including commercially and culturally important groupers. To better understand how S. agalactiae are pathogenic in fish, we investigated interactions between isolates from fish and terrestrial hosts and the cellular immune system of Queensland grouper Epinephelus lanceolatus using flow cytometry. Adherent head-kidney leucocytes (HKL) from Queensland grouper displayed two main cell populations with distinct forward and side scatter by flow cytometry. The population of smaller and less complex cells (P1) was composed of monocytes, lymphocytes and thrombocytes, while the population of primarily larger and more complex cells (P2) comprised predominantly of macrophages and neutrophils. The cells in P2 had higher phagocytic index and capacity when incubated with fluorescent latex beads. HKL were activated by phorbol myristate acetate (PMA) but were unresponsive to lipopolysaccharide (LPS) and peptidoglycan (PTG), suggesting the absence of specific receptors on the surface of these cells for these ligands or a requirement for intermediates. In in vitro phagocytosis assays, all fish isolates of GBS activated a respiratory burst in P2 indicated by significant production of intracellular reactive oxygen species (ROS). Similarly, dog and cat isolates of different serotype and sequence type also induced ROS production in grouper HKL. However, human, crocodile and bovine isolates of GBS did not elicit significant ROS in HKL although they coincided with the highest phagocytic index. This suggests that these strains are capable of quenching ROS production. Terrestrial isolates significantly increased mortality of Queensland grouper leucocytes in vitro, aligned with a more diverse repertoire of cellular toxins in these strains. Opsonisation of a marine strain and terrestrial strain of GBS with antiserum raised against the marine strain resulted in an increase in ROS production by HKL in both cases although there was low antigenic cross reactivity between the two strains by flow cytometry, reflecting their diverse serotypes (Ib vs III). However, pre-incubation of either strain with normal serum from grouper also increased ROS production of HKL suggesting other opsonins may be involved. Based on these results it appears that piscine and terrestrial GBS isolates have contrasting strategies when interacting with the cellular immune system of Queensland grouper; the former seemingly evading phagocytosis, whilst the latter are readily phagocytosed but counteract ROS production.

Surface Structures of Group B Streptococcus Important in Human Immunity

The surface of the Gram-positive opportunistic pathogen Streptococcus agalactiae, or group B Streptococcus (GBS), harbors several carbohydrate and protein antigens with the potential to be effective vaccines. Capsular polysaccharides of all clinically-relevant GBS serotypes coupled to immunogenic proteins of both GBS and non-GBS origin have undergone extensive testing in animals that led to advanced clinical trials in healthy adult women. In addition, GBS proteins either alone or in combination have been tested in animals; a fusion protein construct has recently advanced to human clinical studies. Given our current understanding of the antigenicity and immunogenicity of the wide array of GBS surface antigens, formulations now exist for the generation of viable vaccines against diseases caused by GBS.

Streptococcus agalactiae Non-Pilus, Cell Wall-Anchored Proteins: Involvement in Colonization and Pathogenesis and Potential as Vaccine Candidates

Group B Streptococcus (GBS) remains an important etiological agent of several infectious diseases including neonatal septicemia, pneumonia, meningitis, and orthopedic device infections. This pathogenicity is due to a variety of virulence factors expressed by Streptococcus agalactiae. Single virulence factors are not sufficient to provoke a streptococcal infection, which is instead promoted by the coordinated activity of several pathogenicity factors. Such determinants, mostly cell wall-associated and secreted proteins, include adhesins that mediate binding of the pathogen to host extracellular matrix/plasma ligands and cell surfaces, proteins that cooperate in the invasion of and survival within host cells and factors that neutralize phagocytosis and/or modulate the immune response. The genome-based approaches and bioinformatics tools and the extensive use of biophysical and biochemical methods and animal model studies have provided a great wealth of information on the molecular structure and function of these virulence factors. In fact, a number of new GBS surface-exposed or secreted proteins have been identified (GBS immunogenic bacterial adhesion protein, leucine-rich repeat of GBS, serine-rich repeat proteins), the three-dimensional structures of known streptococcal proteins (αC protein, C5a peptidase) have been solved and an understanding of the pathogenetic role of "old" and new determinants has been better defined in recent years. Herein, we provide an update of our current understanding of the major surface cell wall-anchored proteins from GBS, with emphasis on their biochemical and structural properties and the pathogenetic roles they may have in the onset and progression of host infection. We also focus on the antigenic profile of these compounds and discuss them as targets for therapeutic intervention.

Human IgA-binding peptides selected from random peptide libraries: affinity maturation and application in IgA purification

Phage display system is a powerful tool to design specific ligands for target molecules. Here, we used disulfide-constrained random peptide libraries constructed with the T7 phage display system to isolate peptides specific to human IgA. The binding clones (A1-A4) isolated by biopanning exhibited clear specificity to human IgA, but the synthetic peptide derived from the A2 clone exhibited a low specificity/affinity (K(d) = 1.3 μm). Therefore, we tried to improve the peptide using a partial randomized phage display library and mutational studies on the synthetic peptides. The designed Opt-1 peptide exhibited a 39-fold higher affinity (K(d) = 33 nm) than the A2 peptide. An Opt-1 peptide-conjugated column was used to purify IgA from human plasma. However, the recovered IgA fraction was contaminated with other proteins, indicating nonspecific binding. To design a peptide with increased binding specificity, we examined the structural features of Opt-1 and the Opt-1-IgA complex using all-atom molecular dynamics simulations with explicit water. The simulation results revealed that the Opt-1 peptide displayed partial helicity in the N-terminal region and possessed a hydrophobic cluster that played a significant role in tight binding with IgA-Fc. However, these hydrophobic residues of Opt-1 may contribute to nonspecific binding with other proteins. To increase binding specificity, we introduced several mutations in the hydrophobic residues of Opt-1. The resultant Opt-3 peptide exhibited high specificity and high binding affinity for IgA, leading to successful isolation of IgA without contamination.

A conserved host and pathogen recognition site on immunoglobulins: structural and functional aspects

A common site in the constant region (Fc) of immunoglobulins is recognized by host receptors and is a frequent target of proteins expressed by pathogens. This site is located at the junction of two constant domains in the antibody heavy chains and produces a large shallow cavity formed by loops of the CH2 and CH3 domains in IgG and IgA (CH3 and CH4 domains in IgM). Crystal structures have been determined for complexes of IgG-Fc and IgA-Fc with a structurally diverse set of host, pathogen and in vitro selected ligands. While pathogen proteins may directly block interactions with the immunoglobulins thereby evading host immunity, it is likely that the same pathogen molecules also interact with other host factors to carry out their primary biological function. Herein we review the structural and functional aspects of host and pathogen molecular recognition of the common site on the Fc of immunoglobulins. We also propose that some pathogen proteins may promote virulence by affecting the bridging between innate and adaptive immunity.

Human Siglec-5 inhibitory receptor and immunoglobulin A (IgA) have separate binding sites in streptococcal beta protein

Sialic acid-binding immunoglobulin-like lectins (Siglecs) are receptors believed to be important for regulation of cellular activation and inflammation. Several pathogenic microbes bind specific Siglecs via sialic acid-containing structures at the microbial surface, interactions that may result in modulation of host responses. Recently, it was shown that the group B Streptococcus (GBS) binds to human Siglec-5 (hSiglec-5), an inhibitory receptor expressed on macrophages and neutrophils, via the IgA-binding surface β protein, providing the first example of a protein/protein interaction between a pathogenic microbe and a Siglec. Here we show that the hSiglec-5-binding part of β resides in the N-terminal half of the protein, which also harbors the previously determined IgA-binding region. We constructed bacterial mutants expressing variants of the β protein with non-overlapping deletions in the N-terminal half of the protein. Using these mutants and recombinant β fragments, we showed that the hSiglec-5-binding site is located in the most N-terminal part of β (B6N region; amino acids 1–152) and that the hSiglec-5- and IgA-binding domains in β are completely separate. We showed with BIAcore^TM analysis that tandem variants of the hSiglec-5- and IgA-binding domains bind to their respective ligands with high affinity. Finally, we showed that the B6N region, but not the IgA-binding region of β, triggers recruitment of the tyrosine phosphatase SHP-2 to hSiglec-5 in U937 monocytes. Taken together, we have identified and isolated the first microbial non-sialic acid Siglec-binding region that can be used as a tool in studies of the β/hSiglec-5 interaction.

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 beta 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 beta 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.

Development of specific and rapid detection of bacterial pathogens in dairy products by PCR

A simple and specific method for direct detection of bovine mastitis pathogens (Streptococcus agalactiae (GBS), Staphylococcus aureus and Escherichia coli) in milk products, bacterial samples from milk and isolated bacterial DNA was developed. The method is based on polymerase chain reaction (PCR) using sequence-specific primers only for GBS and species-specific primers derived from 16S and 23S rRNA for all chosen species. The presence of the gene of surface immunogenic protein (Sip) in bovine GBS isolates, described previously only in human GBS isolates was confirmed. The GBS detection was performed with the sequence coding for surface immunogenic protein from GBS human isolates designated as Sip specific sequence (SSS); this sequence was selected for specific primer design. The sequence is unique for GBS and was designed from a consensus of all known sip genes. The specific identification was shown on a collection of 75 GBS bovine isolates from different localities in Slovakia. All isolates were positive to SSS, 16S and 23S rRNA sequence. The 16S and 23S rRNA PCR detection was also performed with S. aureus and E. coli isolates and specific PCR products were also detected. The detection limit of this assay for milk products was 6 CFU/microL (i.e. 6000 CFU/mL) for GBS and E. coli, and 16 CFU/microL for S. aureus. This rapid, sensitive and specific diagnostic method can be performed within hours and represents an innovative diagnostic tool for the detection of milk pathogens in dairy products.

BibA: a novel immunogenic bacterial adhesin contributing to group B Streptococcus survival in human blood

By the analysis of the recently sequenced genomes of Group B Streptococcus (GBS) we have identified a novel immunogenic adhesin with anti-phagocytic activity, named BibA. The bibA gene is present in 100% of the 24 GBS strains analysed. BibA-specific IgG were found in human sera from normal healthy donors. The putative protein product is a polypeptide of 630 amino acids containing a helix-rich N-terminal domain, a proline-rich region and a canonical LPXTG cell wall-anchoring domain. BibA is expressed on the surface of several GBS strains, but is also recovered in GBS culture supernatants. BibA specifically binds to human C4-binding protein, a regulator of the classic complement pathway. Deletion of the bibA gene severely reduced the capacity of GBS to survive in human blood and to resist opsonophagocytic killing by human neutrophils. In addition, BibA expression increased the virulence of GBS in a mouse infection model. The role of BibA in GBS adhesion was demonstrated by the impaired ability of a bibA knockout mutant strain to adhere to both human cervical and lung epithelial cells. Furthermore, we calculated that recombinant BibA bound to human epithelial cells of distinct origin with an affinity constant of approximately 10(-8) M for cervical epithelial cells. Hence BibA is a novel multifunctional protein involved in both resistance to phagocytic killing and adhesion to host cells. The identification of this potential new virulence factor represents an important step in the development of strategies to combat GBS-associated infections.

The IgD-binding domain of the Moraxella IgD-binding protein MID (MID962-1200) activates human B cells in the presence of T cell cytokines

Moraxella catarrhalis immunoglobulin D (IgD)-binding protein (MID) is an outer membrane protein with specific affinity for soluble and cell-bound human IgD. Here, we demonstrate that mutated M. catarrhalis strains devoid of MID show a 75% decreased activation of human B cells as compared with wild-type bacteria. In contrast to MID-expressing Moraxella, the MID-deficient Moraxella mutants did not bind to human CD19+ IgD+ B cells. The smallest MID fragment with preserved IgD-binding capacity comprises 238 amino acids (MID(962-1200)). To prove the specificity of MID(962-1200) for IgD, a Chinese hamster ovary (CHO) cell line expressing membrane-anchored human IgD was manufactured. MID(962-1200) bound strongly to the recombinant IgD on CHO cells. Moreover, MID(962-1200) stimulated peripheral blood lymphocyte (PBL) proliferation 5- and 15-fold at 0.1 and 1.0 microg/ml, respectively. This activation could be blocked completely by antibodies directed against the CD40 ligand (CD154). MID(962-1200) also activated purified B cells in the presence of interleukin (IL)-2 or IL-4. An increased IL-6 production was seen after stimulation with MID(962-1200), as revealed by a human cytokine protein array. MID(962-1200) fused to green fluorescent protein (GFP) bound to human B cells and activated PBL to the same degree as MID(962-1200). Taken together, MID is the only IgD-binding protein in Moraxella. Furthermore, the novel T cell-independent antigen MID(962-1200) may, together with MID(962-1200)-GFP, be considered as promising reagents in the study of IgD-dependent B cell activation.

Surface proteins of Streptococcus agalactiae and related proteins in other bacterial pathogens

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.

The group B streptococcal beta and pneumococcal Hic proteins are structurally related immune evasion molecules that bind the complement inhibitor factor H in an analogous fashion

Complement evasion by different mechanisms is important for microbial virulence and survival in the host. One strategy used by pathogenic bacteria is to bind the soluble complement inhibitor factor H (fH) to their surfaces. In group B streptococci and pneumococci, fH binding has been shown to be mediated by the surface proteins beta and Hic, respectively. We showed previously that Hic binds to the middle region of fH and protects the pneumococcus from opsonophagocytosis. As the beta protein and Hic are structurally closely related, we wanted to compare the fH binding characteristics of these two proteins. By using direct binding assays with radiolabeled proteins and surface plasmon resonance analysis we show that both beta and Hic bind to the short consensus repeats 8-11 and 12-14 in the middle region of fH. Peptide mapping analysis suggested that the fH-binding sites on beta and Hic were composed of discontinuous and partially homologous sequences. Thus, the bacterial virulence proteins use multiple binding sites on fH to secure high avidity. Also, the functionally active sites on fH are thereby left free to inhibit C3b deposition and opsonophagocytosis. These results reveal the evolutionary conservation of an analogous immune evasion strategy in different types of pathogenic streptococci. Importantly, the respective virulence factors could be exploited in the development of protein-based vaccines against these pathogens.

A proline-rich region with a highly periodic sequence in Streptococcal beta protein adopts the polyproline II structure and is exposed on the bacterial surface

Proline-rich regions have been identified in many surface proteins of pathogenic streptococci and staphylococci. These regions have been suggested to be located in cell wall-spanning domains and/or to be required for surface expression of the protein. Because little is known about these regions, which are found in extensively studied and biologically important surface proteins, we characterized the proline-rich region in one such protein, the beta protein of group B streptococci. The proline-rich region in beta, designated the XPZ region, has a proline at every third position, and the sequence is highly periodic in other respects. Immunochemical analysis showed that the XPZ region was not associated with the cell wall but was exposed on the bacterial surface. Moreover, characterization of a beta mutant lacking the XPZ region demonstrated that this region was not required for surface expression of the beta protein. Comparison of the XPZ region in different beta proteins showed that it varied in size but always retained the typical sequence periodicity. Circular dichroism spectroscopy indicated that the XPZ region had the structure of a polyproline II helix, an extended and solvent-exposed structure with exactly three residues per turn. Because of the three-residue sequence periodicity in the XPZ region, it is expected to be amphipathic and to have distinct nonpolar and polar surfaces. This study identified a proline-rich structure with unique properties that is exposed on the surface of an important human pathogen.

High expression of a C protein beta antigen gene among invasive strains from certain clonally related groups of type Ia and Ib group B streptococci

Serotyped strains of group B streptococci can be divided into subtypes based on restriction endonuclease digestion patterns (RDP). Profiles of cell-bound proteins were compared among RDP types. Proteins that showed a remarkable difference in the degree of expression were found among strains of RDP Ia-3, which has been considered potentially virulent, as well as of RDP Ib-1. For RDP Ia-3 strains, the protein was predominant in strains from cerebrospinal fluid (CSF) but was mostly a minor component in other strains. For RDP Ib-1 strains, the protein was predominant in strains from CSF, showed diversity in strains from blood, and was mostly a minor component in other strains. By N-terminal sequencing analysis, the protein was identified as a C protein beta antigen. The level of bound immunoglobulin A (IgA) or anti-beta antigen monoclonal antibody correlated with the level of expressed beta antigen, and invasive strains showed remarkably high levels of binding; the exception was a CSF-derived strain of RDP Ib-1 which produced a large amount of beta antigen and showed a high level of binding of anti-beta antigen monoclonal antibody but no IgA binding. PCR-based amplification revealed that the beta antigen gene was detected in all RDP Ia-3 and Ib-1 strains but was not found in any strains of other RDP types. Competitive reverse transcriptase PCR demonstrated that the difference in the amount of protein produced was due to the difference in the level of expression of the beta antigen mRNA. Our findings imply that differences in gene expression for a protein may contribute to the invasiveness of RDP Ia-3 and Ib-1 strains for the host.

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 approximately 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.

Bovine immunoglobulin A (IgA)-binding activities of the surface-expressed Mig protein of Streptococcus dysgalactiae

The Mig protein of Streptococcus dysgalactiae is a type III immunoglobulin G (IgG)-binding protein, expressing IgG- and alpha2-macroglobulin (alpha2-M)-binding receptors. This study showed that the Mig protein also displays binding activities to bovine immunoglobulin A (B-IgA). Biotin-labelled bovine serum IgA bound immobilized recombinant Mig and alpha2-M receptors derived from Mig, as well as the native Mig extracted from the surface of S. dysgalactiae strain SDG8 and the alpha(2)-M receptor released from the isogenic mig mutant strain Mig8-Mt, as determined by Western blotting and ELISA. There was no B-IgA binding activity to the immobilized IgG receptor derived from Mig or the proteins in the culture supernatant from the mig mutant strain Mig7-Mt, in which expression of Mig or Mig-related peptides on the cell surface was completely abolished. In a reciprocal experiment, biotin-labelled Mig was found to bind immobilized bovine serum IgA but not human IgA (H-IgA). The binding of Mig to bovine serum IgA was competitively inhibited by unlabelled Mig, intact and truncated alpha(2)-M receptors, and bovine serum IgA, but not by the Mig-IgG receptor, H-IgA or B-IgG. The binding of Mig and partially purified bovine secretory IgA (B-sIgA) was also characterized by Western blotting. Membrane-immobilized B-sIgA did not react with the biotin-labelled Mig, whereas soluble B-sIgA showed binding activity to the immobilized alpha2-M receptor of Mig. It is therefore concluded that the 11 kDa N-terminal region of the alpha2-M receptor of the S. dysgalactiae Mig protein specifically binds soluble and immobilized bovine serum IgA, as well as soluble B-sIgA. This is believed to be the first report of a B-IgA-binding protein in S. dysgalactiae.

Human immunoglobulin A (IgA)-specific ligands from combinatorial engineering of protein A

Affinity reagents capable of selective recognition of the different human immunoglobulin isotypes are important detection and purification tools in biotechnology. Here we describe the development and characterization of affinity proteins (affibodies) showing selective binding to human IgA. From protein libraries constructed by combinatorial mutagenesis of a 58-amino-acid, three-helix bundle domain derived from the IgG-binding staphylococcal protein A, variants showing IgA binding were selected by using phage display technology and IgA monoclonal antibodies (myeloma) as target molecules. Characterization of selected clones by biosensor technology showed that five out of eight investigated affibody variants were capable of IgA binding, with dissociation constants (K(d)) in the range between 0.5 and 3 microm. One variant (Z(IgA1)) showing the strongest binding affinity was further analyzed, and showed that human IgA subclasses (IgA(1) and IgA(2)) as well as secretory IgA were recognized with similar efficiencies. No detectable cross-reactivity towards human IgG, IgM, IgD or IgE was observed. The potential use of the Z(IgA1) affibody as a ligand in affinity chromatography applications was first demonstrated by selective recovery of IgA protein from a spiked Escherichia coli total cell lysate, using an affinity column containing a divalent head-to-tail Z(IgA1) affibody dimer construct as a ligand. In addition, efficient affinity recovery of IgA from unconditioned human plasma was also demonstrated.

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 beta protein, is known to bind human IgA-Fc. Here, we show that the beta 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 beta protein but not to an isogenic beta-negative mutant. This binding was due to a direct interaction between beta and FH, as shown by experiments with purified proteins. Inhibition tests and studies with beta fragments demonstrated that FH and IgA-Fc bind to separate and nonoverlapping regions in beta. Heparin, a known ligand for FH, specifically inhibited the binding between beta and FH, suggesting that FH has overlapping binding sites for beta and heparin. Bacteria-bound FH retained its complement regulatory activity, implying that beta-expressing GBS may use bound FH to evade complement attack. The finding that beta 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.

Isolated hypervariable regions derived from streptococcal M proteins specifically bind human C4b-binding protein: implications for antigenic variation

Antigenic variation in microbial surface proteins represents an apparent paradox, because the variable region must retain an important function, while exhibiting extensive immunological variability. We studied this problem for a group of streptococcal M proteins in which the approximately 50-residue hypervariable regions (HVRs) show essentially no residue identity but nevertheless bind the same ligand, the human complement regulator C4b-binding protein (C4BP). Synthetic peptides derived from different HVRs were found to retain the ability to bind C4BP, implying that the HVR corresponds to a distinct ligand-binding domain that can be studied in isolated form. This finding allowed direct characterization of the ligand-binding properties of isolated HVRs and permitted comparisons between different HVRs in the absence of conserved parts of the M proteins. Affinity chromatography of human serum on immobilized peptides showed that they bound C4BP with high specificity and inhibition experiments indicated that different peptides bound to the same site in C4BP. Different C4BP-binding peptides did not exhibit any immunological cross-reactivity, but structural analysis suggested that they have similar folds. These data show that the HVR of streptococcal M protein can exhibit extreme variability in sequence and immunological properties while retaining a highly specific ligand-binding function.

Isolation and characterization of a novel IgD-binding protein from Moraxella catarrhalis

A novel surface protein of the bacterial species Moraxella catarrhalis that displays a high affinity for IgD (MID) was solubilized in Empigen and isolated by ion exchange chromatography and gel filtration. The apparent molecular mass of monomeric MID was estimated to approximately 200 kDa by SDS-PAGE. The mid gene was cloned and expressed in Escherichia coli. The complete mid nucleotide gene sequence was determined, and the deduced amino acid sequence consists of 2123 residues. The sequence of MID has no similarity to other Ig-binding proteins and differs from all previously described outer membrane proteins of M. catarrhalis. MID was found to exhibit unique Ig-binding properties. Thus, in ELISA, dot blots, and Western blots, MID bound two purified IgD myeloma proteins, four IgD myeloma sera, and finally one IgD standard serum. No binding of MID was detected to IgG, IgM, IgA, or IgE myeloma proteins. MID also bound to the surface-expressed B cell receptor IgD, but not to other membrane molecules on human PBLs. This novel Ig-binding reagent promises to be of theoretical and practical interest in immunological research.

Streptococcal IgA-binding proteins bind in the Calpha 2-Calpha 3 interdomain region and inhibit binding of IgA to human CD89

Certain pathogenic bacteria express surface proteins that bind to the Fc part of human IgA or IgG. These bacterial proteins are important as immunochemical tools and model systems, but their biological function is still unclear. Here, we describe studies of three streptococcal proteins that bind IgA: the Sir22 and Arp4 proteins of Streptococcus pyogenes and the unrelated beta protein of group B streptococcus. Analysis of IgA domain swap and point mutants indicated that two loops at the Calpha2/Calpha3 domain interface are critical for binding of the streptococcal proteins. This region is also used in binding the human IgA receptor CD89, an important mediator of IgA effector function. In agreement with this finding, the three IgA-binding proteins and a 50-residue IgA-binding peptide derived from Sir22 blocked the ability of IgA to bind CD89. Further, the Arp4 protein inhibited the ability of IgA to trigger a neutrophil respiratory burst via CD89. Thus, we have identified residues on IgA-Fc that play a key role in binding of different streptococcal IgA-binding proteins, and we have identified a mechanism by which a bacterial IgA-binding protein may interfere with IgA effector function.

Group B streptococcal surface proteins as targets for protective antibodies: identification of two novel proteins in strains of serotype V

Strains of group B streptococcus (GBS) express surface proteins that confer protective immunity. In particular, most strains of the four classical capsular serotypes (Ia, Ib, II, and III) express either of the Rib and alpha proteins, two members of the same protein family. Here, we report a study of surface proteins expressed by strains of serotype V, which has recently emerged as an important serotype among GBS strains causing serious disease. Two novel GBS proteins were identified, purified, and characterized. One of these proteins, designated Fbs, was immunologically unrelated to other GBS surface proteins. This approximately 110-kDa protein was found in 15 of 49 (31%) type V isolates but in few strains of other serotypes. The Fbs proteins expressed by different strains showed limited variation in size. The most common surface protein among type V strains, found in 29 of 49 (59%) isolates, was designated Rib-like, since it cross-reacted with Rib but was not immunologically identical to Rib. Characterization of this Rib-like protein showed that the N-terminal sequence (12 residues) was identical to that of alpha, although these two proteins lacked cross-reactivity. The biochemical and immunological properties of the Rib-like GBS protein indicate that it is closely related to the R28 protein of Streptococcus pyogenes. Importantly, passive and active immunization experiments with mice showed that the Fbs and Rib-like proteins are targets for protective antibodies. These two proteins are therefore of interest for analysis of pathogenic mechanisms and for vaccine development.

An Escherichia coli-Enterococcus faecalis shuttle vector as a tool for the construction of a group B Streptococcus heterologous mutant expressing the beta antigen (Bac) of the C protein complex

Group B streptococci (GBS) represent a very important group of human pathogens. So far little is known about the mechanisms by which these bacteria can cause disease and the bacterial factors involved. One putative virulence factor is the beta antigen of the C protein complex (Bac), which can bind to the Fc region of human IgA. Its binding function might represent an important virulence mechanism. However, the genetic manipulation of this group of bacteria, necessary to prove involvement of bacterial factors in pathogenesis, is still in its infancy. We therefore tested the pAM401 vector system for its suitability in the construction of a heterologous expression mutant using the Bac protein as a model antigen. The bac gene, including its own promoter, was cloned into the Escherichia coli-Enterococcus faecalis shuttle vector pAM401 and was stably maintained extrachromosomally in the bac-deficient GBS strain 335. Expression of Bac was assessed by extracting the protein from transformed 335(pPJTU1) cells, negative controls (335 wild-type, 335(pAM401)) and other Bac-expressing GBS strains (A909, LA239). Blots of the extracted proteins probed with IgA, polyclonal sera and a monoclonal antibody raised against Bac clearly revealed expression of the 130-kDa protein in the transformed GBS 335(pPJTU1) cells. The correct processing and surface anchoring of the expressed Bac was demonstrated by binding of (125)I-labelled IgA to whole cells. Strain 335(pPJTU1) bound 12 times as much IgA compared to the parental strain LA239 and the GBS 335 negative controls, and a total of 25% compared to the high-level-expressing strain A909. Our studies show that the pAM401 shuttle vector can be used for stable heterologous expression of surface proteins in GBS. Our strategy is also of major importance for the complementation of deletion mutants in GBS and other Gram-positive human pathogens to fulfill Koch's postulates. The Bac mutant constructed in this study, 335(pPJTU1), can be used in animal models to assess the importance of Bac in GBS pathogenesis.

An IgA-binding peptide derived from a streptococcal surface protein

Surface proteins that bind to the Fc part of human IgA are expressed by many strains of Streptococcus pyogenes, a major human pathogen. Studies of these proteins have been complicated by their size and by their ability to bind human plasma proteins other than IgA. Here, we describe a synthetic 50-residue peptide, derived from streptococcal protein Sir22, that binds human IgA but not any of the other plasma proteins known to bind to Sir22. The peptide binds serum IgA and secretory IgA and binds IgA of both subclasses. Evidence is presented that the peptide folds correctly both in solution and when it is immobilized and that it readily renatures after denaturation. Together, these data indicate that the peptide corresponds to a protein domain that binds IgA with high specificity. This is the first report of an IgA-binding domain that retains its properties in isolated form.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Inactivation of the alpha C protein antigen gene, bca, by a novel shuttle/suicide vector results in attenuation of virulence and immunity in group B Streptococcus

The alpha C protein of group B Streptococcus (GBS) is a major surface-associated antigen. Although its role in the biology and virulence of GBS has not been defined, it is opsonic and capable of eliciting protective immunity. The alpha C protein is widely distributed among clinical isolates and is a potential protein carrier and antigen in conjugate vaccines to prevent GBS infections. The structural gene for the alpha C protein, bca, has been cloned and sequenced. The protein encoded by bca is related to a class of surface-associated proteins of gram-positive cocci involved in virulence and immunity. To investigate the potential roles of the alpha C protein, bca null mutants were generated in which the bca gene was replaced with a kanamycin resistance cassette via homologous recombination using a novel shuttle/suicide vector. Studies of lethality in neonatal mice showed that the virulence of the bca null mutants was attenuated 5- to 7-fold when compared with the isogenic wild-type strain A909. Significant differences in mortality occurred in the first 24 h, suggesting that the role of the alpha antigen is important in the initial stages of the infection. In contrast to A909, bca mutants were no longer killed by polymorphonuclear leukocytes in the presence of alpha-specific antibodies in an in vitro opsonophagocytic assay. In contrast to previous studies, alpha antigen expression does not appear to play a role in resistance to opsonophagocytosis in the absence of alpha-specific antibodies. In addition, antibodies to the alpha C protein did not passively protect neonatal mice from lethal challenge with bca mutants, suggesting that these epitopes are uniquely present within the alpha antigen as expressed from the bca gene. Therefore, the alpha C protein is important in the pathogenesis of GBS infection and is a target for protective immunity in the development of GBS vaccines.

Identification of a family of streptococcal surface proteins with extremely repetitive structure

The group B Streptococcus (GBS) causes the majority of life-threatening bacterial infections in newborn children. Most GBS strains isolated from such infections express a surface protein, designated Rib, that confers protective immunity and therefore is of interest for analysis of pathogenetic mechanisms. Sequence analysis demonstrated that Rib has an exceptionally long signal peptide (55 amino acid residues) and 12 repeats (79 amino acid residues each) that account for >80% of the sequence of the mature protein. The repeats are identical even at the DNA level, indicating that an efficient mechanism operates to maintain a highly repetitive structure in Rib. The structure of Rib is similar to that of alpha, a previously characterized surface protein that is common among GBS strains lacking Rib. However, highly purified preparations of Rib and alpha did not cross-react immunologically, although the two proteins show extensive amino acid residue identity (47% in the repeat region). When analyzed in Western blots, Rib and alpha give rise to a regularly spaced ladder pattern, apparently due to hydrolysis of acid-labile Asp-Pro bonds in the repeats. We conclude that Rib and alpha are members of a novel family of streptococcal surface proteins with unusual repetitive structure.

Identification of an immunoglobulin A binding motif located in the beta-antigen of the c protein complex of group B streptococci

The beta-antigen of the c protein complex of group B streptococci contains two immunoglobulin A (IgA)-binding domains called A and B. A 73-amino-acid segment in domain A is responsible for most of the IgA-binding activity. To identify the IgA binding motif, the 73-amino-acid domain was divided into 60 14-amino-acid overlapping peptides spot synthesized onto a cellulose membrane. A 20-residue putative antigenic epitope was identified and expressed as a fusion protein. The fusion protein was purified by fast protein liquid chromatography and used to raise rabbit antiserum. By use of a membrane with spot-synthesized peptide amino acids of decreasing length (from 14 to 6 amino acids), the major antigenic epitope recognized by the anti-fusion protein antibodies was mapped to motif MLKKIE. Anti-fusion protein antibodies inhibited the binding of IgA to group B streptococci. This inhibition could be blocked by the peptide containing the motif MLKKIE. These results indicate that the motif MLKKIE is located in the IgA-binding site. The IgA-binding domain of beta-antigen from three group B streptococcal strains reacted with the anti-fusion protein antibodies, and their coding sequences gave positive signals in Southern hybridization. The sequences of beta-antigen from these strains were amplified by PCR, and sequence analysis showed them to be identical. The results indicate that the motif MLKKIE is required for IgA binding and is present in different group B streptococcal strains.

Maternal immunization of mice with group B streptococcal type III polysaccharide-beta C protein conjugate elicits protective antibody to multiple serotypes

Group B streptococcal infection is a major cause of neonatal mortality. Antibody to the capsular polysaccharide protects against invasive neonatal disease, but immunization with capsular polysaccharides fails to elicit protective antibody in many recipients. Conjugation of the polysaccharide to tetanus toxoid has been shown to increase immune response to the polysaccharide. In animal models, C proteins of group B streptococci are also protective determinants. We examined the ability of the beta C protein to serve in the dual role of carrier for the polysaccharide and protective immunogen. Type III polysaccharide was covalently coupled to beta C protein by reductive amination. Immunization of rabbits with the polysaccharide-protein conjugate elicited high titers of antibody to both components, and the serum induced opsonophagocytic killing of type III, Ia/C, and Ib/C strains of group B streptococci. Female mice were immunized with the conjugate vaccine and then bred; 93% of neonatal pups born to these dams vaccinated with conjugate survived type III group B streptococcal challenge and 76% survived type Ia/C challenge, compared with 3% and 8% survival, respectively, in controls (P < 0.001). The beta C protein acted as an effective carrier for the type III polysaccharide while simultaneously induced protective immunity against beta C protein--containing strains of group B streptococci.

Prevalence of IgA receptors in clinical isolates of Streptococcus pyogenes and Streptococcus agalactiae: serologic distinction between the receptors by blocking antibodies

Group A and B streptococci (Streptococcus pyogenes and Streptococcus agalactiae) are the only known bacterial pathogens expressing IgA Fc-receptors. However, the IgA binding proteins of the two species have been found genetically unrelated. In the present investigation the binding of human IgA among clinical isolates of group A and group B streptococci was studied and the respective IgA-binding epitopes were compared serologically. Surface binding of radiolabelled, monoclonal human IgA1 occurred in 38% of 115 unselected group A streptococcal isolates. Comparing four predominant T-types, IgA-binding was found in 77% and 85%, respectively, of types T4 and T28 strains but only in 5% and 25%, respectively, of T1 and T12 strains. In group B streptococci, 70% of 58 type Ib strains but only 2% of 399 strains of other serotypes bound IgA. Using rabbit immune sera raised to the two streptococcal species it was found that strains exhibiting IgA Fc-receptors often induced antibodies blocking the binding of IgA to bacteria. Furthermore, the blocking shown by an individual serum was restricted to the streptococcal group used for immunization showing that also the IgA-binding eptiopes in group A and B streptococci are conformationally distinct. Though infections with serotypes often binding IgA, compared to other types, are not known to differ, it is assumed that the non-immune binding of IgA might favour mucosal colonization of the organisms.

Antigenic diversity of IgA receptors in Streptococcus pyogenes

In a previous study, group A and group B streptococcal IgA receptors were shown to differ serologically, in agreement with their known structural unrelatedness. The present study was undertaken to serologically compare the IgA binding epitopes of group A streptococcal strains representing various serotypes by the use of antisera to this species. It was found that blocking antibodies occurred in antisera to IgA binding but not to non-binding strains and that binding of IgA to a streptococcal strain was generally blocked by antiserum to the homologous type. However, cross-testing of a panel of 11 IgA binding strains, representing various M and T serotypes, with 10 different antisera to group A streptococci, demonstrated that IgA receptors were inhibited to a highly variable degree and that inhibition patterns were unique for each type. Comparing solubilized IgA receptors of various strains in immunoblot experiments, a variation in the molecular mass, between approximately 35 and 45 kDa, emerged. The IgA binding epitopes, analogous to protective sites of streptococcal M-protein, thus exhibited hypervariability which may suggest that IgA binding also plays a key role for evading host immune defence mechanisms.

Protein rib: a novel group B streptococcal cell surface protein that confers protective immunity and is expressed by most strains causing invasive infections

The group B Streptococcus, an important cause of invasive infections in the neonate, is classified into four major serotypes (Ia, Ib, II, and III) based on the structure of the polysaccharide capsule. Since the capsule is a known virulence factor, it has been extensively studied, in particular in type III strains, which cause the majority of invasive infections. Two cell surface proteins, alpha and beta, have also been studied in detail since they confer protective immunity, but these proteins are usually not expressed by type III strains. We describe here a cell surface protein, designated protein Rib (resistance to proteases, immunity, group B), that confers protective immunity and is expressed by most strains of type III. Protein Rib was first identified as a distinct 95-kD protein in extracts of a type III strain, and was purified to homogeneity from that strain. Rabbit antiserum to protein Rib was used to demonstrate that it is expressed on the cell surface of 31 out of 33 type III strains, but only on 1 out of 25 strains representing the other three serotypes. Mouse protection tests showed that antiserum to protein Rib protects against lethal infection with three different strains expressing this antigen, including a strain representing a recently identified high virulence type III clone. Protein Rib is immunologically unrelated to the alpha and beta proteins, but shares several features with the alpha protein. Most importantly, the NH2-terminal amino acid sequences of the Rib and alpha proteins are identical at 6 out of 12 positions. In addition, both protein Rib and the alpha protein are relatively resistant to trypsin (and Rib is also resistant to pepsin) and both proteins vary greatly in size between different clinical isolates. Finally, both protein Rib and the alpha protein exhibit a regular ladderlike pattern in immunoblotting experiments, which may reflect a repetitive structure. Taken together, these data suggest that the Rib and alpha proteins are members of a family of proteins with related structure and function. Since protein Rib confers protective immunity, it may be valuable for the development of a protein vaccine against the group B Streptococcus, an encapsulated bacterium.

Distribution of presumptive pathogenicity factors among beta-hemolytic streptococci isolated from Ethiopia

Beta-hemolytic streptococci are known to bind several mammalian proteins, which are presumed to be important in pathogenicity. The distribution of such binding structures was examined for mouse albumin, human serum IgA, human IgG, human fibrinogen, and human plasminogen. A total of 218 group A beta-hemolytic streptococci (GAS) were studied: 5 isolates from children with acute rheumatic fever (ARF), 18 from acute post-streptococcal glomerulonephritis (APSGN), 57 from tonsillitis, 52 from skin infections, and 86 from healthy carriers. Sixty-eight Streptococcus equisimilis and 20 group G streptococci were also included. Most of the S. equisimilis (60/68) and group G (14/20) were obtained from apparently healthy carriers. The results were evaluated with respect to T type, serum opacity reaction (SOR), site of isolation, and disease type. No direct correlation was detected between the protein-binding structures studied. There was no apparent correlation between any particular protein-binding structure and specific T type. Albumin-binding and IgA-binding activities were inversely correlated among skin and nephritis GAS isolates. A strong correlation was demonstrated between IgA-binding activity and SOR production, while albumin-binding activity correlated with SOR-negative strains. Albumin-binding levels in isolates from ARF, APSGN and tonsillitis were significantly higher than in isolates from healthy carriers (P < 0.001). A higher albumin-binding capacity was shown in skin isolates from APSGN than in isolates from impetigo (P < 0.001).

Binding of human IgA to HCl-extracted c protein from group B streptococci (GBS)

The c beta protein of group B streptococci obtained by HCl extraction appears as a ladder-like pattern in SDS-PAGE when detected by a rabbit anti-c beta serum, and a similar picture is seen when the crude extract is incubated with human IgA and an anti-human IgA conjugate. Affinity-purified c beta antigen and IgA receptors from GBS gave identical pictures in Western blots using rabbit anti-c beta serum. Both the c beta antigen and the IgA receptor are exposed on the surface of GBS as demonstrated by immunofluorescence.

FcR may function as a progression factor of nonlymphoid tumors

Tumor progression is a multistep process involving genetic and epigenetic changes in a transformed clone. Some of these changes may be induced by host factors which may also select for transformed cellular variants with a high ability to survive and propagate. In this article we review studies showing that receptors for the Fc portion of IgG may be expressed on cells from human or animal tumors of nonlymphoid origin. We also review data demonstrating that at least with respect to cells transformed in vitro with Polyoma virus, transformation per se is not sufficient for the induction of Fc receptor expression. We also summarize preliminary data showing that Fc receptor expression is causally involved in conferring a high malignancy phenotype upon transformed cells. Possible mechanisms to explain these observations are discussed.

Large, identical, tandem repeating units in the C protein alpha antigen gene, bca, of group B streptococci

Group B Streptococcus (GBS) is the leading cause of neonatal sepsis and meningitis in the United States. The surface-associated C protein alpha antigen of GBS is thought to have a role in both virulence and immunity. We previously cloned the C protein alpha antigen structural gene (named bca for group B, C protein, alpha) into Escherichia coli. Western blots of both the native alpha antigen and the cloned gene product demonstrate a regularly laddered pattern of heterogeneous polypeptides. The nucleotide sequence of the bca locus reveals an open reading frame of 3060 nucleotides encoding a precursor protein of 108,705 Da. Cleavage of a putative signal sequence of 41 amino acids yields a mature protein of 104,106 Da. The 20,417-Da N-terminal region of the alpha antigen shows no homology to previously described protein sequences and is followed by a series of nine tandem repeating units that make up 74% of the mature protein. Each repeating unit is identical and consists of 82 amino acids with a molecular mass of 8665 Da, which is encoded by 246 nucleotides. The size of the repeating units corresponds to the observed size differences in the heterogeneous ladder of alpha C proteins expressed by GBS. The C-terminal region of the alpha antigen contains a membrane anchor domain motif that is shared by a number of Gram-positive surface proteins. The large region of identical repeating units in bca defines protective epitopes and may play a role in generating phenotypic and genotypic diversity of the alpha antigen.

Molecular characterization of an IgA receptor from group B streptococci: sequence of the gene, identification of a proline-rich region with unique structure and isolation of N-terminal fragments with IgA-binding capacity

Certain strains of group B streptococci express a cell surface protein that binds IgA and acts as a virulence factor. This IgA receptor is referred to here as protein Bac. The gene for protein Bac was cloned and expressed in Escherichia coli, and the complete nucleotide sequence was determined. The deduced amino acid sequence of 1134 residues includes a signal sequence of 37 amino acids and a putative membrane anchor region at the C-terminal end. The processed form of the receptor, 1097 residues, has a calculated molecular weight of 123,786. There are no cysteines in protein Bac, suggesting a fibrillar structure. The C-terminal half of the protein includes a 90 residues long region with a novel type of periodic structure, the "XPZ motif", in which every third amino acid is proline. Unlike other bacterial immunoglobulin-binding proteins, there are no long repeats in protein Bac. Clones which express only part of the protein Bac gene were used to show that IgA-binding takes place in the N-terminal part of the molecule. Protein Bac was originally described as an antigen called beta, but N-terminal fragments that bind IgA do not react with a reference serum against the beta antigen. These and other data indicate that protein Bac can be divided into two regions with different functions: an N-terminal IgA-binding region and a C-terminal region corresponding to the beta antigen. The IgA-binding region of protein Bac does not show any homology to protein Arp, the IgA receptor from group A streptococci, although these receptors have similar binding properties. This indicates that convergent evolution has favored the appearance of these two structurally different streptococcal IgA receptors.