Functional and serological analysis of type II immunoglobulin G-binding proteins expressed by pathogenic group A streptococci

Novel Sample Preparation for Mass Spectral Analysis of Complex Biological Samples

The ability to combine a selective capture strategy with on chip MALDI-TOF analysis allows for rapid, sensitive analysis of a variety of different analytes. In this overview a series of applications of capture enhanced laser desorption ionization time of flight (CELDI-TOF) mass spectrometry are described. The key feature of the assay is an off-chip capture step that utilizes high affinity bacterial binding proteins to capture a selected ligand. This allows large volumes of sample to be used and provides for a concentration step prior to transfer to a gold chip for traditional mass spectral analysis. The approach can also be adapted to utilize specific antibody as the basis of the capture step. The direct and indirect CELDI-TOF assays are rapid, reproducible and can be a valuable proteomic tool for analysis of low abundance molecules present in complex mixtures like blood plasma.

Regulation of protein H expression in M1 serotype isolates of Streptococcus pyogenes

Protein H is an immunoglobulin-binding protein expressed by certain M1 serotypes of Streptococcus pyogenes. In a recent study of invasive group A isolates, it was found that none of the 16 M1 serotype isolates analyzed expressed protein H on their surface despite the presence of the protein H gene (sph) in approximately one-third of the isolates. Selection of stable protein H-expressing variants could be achieved by infection of prtH(+) non-expressing strains into a mouse skin and recovering bacteria from the spleen. This effect was independent of the transcription regulator Mga, since a similar effect was noted in an mga(-) mutant. Thus, host passage of S. pyogenes can lead to stable high level expression of Protein H.

A novel, anchorless streptococcal surface protein that binds to human immunoglobulins

We have characterized a novel surface protein from urea extract of whole cells of group A Streptococcus pyogenes (GAS). A major protein band (35kD) was found to hybridize with human IgG by Western blotting. A search of the N-terminal amino acid sequence of this protein by using the GAS genome sequence database revealed an open reading frame that encoded a 38-kDa protein with a signal peptide sequence. We have named this protein streptococcal immunoglobulin-binding protein 35 (Sib35). It was found to be an anchorless protein with no LPXTG motif, distinct from the M protein superfamily exhibiting immunoglobulin-binding activity, and partially secreted in the culture supernatant. Recombinant Sib35 was also shown to bind human IgA and IgM. The sib35 gene was found in all GAS strains examined, but not in oral, group B, C, or G streptococcal strains. These results suggest that Sib35 is a unique immunoglobulin-binding protein in GAS.

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.

Restoration of Mga function to a Streptococcus pyogenes strain (M Type 50) that is virulent in mice

The Mga protein in B514Sm, a Streptococcus pyogenes strain isolated as a mouse pathogen, contains amino acid substitutions at conserved sites that render the protein defective. Replacement of mga50 with the functional homolog mga4.1 restored full expression of Mga-regulated proteins. Restoration of Mga function did not affect fibrinogen binding, nor did it affect virulence in several mouse models of group A streptococcus infection.

Analysis of expression of a cytosolic enzyme on the surface of Streptococcus pyogenes

The normally cytosolic glycolytic enzyme, glyceraldehyde-3-phosphate dehydrogenase, (GAPDH) has been reported to be expressed on the surface of Streptococcus pyogenes, group A, where it can act as a plasmin binding protein (Plr), and potentially a signaling molecule. In studies of wild-type and isogenic mutants, an association between surface expression of antigenic GAPDH/Plr and M and M-related fibrinogen-binding proteins was identified. Inactivation of the mga gene, whose product controls expression of M and M-related proteins also influenced expression of surface GAPDH/Plr. Revertants or pseudorevertants of mga mutants led to concomitant re-expression of surface GAPDH/Plr and M and M-related proteins. Using surface enhanced laser desorption ionization (SELDI) mass spectroscopy, a physical association between GAPDH/Plr and streptococcal fibrinogen-binding proteins was demonstrated. These studies support the hypothesis that surface M and M-related proteins are involved in anchoring GAPDH/Plr on the surface of group A streptococci.

Interaction of group A streptococci with human plasmin(ogen) under physiological conditions

A series of methods for analyzing the interaction of group A streptococci with the human plasminogen system are described. Examples of group A streptococcal isolates capable of assembling surface plasminogen activator activity when grown in human plasma are presented and the key requirements for this process are evaluated. The stabilities of cell-associated plasmin and plasminogen activator complexes are compared and a model for the interaction of group A streptococci with the plasminogen system in an infected host is presented.

Absence of SpeB production in virulent large capsular forms of group A streptococcal strain 64

Passage in human blood of group A streptococcal isolate 64p was previously shown to result in the enhanced expression of M and M-related proteins. Similarly, when this isolate was injected into mice via an air sac model for skin infection, organisms recovered from the spleens showed both increased expression of M and M-related proteins and increased skin-invasive potential. We show that these phenotypic changes were not solely the result of increased transcription of the mRNAs encoding the M and M-related gene products. Rather, the altered expression was associated with posttranslational modifications of the M and M-related proteins that occur in this strain, based on the presence or absence of another virulence protein, the streptococcal cysteine protease SpeB. The phenotypic variability also correlates with colony size variation. Large colonies selected by both regimens expressed more hyaluronic acid, which may explain differences in colony morphology. All large-colony variants were SpeB negative and expressed three distinct immunoglobulin G (IgG)-binding proteins in the M and M-related protein family. Small-colony variants were SpeB positive and bound little IgG through their M and M-related proteins because these proteins, although made, were degraded or altered in profile by the SpeB protease. We conclude that passage in either human blood or a mouse selects for a stable, phase-varied strain of group A streptococci which is altered in many virulence properties.

Attenuated expression of the mga virulence regulon in an M serotype 50 mouse-virulent group A streptococcal strain

The attenuated expression of virulence genes found in a group A streptococcal strain that is naturally pathogenic for mice was postulated to result from a defect in the strain's multigene regulator, Mga. The sequence of the mga gene reveals three amino acid changes in the gene product that might affect protein function. The defect in the mga gene was complemented by providing either the closely similar mga4 allele or a more divergent mga1 allele in trans. Complementation increased the amount of emm50 transcript and the quantity of surface-extractable M protein, restoring virulence function.

Fc-Mediated Nonspecific Binding Between Fibronectin-Binding Protein I of Streptococcus pyogenes and Human Immunoglobulins

Fibronectin-binding protein I (SfbI) from Streptococcus pyogenes plays a key role in bacterial adhesion to, and invasion of, eukaryotic cells. In addition, SfbI exhibits a considerable potential as mucosal adjuvant and can trigger polyclonal activation of B cells. Here, we report that SfbI is also capable of binding human IgG in a nonimmune fashion. SfbI was reactive with IgG1, IgG2, IgG3, and IgG4 isotypes (type IIo IgG-binding profile). The affinity constant (Kd) of the SfbI-IgG interaction was in the range of 1–2 × 10−5 M. Further studies demonstrated that the SfbI binding was mediated by the Fc component of the IgG molecule. Experiments performed using purified recombinant proteins spanning different domains of SfbI showed that the IgG-binding activity was restricted to the fibronectin-binding domains, and in particular to the fibronectin-binding repeats. Finally, the presence of recombinant SfbI resulted in an impairment of both phagocytosis of IgG-coated RBCs and Ab-dependent cell cytotoxicity by macrophages. These results demonstrated for the first time that, in addition to its major role during the colonization process, SfbI may also favor bacterial immune evasion after the onset of the infection by interfering with host clearance mechanisms.

A secreted streptococcal cysteine protease can cleave a surface-expressed M1 protein and alter the immunoglobulin binding properties

Previous studies of recent clinical isolates of serotype M1 group A streptococci indicated that they display two patterns of non-immune human IgG subclass binding reactivity associated with their M1 protein. One group reacted with all four IgG subclasses (type IIo), while the second group expressed an M1 protein reacting preferentially with human IgG3 (type IIb). In this study, we have demonstrated that a cysteine protease, SpeB, present in culture supernatants of M1 serotype group A streptococcal isolates expressing type IIb IgG binding protein, can convert a recombinant Emm1 protein from a type IIo functional profile to a type IIb profile by removal of 24 amino acids from the N-terminus of the mature M1 protein. Furthermore, SpeB can convert bacteria expressing IgG binding proteins of the type IIo phenotype into those expressing type IIb proteins. The role of the cysteine protease as the central bacterial enzyme in this posttranslational modification event was confirmed by generation of an isogenic SpeB-negative mutant.

Group A streptococcal isolate 64/14 expresses surface plasmin-binding structures in addition to Plr

A recombinant plasmin receptor (Plr) gene product originally cloned from group A streptococcal isolate 64/14 was analysed for its ability to bind plasmin(ogen) and to account for all the surface plasmin-binding properties of streptococcal isolate 64/14. Functional analysis of recombinant Plr demonstrated that the protein exhibited equal reactivity with human Lys-plasmin and Lys-plasminogen, but significantly lower reactivity with Glu-plasminogen. Plasmin-binding was both inhibitable and elutable by lysine or lysine analogs, and active plasmin bound to recombinant Plr was not neutralized by alpha 2-antiplasmin. Thus, the plasmin-binding properties of recombinant Plr correlated with the plasmin-binding phenotype of the intact streptococcal isolate 64/14. In addition, fluid-phase recombinant Plr could completely inhibit binding of plasmin to either immobilized recombinant Plr or group A streptococcal isolate 64/14 with equal efficiency, indicating that surface-expressed Plr could account for all the plasmin-binding properties of the intact organism. An IgM monoclonal antibody to recombinant Plr that specifically recognized a surface structure on streptococcal isolate 64/14 significantly inhibited the binding of plasmin to the recombinant protein; however, the antibody was not successful at inhibiting plasmin-binding to the intact bacteria, indicating the presence of other plasmin-binding structures on the bacterial surface in addition to Plr.

Evidence for independent binding domains within a group A streptococcal type IIo IgG-binding protein

The gene for a type IIo IgG-binding protein has previously been cloned and sequenced. The approximately 60,000 M(r) recombinant gene product binds all four human IgG subclasses and fibrinogen. Treatment of this recombinant protein with CNBr results in generation of a series of fragments. One fragment, an approximately 32,000 M(r) polypeptide, binds IgG1, IgG2, and IgG4 but neither IgG3 nor fibrinogen. N-terminal amino sequencing of this fragment indicated that this was an internal fragment of the protein starting at amino acid 186 of the mature protein. These findings provide evidence for two distinct domains for binding IgG1, IgG2, and IgG4 and binding IgG3 within a single bacterial IgG-binding protein.

DNA sequencing and gene expression of the emm gene cluster in an M50 group A streptococcus strain virulent for mice

The strain B514, an M serotype 50 strain, is capable of causing a natural upper respiratory infection leading to death in mice, as reported by Hook et al. in 1960 (E. W. Hook, R. R. Wagner, and R. C. Lancefield, Am. J. Hyg. 72:111-119, 1960). Thus, this strain was of interest for use in developing an animal model for group A streptococcal colonization and disease. The emm gene cluster for this strain was examined by PCR mapping and found to contain three emm family genes and cluster pattern 5. PCR-generated fragments corresponding to the SF4 (mrp50), SF2 (emmL50), and SF3 (enn50) genes were cloned and the entire gene cluster was sequenced. The gene cluster has greater than 97% DNA identity to previously sequenced regions of the gene cluster of the M2 strain T2/44/RB4 if two small divergent regions that encode the mature amino terminus of the SF-2 and SF-3 gene products are not included. If expressed, the genes encode proteins which bind human immunoglobulin G (Mrp50 and EmmL50) or immunoglobulin A (Enn50). However, in isolates taken directly after passage in mice, the surface proteins arising from these genes were barely detectable. The transcription of each gene in the B514 strain was investigated by Northern (RNA) hybridization, and mRNA transcripts were detected and quantitated relative to those of the recA gene, a housekeeping gene. Transcription of all three emm family genes was found to be over 30-fold attenuated relative to transcription of the same genes in strain T2/44/RB4. This suggests that the positive regulator, Mga, either is not expressed in this strain or has a different requirement for activation; it also suggests that the capsule may be sufficient to inhibit phagocytosis under these circumstances.

Expression of both M protein and hyaluronic acid capsule by group A streptococcal strains results in a high virulence for chicken embryos

The human pathogenic microorganism Streptococcus pyogenes can resist against phagocytic attack of human granulocytes. Streptococcal M protein and hyaluronic acid were identified as virulence factors involved in this protection. So far, no experiments have been reported which describe the contribution of both components together in one system. We used the chicken embryo as an in vivo phagocytosis model to investigate the role of both components on the virulence of streptococci. For this, isogeneic mutants of group A streptococcal strains (GAS) which lack hyaluronic acid capsule (cap-) or M protein (M-) expression were used for infection and their virulence was compared with laboratory strains which had lost their ability to produce one or both virulence factors after long-time laboratory passages on blood agar. The experiments revealed that strains producing both M protein and hyaluronic capsule were highly virulent. Only 1-10 colony-forming units were enough to cause a 50% lethality of 12-day-old chicken embryos. Those strains lacking one of these components showed a significant decrease in virulence. Finally, strains which failed to express either hyaluronic acid or M protein showed an additional tenfold decrease in virulence. This indicates a partial contribution of both M protein and hyaluronic acid to the virulence of GAS in the chicken embryo.

Characterization of a type II'o group A streptococcal immunoglobulin-binding protein

The opacity factor positive M type 2 group A streptococcal isolate, A207, expresses a unique functional type II'o IgG-binding protein which reacts with all four human IgG subclasses and rabbit IgG. In order to determine the gene product or products responsible for this activity, three genes of the vir regulon from this isolate were cloned, expressed and analysed. The fcr A2 gene coded for a protein binding hyman IgG1, IgG2 and IgG4 but not IgG3. The enn2 gene coded for a protein reacting exclusively with human IgA, while the emmL2 gene product bound IgG1, IgG2, IgG3 and IgG4 as well as rabbit but not horse or pig IgG. The IgG3-binding activity of the EmmL2 protein was functionally indistinguishable from the Form 1 IgG3-binding activity present in heat extracts of group A isolate A207.

Analysis of immunoglobulin G-binding-protein expression by invasive isolates of Streptococcus pyogenes

Invasive group A streptococcal isolates collected as part of a Centers for Disease Control and Prevention surveillance study were analyzed for expression of immunoglobulin G (IgG)-binding proteins. Two IgG-binding phenotypes of group A isolates of the M1 serotype were identified. The first group expressed a surface protein that bound all four human IgG subclasses (type IIo) and was recognized by rabbit anti-serotype M1-specific antiserum but not by normal rabbit serum. The second group expressed an IgG-binding protein that was also recognized by the anti-serotype M1 antiserum but demonstrated significant nonimmune reactivity only with human IgG3 (type IIb). Analysis of extracts of the isolates for reactivity with human IgA, fibrinogen, and albumin was also performed. The importance of the binding of human plasma proteins to pathogenic group A streptococci remains to be established.

Distinct profiles of immunoglobulin G-binding-protein expression by invasive serotype M1 isolates of Streptococcus pyogenes

Analysis of immunoglobulin G (IgG)-binding-protein expression by invasive group A streptococcal isolates of the M1 serotype collected as part of a Centers for Disease Control and Prevention surveillance study revealed two distinct phenotypes. One group of type M1 isolates expressed a surface protein reactive with all four human IgG subclasses (type IIo), while a second group expressed a surface protein demonstrating significant reactivity only with human IgG3 (type IIb). The functional forms of IgG-binding protein were antigenically related, and both were recognized by a rabbit polyclonal antiserum to serotype M1 but not by normal rabbit serum. While the quantities of antigenic M1 protein present in the extracts of representative isolates displaying each phenotype differed, the functional differences were found to be qualitative and not solely quantitative. The IgG-binding properties of these antigenically related M1 proteins could be readily distinguished from those of another IgG-binding protein, protein H. Type M1 isolates of the IIb phenotype differed from those of the IIo phenotype by secreting larger amounts of a casein-hydrolyzing protease into culture supernatants.

Characterization of a gene coding for a type IIo bacterial IgG-binding protein

Two antigenic classes of non-immune IgG-binding proteins can be expressed by group A streptococci. One antigenic group of proteins is recognized by an antibody prepared against the product of a cloned fcrA gene (anti-FcRA). In this study, the immunogen used to prepare the antibody that defines the second antigenic class was shown to be the product of the emm-like (emmL) gene of M serotype 55 group A isolate, A928. The emmL55 gene expressed in E. coli produced an M(r) approximately 58,000 molecule which bound human IgG1, IgG2, IgG3 and IgG4, as well as horse, rabbit and pig IgG in a non-immune fashion. These properties are characteristic of the previously described type IIo IgG-binding protein isolated from this strain. In addition, the recombinant protein was reactive with human serum albumin and fibrinogen. The emmL 55 gene sequence was analysed and found to have the organization and sequence characteristics of a typical class I emm-like gene.

Analysis of the interaction of group A streptococci with fibrinogen, streptokinase and plasminogen

Group A streptococci demonstrate a number of distinct ways to interact with the human fibrinolytic system to acquire unregulatable cell-surface enzymatic activity. Interactions between bacteria, fibrinogen, streptokinase and plasminogen resulted in acquisition of cell-associated enzymatic activity that can lyse fibrin clots despite the presence of the major physiological plasmin inhibitor, alpha 2-antiplasmin. Western blot analysis of extracted streptococcal surface proteins suggested that binding of fibrinogen to M or M-related proteins mediated the capture of streptokinase-plasminogen complexes to the bacteria. The enzymatic complex formed by reaction of bacteria with fibrinogen, streptokinase and plasminogen was found to be more stable in human plasma than pre-formed plasmin bound directly to the same bacteria strain.

M-like, immunoglobulin-binding protein of Streptococcus pyogenes type M15

An M-like protein from Streptococcus pyogenes type M15 strain EF1949 (EMML15) was cloned in Escherichia coli and sequenced. Recombinant EMML15 protein revealed a unique binding pattern for human IgG subclasses not described previously. Comparative analysis of the EMML15 amino acid sequence with those of other M-like proteins of opacity factor positive (OF+) serotypes and protein H, and IgG receptor from OF- serotype M1, showed that IgG-binding proteins with common binding of IgG3 were closely related and distinct from streptococcal IgG receptors not binding IgG3. Thus, the Ig-binding proteins from S. pyogenes were subdivided into two main categories according to binding pattern, protein structure, and gene location.

A group A streptococcal Enn protein potentially resulting from intergenomic recombination exhibits atypical immunoglobulin-binding characteristics

The gene encoding the Enn protein (enn) of the M untypeable group A streptococcal (GAS) strain 64/14 was amplified by polymerase chain reaction, cloned into the expression vector pJLA602 and expressed in Escherichia coli DH5 alpha. Unlike other GAS-Enn proteins, which exhibit IgA-binding activity, the recombinant Enn enn64/14 protein reacted preferentially with human IgG3. The 1050 bp open reading frame comprising the enn64/14 gene was completely sequenced. The region of the gene encoding the signal peptide and the C-terminus exhibited > 95% homology to corresponding sections of other enn genes. The region of enn64/14 encoding the N-terminus of the mature Enn protein was found to be highly homologous to the corresponding section of the gene encoding the M-like protein of GAS serotype M9 (emmL9). The recombinant protein encoded by emmL9 was found to react with all four human IgG subclasses. About 30% of the 1152 bp open reading frame of emmL9 encoding the N-terminus was found to display > 90% homology to the corresponding section of enn64/14 but was < 50% homologous in the remainder of the gene sequence. The functional analysis of the subcloned N-terminal section of emmL9 demonstrated a polypeptide exhibiting selective binding to human IgG3. These findings suggested that enn64/14 was a hybrid gene formed by recombination of an enn gene and an emmL9 gene. The putative recombinational event could have involved a set of flanking 7 bp direct repeats. Since enn64/14 and emmL9 are genes from different phylogenetic lineages of GAS, this report provides evidence that intergenomic recombinations between different types of GAS genes can occur and could lead to hybrid proteins with unique Ig-binding characteristics.

Identification of two functional forms of immunoglobulin G3-binding protein expressed by group A streptococci

Analysis of group A streptococcal immunoglobulin G (IgG)-binding protein reactivity with different human IgG3-myeloma proteins provided evidence for at least two functional forms of these molecules. Representative IgG3-binding molecules were isolated, biotinylated, and used as tracers in competitive binding assays. Cross-inhibition studies demonstrated the existence of two distinct patterns of IgG3-binding activity. Proteins of one form could be inhibited from binding to an IgG3-myeloma protein by streptococcal protein G while binding of the second form was not inhibited. These studies further underscore the extent of heterogeneity among immunoglobulin-binding proteins expressed by group A streptococci.

Association of type II immunoglobulin G-binding protein expression and survival of group A streptococci in human blood

Expression of immunoglobulin G (IgG)-binding proteins on group A streptococcus strain 64 was monitored on bacteria subjected to sequential passage in human blood. After approximately 10 cycles through human blood, strain 64 demonstrated enhanced levels of IgG-binding protein, including the expression of a type IIa binding molecule with an M(r) of approximately 47,000 present only at very low levels on the parent isolate. Changes in the expression of IgG-binding proteins after passage in human blood were similar to those observed when the same organism was passaged sequentially intraperitoneally in mice. Strain 64, passaged in human blood 23 times, was found to be more virulent than the parent isolate when used to infect mice either intraperitoneally or in a skin air sac. These findings suggest that the expression of IgG-binding proteins may be a common response of group A organisms to pressures exerted by distinct host defense mechanisms.

Association between expression of immunoglobulin G-binding proteins by group A streptococci and virulence in a mouse skin infection model

In this study, we developed a mouse model of skin infection to test the association between expression of immunoglobulin-binding proteins by and infectivity of group A streptococci. Group A streptococci capable of crossing tissue barriers and establishing a lethal systemic infection in mice showed a higher level of immunoglobulin-binding protein expression. The group A streptococci recovered from the spleen of a mouse that died following a skin infection were found to be more virulent when injected into the skin of naive mice. Together, these results suggest that immunoglobulin-binding protein expression by group A streptococci correlates with their ability to establish invasive skin infections.