Snmmal~The group B Streptococcus, an important cause of invasive infections in the neonate, is dassified 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, a and fl, 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 immunologicaUy unrelated to the e~ and fl proteins, but shares several features with the oe protein. Most importantly, the NH2-terminal amino acid sequences of the Rib and o~ proteins are identical at 6 out of 12 positions. In addition, both protein Rib and the a 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 a protein exhibit a regular ladderlike pattern in immunoblotting experiments, which may reflect a repetitive structure. Taken together, these data suggest that the Rib and ol 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.

No abstract

Most group A streptococcal strains are able to bind immunoglobulin (Ig) in a non-immune manner, and the majority of these strains bind both IgA and IgG. Using molecular cloning and immunochemical techniques, we have purified and characterized the Ig Fc-receptors expressed by four such strains. Two of the strains express a novel type of receptor, designated protein Sir, which binds IgA and IgG of all subclasses, and therefore has broader reactivity than any Fc-receptor previously described. The other two strains express protein Arp, a receptor that binds IgA of both subclasses, and also binds polyclonal IgG weakly. Characterization of the weak IgG-binding ability of protein Arp shows that it binds only some monoclonal IgG proteins, in particular those of the IgG3 subclass. The four strains studied here were unexpectedly found to also express a second Ig-receptor, called protein Mrp, encoded by a gene closely linked to the gene for the first receptor. The Mrp protein does not bind IgA, but it binds IgG molecules of the IgG1, IgG2 and IgG4 subclasses, and it also binds fibrinogen. Binding of fibrinogen has been reported to be a characteristic property of streptococcal M proteins, which suggests that the Mrp protein may be an M protein that also binds Ig. Taken together, all available evidence now indicates that most strains of group A streptococci express two different Ig-binding proteins, encoded by closely linked genes.

The M protein family of molecules in the group A streptococcus comprises a number of cell surface proteins that interact with the immune system of the host. One of the proteins in this family is the IgA receptor Arp4, which has C repeats similar to those that characterize the known M proteins. The streptococcal strain expressing Arp4 also expresses a second immunoglobulin-binding protein, Mrp4, which is shown here to be encoded by a gene located immediately upstream of the gene for Arp4. In addition to binding IgG, Mrp4 also binds fibrinogen, a property ascribed to M proteins. DNA sequence analysis demonstrated that the Mrp4 protein indeed is a member of the M protein family, but it was unexpectedly found to have a type of repeat that is identical to the A repeat described for FcRA76, a partially sequenced streptococcal Fc receptor. Purified FcRA76 was shown to bind fibrinogen and IgG, like Mrp4. These data show that the known molecules in the M protein family can be divided into two classes, A and C, according to the type of repeat region found. Hybridization studies with a panel of clinical isolates indicate that many streptococcal strains express class A and class C proteins, whereas some strains express only class C proteins. Class A molecules show amino-terminal sequence variation, like class C molecules, which suggests that proteins of both classes are targets for the immune response.

Some strains of group B streptococci express a cell surface protein which binds IgA. This report describes some properties of such an IgA receptor and compares it with a previously described IgA receptor from group A streptococci. The group B receptor was released in an almost pure form from bacteria incubated at elevated pH, and could be isolated by IgA-Sepharose affinity chromatography. The sequence of the N-terminal 19 amino acid residues was unique. The receptor preferentially binds IgA of human origin, as shown in immunoblotting experiments with purified IgA from nine different species. The affinity constant of the purified receptor for serum IgA was determined to be 3.5 x 10(8) M-1, but for secretory IgA it was too low to allow determination. This result indicates that secretory component and/or J chain interferes with the binding of IgA to this type of bacterial receptor, which may be one of the physiological functions of these polypeptides. A reduction in affinity was also observed for another complexed form of IgA, alpha 1-microglobulin-IgA. The group B receptor is antigenically unrelated to the IgA receptor from group A streptococci (protein Arp), but competitive inhibition experiments indicate that they bind to the same region in IgA. The implications of these findings, and the biological role of bacterial IgA receptors, are discussed.