Towards developing a vaccine for rheumatic heart disease

Rheumatic heart disease (RHD) is the most serious manifestations of rheumatic fever, which is caused by group A Streptococcus (GAS or Streptococcus pyogenes) infection. RHD is an auto immune sequelae of GAS pharyngitis, rather than the direct bacterial infection of the heart, which leads to chronic heart valve damage. Although antibiotics like penicillin are effective against GAS infection, improper medical care such as poor patient compliance, overcrowding, poverty, and repeated exposure to GAS, leads to acute rheumatic fever and RHD. Thus, efforts have been put forth towards developing a vaccine. However, a potential global vaccine is yet to be identified due to the widespread diversity of S. pyogenes strains and cross reactivity of streptococcal proteins with host tissues. In this review, we discuss the available vaccine targets of S. pyogenes and the significance of in silico approaches in designing a vaccine for RHD.

The human IgA-Fc α receptor interaction and its blockade by streptococcal IgA-binding proteins

IgA plays a key role in immune defence of the mucosal surfaces. IgA can trigger elimination mechanisms against pathogens through the interaction of its Fc region with FcαRs (receptors specific for the Fc region of IgA) present on neutrophils, macrophages, monocytes and eosinophils. The human FcαR (CD89) shares homology with receptors specific for the Fc region of IgG (FcαRs) and IgE (FcαRIs), but is a more distantly related member of the receptor family. CD89 interacts with residues lying at the interface of the two domains of IgA Fc, a site quite distinct from the homologous regions at the top of IgG and IgE Fc recognized by FcαR and FcαRI respectively. Certain pathogenic bacteria express surface proteins that bind to human IgA Fc. Experiments with domain-swap antibodies and mutant IgAs indicate that binding of three such proteins (Sir22 and Arp4 of Streptococcus pyogenes and β protein of group B streptococci) depend on sites in the Fc interdomain region of IgA, the binding region also used by CD89. Further, we have found that the streptococcal proteins can inhibit interaction of IgA with CD89, and have thereby identified a mechanism by which a bacterial IgA-binding protein may modulate IgA effector function.

The In Vitro Interaction of Streptococcus pyogenes with Human Pharyngeal Cells Induces a Phage-Encoded Extracellular DNase

ABSTRACT The role lysogenic bacteriophage play in the pathogenesis of the host bacterium is poorly understood. In a previous study, we found that streptococcal coculture with human pharyngeal cells resulted in the induction of lysogenic bacteriophage as well as the phage-associated streptococcal pyrogenic exotoxin C (SpeC). In this study, we have determined that in addition to SpeC induction, a number of other streptococcal proteins are also released by the bacteria during coculture with pharyngeal cells. Among these, we identified and characterized a novel 27-kDa secreted protein. Sequence analysis of this novel protein demonstrated it to be encoded by the same lysogenic bacteriophage which harbors speC. Protein sequence analysis revealed varied homologies with several streptococcal DNases. Further biochemical characterization of the recombinantly expressed protein verified it to be a divalent cation-dependent streptococcal phage-encoded DNase (Spd1). Although functionally distinct, SpeC and Spd1 are associated by a number of parameters, including genetic proximity and transcriptional regulation. Finally, we speculate on the induction of phage-encoded DNase (Spd1) enhancing the fitness of both bacteria and phage.

M1 protein and protein H: IgGFc- and albumin-binding streptococcal surface proteins encoded by adjacent genes

M1 protein and Protein H are surface proteins simultaneously present at the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structure, protein-binding properties and relationship between these two molecules. The gene encoding M1 protein (emm1) was found immediately upstream of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similarity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, whereas the N-terminal sequences showed no significant similarity. Protein H has affinity for the Fc region of IgG antibodies. Also M1 protein, isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested against polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 protein also reacted with guinea-pig IgG, whereas both streptococcal proteins were negative in binding experiments with rat, mouse, bovine and horse IgG. The two proteins were also tested against other members of the immunoglobulin super family: human IgM, IgA, IgD, IgE, beta 2-microglobulin, and major histocompatibility complex (MHC) class-I and class-II antigens. M1 protein showed no affinity for any of these molecules whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M1 protein. Thus albumin bound to the C repeats and IgGFc to a region (S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstrated that the two streptococcal proteins bound to overlapping sites on IgGFc.