Streptococcus Group A Vaccines

Clinical Description and Complications

Pharyngitis and pyoderma (impetigo) are the most common uncomplicated manifestations of GAS infection. Life threatening infections result when GAS spreads to contiguous sites or disseminates to normally sterile spaces or deep tissues, which may lead to necrotizing fasciitis, STSS, or both. Major nonsuppurative complications of GAS infections are ARF and poststreptococcal glomerulonephritis (PSGN), both considered immune-mediated. ^, Rheumatic heart disease (RHD) results when ARF (often repeated episodes) leads to long-term damage to heart and heart valves.

Virulence Factors and Mechanisms for Adherence and Persistence

An assortment of secreted and surface-expressed molecules mediates the pathogenic processes that characterize acute GAS infection: adhesion, immune evasion, inflammation, and tissue invasion ( Fig. 57.1 ). Several virulence factors that are in preclinical development as possible vaccine antigens are highlighted below ( Table 57.1 ) .

GAS adhere to epithelial cells in the pharynx, cervix, or skin by binding to plasma and matrix proteins, forming microbial aggregates and producing a biofilm that protects against host defenses and nutrient deprivation. Among the best described adhesins are M protein, hyaluronic acid capsule, and fibronectin binding proteins such as SfbI, SOF, PrtF2 and FBP54. R28 is found in M28 GAS strains associated with puerperal fever and is thought to promote binding to cervical epithelium. GAS pilus contributes to adhesion and biofilm formation. ^, These filamentous structures protruding from the cell surface bear the T antigens described by Lancefield and thus present a potential avenue for vaccine development. All GAS isolates tested thus far carry and express one of the 21 known bp/tee backbone pilus genetic locus types and sub-types. ^,

After colonization, GAS persists by deploying a multi-pronged approach to disarm the innate immune response. C5a peptidase (also designated SCPA, or surface bound C5a peptidase) inactivates a chemokine of the complement system, ^, while SpyCEP (also called ScpC or Spy 0416), a cell envelope serine protease, cleaves the neutrophil attractant interleukin-8 (IL-8), both contributing to inhibition of neutrophil recruitment. Extracellular DNases degrade chromatin webs produced by neutrophils to entrap GAS bacteria. ^, The secreted serine esterase (SsE, or Spy 1718) inhibits neutrophil recruitment and contributes to skin invasion and systemic dissemination. The secreted toxin streptolysin O (SLO) kills host cells by forming membrane pores, including in white blood cells. Streptococcal inhibitor of complement (SIC), found in serotype M1 and M57, inhibits complement-mediated lysis. The hyaluronic capsule promotes resistance to host defense peptides and phagocyte killing within the extracellular neutrophil traps. ^, SCPA, SpyCEP, SsE and SLO are candidate GAS vaccine antigens. ^,

M protein, encoded by emm , impedes neutrophil phagocytosis in the absence of type-specific antibody by binding plasma proteins and inhibiting opsonization via the alternate complement pathway. Antigenic heterogeneity of the N-terminus of M protein enables GAS to escape recognition by specific antibody. Many GAS strains express M-like proteins (encoded by mrp and enn genes) which may also play a role in immune evasion. ^, M proteins and M-related proteins (Mrp) have been found to generate protective immunity in animal models. ^,

An appreciation of the structure of M protein helps to explain its function as the major virulence factor of GAS and also as a candidate protective antigen ( Fig. 57.1 ). These alpha-helical coiled-coil fibrillar rods consist of two polypeptide chains anchored in the cell wall peptidoglycan by a C-terminus LPxTG motif. Each polypeptide contains up to four segments of repeating amino acids (labeled A-D). The C repeats contain epitopes which are exposed on the surface of the cell wall and are largely conserved among different M types. During infection, opsonic antibodies that confer protection are directed predominantly at the N-terminus, the region that serves as the basis for Lancefield’s M serotyping classification. Currently GAS strains are molecularly classified based on sequence analysis of the portion of the emm gene that encodes M serospecificity. The >220 distinct emm types have been further classified into 48 distinct emm clusters based on phylogenetic, structural, and binding properties of M protein. ^,

GAS superantigens or streptococcal pyrogenic exotoxins have potent pro-inflammatory properties. ^, This family of superantigens induce profound immunologic changes ^, as well as fever, tissue damage, and clinical manifestations of endotoxic shock. SpeA and SpeC, also known as erythrogenic toxins because of their association with the rash of scarlet fever, are examples of bacteriophage-encoded moieties and contain distinct T cell receptor and class II MHC binding sites. Gene carriage for SpeC and superantigen SSA, and higher SpeA levels of expression, are epidemiologically linked to recent scarlet fever epidemics. SpeA and SpeC enhance colonization in mouse infection models and are potential GAS vaccine components.

GAS elaborate multiple other surface and extracellular products that promote virulence, including streptolysin S, deoxyribonucleases, cysteine protease SpeB, Endo-S, Mac-1/2, streptokinase, several other distinct superantigens and hyaluronidase to name a few, which have been reviewed elsewhere. ^,

Potential Role of GAS Vaccine Antigen Candidates in Autoimmunity

The pathogenesis of the nonsuppurative postinfectious sequelae of GAS is not well understood. ARF follows ∼3% of episodes of untreated GAS pharyngitis under epidemic conditions but is considerably less frequent in endemic situations. Observations of Aboriginal populations experiencing high rates of ARF have led some investigators to suggest that skin strains may lead to ARF in these communities, either directly or by initiating throat infection ; however, these findings remain inconclusive. Strains causing pharyngitis and ARF were long thought to bear different M types and other antigenic and virulence properties than those that cause pyoderma, although some overlap exists. ^, Should a causal link between pyoderma and ARF be proven, alteration of the composition of serotype-specific M protein-based vaccines may be required.

The concept of molecular mimicry has been invoked to explain the pathogenesis of ARF and PSGN. M protein bears antigenic similarities with human fibrillar proteins, such as cardiac muscle myosin and molecules of the extracellular matrix of joints and kidneys. Other evidence suggests that the antibodies that target the valves in ARF instead target the group A carbohydrate (GAC) from the offending GAS strain. An alternative hypothesis is the neo-antigen theory that M protein binds to subendothelial collagen and creates a new antigen that triggers an inflammatory autoimmune response. ^, The antibodies that are generated do not cross-react with M protein and thus do not involve classic molecular mimicry. Either way, a mechanism for carditis is postulated whereby GAS infection stimulates cross-reactive antibodies that bind to the valvular endothelium and cause inflammation and infiltration by CD4+ T cells that recognize GAS M protein and cardiac antigens. Antibodies generated by the dominant epitope of GAC, N -acetylglucosamine, cross-react with GM1 gangliosides on the surface of neuronal and valvular endothelial cells and have also been proposed in the pathogenesis of Sydenham’s chorea and carditis, respectively. Sero-reactivity with components of the cell membrane, SpeB, and other GAS antigens has been suggested in the etiology of PSGN. ^, Genetics may also play a role. Notably, antibodies which cross-react with human tissues and somatic components of GAS are found in many healthy children, so their presence alone does not explain the nonsuppurative sequelae of GAS. Further elucidation of the pathogenesis of these sequelae will facilitate rational development of a safe GAS vaccine.

Treatment and Prevention with Antimicrobials

In the 1950s, Denny et al. demonstrated that penicillin prevents more than 90% of ARF episodes (primary prevention) if initiated within 9 days of the onset of GAS phayryngitis. Furthermore, the observation that nearly 30% of patients with rheumatic carditis experience additional valvular injury when re-infected with GAS prompted recommendations to administer prophylactic penicillin during the years when the risk of exposure to GAS is greatest (secondary prevention). In contrast, although antibiotics speed recovery of skin lesions, they do not appear to prevent glomerulonephritis following either pharyngitis or impetigo. Severe invasive infections are treated with a combination of penicillin and an antibiotic that inhibits protein synthesis such as clindamycin plus surgical debridement of necrotic tissue. Intravenous immunoglobulin therapy has been advocated as ancillary therapy for STSS based on its ability to neutralize superantigen toxins, but its efficacy remains inconclusive.

There are well-recognized limitations to the treatment of GAS disease. Nearly one-third of episodes of ARF are preceded by a GAS infection that does not receive medical attention. In low resource settings, antibiotics frequently are not available for effective ARF primary and secondary prevention. Even in the face of adequate therapy, the outcome of invasive disease may be poor. ^, GAS infections are a strong driver of antibiotic use. While resistance of GAS to macrolides and lincosamides is well described in many parts of the world, ^, β-lactams remain the first line of therapy and susceptibility to these agents has been considered universal. Application of whole genome sequencing has identified amino acid replacements in some penicillin-binding proteins (PBP2X) that conferred decreased susceptibility to some β-lactams, including penicillin G. ^, These mutations are geographically disseminated and exist in strains that commonly cause pharyngitis and invasive infections. While not yet achieving a minimal inhibitory concentration associated with resistance, they represent a potential public health threat, and a further justification for development of a GAS vaccine.