Bacterial Infections
Group A Streptococcus
Disease burden
Streptococcus pyogenes (group A streptococcus, GAS) is an important species of Gram-positive extracellular bacterial pathogen which colonizes the throat or skin and is responsible for a broad spectrum of diseases that range from simple and uncomplicated pharyngitis and skin infections (impetigo, erysipelas, and cellulitis) to scarlet fever and life-threatening invasive illnesses including pneumonia, bacteremia, necrotizing fasciitis, streptococcal toxic shock syndrome (TSS), and nonsuppurative sequelae such as acute rheumatic fever, reactive arthritis and glomerulonephritis
[113]
[114]
. Streptococcal pharyngitis continues to be one of the most common childhood illnesses throughout the world, with an estimated 7.3 million outpatient physician visits each year among children in the USA, 15% to 36% of which are due to GAS
[115]
[116]
. Rheumatic fever (RF) is a delayed sequel to GAS pharyngitis. The disease seems to be of an autoimmune nature, resulting from the production of autoreactive T cells and antibodies that recognize myosin, tropomyosin, keratin and N-acetylglucosamine as well as streptococcal M protein and streptococcal membranes
[117]
[118]
. The incidence of RF has declined in industrialized countries since the 1950s to reach today an annual figure of around 0.5 cases per 100 000 school age children. In contrast, it remains an endemic disease in developing countries with annual incidence rates ranging from 100 to 200 cases per 100 000 school-age children. It also is a major cause of cardiovascular mortality in these countries
[119]
. The WHO has estimated that 12 million people worldwide have rheumatic heart disease, of whom 400 000 die every year
[120]
. Australia's aboriginal population suffers the highest incidence worldwide
[121]
[122]
.
GAS also is an important cause of severe infection such as streptococcal TSS and necrotizing fasciitis. Approximately 9700 cases of invasive disease and 1300 deaths are attributed to GAS each year in the USA
[123]
[124]
. Rates of severe GAS infection reach 2.5 to 3/100 000 population in the northern European countries
[125]
[126]
. It has recently been estimated that there currently are more than 18 million cases of severe group A streptococcal disease such as rheumatic heart disease in the world, with more than 500,000 deaths each year. Considerable overlap has been observed between GAS strains that cause pharyngitis in children and those associated with invasive disease in the community, suggesting that shool-age children serve as a reservoir of infection for the community
[127]
[128]
. Prospective, longitudinal studies are clearly needed to better understand the epidemiology of streptococcal infections in developing countries and implement more effective public health prevention programs.
Bacteriology
Group A streptococci are gram-positive bacteria covered with an outer hyaluronic acid capsule and a layer of group A carbohydrate, a polymer of rhamnose with N-acetylglucosamine side chains. In addition, molecules of M protein attached to the bacterial membrane extend from the cell surface as coiled-coil fibers that appear as fibrils on the surface of the bacterium
[129]
. More than 60 years ago, Lancefield and Dole described serotyping of GAS based on a trypsin-sensitive surface antigen, the M protein, and a variable trypsin-resistant antigen, the T antigen, which turned out to be pilus structures made of adhesion proteins
[130]
[131]
that promote pharyngeal cell adhesion and biofilm formation
[132]
. Some 20 T serotypes have been identified.
The M protein is a major surface protein of GAS, with more than 130 distinct serotypes identified
[133]
. The cloning and sequencing of the corresponding emm genes revealed repeating sequence motifs in the N-ter region of the protein, called the A region, which confers serotype specificity on the bacterial strain and induces strain-specific protective immunity against GAS infection. GAS clinical isolates emm typing in various settings did not show significant association between emm type and throat or skin isolates
[134]
[135]
[136]
. The M protein also is a virulence factor as it binds complement regulatory protein factor H and inhibits phagocytosis.
Another important virulence factor found at the surface of GAS is the C5a peptidase, an endopeptidase that cleaves the complement-derived chemotaxin C5a, inhibiting the recruitment of phagocytic cells to the site of infection
[137]
[138]
. Serum opacity factor (SOF) is yet another virulence factor expressed at the surface of S. pyogenes. It binds apolipoprotein A1 and disrupts the structure of high density lipoproteins. It also binds fibronectin and fibrinogen
[139]
. Among other surface proteins are the fibronectin-binding proteins Sfb1, FBP54 and R28, which, together with the M protein and the hyaluronic acid capsule, allow the bacterium to adhere to, colonize and invade human skin and mucus membranes
[140]
[141]
[142]
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.
Attachment of GAS to pharyngeal or dermal epithelial cells is the key initial step in colonization of the host. The attachment process actually involves multiple GAS proteins
[144]
including lipoteichoic acid (LTA), which binds fibronectin, the M protein, which binds CD46 on keratinocytes, the fibronectin-binding protein (FBP54), the F protein (SfbI), the serum opacity factor, and any number of other factors. The M protein and SfbI also are described as invasins, as they help intracellular invasion of epithelial cells by the bacteria. Bacterial invasion and movement through normal tissue barriers also involves binding of host plasminogen /plasmin by surface proteins such as glyceraldehyde-3-phosphate dehydrogenase, enolase, and strepto kinase, a fibrinolytic plasminogen activator that has been associated with the pathogenesis of acute poststreptococcal glomerulonephritis.
Several virulence factors of GAS have been identified which play a major role in the pathogenesis of scarlet fever, TSS, invasion of soft tissues and skin and necrotizing fasciitis. These are the extracellular pyrogenic exotoxins A, B, and C as well as exotoxin F and streptococcal superantigen SSA. All these toxins trigger massive nonspecific activation of T cells and production of inflammatory interleukins and cytokines (review in
[113]
). GAS also secrete a variety of proteins that play a major role in tissue invasion, including hydrolases that degrade proteins and nucleic acids, and esterases such as carboxylic esterase (Sse)
[145].
Vaccines
Group A streptococcal vaccine development faces substantial obstacles. Firstly, the widespread diversity of circulating GAS strains and M protein types is a major obstacle
[146]
. Opsonizing antibodies directed against the M protein are serotype-specific and there are more than 130 identified M serotypes. Secondly, immunological cross-reactivity has been demonstrated between epitopes in the M protein and several human tissues, including heart, kidney, and cartilage. Although the pathogenesis of RF is not yet fully understood, increasing evidence indicates the existence of an autoimmune process. And, thirdly, as humans are the only hosts for group A streptococci, no really relevant animal model is available.
Numerous experimental M protein-based candidate vaccines ranging from crude cell walls to highly purified M proteins were evaluated in the 1960s and 1970s but these approaches were limited by the observation of serologic cross-reactivity between epitopes in the M protein and human tissues including the heart, joints and brain
[113]
. Clinical studies led to a multi-decade setback in the use of M protein-based vaccines
[147]
. The eventual discovery that type-specific, N-terminal regions of the M protein elicited strong bactericidal immune responses and were devoid of potentially harmful cross-reactive epitopes led to the reopening of the development of multivalent M protein-based GAS vaccines.
Recent vaccine strategies have targeted either the type specific N-terminal region of the M protein or the highly conserved C-terminal region of the molecule. Immunization with the N-ter type-specific region induced protective bactericidal and opsonic antibody against the specific GAS serotype, whereas immunization with the C-ter region of the M protein protected against multiple serotypes and prevented colonization at mucosal surfaces
[148]
. To develop a suitable vaccine candidate, noncross-reactive, serotype-specific M protein epitopes were selected and linked to a conserved 14 amino acid-long epitope in the C-terminal half of the protein, J14, which is shared by about 70% of isolates. Prototype vaccine constructs have demonstrated excellent immunogenicity and protection in mice
[149]
[150]
[151]
and tolerance in human volunteers, including a hexavalent, a heptavalent and a 26-valent M protein vaccine
[152]
[153]
[154]
. The 26-valent M peptide vaccine was recently evaluated in a Phase II clinical trial in Canada.
In parallel, other attempts were made at developing M protein-based subunit GAS vaccines
[155]
. One such approach was based on the use of a lipid core peptide (LCP) system that was chemically linked to N-ter and/or C-ter peptides from the M protein and which induced long-lasting protection against GAS infection in mice, based on the induction of opsonic antibody
[146]
[156]
[157]
[158]
[159]
. The candidate vaccine was also effective in mice when administered by the intra-nasal route in the presence of cholera toxin B subunit
[160]
. Another candidate vaccine was based on the J14 peptide from the M protein incorporated into a lipoprotein construct with a universal T cell epitope and a self-adjuvanting lipid moiety, Pam(2)Cys
[151]
.
Again, the major problem faced with M protein-based vaccines is the very high diversity of S pyogenes strains in the field
[161]
[162]
. To bypass this obstacle, several attempts are being made at developing vaccines based on other bacterial proteins. Active and passive intranasal immunization with GAS surface protein C5a peptidase (SCPA) was shown to prevent infection of nasal mucosa-associated lymphoid tissue in mice
[163]
. The C5a peptidase is 95% to 98% identical among different GAS serotypes. A strong immune response to SCPA was observed in serum samples from children with GAS pharyngitis
[164]
. Similar observations were made with the streptococcal esterase Sse
[145]
. Other candidate vaccine antigens include the fibronectin-binding proteins
[165]
[166]
[167]
, exotoxins SPE A and SPE C that are involved in TSS and scarlet fever
[168]
[169]
, streptococcal immunoglobulin-binding protein Sib 35
[170]
, group A carbohydrate-protein conjugates
[171]
and extracellular lipoproteins
[172]
. Other surface proteins have recently been identified through a reverse vaccinology approach
[173]
, including the Spy 1325 protein
[174]
and the serine protease Spy 0416
[175].
Efficacy and long-term safety trials of group A streptococcal vaccines are expected to be difficult, as they will require both time and large sample sizes, especially if efficacy endpoints are clinical endpoints. WHO is currently developing standard protocols for the clinical evaluation of group A streptococcal vaccines.