SpeA

SpeA, or streptococcal pyrogenic exotoxin A, is a secreted superantigen produced by Streptococcus pyogenes (group A Streptococcus). The speA gene is carried by lysogenic bacteriophages that can integrate into the GAS genome; strains expressing SpeA are often linked to increased virulence and to outbreaks of diseases such as scarlet fever and streptococcal toxic shock syndrome (STSS).

Molecular characteristics and regulation: SpeA is a small protein, typically around 20–22 kilodaltons. As a superantigen, it binds simultaneously to MHC class II molecules on antigen-presenting cells and to the beta chain of T-cell receptors, outside the conventional antigen-presenting groove. This cross-linking triggers widespread, nonspecific T-cell activation and a massive release of cytokines (including IL-2, IFN-γ, and TNF-α), leading to fever, rash, hypotension, and potentially multiorgan dysfunction. Expression of speA is influenced by phage biology and regulatory networks within GAS, such as the Mga regulon.

Clinical relevance and detection: SpeA-associated strains have been implicated in severe disease manifestations, notably streptococcal toxic shock syndrome and scarlet fever outbreaks. Immunity is variable, and antibodies against SpeA can neutralize its activity in some individuals. Detection methods include PCR assays targeting the speA gene, with toxin presence correlating to virulence potential in epidemiological studies. Management of SpeA-related disease follows general guidelines for GAS infections, emphasizing appropriate antibiotic therapy and supportive care; understanding SpeA contributes to insights into GAS virulence and outbreak dynamics.