Systematic Assessment of the Contribution of Superantigens to Nasopharyngeal Colonization in a Mouse Model of Streptococcal Infection

Streptococcus pyogenes is adapted for persistence in humans. It typically colonizes the tonsils and skin, and humans are the only known reservoir. S. pyogenes can cause a wide range of mild to serious infections. Most streptococci-related deaths are due to complications of rheumatic fever and invasive infections. S. pyogenes produces virulence factors that contribute to the pathogen’s ability to colonize and cause disease, including streptococcal superantigens (SAgs), also known as streptococcal pyrogenic exotoxins (Spes). SAgs function by cross-linking T cells and antigen presenting cells (APC) which may cause a massive inflammatory response, and as such have been found to contribute to streptococcal toxic shock syndrome (STSS). The role of SAgs in adaptation of S. pyogenes to its target niche has not been investigated. S. pyogenes experimentally colonizes mouse complete nasal turbinates (cNT). Murine models of streptococcal infection are imperfect systems due to inherent host-specific tropism. In this thesis, the colonization potential of S. pyogenes MGAS8232, associated with rheumatic heart disease, and S. pyogenes MGAS5005, associated with invasive infections, were assessed to explore the contribution of SAgs to nasal colonization in a mouse model. Colonization was tested in multiple mouse strains expressing human or mouse major histocompatibility complex (MHC) class II. C57BL/6 (B6) mice expressing human leukocyte antigen (HLA) transgenes showed enhanced colonization, up to ~100,000-fold at 48 hours post-inoculation. Individual and combined SAg deletions were assessed for nasal colonization. S. pyogenes MGAS8232 deletion colonization in mice expressing HLA-DR4 and HLA-DQ8 (DR4/DQ8) determined that SpeA was the major contributing SAg to the establishment of colonization, with minor contributions from SpeL and streptococcal mitogenic exotoxin Z (SmeZ). Colonization of S. pyogenes MGAS5005 was assessed in FVB mice, which express murine MHC class II q (H-2q), and SpeJ and SmeZ were the main contributing SAgs to the establishment of nasal colonization. SAgs contribute to 48-hour streptococcal recovery, but do not change the kinetics of bacterial clearance from cNT. These findings support the hypothesis that SAgs play an important role in niche adaptation and the establishment of colonization in mild or asymptomatic streptococcal infections in a murine model of nasal infection