Mechanisms of host recognition and immune evasion of members of the Streptococcus anginosus/milleri group.

Abstract

The Streptococcus Anginosus/Milleri Group (SMG) is made up of three closely related but distinct bacterial species: Streptococcus intermedius, Streptococcus constellatus, and Streptococcus anginosus. The SMG are recovered from about one-third of healthy, asymptomatic individuals. Despite this, the SMG cause more incidences of invasive streptococcal disease than Group A and Group B Streptococcus combined. Members of this group are somehow able to live a dual lifestyle. Little work has been conducted on the molecular pathogenicity of the SMG and host factors that contribute to host susceptibility to this group have been under-investigated. My research works towards discovering how the host recognizes the SMG as well as what enables the SMG to evade clearance by the immune system. I hypothesize that: 1) recognition of the SMG by toll-like receptor 2 (TLR2) plays a key role in triggering a cytokine response by the innate immune branch (which coordinates the immune response to the SMG), 2) the expression of cytolysins and extracellular polysaccharides by members of the SMG enables evasion of innate immune recognition and cytokine responses. hTLR2 reporter and monocyte-like cell lines as well as human blood samples from healthy donors were used to investigate the host factors that contribute to SMG infection. Five clinical reference SMG strains and a transposon mutant library were used to probe the contributing bacterial factors. It was found that TLR2 activation plays an important role in the cytokine response to the SMG, but there is heterogeneity between strains in their ability to activate TLR2. It was also found that intermedilysin expression by S. intermedius strains enables evasion of recognition; however, different hosts display varying susceptibility to this cytolysin. This study reveals that investigation of both host and microbial factors is essential to build an understanding of the mechanisms of SMG transition from commensalism to pathogenicity.