Protection acquired upon intraperitoneal group a Streptococcus immunization is independent of concurrent adaptive immune responses but relies on macrophages and IFN-γ.

Group A Streptococcus (GAS; Streptococcus pyogenes) is an important bacterial pathogen causing over 700 million superficial infections and around 500.000 deaths due to invasive disease or severe post-infection sequelae yearly. In spite of this major impact on society, there is currently no vaccine available against this bacterium. GAS strains can be separated into >250 distinct emm (M)-types, and protective immunity against GAS is believed to in part be dependent on type-specific antibodies. Here, we analyse the nature of protective immunity generated against GAS in a model of intraperitoneal immunization in mice. We demonstrate that multiple immunizations are required for the ability to survive a subsequent lethal challenge, and although significant levels of GAS-specific antibodies are produced, these are redundant for protection. Instead, our data show that the immunization-dependent protection in this model is induced in the absence of B and T cells and is accompanied by the induction of an altered acute cytokine profile upon subsequent infection, noticeable e.g. by the absence of classical pro-inflammatory cytokines and increased IFN-γ production. Further, the ability of immunized mice to survive a lethal infection is dependent on macrophages and the macrophage-activating cytokine IFN-γ. To our knowledge these findings are the first to suggest that GAS may have the ability to induce forms of trained innate immunity. Taken together, the current study proposes a novel role for the innate immune system in response to GAS infections that potentially could be leveraged for future development of effective vaccines.