Innate immune responses to Streptococcus pyogenes

Recognition of pathogens by host immune cells is the key step in defense against infectious microorganisms. An inappropriate recognition may result in insufficient immune responses, yet an overactivation of the immune system may be equally deleterious. Pharmacological modulation of the pathogen recognition represents a promising so far largely unexploited approach to adjust the strength of immune responses. In the proposed project, molecules involved in the recognition of Streptococcus pneumoniae (pneumococcus) and Streptococcus pyogenes (Group A Streptococcus, GAS) by the host innate immune cells will be studied on a whole-genome scale, and pharmacological intervention with the recognition process will be exploited. The Gram-positive bacteria S. pneumoniae and S. pyogenes are important human pathogens whose estimated combined death toll is as high as that of Mycobacterium tuberculosis (1). Despite the availability of effective antibiotics infections due to S. pneumoniae and S. pyogenes are still a major cause of mortality in the third world countries. In the developed countries these pathogens are leading agents of both mild and severe diseases that represent a high economical burden to the health care systems. The widespread emergence of resistance to antibiotics (particularly in the case of S. pneumoniae) has become a major concern in recent years. Both pathogens cause an extraordinary wide range of pathological conditions (e.g. otitis, pneumonia, meningitis, pharyngitis, acute rheumatic fever, rheumatic heart disease or sepsis). The high variability of the clinical manifestations is on one side due to differences in the virulence of the bacterial strains within the S. pneumoniae and S. pyogenes species, and on the other side due to different susceptibility of the infected humans. The host susceptibility is determined by genetic factors (mostly genes of the innate and adaptive immune system) and factors affecting the condition of the immune system (e.g. age, immunosuppressive medications, co-infections with other pathogens). The initial recognition of the pathogens by the innate immune system plays a major role in launching innate and adaptive immune responses and, consequently, in the outcome of the infection. Findings from our laboratories indicate that yet unidentified molecules are involved in the recognition of S. pneumoniae and S. pyogenes ((2) and Gratz et al., JBC in press). In case of S. pyogenes the situation is particularly striking since none of the most frequently used pattern recognition receptors (PRRs) of the TLR family is used, yet the activation of macrophages is dependent on MyD88, a molecule downstream of TLRs. Our data also show that S. pyogenes induces interferon production ((3) and Gratz et al., submitted) which was unexpected for a Gram-positive extracellular bacterium. These findings implicate that the pathogen-associated molecular patterns (PAMPs) remain to be identified as well. We will use whole genome siRNA-based functional genomics screens to identify the murine receptors for S. pneumoniae and S. pyogenes as well as the components of signaling pathways downstream of those receptors. The function and relevance of the corresponding human genes will be studied in a more targeted approach by analyzing the relevant genes individually. To characterize the pathogen-associated molecular patterns a transposon insertion mutant library of S. pyogenes will be employed in in vitro infection assays. The contribution of the quality of recognition of the pathogens to the virulence of various strains of S. pneumoniae and S. pyogenes will be investigated using a collection of clinical isolates in in vitro infection assays. To quantitatively modulate the host-pathogen interactions, compounds and antibodies will be developed that interfere with the identified PAMPs, PRRs, and components of the host signaling cascades. The overall approach will enable us to identify new key players in streptococcal diseases and develop strategies leading to boosting (in case of insufficient activation of the patients immune system) or attenuating immune responses (e.g. in case of toxic shock).

Streptococcus pyogenes is an important human pathogen that causes a broad range of diseases. The bacterium colonizes preferentially the throat epithelium where it can evoke usually mild or antibiotics treatable illness such as strep throat or scarlet fever. Systemic infections with S. pyogenes are less frequent but can develop into life-threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. The immune system launches a usually successful response that is initiated by a so far not understood recognition of this pathogen by the cells of the innate immune system. These cells produce upon infection a variety of cytokines which are small secreted proteins orchestrating a full blown protective response. Among these cytokines, type I interferons play a critical role as demonstrated by our study. We further showed that interferon-beta, the key type I interferon, is produced only after macrophages and dendritic cells have taken up the pathogen and liberated the bacterial nucleic acids for recognition in the intracellular vesicles. Importantly, macrophages and dendritic cells recognize different nucleic acids and employ different signaling pathways to respond. Our data reveal that the pathogenic bacterium S. pyogenes avoids recognition by the standard recognition mechanisms of the immune system. However, we show that immune cells possess additional systems to detect such pathogens and to launch efficient immune response. These novel recognition mechanisms are currently being investigated in our laboratory.