Innate immune response

Signalling in the innate immune response

Invading pathogens are rapidly sensed by the hosts' innate immune system. The molecular mechanisms underlying pathogen detection and the subsequent cellular responses are rapidly being unveiled, but major knowledge gaps still remain. We focus on two aspects of the innate immune response: the initial steps after activation of the Toll-like receptors and the cellular response on viral infection.

1. The early steps following activation of Toll-like receptors

Toll-like receptors (TLRs) are transmembrane receptors that recognize pathogen-associated molecular patterns (PAMPs), molecular signatures which are conserved in pathogens, such as lipopolysaccharides of gram-negative bacteria or viral double-stranded RNA. TLRs play a central role in innate immunity: recognition of specific PAMPs leads to the activation of immune cell responses including phagocytosis, apoptosis, or the production of cytokines or interferons.

TLRs function as homo- or heterodimers or possibly as oligomers. They are characterized by the presence of extracellular Leucine Rich Repeats involved in PAMP recognition and intracellular Toll/IL-1 receptor-like (TIR) domains. Ligand-dependent activation of the TLRs leads to a cascade of intracellular protein-protein interactions involving a set of adapter proteins that ultimately leads to the appropriate cellular effects. Despite the importance of TLR signalling in the innate immune system, little is known about the molecular events that trigger the signalling cascade.

We combine MAPPIT with mutagenesis and structural data to map the interfaces of protein interactions in the TLR pathway. With FACS- and Array-MAPPIT, we screen for new interaction partners in the TLR signalling pathways.

2. Cellular responses on viral infection

Type I Interferons (IFNs) are involved in host defence mechanisms, particularly against viral infections.

Activation of the type I IFN receptor complex induces a so-called "antiviral state" primarily by activating Stat1 and Stat2. Fosforylation of Stat3 also occurs, but remarkably, in contrast to Stat3 activation via other receptor systems, this does not result in activation of Stat3 reporter genes. We obtained evidence for a negative regulatory mechanism and use genome-wide RNAi screening to identify the inhibitory complex.

Interferon-Stimulated Gene 15 (ISG15) is an Ubiquitin-Like molecule, highly induced upon type I IFN treatment. Upon the cooperative activity of specific Ubiquitinating enzymes, ISG15 can be conjugated to its substrates. Although the involvement of ISG15 in antiviral defence against several RNA viruses has been established, many aspects of the mode of action of ISG15 remain enigmatic. We have recently mapped residues in ISG15 critical for the ISGylation process and created ISG15 variants with significantly enhanced ISGylation efficiency qualities. These variants are used to study how ISGylation contributes to antiviral defence.

Upon viral infections, cellular factors are induced that interact with viral proteins and actively interfere with their function. Many of these â??restriction factors' are IFN-induced. With MAPPIT, the interplay between such cellular factors and viral components are being studied.