Jak-Stat signalling: virus-host interactions

Interferons are the major cytokines in the host defense against viral and microbial infections. They transduce their signals via the Jak-Stat signalling pathway, which is also implicated in the response to many other cytokines and some growth factors. The major components of the pathway are receptor-associated tyrosine kinases, the Jaks, and signal activated transcription factors, the Stats. Although analysis of knock-out mice lacking individual Jaks/Stats demonstrated their essential role in the host defense, the molecular mechanisms by which the individual components contribute to the responses are poorly understood. The aim of this project is the in vitro characterisation of the relative contribution of the specific components of the Jak/Stat pathway to the antiviral response and specificity. This will be achieved by establishing stable in vitro systems from different knock-out mice. Immortalised cells will be complemented with the respective protein in a tightly regulated inducible manner, thereby providing the first system described so far to study time- and dose- dependent functions of Jaks and Stats. In particular, we will focus on the role of Tyk2, Stat1 and Stat3 upon infection with a number of different viruses utilising different replication strategies. Host defense of knock-out and wildtype cells will be analysed in a comparative manner using two approaches. Firstly, virus growth and replication will be analysed in detail, thereby defining the exact step of the viral replication cycle, which is affected by the antiviral responses. Secondly, host gene-induction and cytokine expression upon infection will be characterised. To our knowledge this project will provide the first comprehensive and comparative analysis of the role of Tyk2, Stat3 and Stat1 for the host defense against viral infections.

The Jak-Stat (Janus kinase - signal transducer and activator of transcription) pathway is a signal transducing cascade that enables cells to respond to extracellular signals with the appropriate re-programming of gene expression. Many of these extracellular signals (e.g. cytokines) are essential components of the host response and defense pathways against pathogens. In line with these crucial roles, lack of functional Jak-Stat signaling leads to increased sensitivity to most viral and bacterial infections. Aim of the project was to further elucidate the molecular functions of individual members of the Jak and Stat family for the defense against viral infections. We identified essential functions of Tyk2 in the defense against Murine Cytomegalovirus (MCMV) in vitro and in vivo. Interestingly, requirement for Tyk2 for efficient control of MCMV replication is cell type specific, arguing for fundamental differences in host responses. A similar phenomenon was demonstrated fo the defense against Vesicular Stomatitis Virus (VSV). Within these studies we also describe an as yet unappreciated modifying function of Tyk2 in the host transcriptional responses to MCMV infection. The molecular pathways involved in this specific functions of Tyk2 in gene regulation will be part of future projects. In summary, these studies contribute on the one hand to the emergence of a broader picture about the multi-faced functions of Tyk2 in host defense and on the other hand to the understanding of the complex requirements for efficient host defense in vitro and in vivo against viruses and specificities therein.