My Master study has been divided into two different parts. Both projects aim to provide a better understanding of virus-cell interactions. During viral infections, innate immunity allows the host to detect the virus presence and then to fight against it. The first part consists of a study about the newly identified tetherin antiviral protein and influenza virus, the causative agent of flu. We wanted to assess whether the influenza virus was a target of tetherin and if so how influenza manages to alleviate this restriction. In March 2012 an article concerning that study was published by our group, in which I actively participated. The second part focuses on the MDA5 protein, which senses the presence of several different viruses in order to trigger host defenses. We have been interested in understanding its regulation in the cell and more specifically a putative negative feedback pathway involving the E3 ubiquitin ligase β-TrCP. Such pathway could lead to a degradation of MDA5 which might be required to avoid a cytokines overproduction.