Visualisation of direct interaction of MDA5 and the dsRNA replicative intermediate form of positive strand RNA viruses.

The innate immune system is a vital part of the body's defences against viral pathogens. The proteins retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation associated gene 5 (MDA5) function as cytoplasmic pattern recognition receptors that are involved in the elimination of actively replicating RNA viruses. Their location and their differential responses to RNA viruses emphasises the complexity of the innate detection system. Despite the wealth of information on the types of RNA that trigger RIG-I, much less is known about the nature of the RNAs that act as agonists for MDA5. In order to identify which RNA species triggers MDA5 activation during infection, we isolated viral ssRNA and replicative intermediates of RNA from positive sense ssRNA viruses. We reveal that MDA5 recognises not the genomic ssRNA but the dsRNA generated by the replication of these viruses. Furthermore, using fluorescent imaging we present the first report of the visualisation of dsRNA and MDA5, which provides unique evidence of the relationship between viral dsRNA and MDA5 and proves without a doubt that MDA5 is the key sensor for the dsRNA replicative intermediate form of positive sense ssRNA viruses.

Anion homeostasis is important for non-lytic release of BK polyomavirus from infected cells.

BK polyomavirus (BKPyV) is a member of a family of potentially oncogenic viruses, whose reactivation can cause severe pathological conditions in transplant patients, leading to graft rejection. As with many non-enveloped viruses, it is assumed that virus release occurs through lysis of the host cell. We now show the first evidence for a non-lytic release pathway for BKPyV and that this pathway can be blocked by the anion channel inhibitor DIDS. Our data show a dose-dependent effect of DIDS on the release of BKPyV virions. We also observed an accumulation of viral capsids in large LAMP-1-positive acidic organelles within the cytoplasm of cells upon DIDS treatment, suggesting potential late endosome or lysosome-related compartments are involved in non-lytic BKPyV release. These data highlight a novel mechanism by which polyomaviruses can be released from infected cells in an active and non-lytic manner, and that anion homeostasis regulation is important in this pathway.

Human rhinovirus recognition in non-immune cells is mediated by Toll-like receptors and MDA-5, which trigger a synergetic pro-inflammatory immune response.

The early detection of invading viruses by the host depends on their identification by pathogen sensors. These include Toll-like receptors (TLRs) as well as cytoplasmic RNA helicases such as retinoic acid inducible protein I (RIG-I) and melanoma differentiation associated gene 5 (MDA-5). These pathogen sensors recognize specific molecular patterns found in viruses and trigger inflammatory and antiviral responses that result in the eradication of invading pathogens. In this study we investigated the specific recognition of Human rhinovirus 6 (HRV6) the common cold pathogen by the innate immune response in lung epithelial cells. Our experiments established that in the first stages on infection the TLRs play a crucial role in HRV recognition and that different constituents of HRV6 are recognized by different TLRs, while upon viral replication and generation of dsRNA the type I IFN inflammatory response is mediated by MDA-5. The HRV6 capsid is recognized via TLR2, whereas upon HRV6 ssRNA internalization the virus genome is recognized by TLR7 and TLR8. Upon generation of dsRNA the type I IFN response is mediated by MDA-5. The combined recognition by different TLRs and MDA5 and their upregulation concurs with the huge inflammatory response seen in the common cold caused by human rhinoviruses.