RESEARCH SUMMARY

Our general objective is to study different biological aspects of the toroviruses, emergent viruses that are practically ignored despite they have the potentiality to infect and cause enteric diseases in different animal species and in man. They are enveloped viruses with a single-stranded RNA genome, which belong to the Coronaviridae (Nidovirales order). Four genotypes or species have been identified so far: equine torovirus (EqToV or BEV), bovine torovirus (BToV), porcine torovirus (PToV) and human torovirus (HToV). The impossibility to grow these viruses in culture cells, with the only exception of the equine torovirus BEV, has probably contributed to the insufficient study of these viruses.

Our group has developed specific assays for the diagnostic of porcine torovirus (PToV): an ELISA assay to determine the presence of antibodies to torovirus in serum samples, and a real time RT-PCR for the detection and quantitation of PToV in clinical samples. The results obtained using these assays show that porcine torovirus is highly prevalent in Spanish porcine livestock. This was the first study showing the presence of torovirus in Spain. We have later performed a longitudinal study analyzing samples taken from animals at different time points during piglet’s life. This study revealed that the animals become infected soon after weaning, when maternal protection transferred through the colostrum declines. Currently we are performing an extensive epidemiological study and the genetic characterization of the viral strains identified.

Another line of research is focused in the study of virus-host interactions. We have observed that BEV causes apoptosis at late times post-infection, once the new viral progeny has been produced. In addition, we have proven that the structural proteins S and M are capable of triggering this process when individually expressed. Thus, the induction of apoptosis in the infected cells, specifically in the enterocytes and in the crypts of the enteric tract could be the cause of the diarrhea exhibited by infected animals.

A third area of interest in our group is the characterization of the torovirus replication/transcription complexes. Positive-strand RNA viruses utilize virus-modified cellular membranes to build their replication machineries. By electron microscopy we have observed clusters of double membrane vesicles (DMVs) in the cytoplasm of cells infected with BEV, the prototype torovirus. They were observed both in thin sections of infected cells that had been embedded on conventional epoxy resin and in cryosections, that were later subjected to immunogold labeling with antibodies directed against double-stranded RNA. We are currently trying to determine the composition of these structures and identify the origin of the associated membranes.