Noroviruses and Epidemic Gastroenteritis

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications & trials

Abstract

The Caliciviridae is a family of nonenveloped, positive-strand RNA viruses and now consists of 11 genera: Norovirus, Sapovirus, Bavovirus, Lagovirus, Minovirus, Nacovirus, Nebovirus Recovirus, Salovirus, Valovirus, and Vesivirus. The diseases caused by caliciviruses vary, according to the virus and its host species. Members of the Caliciviridae causing diarrheal disease in humans belong to the genera Norovirus and Sapovirus. The Caliciviruses Section (CS) in LID conducts research related to caliciviruses, with an emphasis on the noroviruses. Noroviruses are markedly diverse and strains infecting humans belong to either Genogroup I (GI) or Genogroup II (GII), with multiple genotypes within each group. The role of genetic and antigenic diversity in the epidemiology of these viruses has been an ongoing area of active research in our laboratory because this information is essential in the design of vaccines and therapeutics. This year we completed a large study spanning several years in which we have analyzed the genomes of noroviruses of varying genotypes associated with chronic infection in immunocompromised patients. We discovered that such infections were likely established from a single founder virus infection that could, in many cases, form discrete subpopulations of viruses over time. We explored the effect of treatment, such as immunoglobulin therapy, on the evolution of these distinct populations. Sequencing data showed that each subpopulation evolved independently under selective pressure in the host, and that the antigenic sites of the viral capsid in each subpopulation sustained unusually high numbers of amino acid substitutions over time. These findings suggest that noroviruses associated with chronic infection have the ability to escape neutralization by the antibodies present in pooled gamma globulin therapy, consistent with the poor performance of this therapy in controlling norovirus infection. Moreover, noroviruses associated with chronic infection in many immunocompromised patients are evolving as multiple subpopulations, indicating that therapeutics must be broadly-reactive in order to cover a number of variants within the host. This high resolution genomics approach will be applied to the analysis of norovirus therapeutic outcomes in our future clinical studies in order to assess potential mechanisms responsible for treatment success or failure. Our laboratory has continued to work in the improvement of tools for preclinical studies of vaccines and therapeutics. We are modifying and optimizing cell culture systems, including intestinal enteroids, so that we can increase the replicative capacity of human noroviruses in vitro. For example, our discovery that enteroendocrine cells (EECs) are targets of norovirus infection in the intestinal tract has prompted us to explore how this cell type can be incorporated into cell culture experiments. We are currently using technologies such as single cell RNAseq to define specific host factors in cells that are essential for norovirus replication. An area of active research in our laboratory is to develop platforms in which to conduct high throughput screening (HTS) of functional antibodies and antiviral drugs. We have made progress in this area by the development of a new immunoassay (LIPS-Blockade) as well as the establishment of human cell lines that support the replication machinery of a mouse norovirus. Until we have achieved efficient, lytic replication of the human noroviruses in continuous cell lines, these tools should enhance HTS studies in the norovirus field.