Host Factors Required for Alphavirus Infection: The Identification of Mxra8 as an Entry Receptor
Rong Zhang1, Arthur S. Kim1, Katherine Basore1, Julie M. Fox1, William B. Klimstra2, Rebecca Rimkunas3, Rachel H. Fong3, James E. Crowe, Jr4, Benjamin J. Doranz3, Daved H. Fremont1, Michael S. Diamond1
1Washington University School of Medicine St. Louis, MO, USA; 2University of Pittsburgh, Pittsburgh, PA, USA; 3Integral Molecular, Inc., Philadelphia, PA USA, 4Vanderbilt University, Nashville, TN, USA;
Arthritogenic alphaviruses comprise a group of enveloped RNA viruses that are transmitted to humans by mosquitoes and cause debilitating acute and chronic musculoskeletal disease, which has substantial impact on quality of life. To define host factors contributing to the entry and pathogenesis of arthritogenic alphaviruses, we conducted a genome-wide CRISPR-Cas9 screen using a chikungunya (CHIKV) reporter virus. We identified the cell adhesion molecule Mxra8 as an entry receptor for multiple emerging arthritogenic alphaviruses, including chikungunya, Ross River, Mayaro and O'nyong nyong viruses. Gene editing of mouse Mxra8 or human MXRA8 resulted in reduced levels of viral infection of cells and, reciprocally, ectopic expression of these genes resulted in increased infection. Mxra8 bound directly to chikungunya virus particles and enhanced virus attachment and internalization into cells. Consistent with these findings, Mxra8-Fc fusion protein or anti-Mxra8 monoclonal antibodies blocked chikungunya virus infection in multiple cell types, including primary human synovial fibroblasts, osteoblasts, chondrocytes and skeletal muscle cells. Mutagenesis and structural experiments suggest that Mxra8 binds to a surface-exposed region across the A domain of chikungunya virus E2 protein Finally, administration of the Mxra8-Fc protein or anti-Mxra8 blocking antibodies to mice reduced chikungunya and O'nyong nyong virus infection as well as associated foot swelling. Results which have been confirmed in newly generated Mxra8 KO mice. Ongoing efforts are aimed at defining the exact binding footprint of Mxra8 and different alphavirus E proteins. The identification and characterization of host factors required for CHIKV infection, such as Mxra8, may help to develop countermeasures for this globally expanding family of arthritogenic alphaviruses.