DESCRIPTION: Emerging viral diseases sometimes arise when viruses mutate to acquire the ability to infect cells of a new species. Coronaviruses generally cause disease in only one animal species. The viruses are readily transmitted to other members of the same species, but they are transmitte poorly or not at all to other species. This species specificity of coronavirus infection and disease is determined in part by species-specific differences among the cell membrane glycoproteins that coronaviruses use as receptors to enter susceptible host cells. Receptors for mouse, human, pig, dog and cat coronaviruses have been identified and their cDNAs cloned. When the recombinant receptor proteins are expressed in virus-resistant cell lines, they become susceptible to infection with the appropriate coronavirus. Rarely, host range mutants of viruses occur that have gained the ability to infect cells from another animal species. Because not only the major receptor but a large number of alternative receptors for murine coronaviruse MHV have been identified and host range mutants of the virus have been selected, this is an excellent model system for the study of how mutations that affect virus species specificity may be selected. The proposed experiments will examine how murine coronaviruses acquire the ability to infect cells from many other species, and determine whether such host range mutants can infect the new species in vivo and cause disease. The hypothesis that host range mutants of viruses are selected in cells or tissues that express low levels of the natural virus receptor in addition to alternative receptors of lower efficiency will be tested. Using a combination of genetic, biochemical, and molecular techniques with reagents that have been developed to study MHV receptors, mutations will be identified in the virus attachment glycoprotein that affect receptor specificity and identify what receptors these mutant viruses use to infect cells of a different species. The molecular mechanism by which expression of viru receptors is down-regulated in persistently infected cells will also be studied.