Attenuated reovirus type 3 strains generated by selection of haemagglutinin antigenic variants

Abstract

Infection of suckling mice with mammalian reovirus type 3 results in fatal encephalitis associated with marked destruction of neurones^1–4. The viral tropism specific to neural cells is determined by the reovirus haemagglutinin^3,4. This haemagglutinin (HA), the σ1 polypeptide, is encoded in the S1 dsRNA segment^5,6. In addition to determining cell and tissue tropism, the viral haemagglutinin determines humoral and cellular immune specificity^7–10, binding to cellular micro-tubules¹¹ and inhibition of cellular DNA synthesis¹². To elucidate the structural basis for the functions of the reovirus type 3 HA, we have recently isolated and characterized several anti-HA monoclonal antibodies, which we have used to show that there are distinct functional domains on the reovirus type 3 HA¹³. One group of monoclonal antibodies neutralizes viruses while a second inhibits haemagglutination. Thus there are distinct HA functional domains involving haemagglutination and neutralization. One of the neutralizing monoclonal antibodies was used to select reovirus antigenic variants that were no longer neutralized by the monoclonal antibodies. These ‘variant’ viruses were tested for their neurovirulence by determining their capacity to replicate in mouse brains, leading to fatal encephalitis. We report here that all variants are markedly less virulent then the parental type 3 virus from which they were selected. These findings demonstrate directly the critical role of the viral haemagglutinin in neurovirulence and show that variant viruses that are altered at a site recognized by neutralizing antibody cannot grow efficiently in neural tissue.