Evidence of the recombinant origin of a bat severe acute respiratory syndrome (SARS)-like coronavirus and its implications on the direct ancestor of SARS coronavirus.

Bats have been identified as the natural reservoir of severe acute respiratory syndrome (SARS)-like and SARS coronaviruses (SLCoV and SCoV). However, previous studies suggested that none of the currently sampled bat SLCoVs is the descendant of the direct ancestor of SCoV, based on their relatively distant phylogenetic relationship. In this study, evidence of the recombinant origin of the genome of a bat SLCoV is demonstrated. We identified a potential recombination breakpoint immediately after the consensus intergenic sequence between open reading frame 1 and the S coding region, suggesting the replication intermediates may participate in the recombination event, as previously speculated for other CoVs. Phylogenetic analysis of its parental regions suggests the presence of an uncharacterized SLCoV lineage that is phylogenetically closer to SCoVs than any of the currently sampled bat SLCoVs. Using various Bayesian molecular-clock models, interspecies transfer of this SLCoV lineage from bats to the amplifying host (e.g., civets) was estimated to have happened a median of 4.08 years before the SARS outbreak. Based on this relatively short window period, we speculate that this uncharacterized SLCoV lineage may contain the direct ancestor of SCoV. This study sheds light on the possible host bat species of the direct ancestor of SCoV, providing valuable information on the scope and focus of surveillance for the origin of SCoV.

Demonstration of receptor binding properties of VP2 of very virulent strain infectious bursal disease virus on Vero cells.

Tissue culture adaptation of infectious bursal disease virus (IBDV) results in alternation of three residues on its major capsid protein VP2 and these residues may engage in receptor binding. Although the key of successful infection of tissue culture adapted IBDV in tissue cultures was defined as the virus entering steps, mechanism of the adaptation is poorly understood. In this study, recombinant VP2s of an attenuated strain (D78) and a very virulent strain (HK46) of IBDV tagged with rabbit immunoglobulin G heavy chain were expressed in mammalian cells, generating RAVP2 and RVVP2, respectively, in high purity. Using flow cytometry, both RAVP2 and RVVP2 were demonstrated to bind with Vero cells while these bindings were blocked by D78 viral particles, implying both very virulent IBDVs (vvIBDVs) and attenuated IBDVs bind to Vero cells through the same receptor(s). Since vvIBDVs cannot be propagated directly in tissue cultures, the specific binding between RVVP2 and Vero cells suggests the barrier for tissue culture adaptation may be beyond the virus attachment process.

Phylogenetic analysis reveals a correlation between the expansion of very virulent infectious bursal disease virus and reassortment of its genome segment B.

Infectious bursal disease virus (IBDV) is a birnavirus causing immunosuppressive disease in chickens. Emergence of the very virulent form of IBDV (vvIBDV) in the late 1980s dramatically changed the epidemiology of the disease. In this study, we investigated the phylogenetic origins of its genome segments and estimated the time of emergence of their most recent common ancestors. Moreover, with recently developed coalescence techniques, we reconstructed the past population dynamics of vvIBDV and timed the onset of its expansion to the late 1980s. Our analysis suggests that genome segment A of vvIBDV emerged at least 20 years before its expansion, which argues against the hypothesis that mutation of genome segment A is the major contributing factor in the emergence and expansion of vvIBDV. Alternatively, the phylogeny of genome segment B suggests a possible reassortment event estimated to have taken place around the mid-1980s, which seems to coincide with its expansion within approximately 5 years. We therefore hypothesize that the reassortment of genome segment B initiated vvIBDV expansion in the late 1980s, possibly by enhancing the virulence of the virus synergistically with its existing genome segment A. This report reveals the possible mechanisms leading to the emergence and expansion of vvIBDV, which would certainly provide insights into the scope of surveillance and prevention efforts regarding the disease.