Avian reovirus major μ-class outer capsid protein influences efficiency of productive macrophage infection in a virus strain-specific manner

We determined that the highly pathogenic avian reovirus strain 176 (ARV-176) possesses an enhanced ability to establish productive infections in HD-11 avian macrophages compared to avian fibroblasts. Conversely, the weakly pathogenic strain ARV-138 shows no such macrophagotropic tendency. The macrophage infection capability of the two viruses did not reflect differences in the ability to either induce or inhibit nitric oxide production. Moderate increases in the ARV-138 multiplicity of infection resulted in a concomitant increase in macrophage infection, and under such conditions the kinetics and extent of the ARV-138 replication cycle were equivalent to those of the highly infectious ARV-176 strain. These results indicated that both viruses are apparently equally capable of replicating in an infected macrophage, but they differ in the ability to establish productive infections in these cells. Using a genetic reassortant approach, we determined that the macrophagotropic property of ARV-176 reflects a post-receptor-binding step in the virus replication cycle and that the ARV-176 M2 genome segment is required for efficient infection of HD-11 cells. The M2 genome segment encodes the major μ-class outer capsid protein (μB) of the virus, which is involved in virus entry and transcriptase activation, suggesting that a host-specific influence on ARV entry and/or uncoating may affect the likelihood of the virus establishing a productive infection in a macrophage cell.