A Cell-Type-Specific Defect in Assembly of Influenza A Virus Particles in Primary Human Macrophages

The primary target of Influenza A virus (IAV) is epithelial cells in the respiratory tract. In contrast to epithelial cells, productive virus infection of most IAV strains is either completely blocked or highly inefficient in macrophages. The exact nature of the defect in IAV replication that leads to inefficient production of progeny virus particles in human macrophages remains to be determined. In this study, we showed that primary human monocyte-derived macrophages (MDM) are inefficient in IAV release even when compared to a macrophage-like cell line, despite comparable levels of viral protein translation and expression of viral glycoproteins at the plasma membrane. Correlative fluorescence scanning electron microscopy revealed that formation of budding structures at the cell surface is inefficient in MDM even though clustering of a viral glycoprotein, hemagglutinin (HA), is observed, suggesting that a step in the assembly process is blocked in this cell type. Using an in situ proximity ligation assay, we further determined that association between HA and the viral ion channel protein M2 is defective in MDM. In contrast, HA and another glycoprotein neuraminidase (NA) associate with each other on the MDM surface efficiently. Altogether, our study shows that association between HA with M2 on the plasma membrane is a discrete step in the IAV assembly process, which is separable from the association between HA with NA. Overall, our study lends key insights into cell-type-specific molecular events important for completion of IAV assembly in host cells.