Serial Section Array Scanning Electron Microscopy Analysis of Cells from Lung Autopsy Specimens following Fatal A/H1N1 2009 Pandemic Influenza Virus Infection.

A/H1N1 2009 pandemic influenza virus (A/H1N1/pdm09) was first identified as a novel pandemic influenza A virus (IAV) in 2009. Previously, we reported that many viral antigens were detected in type II alveolar epithelial cells (AEC-IIs) within autopsied lung tissue from a patient with A/H1N1/pdm09 pneumonia. It is important to identify the association between the virus and host cells to elucidate the pathogenesis of IAV pneumonia. To investigate the distribution of virus particles and morphological changes in host cells, the autopsied lung specimens from this patient were examined using transmission electron microscopy (TEM) and a novel scanning electron microscopy (SEM) method. We focused on AEC-IIs as viral antigen-positive cells and on monocytes/macrophages (Ms/Mϕs) and neutrophils (Neus) as innate immune cells. We identified virus particles and intranuclear dense tubules, which are associated with matrix 1 (M1) proteins from IAV. Large-scale two-dimensional observation was enabled by digitally "stitching" together contiguous SEM images. A single whole-cell analysis using a serial section array (SSA)-SEM identified virus particles in vesicles within the cytoplasm and/or around the surfaces of AEC-IIs, Ms/Mϕs, and Neus; however, intranuclear dense tubules were found only in AEC-IIs. Computer-assisted processing of SSA-SEM images from each cell type enabled three-dimensional (3D) modeling of the distribution of virus particles within an ACE-II, a M/Mϕ, and a Neu. Generally, it is difficult to observe IAV particles in postmortem samples from patients with seasonal influenza. In fact, only a few viral antigens are detected in bronchial epithelial cells from autopsied lung sections. Previously, we detected many viral antigens in AEC-IIs from the lung. This was because the majority of A/H1N1/pdm09 in the lung tissue harbored an aspartic acid-to-glycine substitution at position 222 (D222G) of the hemagglutinin protein. A/H1N1/pdm09 harboring the D222G substitution has a receptor-binding preference for α-2,3-linked sialic acids expressed on human AECs and infects them in the same way as H5N1 and H7N9 avian IAVs. Here, we report the first successful observation of virus particles, not only in AEC-IIs, but also in Ms/Mϕs and Neus, using electron microscopy. The finding of a M/Mϕ harboring numerous virus particles within vesicles and at the cell surface suggests that Ms/Mϕs are involved in the pathogenesis of IAV primary pneumonia.