Pathology, molecular biology, and pathogenesis of avian influenza A (H5N1) infection in humans

Abstract

H5N1 avian influenza is a highly fatal infectious disease that could cause a potentially devastating pandemic if the H5N1 virus mutates into a form that spreads efficiently among humans. Recent findings have led to a basic understanding of cell and organ histopathology caused by the H5N1 virus. Here we review the pathology of H5N1 avian influenza reported in postmortem and clinical studies and discuss the key pathogenetic mechanisms. Specifically, the virus infects isolated pulmonary epithelial cells and causes diffuse alveolar damage and hemorrhage in the lungs of infected patients. In addition, the virus may infect other organs, including the trachea, the intestines, and the brain, and it may penetrate the placental barrier and infect the fetus. Dysregulation of cytokines and chemokines is likely to be one of the key mechanisms in the pathogenesis of H5N1 influenza. We also review the various molecular determinants of increased pathogenicity that have been identified in recent years and the role of avian and human influenza virus receptors in relation to the transmissibility of the H5N1 virus. A comprehensive appreciation of H5N1 influenza pathogenetic mechanisms should aid in the design of effective strategies for prevention, diagnosis, and treatment of this emerging disease.

Figure 1

Examples of results of in situ hybridization, IHC, and lectin staining in various organs of H5N1 autopsies. A: Lung tissue showing severe damage, hyaline membrane formation, edema, fibrin exudation, and cellular infiltration (H&E staining). B: Double labeling with in situ hybridization (NP anti-sense probe) (purple-blue signals) and IHC with antibody to tubulin β (red signals, arrowheads) show positive in situ hybridization signals in the cytoplasm of both a tubulin-negative nonciliated cell (arrow) and a tubulin-positive (asterisk) ciliated cell in the trachea. C: Positive IHC staining (anti-NP antibody) in the nuclei and cytoplasm of some pneumocytes (arrows). D: Double labeling with in situ hybridization (NP sense probe) and IHC with antibodies to surfactant antibody A showing both dark blue nuclear in situ hybridization signals (arrows) and brownish-red cytoplasmic IHC signals (arrowheads) in a single cell in the lung. E: Positive in situ hybridization signals (NP sense probe) in the cytoplasm of some cells (arrows) in brain tissue taken from the parietal lobe. Double labeling with antibodies to neuron-specific enolase identifies these cells as neurons (not shown). F: Positive IHC signals (anti-NP antibody) in large mononuclear cells (arrows) with morphological features of macrophages within the core of a chorionic villus (arrows). IHC with antibody to CD68 on consecutive sections shows that these cells are most likely Hofbauer cells (fetal macrophages) (not shown). G: Positive IHC signals (anti-HA antibody) in the cytoplasm of some pneumocytes in fetal lung tissue. H: IHC with antibodies to macrophage inflammatory protein-1α shows a large number of positive cells in lung tissue. I: Staining with Maackia amurensis lectin II (specific for α-2,3-linked sialic acids) detects the presence of avian influenza virus receptors on pneumocytes. A, C, D, and F involve tissues taken from a 24-year-old pregnant female infected with H5N1 virus who died 9 days after disease onset. B, E, H, and I are taken from a 35-year-old male H5N1 patient who died 27 days after disease onset. G is lung tissue of the fetus carried by the 24-year-old pregnant female. B, D, and E: The in situ hybridization probes were labeled with digoxigenin and a NBT/BCIP substrate chromogen kit (Promega Corp., Madison, WI) was used to visualize the in situ hybridization signals, resulting in a purplish blue color. Anti-HA and anti-NP antibodies were purchased from Beijing Perfect Biotechnology Ltd. (Beijing, China) and VivoStat Inc (Portland, ME), respectively. B–D and F: The IHC reaction products were detected with 3-amino-9-ethylcarbazole (AEC) (Sigma, St. Louis, MO), which gives a brownish-red color. G–I: The IHC reaction products were colorized with diaminobenzidine (Zymed Laboratories, South San Francisco, CA), which gives a brown reaction color. C and F–I are counterstained with hematoxylin. B and E are lightly counterstained with methyl green. Scale bars: 25 μm (A, C, D, F, G); 10 μm (B); 12.5 μm (E, I); 20 μm (H).

Figure 2

Proposed pathogenesis of human H5N1 infection. Diagram depicting the key pathogenetic mechanisms, viral genes, and gene products that may be involved in H5N1 influenza virus infection. CTLs, cytotoxic T lymphocytes.