Infection of Human Tracheal Epithelial Cells by H5 Avian Influenza Virus Is Regulated by the Acid Stability of Hemagglutinin and the pH of Target Cell Endosomes

Abstract

Despite the possible relationships between tracheal infection and concomitant infection of the terminal part of the lower respiratory tract (bronchioles/alveoli), the behavior of avian influenza viruses (AIVs), such as H5N1, in the conducting airways is unclear. To examine the tropism of AIVs for cells lining the conducting airways of humans, we established human tracheal epithelial cell clones (HTEpC-Ts) and examined their susceptibility to infection by AIVs. The HTEpC-Ts showed differing susceptibility to H5N1 and non-zoonotic AIVs. Viral receptors expressed by HTEpC-Ts bound all viruses; however, the endosomal pH was associated with the overall susceptibility to infection by AIVs. Moreover, H5N1 hemagglutinin broadened viral tropism to include HTEpC-Ts, because it had a higher pH threshold for viral–cell membrane fusion. Thus, H5N1 viruses infect human tracheal epithelial cells as a result of their higher pH threshold for membrane fusion which may be one mechanism underlying H5N1 pathogenesis in human airway epithelia. Efficient replication of H5N1 in the conducting airways of humans may facilitate infection of the lower respiratory tract.

Hemagglutinin Stability and Its Impact on Influenza A Virus Infectivity, Pathogenicity, and Transmissibility in Avians, Mice, Swine, Seals, Ferrets, and Humans

“…The avian viruses that had lower HA activation pH values were more restricted in replication. Similar results were obtained in a follow-up study using human tracheal epithelial cell clones (HTEpC-Ts) [ 199 ].…”

H9N2 Influenza Virus Infections in Human Cells Require a Balance between Neuraminidase Sialidase Activity and Hemagglutinin Receptor Affinity

“…Images were captured with an IN Cell analyzer 2000 automated microscope (GE Healthcare). Fields containing both virus antigen-positive cells and cell nuclei were counted by IN Cell Developer Toolbox software (GE Healthcare), as described previously (37). Infection rates were calculated by dividing the number of infected cells by the total number of nuclei in the same field.…”

“…The multiplicities of infection (MOIs) for the virus infections were equalized based on FFU assays on MDCK cells. The infectivity of these viruses was measured with an IN Cell analyzer automated microscope, which enabled us to accurately quantify high numbers of foci in image fields (37,38). In this study, after virus adsorption, the cells were washed and then overlaid with 1% methylcellulose in medium without trypsin to prevent the release and spread of progeny virus, thereby enabling us to observe only foci that were a result of the initial input virus infection.…”

Avian Influenza Virus Tropism in Humans

“…AIV enters the respiratory tract through the nose, mouth, or even eyes in humans and continuously travels along the respiratory tract to search for suitable host cells [4]. Types of human cells infected by AIV are respiratory epithelial cells [11,12], intestinal epithelial cells [13,14], immune cells [15,16] and nerve cells [17]. AIV does not typically transmit directly to humans and is assumed to require consecutive adaptation before transmitting efficiently among humans [18].…”