Mikhail N. Matrosovich ^*  , Tatyana Y. Matrosovich ^*, Thomas Gray , Noel A. Roberts 

^*Institute of Virology, Philipps University, 35037 Marburg, Germany; M. P. Chumakov Institute of Poliomyelitis and Viral Encephalitides, Moscow 142782, Russia; Laboratory of Molecular Carcinogenesis, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709; and ^{?/SUP>Roche Products, Ltd., Welwyn Garden City, Herts AL7 3AY, United Kingdom}

Edited by Peter Palese, Mount Sinai School of Medicine, New York, NY, and approved January 30, 2004 (received for review December 3, 2003)

The recent human infections caused by H5N1, H9N2, and H7N7 avian influenza viruses highlighted the continuous threat of new pathogenic influenza viruses emerging from a natural reservoir in birds. It is generally believed that replication of avian influenza viruses in humans is restricted by a poor fit of these viruses to cellular receptors and extracellular inhibitors in the human respiratory tract. However, detailed mechanisms of this restriction remain obscure. Here, using cultures of differentiated human airway epithelial cells, we demonstrated that influenza viruses enter the airway epithelium through specific target cells and that there were striking differences in this respect between human and avian viruses. During the course of a single-cycle infection, human viruses preferentially infected nonciliated cells, whereas avian viruses as well as the egg-adapted human virus variant with an avian virus-like receptor specificity mainly infected ciliated cells. This pattern correlated with the predominant localization of receptors for human viruses (2-6-linked sialic acids) on nonciliated cells and of receptors for avian viruses (2-3-linked sialic acids) on ciliated cells. These findings suggest that although avian influenza viruses can infect human airway epithelium, their replication may be limited by a nonoptimal cellular tropism. Our data throw light on the mechanisms of generation of pandemic viruses from their avian progenitors and open avenues for cell level-oriented studies on the replication and pathogenicity of influenza virus in humans.

Abbreviations: HA, hemagglutinin; HRP, horseradish peroxidase; SNA, Sambucus nigra agglutinin; MAA, Maackia amurensis agglutinin; HTBE, differentiated human tracheobronchial epithelial cells; Mem96-M, A/Memphis/14/96-M (H1N1); Mem96-E1, A/Memphis/14/96-E1 (H1N1); DkH1, A/Mallard/Alberta/119/98 (H1N1); moi, multiplicity of infection.

To whom correspondence should be addressed. E-mail: mikhail.matrosovich@med.unimarburg.de

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