Receptor binding and transmission studies of H5N1 influenza virus in mammals

OPEN Emerging Microbes and Infections (2013) 2, e85; doi:10.1038/emi.2013.89 ß 2013 SSCC. All rights reserved 2222-1751/13 www.nature.com/emi REVIEW Receptor binding and transmission studies of H5N1 influenza virus in mammals Hanjun Zhao1,2, Jie Zhou1, Shibo Jiang3,4 and Bo-Jian Zheng1 The H5N1 influenza A virus that is currently circulating in Asia, Africa and Europe has resulted in persistent outbreaks in poultry with sporadic transmission to humans. Thus far, it is believed that H5N1 does not possess sufficient ability for human-to-human transmission and subsequent pandemic infection. Both receptor binding specificity and virus infectivity are key factors in determining whether influenza A virus becomes pandemic. The use of human viral isolates in various studies has helped to illustrate the changes in receptor binding specificity and virulence as a result of adaptation in humans. In this review, we highlight the important amino acids and domains of viral proteins related to receptor binding specificity that have been reported for humans and avians using mammalian models. Thus, this review will consolidate findings from studies that have shed light on the receptor binding and transmission characteristics of the H5N1 influenza virus, with the goal of improving our ability to predict the transmission efficiency or pandemic potential of new viral strains. Emerging Microbes and Infections (2013) 2, e85; doi:10.1038/emi.2013.89; published online 18 December 2013 Keywords: receptor binding; transmission; H5N1; mammal; influenza A virus INTRODUCTION The highly pathogenic avian influenza (HPAI) H5N1 virus was first isolated from sick geese in southern China in 1996.1 As of October 8th, 2013, outbreaks of H5N1 influenza across the globe have resulted in millions of deaths in birds and 377 deaths (mortality rate of 59.3%) in humans.2 The first reported cases of H5N1 virus infection in humans occurred in Hong Kong in 1997, which coincided with an outbreak of the virus in the territory’s chicken farms.3,4 More than 15 years after this outbreak, other avian H5N1 virus strains have not acquired the level of transmissibility and replication required to cause a human pandemic. Nonetheless, in anticipation of this possibility, much attention has been focused on transmission and adaptation of avian H5N1 virus in humans and the potential increase in virulence caused by amino acid mutations in viral proteins. Recently, the geographic distribution of avian H5N1 virus infection has expanded to Kazakstan, Mongolia, Djibouti, Egypt, Turkey and Russia, indicating that more of the world’s population is at risk.5 However, the determinants of infection and susceptibility of humans to H5N1 virus have not been fully elucidated. To become a pandemic virus, the viral strain should be able to transmit efficiently between humans, which is determined by factors such as whether the virus grows to high enough titers in the human lungs.6,7 Fortunately, the current H5N1 virus strains have not acquired such abilities in humans. However, two recent experimental studies have shown that reassortant H5N1 viruses with four mutations in hemagglutinin (HA) were capable of droplet transmission in a ferret model.8,9 Therefore, the potential risk does exist for a natural H5N1 virus to evolve into a pandemic virus after continuous circulation between avian and human hosts, which will provide opportunities for the acquisition of the necessary amino acid mutations in viral proteins, particularly HA. This review will focus on studies reporting on the receptor binding of H5N1 virus in humans and other mammals, as well as the amino acid mutations in viral proteins related to transmission of the virus in humans and experimental animals. In virus binding studies, the attachment of viruses can be directly measured by labeling viruses and then applying them to tissue sections in a method coined virus histochemistry, which has been used to study the pattern of virus attachment in different tissues. By comparing virus binding and transmission in different animal models, this knowledge will help to elucidate potential transmission routes and will provide the genetic basis for predicting whether a mutated virus strain may evolve into a pandemic virus. VIRAL RECEPTOR BINDING AND TRANSMISSION IN HUMANS One important aspect of influenza virus infection is the interaction between the viral surface glycoprotein HA and the corresponding receptor on host cells. To infect host cells, influenza virus utilizes HA to bind to complex glycans on the host cell surface via a terminal sialic acid (SA). Influenza viruses have different preferences for SAs with different linkages. For example, human influenza virus prefers sialic acid linked to galactose via an a-2,6 bond (SAa-2,6Gal), whereas avian influenza virus prefers the terminus with sialic acid linked to galactose via an a-2,3 bond (SAa-2,3Gal).10 SAa-2,6Gal is the major linkage for vicinal galactose in the human upper respiratory epithelium.11 Epithelial cells in the paranasal sinuses, pharynx, trachea and bronchi mainly express SAa-2,6Gal, which is also expressed in ciliated 1 Department of Microbiology, the University of Hong Kong, Hong Kong 999077, China; 2Department of Clinical Laboratory, Second People’s Hospital of Wuhu, Wuhu 241000, Anhui Province, China; 3Key Laboratory of Medical Molecular Virology of Ministries of Education and Health, Shanghai Medical College and Institute of Medical Microbiology, Fudan University, Shanghai 200032, China and 4Lindsley F. Kimball Research Institute, New York Blood Center, New York, NY 10065, USA Correspondence: BJ Zheng, E-mail: Received 8 August 2013; revised 10 November 2013; accepted 18 November 2013 Receptor binding and transmission of H5N1 virus H Zhao et al 2 and goblet cells in the human lung.12,13 Apart from SAa-2,6Gal, the human respiratory tract also expresses SAa-2,3Gal on non-ciliated cuboidal bronchiolar cells, which are situated at the junction between the respiratory bronchiole and alveolus.11 Highly pathogenic avian H5N1 virus labeled by fluorescein isothiocyanate (FITC) was shown to preferentially attach to type-II pneumocytes, alveolar macrophages, and non-ciliated cuboidal epithelial cells in the terminal bronchioles of the human lower respiratory tract.14 The binding of H5N1 virus rarely occurs at the trachea and upper respiratory tract, which is consistent with pathological findings observed at autopsy, such as diffuse alveolar damage, interstitial pneumonia, focal hemorrhage, and bronchiolitis.15,16 Some strains of the highly pathogenic H5N1 virus, such as A/Hong Kong/486/97 (HK486) or A/Duck/Hong Kong/200/01, show high binding affinity to SAa-2,3Gal but not SAa-2,6Gal. However, some other strains isolated from patients, such as A/Hong Kong/212/03 and A/Hong Kong/213/03 (HK213), can recognize both SAa-2,3Gal and SAa-2,6Gal.17,18 These findings implicate the ability of H5N1 virus, which preferentially binds to SA (...truncated)