Science in China Series C: Life Sciences www.scichina.com life.scichina.com © 2009 SCIENCE IN CHINA PRESS www.springer.com/scp www.springerlink.com Review Tissue and host tropism of influenza viruses: Importance of quantitative analysis ZHANG Hong1,2 1 Z-BioMed, Inc., Rockville, MD 20855, USA; 2 Department of Respiratory Medicine, Affiliated Hospital of Zunyi Medical College, Zunyi 563003, China It is generally accepted that human influenza viruses preferentially bind to cell-surface glycoproteins/ glycolipids containing sialic acids in α2,6-linkage; while avian and equine influenza viruses preferen- tially bind to those containing sialic acids in α2,3-linkage. Even though this generalized view is accurate for H3 subtype isolates, it may not be accurate and absolute for all subtypes of influenza A viruses and, therefore, needs to be reevaluated carefully and realistically. Some of the studies published in major scientific journals on the subject of tissue tropism of influenza viruses are inconsistent and caused confusion in the scientific community. One of the reasons for the inconsistency is that most studies were quantitative descriptions of sialic acid receptor distributions based on lectin or influenza virus immunohistochemistry results with limited numbers of stained cells. In addition, recent studies indicate that α2,3- and α2,6-linked sialic acids are not the sole receptors determining tissue and host tropism of influenza viruses. In fact, determinants for tissue and host tropism of human, avian and animal influenza viruses are more complex than what has been generally accepted. Other factors, such as glycan topology, concentration of invading viruses, local density of receptors, lipid raft microdomains, coreceptors or sialic acid-independent receptors, may also be important. To more efficiently control the global spread of pandemic influenza such as the current circulating influenza A H1N1, it is crucial to clarify the determinants for tissue and host tropism of influenza viruses through quantitative analysis of experimental results. In this review, I will comment on some conflicting issues related to tissue and host tropism of influenza viruses, discuss the importance of quantitative analysis of lectin and influenza virus immunohistochemistry results and point out directions for future studies in this area, which should lead to a better understanding of tissue and host tropism of influenza viruses. tissue and host tropism, influenza viruses, sialic acid receptors, quantitative analysis The influenza viruses belong to the Orthomyxoviridae associated with high morbidity and mortality in people family and are enveloped animal viruses containing a of all ages, caused millions of deaths in the previous segmented single-stranded RNA genome[1,2]. They are three influenza pandemics, is the cause of the current classified into three types, A, B and C, based on their influenza H1N1 pandemic and has the potential to cause immunologically distinct nucleoprotein and matrix more pandemics in the future[3,4]. protein antigens. Type A influenza viruses are further A new human influenza virus could initiate as an grouped into antigenic subtypes according to their avian or animal influenza virus that adapts to humans specific surface glycoproteins hemagglutinin (HA) and through accumulation of mutations or as a hybrid influ- neuraminidase (NA). Influenza A occurs in humans, other mammals, and birds and sixteen distinct HA Received October 1, 2009; accepted November 11, 2009 subtypes and nine NA subtypes are currently recognized. doi: 10.1007/s11427-009-0161-x Influenza A attracts the most attention because it is †Corresponding author (email: [email protected]) Citation: ZHANG Hong. Tissue and host tropism of influenza viruses: Importance of quantitative analysis. Sci China Ser C-Life Sci, 2009, 52(12): 1101-1110, doi: 10.1007/s11427-009-0161-x enza virus containing a combination of genes derived At this stage, the influenza H1N1 pandemic can still be from an avian, an animal and a human influenza virus. characterized as being moderate in terms of severity and Therefore, influenza pandemics arise when a new mortality rate. The epidemiology, molecular biology, influenza virus emerges from human, avian or animal pathology, and pathogenic factors of influenza viruses influenza viruses or from reassortants of them, infects including H5N1 in humans have been comprehensively humans, and spreads efficiently among people. The first reviewed recently[8−14]. This article will briefly review transmission of the highly pathogenic H5N1 avian the generalized view of receptors for influenza viruses, influenza A virus directly from chickens to humans in comment on some conflicting issues related to tissue and Hong Kong was reported in 1997[5]. As of 31 August host tropism of influenza viruses, discuss the importance 2009, a total of 440 laboratory-confirmed human cases of quantitative analysis of lectin and influenza virus of avian influenza A (H5N1) virus infections from 15 immunohistochemistry results and point out directions countries have been reported to WHO and 262 of them for future studies in this area. (59.5%) have been fatal (www.who.int). In China, 25 of the 38 confirmed cases (65.8%) have been fatal. Among 1 Generalized view of receptors for the 45 confirmed cases (12 deaths) reported to WHO in influenza viruses 2009, 7 were from China and 4 of them were fatal. The generalized view is that avian influenza viruses These events demonstrate that avian influenza viruses replicate in the gastrointestinal tract of the host and can infect human directly and China is an important preferentially bind to cell-surface glycoproteins or country for the surveillance, prevention and control of glycolipids containing sialic acids in α2,3-linkage potential influenza pandemics. Continued circulation of (SAα2,3); while human influenza viruses replicate in the H5N1 and other avian influenza viruses in Asia shows host’s respiratory tract and bind to respiratory epithelial that a future pandemic from avian influenza is a real cells via sialic acids attached to glycoproteins or glycoli- threat. pids containing sialic acids in α2,6-linkage (SAα2,6). The current circulating influenza A H1N1 viruses Hundreds of articles including many reviews related to were probably originated from the combination of six influenza virus receptors have been published during last gene segments (PB2, PB1, PA, HA, NP, and NS1) from 20 years. Many review articles and several articles swine influenza H1N2 viruses circulating in the USA published in major scientific journals such as Science, from 1999 to 2001 (nucleotide sequence similarities Nature and Nature Medicine made generalized statemen- >96%) and two gene segments (NA and M1) from swine ts about influenza virus receptors and cited two original influenza viruses circulating in Europe from 1985 to articles published in 1983[15,16]. Here are some examples: 1999 (nucleotide sequence similarities >93%)[6]. The “Human and avian influenza A viruses differ in their gene segments of these influenza A H1N1 viruses could recognition of host cell receptors: the former preferential- also be traced back to human or avian origins if those ly recognize receptors with saccharides terminating in nucleotide sequences with similarities lower than 93% were considered. Since the first report of the swine sialic acid-α2,6-galactose (SAα2,6Gal), whereas the [17] influenza H1N1 in April 2009[7], there have been over latter prefer those ending in SAα2,3Gal” ; “Avian 526060 cases and at least 6770 deaths (1.29%) reported influenza viruses bind to cell-surface glycoproteins or to the WHO from all regions of the world (www. who. glycolipids containing terminal sialyl-galactosyl resi- int). The WHO Director-General raised the level of dues linked by 2-3-linkages [Neu5Ac(α2-3)Gal], whereas influenza pandemic alert from phase 5 to phase 6 on human viruses, including the earliest available isolates June 11, 2009 following the advice from the Emergency from 1957 and 1968 pandemics, bind to receptors that Committee which concluded that the criteria for a contain terminal 2-6-linked sialyl-galactosyl moieties [18] pandemic had been met. Currently, pandemic H1N1 [Neu5Ac(α2-6)Gal]” ; “HA binds to receptors influenza virus continues to be the predominant circula- containing glycans with terminal sialic acids, where ting virus of influenza in the world and all pandemic their precise linkage determines species preference. A H1N1 2009 influenza viruses sequenced so far have switch in receptor specificity from sialic acids connected been antigenically and genetically very similar to the to galactose in α2-3 linkages (avian) to α2-6 linkages first sequenced new influenza virus (A/California/7/2009). (human) is a major obstacle for influenza A viruses to 1102 ZHANG H. Sci China Ser C-Life Sci | Dec. 2009 | vol. 52 | no. 12 | 1101-1110 cross the species barrier and to adapt to a new host”[19]; extent, to be dependent on the species…” (true for H3 “Human viruses of the H1, H2 and H3 subtypes that are subtype isolates only) could have evolved to become known to have caused pandemics in 1918, 1957, and generalized statements such as “human influenza viruses 1968, respectively, recognize α2,6-linked sialic acid”[20]; preferentially recognize receptors with SAα2,6Gal, and “Human influenza strains preferentially bind to whereas avian influenza viruses preferentially recognize sialic acid residues linked to galactose by the α2,6 receptors with SAα2,3Gal” (implying isolates of all HA linkage, while avian and equine influenza strains subtypes). recognize sialic acid linked to galactose by α2,3 In another article of Rogers et al., the sequencing linkage.”[21] bands showing mutations of CTG-leucine (human specific) In one of their original articles published in 1983, to CAG-glutamine (avian specific) or ATG-methionine Rogers et al.[15] modified human type B erythrocytes to (specific for both human and avian receptors) at amino contain sialyloligosaccharides of defined sequence with acid 226 of HA1 were not clear enough to make defini- different sialyltransferases purified from porcine tive conclusions[16].