Viral characteristics of influenza.

The viruses that lead to influenza are members of the Orthomyxoviridae family, which consists of enveloped viruses with a single, segmented, negative-strand ribonucleic acid (RNA) genome. Four genera have been defined, including influenza A, B, and C and Thogotovirus (or influenza D). Although these viruses possess similar proteins, each virus encodes proteins by different methods. This review focuses on the clinically relevant viruses, influenza A and B, which are briefly summarized in Table 1. The unique genetics of influenza A has led to additional divisions into subtypes on the basis of the hemagglutinin (HA) and neuraminidase (NA) antigens present in a given virus. The current nomenclature for influenza A provides the host of origin, the geographic location of the first isolation, the strain number, and the year of isolation. (1) The antigenic description may be provided parenthetically (eg, Equine/Miami/l/63 [H3/N8]). Influenza A differs from influenza B and C in its ability to infect multiple species, including avian and mammalian species. Humans, horses, and swine have been infected most frequently. Influenza B infects only humans. In addition, the HA and NA proteins derived from influenza A viruses have greater sequence variability than influenza B in their amino acids. (2)

Structure and Replication

Influenza A and B are morphologically indistinguishable and have a rather prominent array of surface spikes projecting over the viral surface that correspond to the HA and NA proteins. (3) Each of these viruses contains eight segments of single-stranded RNA that encode the viral polymerase complex, HA, NA, nucleocapsid proteins, matrix proteins, and nonstructural proteins involved in various aspects of RNA processing and transport (Fig. 1). (4)

[FIGURE 1 OMITTED]

The HA glycoprotein contains the receptor binding site for these viruses and can bind to sialic acid residue on cell surface glycoproteins and/or glycotipids. The virus, which is absorbed into vesicles by receptor-mediated endocytosis, begins to uncoat as these vesicles fuse into low-pH endosomes. The HA glycoprotein undergoes a unique conformational change in this low-pH environment that allows it to fuse the viral membrane with host cellular membrane and release viral ribonucleic particles (RNPs) into the cytosol. (5)

Viral RNPs enter the host nucleus through a nuclear pore, at which point the virus must use host cell transcripts to provide capped RNA primers to initiate viral messenger RNA (mRNA) synthesis. This capped, positive-strand mRNA serves as a template for the production of new negative-strand mRNA that ultimately has dual functions: 1) the production of positive-strand template mRNA to produce new viral protein components and 2) the production of negative-strand virion RNA to be packaged into nascent viral particles. (6) The genome, being segmented, may use a random packaging mechanism in which any of the eight segments may be packaged in a given virion. This process results in many noninfectious virions, a finding that is documented in virion preparations. (7)

The budding of fully packaged viruses is visible by performing electron microscopy. Studies using mutant viruses have demonstrated an important role for the cytoplasmic tails of both HA and NA, which are highly conserved. (8) It is hypothesized that these proteins are incorporated into lipid rafts that serve as the sites for virion budding. (9) Such rafts permit the exclusion of host membrane proteins from the budding virus and the enrichment of the bud with cholesterol and sphingolipids. Some data suggest that viral NA activity is necessary to prevent the aggregation of virus both to the cell surface and to itself as budding occurs. (10)

Virulence Factors

Influenza viruses lead to excess morbidity and mortality in infected patients, with both host and viral factors contributing to the degree of virulence. Perhaps the greatest determinant of the virulence of a given infection is the level of preexisting host immunity to influenza virus. (11) Naive individuals have the most severe illness, with primary infections in infants and children leading to high morbidity. (12) Despite this, mortality remains highest in elderly patients (13) and in individuals with underlying medical conditions. (14) These conditions include especially preexisting pulmonary or cardiac disease but also pregnancy, malignancy, and metabolic disease.

Antigenic Drift and Shift

The introduction of new, pandemic influenza A viruses (as seen in the outbreaks of 1918, 1957, and 1968) led to greater virulence in all age groups. (15) Mortality appeared highest in the first year after the introduction of the new subtypes. Such new viruses occur because influenza viruses exhibit a unique ability to undergo antigenic variation, most notably the influenza A virus. Both antigenic shift, involving minor antigenic changes in the HA and NA glycoproteins, and antigenic drift, involving major changes, have been described. (16,17)

Antigenic drift occurs because of the accumulation of point mutations that eventually results in amino acid substitutions. (18) These mutations may occur naturally or under pressure from antibodies produced in response to the virus in infected individuals. For the HA glycoprotein, this results in differences in key antigenic sites at which host antibody binds and in doing so facilitates viral escape and the progression of the infection. Antigenic drift also occurs with the NA protein to some extent.

Antigenic shift involves the replacement of entire gene segments within influenza A viruses and results in novel viruses. Influenza B viruses do not undergo antigenic shifts. Major antigenic shifts in influenza A occurred in 1957, 1968, and 1977, with each shift occurring suddenly in association with pandemics. The genetic reassortment that occurs with antigenic shifts probably is a result of interactions between human and animal influenza A viruses. (19) For example, the novel HA and NA genes that appeared in the Asian pandemic of 1957 were derived from an avian influenza virus. (20)

It is likely that not only the reassortment of genes between viruses but also the direct transfer of viruses can occur. (21) Such direct infections from mammals or birds to humans should have little capacity to be spread to other humans, although phylogenetic studies suggest that the great Spanish influenza pandemic of 1918 may have been the result of a direct transmission. In 1997, the direct transmission of H5N1 influenza viruses from birds to humans in Hong Kong was documented. These viruses infected 18 humans and caused the deaths of 6 of them. (22)

Influenza Strain Variability

Several lines of evidence suggest that a given influenza strain may be associated with greater virulence. Clearly, the HIN1 virus that was pandemic in 1918 was associated with significant excess mortality in young adults. Its reintroduction in 1977 was associated with relatively mild illness. (23) This more virulent strain has been linked to encephalitis as well. (24) H3N2 viral strains have been associated with croup in at least one prospective study. (25)

Viral Pathogenesis and Immune Response to Infection

Transmission

Influenza viruses do not have the ability to induce latency or persistence. As such, all infections occur because of direct person-to-person spread during periods of acute infection. Studies have documented that these viruses can be isolated throughout the world in any given month, although the majority of U.S. epidemics have occurred between December and May. (18) The winter months are characterized by periods of low humidity that may facilitate the ability of influenza to survive in aerosol droplets. (26) Influenza viruses are transmitted by droplets that are generated during sneezing and coughing. The particles are smaller than 2 [micro]m in size and can be deposited in the lower respiratory tract. Children seem to be a major vector for transmission. (27) Infectious virus can be recovered from the upper and lower respiratory tracts of infected individuals. Experimental infections with influenza A have suggested that viral shedding peaks approximately 48 hours after exposure and then slowly decreases until cessation at Days 6 through 8. (28) Although infectious virus can no longer be isolated, viral antigen can be detected in infected individuals for several more days.

Pathogenesis

Influenza viruses have the ability to replicate primarily in cells lining the respiratory tract. Viruses are released only on the apical surface of the cells, perhaps facilitating transmission because of accumulation in the lumen of the respiratory tract. (18) Upon infection, epithelial cells produce two major inflammatory cytokines, interferon-[alpha] and interleukin-6, the levels of which correlate with the severity of clinical symptoms and seem to peak by Day 2 of infection. (29) Other implicated cytokines include interleukin-8 and tumor necrosis factor [alpha], although these entities do not appear until later in the clinical illness. Infection with influenza viruses leads to dramatic changes throughout the respiratory tract. (30) Viruses induce not only epithelial cell necrosis but also cellular apoptosis as a result of the inhibition of cell protein synthesis. (31) Columnar ciliated cells begin to desquamate soon after the onset of symptoms, and infiltration of the submucosa by neutrophils and mononuclear cells proceeds and is associated with edema and hyperemia. Viral antigen has been detected in epithelial cells of the entire respiratory tract, including Types I and II alveolar epithelial cells. These cells are affected more significantly during primary influenza pneumonia, with denudement of the alveolar walls, edema, the production of exudate, and hemorrhage leading to alveolar rupture. (32)

Host Immune Responses

Both humoral and cellular immune responses are detected in the setting of influenza infection. The majority of individuals infected with influenza develop a primary humoral antibody response consisting of immunoglobulin M (IgM), IgA, and IgG, and this response does seem to be protective. (28) Maternally transmitted antibody, for example, offers protection from influenza A infection in newborn infants. (33) In addition, the levels of antibody to the HA and NA glycoproteins correlate with resistance to infection and to viral replication. (34) Interestingly, antibodies to HA seem to be neutralizing, whereas those to NA seem to prevent spread once infection has begun. (35)

The host's local, mucosal IgA response is likely key to both protection against infection and effective viral clearance after infection with influenza. (36) IgA antibodies to NA and especially to HA are detectable in respiratory secretions shortly after infection, and they persist for several months. (28) Rechallenge with virus leads again to mucosal IgA secretion as well as a proportional serum IgA response. In studies of attenuated influenza viruses, local IgA levels at the time of inoculation correlated with resistance to infections. (37)

The host's cellular immune responses to influenza may be redundant. Both C[D.sup.4+] T helper cells and C[D.sup.8+] cytotoxic T cells (CTLs) can independently mediate viral clearance in infected animals, but the absence of both renders the host unable to recover from experimental infection. (18) The C[D.sup.4+] T cell seems to offer help primarily to B cells for the production of antiviral antibodies as its means of providing protection. (18,38) CTLs can clear both infectious virus and RNA from infected tissues (39) in murine models, but the mechanism by which this is accomplished has not yet been elucidated. Possibilities include the secretion of antiviral cytokines or direct cytolysis of infected cells. Notably, immunization with influenza A has not yielded protective C[D.sup.8+] CTL responses. (40)

Table 1. Comparison of influenza viruses (a)

Characteristic              Influenza A              Influenza B

Genome                Negative-strand RNA,       Negative-strand RNA,
                        eight segmented genes      eight segmented
                                                   genes
Sequence              High                       Moderate
  variability in HA
  and NA
Mode of antigen       Antigenic drift and        Antigenic drift only
  variation             antigenic shift
Hosts                 Humans, avian, swine,      Humans only
                        equine, marine mammals
Epidemiology          Epidemic, pandemic         Epidemic,
                                                   nonpandemic

(a) RNA, ribonucleic acid; HA, hemagglutinin; NA, neuraminadase.

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From the James H. Quillen VA Medical Center and the Department of Internal Medicine, James H. Quillen College of Medicine, East Tennessee State University, Johnson City, TN.

No financial support was obtained for this manuscript. The authors of this manuscript do not have any financial, commerical, or proprietary interest in any drug, device, or equipment mentioned in this article. The views contained in this article do not necessarily reflect those of the Department of Veterans Affairs of the United States.

Reprint requests to Felix A. Sarubbi, MD, Department of Internal Medicine, James H. Quillen College of Medicine. East Tennessee State University, Box 70622, Johnson City, TN 37614. Email: [e-mail]

Accepted June 19, 2003.