Swine Flu H1N1 - Pandemic History

There is pandemonium surrounding Swine Flu H1N here in India (where we love to ignore the deadly Malaria and TB). The number of deaths due to to Swine Flu is fraction of what every year malaria alone causes. But who cares about mosquito bites and the filth around.. Swine Flu is the 'In Thing'. Everyone from second class traveller in train to the daily Sea Link traveller is bothered about Swine Flu and has his/her personal opinion on it, given the Dailies are trying to educate generality of men through articles published. But we cant afford to know little. I am publishing this article about H1N1, its history and some properties etc... I am typing out this excerpt from textbook of immunology by Kuby. This is definitely more comprehensive than articles from newspaper and requires one to have some prior knowledge of viruses.

'The influenza virus infects the upper respiratory tract andmajor central airways in humans, horses, birds, pigs, andeven seals. In 1918–19, an influenza pandemic (worldwide epidemic) killed more than 20 million people, a toll surpassingthe number of casualties in World War I. Some areas,such as Alaska and the Pacific Islands, lost more than half oftheir population during that pandemic.'

PROPERTIES OF THE INFLUENZA VIRUS

Influenza viral particles, or virions, are roughly spherical or ovoid in shape, with an average diameter of 90–100 nm. Thevirions are surrounded by an outer envelope—a lipid bilayer acquired from the plasma membrane of the infected host cell during the process of budding. Inserted into the envelope are two glycoproteins, hemagglutinin (HA) and neuraminidase (NA), which form radiating projections that are visible in electron micrographs . The hemagglutinin projections, in the form of trimers, are responsible for the attachment of the virus to host cells. There are approximately1000 hemagglutinin projections per influenza virion. The hemagglutinin trimer binds to sialic acid groups on host-cellglycoproteins and glycolipids by way of a conserved amino acid sequence that forms a small groove in the hemagglutinin molecule.Neuraminidase, as its name indicates, cleavesN-acetylneuraminic (sialic) acid from nascent viral glycoproteins and host-cell membrane glycoproteins, an activity that presumably facilitates viral budding from the infected host cell.Within the envelope, an inner layer of matrix protein surrounds the nucleocapsid, which consists of eight dif-ferent strands of single-stranded RNA (ssRNA) associated with protein and RNA polymerase . Each RNA strand encodes one or more different influenza proteins.

Three basic types of influenza (A, B, and C), can be distinguished by differences in their nucleoprotein and matrix proteins.

Type A, which is the most common, is responsible for the major human pandemics. Antigenic variation in hemagglutinin and neuraminidase distinguishes subtypes of type A influenza virus. According to the nomenclature of the World Health Organization, each virus strain is defined by its animalhost of origin (specified, if other than human), geographical origin, strain number, year of isolation, and antigenic description

of HA and NA (Table 17-2). For example, A/Sw/Iowa/15/30 (H1N1) designates strain-A isolate 15 that arose in swine in Iowa in 1930 and has antigenic subtypes 1 of HA and NA. Notice that the H and N spikes are antigenically distinct in these two strains. There are 13 different hemagglutinins and 9 neuraminidases among the type A influenza viruses. The distinguishing feature of influenza virus is its variability. The virus can change its surface antigens so completely that the immune response to infection with the virus that caused a previous epidemic gives little or no protection against the virus causing a subsequent epidemic. The antigenic variation results primarily from changes in the hemagglutinin and neuraminidase spikes protruding from the viral

envelope. Two different mechanisms generate antigenic variation in HA and NA: antigenic drift and antigenic shift. Antigenic drift involves a series of spontaneous point mutations that occur gradually, resulting in minor changes in HA and NA. Antigenic shift results in the suddenemergence of a new subtype of influenza whose HA and possibly also NA are considerably different from that of the virus present in a preceding epidemic. The first time a human influenza virus was isolated was in 1934; this virus was given the subtype designation H0N1 (where H is hemagglutinin and N is neuraminidase). The H0N1 subtype persisted until 1947, when a major antigenicshift generated a new subtype, H1N1, which supplanted the previous subtype and became prevalent worldwide until1957, when H2N2 emerged. The H2N2 subtype prevailed for the next decade and was replaced in 1968 by H3N2.Antigenicshift in 1977 saw the re-emergence of H1N1. The most recentantigenic shift, in 1989, brought the re-emergence of H3N2, which remained dominant throughout the next several years. However, an H1N1 strain re-emerged in Texas in 1995, and current influenza vaccines contain both H3N2 and H1N1 strains. With each antigenic shift, hemagglutinin and neuraminidase undergo major sequence changes, resulting in major antigenic variations for which the immune systemlacks memory. Thus, each antigenic shift finds the populationimmunologically unprepared, resulting in major outbreaks of influenza, which sometimes reach pandemic proportions. Between pandemic-causing antigenic shifts, the influenzavirus undergoes antigenic drift, generating minor antigenic variations, which account for strain differences within a subtype.The immune response contributes to the emergence of these different influenza strains. As individuals infected with a given influenza strain mount an effective immune response, the strain is eliminated. However, the accumulation of point mutations sufficiently alters the antigenicity of some variants so that they are able to escape immune elimination . These variants become a new strain of influenza, causing another local epidemic cycle. The role ofantibody in such immunologic selection can be demonstratedin the laboratory by mixing an influenza strain with a monoclonal antibody specific for that strain and then culturing the virus in cells. The antibody neutralizes all unaltered viral particles and only those viral particles with mutations resulting in altered antigenicity escape neutralization and are able to continue the infection.Within a short time in culture, a new influenza strain can be shown to emerge. Antigenic shift is thought to occur through genetic reassortment between influenza virions from humans and from various animals, including horses, pigs, and ducks. The fact that influenza contains eight separate strands of ssRNA makes possible the reassortment of the RNA strands of human and animal virions within a single cell infected with both viruses. Evidence for in vivo genetic reassortment between influenza A viruses from humans and domestic pigs was obtained in 1971. After infecting a pig simultaneously with human Hong Kong influenza (H3N2) and with swine influenza (H1N1), investigators were able to recover virions expressing H3N1. In some cases, an apparent antigenic shift may represent the re-emergence of a previous strain that has remained hidden for several decades. In May of 1977, a strain of influenza, A/USSR/77 (H1N1), appeared that proved to be identical to a strain that had caused an epidemic 27 years earlier. The virus could have been preserved over the years in a frozen state or in an animal reservoir.

When such a re-emergence occurs, the HA and NA antigens expressed are not really new; however, they will be seen by the immune system of anyone not previously exposed to that strain (people under the age of twenty-seven in the 1977epidemic, for example) as if they were new because no memory cells specific for these antigenic subtypes will exist in the susceptible population. Thus, from an immunologicpoint of view, the re-emergence of an old influenza A strain. 

HOST RESPONSE TO INFLUENZA INFECTION

Humoral antibody specific for the HA molecule is produced during an influenza infection. This antibody confers protection against influenza, but its specificity is strain-specific and is readily bypassed by antigenic drift. Antigenic drift in the HA molecule results in amino acid substitutions in several antigenic domains at the molecule’s distal end. Two of these domains are on either side of the conserved sialic-acid–binding cleft, which is necessary for binding of virions to target cells. Serum antibodies specific for these two regions are important in blocking initial viral infectivity. These antibody titers peak within a few days of infection and then decrease over the next 6 months; the titers then plateau and remain fairly stable for the next several years. This antibody does not appear to be required for recovery from influenza, as patients with agammaglobulinemia recover from the disease. Instead, the serum antibody appears to play a significant role in resistance to reinfection by the same strain. When serum-antibody levels are high for a particular HA molecule, both mice and humans are resistant to infection by virions expressing that HA molecule. If mice are infected with influenza virus and antibody production is experimentally suppressed, the mice recover from the infection but can be reinfected with the same viral strain. In addition to humoral responses, CTLs can play a role in immune responses to influenza.