Influenza Viruses

Characteristics

causes Influenza (severe upper respiratory tract disease)

only members of the orthomyxovirus family (myxo=mucus), these pleomorphic, single-stranded RNA (negative sense), lipoprotein enveloped viruses have segmented (multiple strands) genomes. There are three serotypes A, B, and C

Serotypes A and B have 8 RNA strands, while serotype C has 7 RNA strands. Humans are major host; also circulate among many mammalian and avian species (i.e., Spanish flu)

The envelope is covered by the two major antigens: hemagglutinin (HA), and neuraminidase (NA). Located underneath the lipoprotein layer is M Protein which binds both HA and NA, stabilizes the envelope, and is an important ion channel.

(1) Hemagglutinin (HA): allows the virus to bind to target cells, as well as to the endosome containing the virus after uptake. It is the most important antigen on the virus. HA functions by binding sialic acid-containing receptors on cell surfaces of RBCs and epithelial respiratory cells, thus allowing penetration of virus into target cell cytoplasm. HA is composed of two subunits, HA1 and HA2 which are connected by a disulfide bond. HA2 contains a hydrophobic region (fuschin) which plays a role in the replicative cycle (see below).
(2) Neuraminidase (NA): has two functions. During attack it degrades the protective layer of mucus in the respiratory tract allowing HA to bind, and during the end of infection it cleaves sialic acid to release progeny virus from infected cell.

Replicative Cycle: The virus adsorbs to the cell as the HA interact with sialic acid receptors on the cell surface. The virus then enters the cell in a vesicle. When the pH inside the vesicle drops, the M protein pumps protons into the virus thus inducing the HA protein to expose its disulfide cleavage domain allowing a cell protease to cleave it and expose the HA2 fuschin region. Because this region is hydrophobic it binds the vesicle’s membrane and allows the genomic material to escape into the cytoplasm Þ nucleus. In the nucleus the 8 viral mRNA segments get transcribed by viral polymerase (see genetics below). Translation happens in the cytoplasm with some of the viral proteins transporting back to the nucleus. Assembly of the nucleocapsids occurs in the nucleus. Finally, the finished product floats to the outer membrane and buds out of the cell with the aid of NA cleavage of sialic acid from viral and cellular glycoproteins.

Genetics: The genetic material of the flu virus undergoes two steps: vRNA (-) Þ mRNA (+) Þ template RNA (-).

While the 3’ and 5’ regions of the vRNA (v=viral) and the template RNA are identical, the 3’ region of the mRNA is longer by 10-13 nucleotides. This is explained by the fact that the virus ‘cannibalizes’ newly transcribed host mRNA, and uses it as a primer. The viral transcription complex (PB1, PB2, and PA) then attaches to the strand, removes the primer, and reanneals the strand fragments. The RNA is then ready to make more virus.

Disease and Pathogenesis

Transmission – airborne respiratory droplets

The ability of influenza A virus to cause epidemics is dependent on antigenic changes in the HA and NA. These changes occur while the virus is in its primary host – birds
The two types of changes are antigenic drifts and antigenic shifts.

(1) Antigenic Drifts – are minor changes based on mutation.

(2) Antigenic Shifts – are major changes based on the reassortment of genome pieces. Occurs when one subtype of virus is replaced by another – responsible for periodic influenza pandemics

Interesting aside: There are 14 different HAs, but subtypes H1, H2, and H3 are the most prevalent. Also, there are 9 different NAs, but subtypes N1 and N2 are the most prevalent. In birds all antigenic subtypes of HA and NA are present. Why haven’t these transferred to humans? No one knows. What will happen when/if they do? Medical disaster.

Pathogenesis and Immunity: After the virus has been inhaled, the NA degrades the protective mucus layer, allowing the virus to gain access to the cells of the upper and lower respiratory tract. The infection is limited primarily to this area, and, despite systemic symptoms, viremia rarely occurs. The systemic symptoms are due to cytokines circulating in the blood. There is necrosis of the superficial layers of the respiratory epithelium. Immunity rests mainly upon secretory IgA in the respiratory tract. IgG is also produced but is less protective. Cytotoxic T cells also play a protective role.

Laboratory Diagnosis: Dx is mostly clinical. Virus can be grown in cell culture and detected by hemadsorption or hemagglutination. It is identified by hemagglutination inhibition or complement fixation. Antibody titer rise in convalescent-phase serum is diagnostic.

Treatment: Amantadine (Symmetrel) or rimantadine (Flumadine). Rarely used.

Prevention: Vaccine (typically consisting of two attenuated A strains, and one attenuated B strain) is reformulated each year to contain the current antigenic strains. The vaccine was originally developed by crossing an attenuated strain of the virus with one that had virulent HA and NA antigens. The result was a strain that was ONLY virulent for HA and NA, thus developing an immune response without making you sick. Note that antibodies against the HA neutralizes the infectivity of the virus (and prevents disease). Because the virus in the vaccine is killed, there is no replication and consequently little secretory IgA appears on the respiratory mucosa. The vaccine does induce IgG, which offers some protection.