Avian Influenza Virus Tropism in Humans

Abstract

An influenza pandemic happens when a novel influenza A virus is able to infect and transmit efficiently to a new, distinct host species. Although the exact timing of pandemics is uncertain, it is known that both viral and host factors play a role in their emergence. Species-specific interactions between the virus and the host cell determine the virus tropism, including binding and entering cells, replicating the viral RNA genome within the host cell nucleus, assembling, maturing and releasing the virus to neighboring cells, tissues or organs before transmitting it between individuals. The influenza A virus has a vast and antigenically varied reservoir. In wild aquatic birds, the infection is typically asymptomatic. Avian influenza virus (AIV) can cross into new species, and occasionally it can acquire the ability to transmit from human to human. A pandemic might occur if a new influenza virus acquires enough adaptive mutations to maintain transmission between people. This review highlights the key determinants AIV must achieve to initiate a human pandemic and describes how AIV mutates to establish tropism and stable human adaptation. Understanding the tropism of AIV may be crucial in preventing virus transmission in humans and may help the design of vaccines, antivirals and therapeutic agents against the virus.

Keywords: antiviral; avian influenza virus; influenza A virus; vaccine; virus tropism in humans.

Figure 1

Influenza A virus virion structure and genome organization. (A) Schematic organization of a gene segment (vRNA). The IAV genes consist of three parts: 1. Untranslated regions (UTRs) comprise the universal primers uni-12 (12nt) and uni-13 (13nt) at the end of 3′ and 5′ of each segment, which are highly conserved among the eight gene segments and among all IAV viral strains; and the segment-specific non-coding regions (ssNCRs); 2. Packaging signals; and 3. Coding region. (B) IAV Genome organization: The eight negative-sense, single-stranded RNA segments of IAV genome code for PB2, PB1, PA, HA, NP, NA, M and NS, respectively (indicated in white boxes). The darker blue colour boxes at the end of each vRNA segment indicate the 3′ and 5′ untranslated regions (UTRs); the lighter blue colour boxes indicate the packaging signals. The numbers inside the blue boxes represent the nucleotide length for the UTRs and the packaging signals. (C) IAV virion structure: IAV comprises three components: an envelope made of a lipid bilayer containing three transmembrane proteins (HA, NA and M2 ion channel); a layer of the inner surface envelope M1; and a core made of eight vRNA segments, packaged together to form vRNP. (Created with BioRender.com).

Figure 2

Schematic diagram showing the host range of influenza A viruses. Reservoirs and interspecies transmission events of influenza A viruses and the subtypes involved in these events. Aquatic wild birds represent the natural reservoir of influenza A viruses, from which they can be transmitted to a variety of other hosts. Circled arrows represent continuous virus circulation among wild birds, domestic birds, domestic animals, bats, horses, pigs and humans. (Created with BioRender.com).

Figure 3

IAV life cycle and the key determinants of AIV tropism in the human host. (1) Upon viral entry in the respiratory tract, AIV attaches to the host sialic acid (SA) receptor by interacting with HA–NA, thus, inducing cell internalization via endocytosis. (2) The acidification of the late endosome (host pH) triggers a conformational change in cleaved HA (where HA is cleaved by host protease), which later causes membrane fusion and vRNP release into the cytoplasm. (3) Following that, vRNPs are transported into the nucleus via the host importin α/β-mediated nuclear import pathway. (4) In the nucleus, replication and transcription of viral RNA are proceeded by viral polymerases with the help of host RNA polymerase II and ANP32A. (5) After transcription, the viral mRNAs leave the nucleus to be transported to the host ribosome for translation. However, the viral pre-mRNA of M and NS undergo a maturation process in which the pre-mRNAs are spliced before the translation process. (6) In the nucleus, upon viral protein translation, NP binds to nascent viral RNA to prevent RNA degradation. PA, PB1 and PB2 are assembled as RdRp complex before binding with RNA-NP, thus, generating nascent vRNP. Then, nascent vRNPs are exported from nucleus to cytoplasm (7) and all nascent viral proteins are trafficked to the budding membrane. (8) The bud formation is initiated by the interaction of M1 with host machinery before (9) the nascent virus is released by the interaction of HA–NA with the host SA receptor. (10) Evasion of the host immune response. (Created with BioRender.com).

Figure 4

E190D and G225D substitution in H1N1. (A) Overall trimeric structure of HA of IAV H1N1. The location of RBS is marked. (B) Comparison of the structural characteristics and amino acid positions of avian (cyan) and human (beige) H1 RBS proteins. (C) The impact of specific receptor-binding variants on viral receptor specificity that facilitated the entry of H1N1 into the human cells. Adapted from ref. [101]. Created using Pymol (Schrodinger LLC) with PDB IDs 1RV0 and 1RVZ, and with BioRender.com.

Figure 5

Q226L and G228S substitution in H3N2. (A) Overall trimeric structure of HA of IAV H3N2. The location of RBS is marked. (B) Comparison of the structural characteristics and amino acid positions of avian (pink) and human (white) H3 RBS proteins. (C) The impact of specific receptor-binding variants on viral receptor specificity that facilitated the entry of H3N2 into the human cells. Adapted from ref. [101]. Created using Pymol (Schrodinger LLC) with PDB IDs 6TZB and 1MQM, and with BioRender.com.

Figure 6

The replication of IAV viral RNA comprises two steps: (A) Replication of vRNA to cRNA. Initiation of replication occurs when PB1 of the RNA polymerase generates pppApg dinucleotide at position 1 and 2 of the 3′ UC region of the vRNA template. Priming loop is truncated and stimulates the elongation of the nascent cRNA by breaking the 5′ to 3′ base pair of the vRNA panhandle region. Next, the resulting cRNP complex is stabilized by new NP and free RdPR. (B) Replication of cRNA to vRNA. With the assistance of a free RdRP, replication of vRNA is activated with synthesis of pppApg at position 4 and 5 of the 3′ UC terminal of cRNA template. Dimerisation between cRNP RNA polymerase and a regulatory polymerase facilitate backtracking of the cRNA and cause realignment of the pppApg dinucleotide from position 4 and 5 to position 1 and 2. This initiates the elongation of the nascent vRNA. The nascent vRNA is stabilized by free RdRP and new NP. (Created with BioRender.com).