Early Alterations of the Receptor-Binding Properties of H1, H2, and H3 Avian Influenza Virus Hemagglutinins after Their Introduction into Mammals

TLDR

Influenza A viruses from wild birds can spill over into mammals, and amino‑acid changes at positions 190 and 225 in HA are linked to the emergence of H1 human pandemic strains. The study aimed to detect early receptor‑binding changes in HA after avian‑to‑mammal transmission and identify the responsible substitutions by examining initial isolates from the 1957, 1968, 1979, and 1992 outbreaks. Binding assays used synthetic sialylglycopolymers 3′SL‑PAA (avian receptor) and 6′SLN‑PAA (human receptor) to evaluate HA affinity. Early after transmission, avian and seal HAs bound human‑type 6′SLN‑PAA weakly, whereas human and swine isolates bound it more strongly; mutations 226Q→L in H2/H3 and 190E→D/225G→E in H1 swine increased affinity, indicating that altered receptor specificity is an early event that may enable efficient epidemic spread.

Abstract

ABSTRACT Interspecies transmission of influenza A viruses circulating in wild aquatic birds occasionally results in influenza outbreaks in mammals, including humans. To identify early changes in the receptor binding properties of the avian virus hemagglutinin (HA) after interspecies transmission and to determine the amino acid substitutions responsible for these alterations, we studied the HAs of the initial isolates from the human pandemics of 1957 (H2N2) and 1968 (H3N2), the European swine epizootic of 1979 (H1N1), and the seal epizootic of 1992 (H3N3), all of which were caused by the introduction of avian virus HAs into these species. The viruses were assayed for their ability to bind the synthetic sialylglycopolymers 3′SL-PAA and 6′SLN-PAA, which contained, respectively, 3′-sialyllactose (the receptor determinant preferentially recognized by avian influenza viruses) and 6′-sialyl( N -acetyllactosamine) (the receptor determinant for human viruses). Avian and seal viruses bound 6′SLN-PAA very weakly, whereas the earliest available human and swine epidemic viruses bound this polymer with a higher affinity. For the H2 and H3 strains, a single mutation, 226Q→L, increased binding to 6′SLN-PAA, while among H1 swine viruses, the 190E→D and 225G→E mutations in the HA appeared important for the increased affinity of the viruses for 6′SLN-PAA. Amino acid substitutions at positions 190 and 225 with respect to the avian virus consensus sequence are also present in H1 human viruses, including those that circulated in 1918, suggesting that substitutions at these positions are important for the generation of H1 human pandemic strains. These results show that the receptor-binding specificity of the HA is altered early after the transmission of an avian virus to humans and pigs and, therefore, may be a prerequisite for the highly effective replication and spread which characterize epidemic strains.