Gradual adaptation of animal influenza A viruses to human-type sialic acid receptors

Influenza A viruses (IAVs) originating from animal reservoirs pose continuous threats to human health as demonstrated by the Spanish flu pandemic. Infection starts by attachment to host receptors, a crucial step that is targeted by immunological, prophylactic, and therapeutic intervention. Fine-tuning of virus hemagglutinin binding to host-specific receptor repertoires needs to remain balanced to receptor-destroying neuraminidase (NA) activity and is a key step in host adaptation. It determines NA-dependent virus motility, enabling IAVs to traverse the mucus layer and to bind to, and migrate over, the epithelial cell surface for reaching a location supporting endocytic uptake. Canonical adaptations in enzootic/zoonotic IAVs enhancing human-type receptor binding are well-known, but the context and timespan required for their selection pose many questions. We discuss recent developments, focusing on the dynamic nature of interactions of IAV with the heterogeneous receptor repertoires present in humans and potential intermediate hosts. Potential pre-adaption toward human-type receptor binding in intermediate hosts will be discussed.