Influenza A virus hemagglutinin trimerization completes monomer folding and antigenicity.

Influenza A virus (IAV) remains an important human pathogen largely because of antigenic drift, the rapid emergence of antibody escape mutants that precludes durable vaccination. The most potent neutralizing antibodies interact with cognate epitopes in the globular "head" domain of hemagglutinin (HA), a homotrimeric glycoprotein. The H1 HA possesses five distinct regions defined by a large number of mouse monoclonal antibodies (MAbs), i.e., Ca1, Ca2, Cb, Sa, and Sb. Ca1-Ca2 sites require HA trimerization to attain full antigenicity, consistent with their locations on opposite sides of the trimer interface. Here, we show that full antigenicity of Cb and Sa sites also requires HA trimerization, as revealed by immunofluorescence microscopy of IAV-infected cells and biochemically by pulse-chase radiolabeling experiments. Surprisingly, epitope antigenicity acquired by HA trimerization persists following acid triggering of the globular domains dissociation and even after proteolytic release of monomeric heads from acid-treated HA. Thus, the requirement for HA trimerization by trimer-specific MAbs mapping to the Ca, Cb, and Sa sites is not dependent upon the bridging of adjacent monomers in the native HA trimer. Rather, complete antigenicity of HA (and, by inference, immunogenicity) requires a final folding step that accompanies its trimerization. Once this conformational change occurs, HA trimers themselves would not necessarily be required to induce a highly diverse neutralizing response to epitopes in the globular domain.