Immune imprinting1 or original antigenic sin2 is a phenomenon whereby the immune system preferentially recalls its initial response to a related, often evolving pathogen after subsequent exposure. Despite its important implications for vaccine development, the causes of imprinting remain unclear. Here, to understand the basis and impact of imprinting by influenza A viruses, we characterized the B cell responses of young children after consecutive first infections with divergent H1N1 and H3N2 strains of influenza. Children had a primary but otherwise similar B cell response to that of adults. Adult B cells commonly cross-reacted with past strains using more stereotyped and mutated immunoglobulin genes, indicating substantial homosubtypic imprinting. In children, after consecutive heterosubtypic primary infections, up to 6% of memory B cells are H1/H3 cross-reactive and bind to the highly conserved central stalk epitope—a lead target for broadly protective vaccine candidates. Over 90% of these B cells had a higher affinity for the imprinting H3N2 strain, resulting in reduced breadth and neutralization potency against H1N1 strains. Mechanistically, the imprinting H3 strains and affected H1 strains shared a residue change in the stalk epitope (D46N) that was central to the nearly universal shift in reactivity, despite differing by only a single atomic group. In conclusion, imprinting by influenza viruses can cause a deleterious shift of nearly the entire memory recall response against key, conserved epitopes.
