Variant proteins stimulate more IgM+ GC B-cells revealing a mechanism of cross-reactive recognition by antibody memory

Abstract

Vaccines induce memory B-cells that provide high affinity secondary antibody responses to identical antigens. Memory B-cells can also re-instigate affinity maturation, but how this happens against antigenic variants is poorly understood despite its potential impact on driving broadly protective immunity against pathogens such as Influenza and Dengue. We immunised mice sequentially with identical or variant Dengue-virus envelope proteins and analysed antibody and germinal-centre (GC) responses. Variant protein boosts induced GCs with a higher proportion of IgM+ B cells. The most variant protein re-stimulated GCs with the highest proportion of IgM+ cells with the most diverse, least mutated V-genes and with a slower but efficient serum antibody response. Recombinant antibodies from GC B-cells showed a higher affinity for the variant antigen than antibodies from a primary response, confirming a memory origin. This reveals a new process of antibody memory, that IgM memory cells with fewer mutations participate in secondary responses to variant antigens, demonstrating how the hierarchical structure of B-cell memory is used and indicating the potential and limits of cross-reactive antibody based immunity.

Higher levels of B‐cell mutation in the early germinal centres of an inefficient secondary antibody response to a variant influenza haemagglutinin

“…c). With the previously reported heterotypic dengue virus proteins, early GC B cells had fewer mutations, compared with the homotypic boost . Further, here, at day 6 after Bris HA boosting, there was a lower proportion of IgM + B cells (Fig.…”

“…b). This contrasted with the sustained response seen after PR8 HA boosting, and the responses after boosting with heterotypic dengue virus proteins, which were still fourfold to eightfold above background at day 17 . Analysis of total VH mutations in Bris HA boosted GC B cells showed the same profile as the PR8 HA boost (Fig.…”

“…After PR8 HA1 priming, we performed boosts with Bris HA, which has 82% identity with PR8 over the HA1 region. Initially, the boosts were not adjuvanted because this was not previously necessary to induce successful heterotypic secondary responses . In the experiments reported below, however, because of the poor durability of the non‐adjuvanted boost response in the GC, we repeated all boosts with an adjuvant.…”

“…Foreign antigens stimulate the formation of memory B cells that have undergone somatic hypermutation (SHM) of their antibody genes and selection in germinal centres (GCs) . In this manner, immunity is established against identical pathogens.…”

“…We have recently shown that antibody memory responses to widely variant proteins involved secondary GCs with a higher proportion of IgM + B cells, with fewer VH mutations, compared with the memory response to the homotypic antigen. These observations support the idea that ‘lower’ layers of the B‐cell memory compartment, with less SHM, could furnish secondary responses against variant antigens …”