Preprint Club
A cross-institutional Journal Club Initiative

Keywords

• Antigen Re-exposure

• Recall Germinal Centers

• Antibody Feedback

Main Findings

Re-exposure to antigen simultaneously elicits a robust antibody response in serum and induces the formation of recall germinal centers (GCs) in tissue. The serum antibody response observed after antigen re-exposure is driven almost entirely by memory B cells (MBCs) expanded from the first antigen encounter while the newly formed recall GCs are almost entirely composed of naïve B cells. Recall immunization is thus defined by both a primary-derived antibody response and a de novo GC response. However, the mechanisms that facilitate selective utilization of primary-derived MBCs for antibody production and selective recruitment of naïve B cells to recall GCs are not well understood.

In this preprint, Schiepers et al. show that the seemingly divergent antibody and GC responses observed in recall immunization are linked by a single antibody-mediated feedback loop. The authors immunized mice with recombinant hemagglutinin in the right hind foot pad and one month later boosted these mice in the contralateral footpad with the same antigen. Intriguingly, they found that B cells recruited to the recall GCs were unable to detectably bind antigen and thus unable to contribute to secondary antibody titers. The authors hypothesized that pre-existing antibody was suppressing the ability of B cells with antigen-binding capacity from entering recall GCs. To test this, they used a tamoxifen-inducible Prdm1-/- mouse to deplete circulating antibody before homologous boosting. Depleting circulating antibody significantly increased antigen-binding in recall GCs. Boosting with variant antigen (10% bp difference) also increased antigen-binding in recall GCs, suggesting antibody feedback steers recall GC B cells towards variant epitopes.

This preprint builds upon the story of two recent papers from the Victoria Lab that established that (1) serum antibodies consistently derive from MBCs from the primary immune response even after repeated boosting with the same antigen and (2) recall GCs preferentially engage B cells with no prior GC experience. Here, these two seemingly contradictory findings are revealed to be linked by antibody feedback suppressing B cells with antigen-binding capacity from entering recall GCs. The authors also showed using hapten-carrier experiments that antibody feedback completely prevents the formation of recall GCs in the absence of memory T cells, suggesting that B cells with little to no antigen binding capacity rely on excess memory T cell help to form recall GCs.

This work supports a model in which the fundamental role of recall GCs is to mature naïve-derived B cells specifically tailored to escape epitopes on pathogens rather than secondary maturation of the existing memory repertoire.

Limitations

• This work was performed using a mouse model. Additional work will need to be performed to determine whether these findings generalize to humans.

• The authors showed that boosting mice with variant antigen (10% amino acid difference from priming antigen) partially rescued the antigen-binding ability of B cells in recall GCs. However, the p-value associated with the result only just met the standard of statistical significance (p = .048). The authors supported this result by showing that greater rescue could be achieved by boosting mice with a variant antigen with 20% amino acid difference from the priming antigen (p = .031); however, many pathogen variants in nature will differ from each other by less than 20% on an amino acid level.

• The depletion of antibody using Prdm1∆GC mice was unable to explain the dominance of naïve-derived B cells in recall GCs. Additional experiments will be required to unravel the mystery as to how recall GCs preferentially recruit naïve B cells and exclude MBCs during antigen re-exposure.

Significance/Novelty

This work provides the first direct evidence that antibody feedback suppresses B cells with antigen-binding capacity from entering recall GCs in mice upon re-exposure to an antigen. The action of antibody-mediated feedback partially explains why serum antibodies consistently derive from MBCs from the primary immune response even after repeated boosting with the same antigen and recall GCs preferentially engage naïve B cells.

This work represents an important advancement in our mechanistic understanding of how recall germinal centers develop in the setting of recall immunization and is directly relevant to the field of vaccine development. These experiments illustrate that the murine immune system is geared to respond to variant challenges rather than homologous challenges. If these results generalize to humans, then it suggests that vaccine boosters could be made more effective by including variant antigens.

Credit

Reviewed by Jake Herb as part of a cross-institutional journal club between the Icahn School of Medicine at Mount Sinai, the University of Oxford, the Karolinska Institute and the University of Toronto.

The author declares no conflict of interests in relation to their involvement in the review.