Defining the role of T cell help in germinal centers by intercellular enzymatic labeling

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PROJECT SUMMARY Generation of high affinity antibodies in germinal centers (GCs) is a critical step in a wide variety of clinically relevant processes, from protection against pathogens by prior infection or vaccination to the development of allergies and autoimmune diseases. Antibody affinity maturation follows a prototypical Darwinian framework, in which GC B cells introduce random mutations into the antigen-binding portions of their immunoglobulin (Ig) genes, generating variations in affinity within their progeny. Rare B cells that acquire affinity-increasing mutation are then selectively expanded within the GC population, thus increasing the average affinity of GC B cells as a whole, in a process we refer to as positive selection. Despite decades of work, the precise cellular mechanisms of positive selection—in other words, how GCs “pick out” B cells with the highest affinity—remains a topic of debate. More than 10 years ago, we provided the first in vivo evidence in mice for a role for T follicular helper (Tfh) cells as arbiters of this selective process. In our model, Tfh cells would sense how much peptide a B cell could present on its surface (which in turn depended on the B cell’s affinity), providing help selectively to the highest-affinity B cells. However, despite accumulating functional evidence for this model, selective delivery of T cell help to B cells based on their affinity has never been directly demonstrated in physiological settings. To achieve this, we developed LIPSTIC, a method that allows us to directly record T cell help to B cells with great precision in vivo. In Aim 1 of this project, we propose to use LIPSTIC as a means to directly test the T cell help model in classic hapten-carrier induced GC selection models. In Aim 2, we will follow up on this by testing our findings from mouse LIPSTIC in human vaccine-induced GCs. In Aim 3, we use the original LIPSTIC in conjunction with two novel versions on this strategy to investigate the dynamics of multi-antigen driven selection in influenza-induced GCs.