Student Theses and Dissertations

Date of Award

2022

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

Germinal centers (GC) are sites of B cell clonal expansion, diversification, and antibody affinity selection. This process is limited and directed by T follicular helper cells that provide helper signals to B cells that endocytose, process, and present cognate antigens in proportion to receptor affinity. GCs play a crucial role in immunity by generating an evolving B cell pool that serves as the origin of protective memory B and plasma cells. Therefore, understanding how the GC reaction is controlled and how high-affinity clones are selected within the GC is fundamental to our understanding of adaptive immunity and of crucial importance to the development of vaccines. Under current models, GC selection is primarily determined by the antigen capture function of the B cell receptor (BCR). However, the BCR can also function as a signaling entity, and it is not well understood how signaling by the B cell receptor contributes to selection. The critical barriers to addressing this question have been a lack of specific BCR signaling reporters and models that do not compromise the initiation and maintenance of the GC reaction. In the first part of my thesis, I developed and characterized a “tracker” to detect active antigen engagement in vivo. Crucially, this tracker does not confer any cognate antigen for presentation, thus uncoupling the signaling and antigen capture functions of the BCR. In the second part of my thesis, I used this tracker in combination with a c-Myc reporter to investigate the role of BCR signaling in positive selection. I found that BCR signaling itself enhanced the ability of cells to receive T cell help, even when antigen presentation had been normalized. Transcriptome analysis showed that continuous BCR engagement was necessary for full induction of positive selection pathways and identified a subset of pre-memory B cells associated with lower magnitudes of positive selection. GC BCR engagement per se also induces metabolic changes that may prime cells to receive T cell help. To investigate the role of BCR selection in negative selection and survival, I developed a Bruton’s tyrosine kinase (BTK) drug-resistant mouse model. I found that continuous BCR engagement was necessary for the survival of light zone B cells and that survival was intrinsic to BCR signaling by inhibiting BTK. Lastly, I investigated the synergy between BCR signaling and T cell help in the presence of antigen targeting and low levels of drug treatment. Dampening of BCR signaling impacted the proliferation of cells after migration, despite normalized antigen presentation capacity. In summary, selection in the GC is dependent on both the signaling and endocytic functions of the BCR.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

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