Signaling circuits involved in the selection of high-affinity antigen-specific B cells in the germinal center

Language
en
Document Type
Doctoral Thesis
Granting Institution
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Naturwissenschaftliche Fakultät
Issue Date
2024
Authors
Kuria, Timothy Chege
Editor
Abstract

The germinal center (GC) serves as the site for somatic hypermutation, affinity maturation, and the selection of high-affinity B cells. Two selection models have been proposed to take place in the GC: a death-limited model and a birth-limited model. In the simplified death-limited model, low-affinity GC B cells undergo apoptosis, while high-affinity B cells do not. In the more pragmatic birth-limited model, selection signals are mediated through increased upregulation of metabolic factors in high-affinity cells, promoting faster proliferation compared to low-affinity GC B cells. Regardless of the selection model, high-affinity B cells consistently outcompete low-affinity B cells in the GC. However, the signaling circuits that mediate this selection are not yet fully understood. An adoptive experimental approach utilizing a 4-hydroxy-3-nitrophenyl (NP) antibody mouse model (B1-8 mouse model) coupled to a non-responsive NP recipient mouse model was employed. Donor high and low-affinity B cells were competitively transferred into the recipient mice. On days 6 and 9 of the GC response, recipient mice were sacrificed, and the GC response was analyzed. RNA was also extracted from the donor B cells, followed by bulk RNA sequencing and differential gene expression analysis to identify potential genes mediating selection in the GC. High-affinity B cells consistently outcompeted low-affinity B cells, even with a starting ratio of high-affinity to low-affinity B cells of 1:138. Differential expression analysis of low-affinity and high-affinity GC B cells revealed a panel of genes, including the B cell co-receptor, CD72. In multiple follow-up experiments, high-affinity B cells downregulated the expression of CD72 not only at the transcriptome level but also at the protein level. CD72 is expressed on all B cells except in antibody-secreting cells, and the downregulation of CD72 likely serves to prepare high-affinity GC B cells for differentiation into antibody-secreting cells. Additionally, CD72 knock-out B cells were outcompeted by wild-type B cells owing to the lack of CD72-CD100 mediated activation and proliferation. In this thesis, novel genes were identified, including the B cell co-receptor CD72, which may play a role in mediating the GC selection process and differentiation into antibody-secreting cells. We have gained meaningful insights through the possible identification of a new GC selection signaling pathway that CD72 mediates.

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