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

Document Type
Doctoral Thesis
Granting Institution
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Naturwissenschaftliche Fakultät
Issue Date
Kuria, Timothy Chege

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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