Investigating the role of EFhd2/Swiprosin-1 in B cell metabolic reprogramming and immune synapse formation

Effective vaccination involves the generation of immunological memory and long-lived plasma cells (LLPCs) secreting antigen-specific, high-affinity antibodies. Memory B cells and LLPCs originate from specialized structures in secondary lymphoid organs, the germinal centers (GCs). The dynamic processes in the GCs depend on cytoskeletal remodeling and metabolic reprogramming, enabling GC B cells to adapt the different requirements such as proliferation, differentiation and migration. The positive selection of high-affinity GC B cells relies on immune synapse (IS) formation with follicular dendritic cells (FDCs) and T follicular helper (Tfh) cells. However, in the B cell follicle, also resting B cells engage membrane-bound antigens presented by dendritic cells. While B cells can recognize both soluble and membrane-bound antigens, repetitive immobile antigen arrays, such as those found on viral surfaces, elicit stronger humoral immune responses than soluble antigens. This highlights the importance of IS formation in B cell activation and raises the question whether cytoskeletal and metabolic reprogramming are connected in B cells. The F-actin binding protein EFhd2/Swiprosin-1 (EFhd2) is upregulated in GC B cells and its deficiency (EFhd2 KO) impairs GC B cell–FDC IS formation in a B cell intrinsic manner, but surprisingly increases GC output without enhancing antibody affinity maturation. Beyond GC B cells, EFhd2 is implicated in IS formation in T cells and is linked to B cell receptor (BCR)-induced mitochondrial apoptosis in cell lines. This thesis aimed to decipher whether the cytoskeleton and metabolism intercalate in B cells. Therefore, I attempted to understand the role of EFhd2 in linking cytoskeletal dynamics to cellular metabolism in activated B cells, particularly after soluble anti-IgM stimulation or during IS formation, which was mimicked here by coated anti-IgM antibodies. Metabolic profiling of soluble anti-IgM stimulated B cells revealed that EFhd2 stabilizes MYC expression, fosters glucose responsiveness of the activated cells and enhances translational capacity. Consequently, in EFhd2 KO B cells metabolism shifted towards higher mitochondrial respiratory capacity with larger, more functional mitochondria. Unlike soluble anti-IgM activation and unlike WT B cells, EFhd2 KO B cells failed completely to increase mitochondrial capacity upon immobilized antigen exposure. However, microtubule stabilization restored the spare respiratory capacity in these cells, indicating that EFhd2 modulates mitochondrial respiration during IS formation by enhancing microtubule stability. Confocal live cell imaging further revealed that EFhd2 KO B cells had a reduced mitochondrial area at the IS and an uneven distribution of mitochondria, microtubules, and BCR clusters. In summary, EFhd2 ensures cytoskeletal integrity and optimizes mitochondrial function during IS formation. Conversely, it fosters glucose responsiveness and anabolic pathways during soluble anti-IgM activation. Hence, EFhd2 equips the BCR with the capacity to rewire B cell metabolism depending on the physical nature of the antigen and this metabolic switch might govern B cell fate decisions.