Shaping the Mitochondrial Powerhouse in Germinal Centre B cells 2309349

Abstract Introduction B cells within the germinal centre (GC) proliferate rapidly. Nonetheless, unlike proliferating T cells, GC B cells conduct minimal glycolysis. Instead, they rely on fatty acid oxidation to fuel mitochondrial OxPhos. Reflecting this, GC B cells show increased mitochondrial mass, transcription and translation. Interestingly, mitochondria also display morphological dynamism dictated by fission and fusion events. However, mitochondrial morphology in B cells is poorly defined. Given the role of mitochondrial metabolism in the GC, we hypothesise that mitochondrial morphology is important in GC B cells. Methods To characterise morphology, mitochondria from different B cell populations were imaged with SIM microscopy. To investigate mitochondrial fusion in GC B cells, we conditionally deleted genes enabling mitochondrial fusion, Mfn1 and Mfn2, in GC B cells using Aicda-Cre (henceforth Aicda-Mfn1&2). Mice were immunised with the model antigen NP-CGG and GC responses and metabolic profiles were analysed. Single cell RNA sequencing was performed on isolated GC B cells from control and Aicda-Mfn1&2 mice. Results Relative to naïve B cells, plasma cells possess fragmented mitochondria, whereas GC B cells have fused, interconnected mitochondria. Post immunisation, Aicda-Mfn1&2 mice developed reduced GC responses and impaired somatic hypermutation and plasma cell differentiation. Loss of MFN1 and MFN2 resulted in a shift towards glucose-dependent metabolism and altered amino acid metabolism. scRNA data highlighted increased transcription of Dhrs3, suggesting a decrease in retinoic acid (RA) signalling. Addition of RA to GC B cells was associated with changes in plasma cell differentiation, mitochondrial mass and ROS production. Conclusion We have shown that mitochondrial morphology is dynamically remodelled in B cell activation and differentiation. In GC B cells, mitochondria are fused, and this fusion is essential for a robust GC response by maintaining proper metabolic pathways and retinoic acid signalling. Funding Source Kennedy trust studentship Topic Categories Lymphocyte Differentiation and Peripheral Maintenance (LYM)