Background In an autoimmune environment, rogue self-reactive B cells escape tolerance, differentiate to a variety of self-antigens (epitope spreading) and are selected into germinal centers (GC) in a T cell dependent manner. We investigated this question using a mixed bone marrow chimera model that combines transgenic B cells from lupus-like mice (564 Igi) with those from wild type (WT) B6 mice. In this model, WT B cells, specific for self-antigens distinct from those targeted by the Tg B cells, expand and dominate in GC.
Methods Autoimmune and WT chimeric mice were prepared using the tamoxifen-inducible Aicda-CreERT2-EYFP mice to track WT cells. WT chimeras were immunized with NP-CGG. At peak GC stage, EYFP+ splenic B cells from both cohorts, were sorted and processed for gene expression using RNAseq.
Results B cells grouped into 4 clusters: GCs (DZ and LZ), plasma cells and memory B cells. DZ, LZ and PC clusters were represented in similar proportions in both conditions; whereas, MemB cells were more expanded in the immunized chimeras. Using the paired single cell BCR sequences and repertoire analysis, we observed clones with clear expansion both in the autoimmune and immunized chimeras. We observed levels of mutation in a similar range though DZ, LZ and MemB from autoimmune mice had a significantly higher number of nucleotide replacement mutations, and the reverse was observed in PCs. Nevertheless, PCs in both conditions reached similar maximum levels of mutation. Interestingly, autoimmune cells showed more isotype diversification in all compartments. Notably, we observed distinct gene expression for the autoreactive B cells such as CXCL10 by GC B cells and SLPI expression by autoreactive plasma cells. Strikingly, we identified DN2- and DN4-like memory B cells in both conditions.
Conclusions We find WT B cells break tolerance, expand in GC and develop into MemB and PCs in a seemingly unrestricted manner, similar to immune mice. Results should open the way to new approaches to control pathogenicity of rogue B cells in autoimmune disease.
Acknowledgements This work was supported by grants from NIH (R01 AI130307 and AR074105).
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