High-throughput sequencing of IgH gene in minor salivary glands from Sjogren's syndrome patients reveals dynamic B cell recirculation between ectopic lymphoid structures

Objective B cells play a central role in Sjogren's syndrome (SS) whereby autoreactive B-cells populate ectopic germinal centres (GC) in SS salivary glands (SG) and undergo somatic hypermutation (SHM) and class-switch recombination of the immunoglobulin genes. However, the capacity of specific B cell clones to seed ectopic GC in different SG and undergo clonal diversification is unclear. To unravel the dynamics of B cell recirculation among minor SG biopsies, we investigated the immunoglobulin heavy chain (IgH) gene usage and the pattern of SHM using a high-throughput sequencing approach. Methods We generated similar to 166,000 reads longer than 350bp and detected 1631 clonotypes across eight samples from four different SS patients, all characterised by the presence of functional ectopic GC as demonstrated by the expression of activation-induced cytidine deaminase. Results A large number of shared clonotypes were observed among paired mSG biopsies from each patient but not across different patients. Lineage tree analysis revealed significant clonal expansion within the mSG with the identification of shared dominant B cell clones suggestive of extensive recirculation across different SG. Several shared clonotypes with high proliferating capacity displayed IgH-VH gene usage common in autoreactive B cells, including VH1-69, which is typical of rheumatoid factor+ B cells representing potential lymphoma precursors. Conclusion The complex dynamic recirculation of B cells that we observed within ectopic GC responses linked with their ability to independently proliferate, undergo ongoing SHM and Ig class-switching within individual glands may explain the difficulty in achieving consistent eradication of ectopic GCs following B cell depleting agents reported in different studies.