Background/Purpose: Rheumatoid arthritis (RA) is a prototypic autoimmune disease characterized by a prominent humoral autoimmunity. Of particular relevance is the local production of autoantibodies and the presence of ectopic lymphoid neogenesis (ELN) in the rheumatoid synovium. However, the mechanisms underlying break of B cell tolerance and local autoantibody production remain poorly understood. In this study we aimed to identify cellular and molecular pathways implicated in RA-specific humoral autoimmunity.

Methods: Gene expression profiling was performed on synovial tissue from individuals with RA and inflammation-matched non-RA controls without known autoantibodies. Expression of Bob1 was validated on three independent cohorts of synovium by qPCR and immunohistochemistry. Bob1-positive cells were analysed by triple-color immunofluorescence. B and T cell subsets were FACS-sorted from tonsils. Effects of Bob1-deficiency on collagen-induced arthritis (CIA) model were examined using Bob1-deficient mice (Bob1^-/-) and RAG-1-null mice after adoptive transfer of WT and Bob1^-/-B and T cells. Anti-collagen type II (CII) antibodies (Abs) were assessed by ELISA. Germinal center (GC) formation was analysed in the inguinal lymph nodes of immunized mice by FACS.

Results: Transcriptional profiling in RA synovium revealed a prominent B cell signature with the B cell-specific transcriptional co-activator Bob1 and its transcriptional target BCMA among the most upregulated genes. Further validation by qPCR and immunohistochemistry confirmed the microarray data and demonstrated elevated expression of Bob1 in RA synovium at the early phase of the disease. Triple immunofluorescence analysis of RA synovium revealed expression of Bob1 in B cells and in a subset of T cells. Subsequent Western blot analysis of tonsillar T cell subsets confirmed expression of Bob1 in T follicular helper (T_FH) cells. Interestingly, expression of Bob1 in RA synovium was strongly correlated (r=0.91; p<0.0001) with expression of CD21L, a molecular marker of ELN, linking highest levels of Bob1 to the presence of GCs. In addition, functional analysis in mice showed that Bob1^-/-mice failed to produce pathogenic anti-CII Abs and were completely protected from CIA. Adoptive transfer of different combination Bob1-deficient and sufficient B and T cells to RAG-1-null mice demonstrated that only mice which received Bob1-sufficient B cells developed CIA. The susceptibility to CIA was mirrored by the presence of anti-CII Abs in serum and GC B cells in lymph nodes of animals. Mice which received Bob1-deficient B cells were resistant to CIA, did not develop anti-CII Abs and did not have GC B cells despite the ability of transferred Bob1-deficient T cells differentiate into T_FHcells and produce IL-21. Finally, increased expression of Bob1 and its correlation with CD21L in salivary glands of patients with Sjögren’s Syndrome, but not in Sicca controls, suggest a general role for Bob1 in B cell-driven autoimmunity.

Conclusion: Aberrant high Bob1 expression in RA synovium and failure of Bob1-deficient B cells to form GCs and produce autoantibodies indicate that Bob1 may contribute to humoral autoimmunity in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/bob1-expression-is-elevated-in-rheumatoid-synovium-and-its-expression-in-b-cells-is-required-for-development-of-collagen-induced-arthritis/