AB0015 Capture of iga immune complexes and enrichment in iga ig gene expression both suggest a role for fcrl4+ b cells in the link between mucosal and joint inflammation
Authors: J Cameron, E Clay, K Amara, N Sippl, A Filer, K Raza, V Malmström, D Scheel-Toellner
Journal: Annals of the Rheumatic Diseases (2017)

<h3>Background</h3> Increasing evidence points to the autoimmune process of rheumatoid arthritis (RA) originating at mucosal surfaces. Previous work from our group described a subset of B cells in the synovium and synovial fluid (SF) of RA patients which can be distinguished by their expression of the Fc-like receptor 4 (FcRL4) and elevated expression of RANKL, indicating a unique pathogenic function<sup>1,2</sup>. B cells expressing FcRL4 were originally described as a distinct memory B cell subset in human tonsils where they accumulate in the epithelium<sup>3</sup>. We have recently shown that they are enriched in cells recognizing citrullinated autoantigens (Amara, K. et al. under revision). Recent <i>in vitro</i> work suggested that FcRL4 is a low affinity receptor for aggregated IgA<sup>4</sup>. <h3>Objectives</h3> 1) To investigate the interaction of RA synovial fluid FCRL4+ B cells with IgA 2) To examine the distribution of Ig classes by flow cytometry and PCR <h3>Methods</h3> SF mononuclear cells were isolated, labelled for FcRL4, IgA and CD19 and analysed by flow cytometry. In experiments identifying IgA B cell receptors, SF mononuclear cells were briefly incubated in an acidic buffer to remove surface receptor bound antibodies before staining. Heat-aggregated purified human IgA was added to assess IgA binding to FcRL4+ B cells. Single B cells were sorted from SF of RA patients and their constant region genes probed for identification of their Ig subclass by PCR. <h3>Results</h3> <i>Ex vivo</i>, FcRL4<sup>+</sup> B cells in SF of RA patients have a higher load of IgA bound to their surface compared to their FcRL4- counterparts. After in vitro removal of surface bound IgA, they can bind heat-aggregated IgA (p=0.0313). We also demonstrate that a significantly higher proportion of FcRL4+ B cells use IgA BCRs (p=0.0061) by flow cytometry and further more by probing constant region genes by PCR an enrichment for Ig genes for coding the IgA1 isotype (p=0.009) was found. <h3>Conclusions</h3> Both their ability to capture IgA immune complexes through binding to FcRL4 and their enrichment in IgA Ig gene expression suggest a potential role for synovial fluid FcRL4<sup>+</sup> B cells in the mucosal origin of joint inflammation. <h3>References</h3> Yeo, L. et al. Cytokine mRNA profiling identifies B cells as a major source of RANKL in rheumatoid arthritis. Ann Rheum Dis 70, 2022–2028, doi:10.1136/ard.2011.153312 (2011). Yeo, L. et al. Expression of FcRL4 defines a pro-inflammatory, RANKL-producing B cell subset in rheumatoid arthritis. Ann Rheum Dis 74, 928–935, doi:10.1136/annrheumdis-2013–204116 (2015). Falini, B. et al. Expression of the IRTA1 receptor identifies intraepithelial and subepithelial marginal zone B cells of the mucosa-associated lymphoid tissue (MALT). Blood 102, 3684–3692, doi:10.1182/blood-2003–03–0750 (2003). Wilson, T. J., Fuchs, A. &amp; Colonna, M. Cutting edge: human FcRL4 and FcRL5 are receptors for IgA and IgG. J Immunol 188, 4741–4745, doi:10.4049/jimmunol.1102651 (2012). <h3>Disclosure of Interest</h3> None declared