Introduction 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 distinguishable by their expression of Fc-like receptor 4 (FcRL4) and elevated expression of RANKL, indicating a unique pathogenic function.^1,2 B cells expressing FcRL4 were originally described as a distinct memory B cell subset in human tonsils where they are localised in the epithelium.³ We have recently shown in RA that they are enriched in clones recognising citrullinated autoantigens.³ Recent in vitro work suggested that FcRL4 is a low affinity receptor for aggregated IgA.⁴

Objectives

• Investigate the interactions between IgA and FcRL4 and FcRL4+ B cells using tonsil and SF B cells, and transfected cell lines.

• Examine the distribution of Ig classes by flow cytometry and PCR.

Methods Mononuclear cells were isolated from SF (n=10) and tonsil, labelled for CD19, FcRL4, IgA, and IgG and analysed by flow cytometry. In experiments identifying BCRs and relative loads of surface bound IgA; SFMCs were washed in an acidic buffer to remove receptor-bound proteins 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.

Results We show that ex vivo SF FcRL4⁺ B cells have a significantly higher load of IgA bound to their surface (p=0.0001) than FcRL4^- B cells. Following in vitro removal of surface bound IgA, FcRL4 +B cells bind heat-aggregated IgA (p=0.03 vs control). We also demonstrate that a significantly higher proportion of FcRL4⁺ B cells use IgA BCRs (p=0.0061) by flow cytometry, and by probing constant region genes by PCR an enrichment for Ig genes for coding the IgA1 isotype (p=0.009) was found. Furthermore, three out of eight of the antibodies recognising citrullinated peptides had been cloned from FcRL4⁺ IgA⁺ B cells.

Conclusions In conclusion, their specificity for citrullinated antigens, ability to capture IgA immune complexes through via FcRL4 and enrichment in IgA1 subclass expression, suggest a role for FcRL4 +B cells in the mucosal origin of joint inflammation.

References

1. . Yeo L, et al. Ann Rheum Dis2011;70:2022–2028. doi:10.1136/ard.2011.153312

2. . Yeo L, et al. Ann Rheum Dis2015;74:928–935. doi:10.1136/annrheumdis-2013-204116

3. . Amara K, et al. J. Autoimmun2017. doi:10.1016/j.jaut.2017.03.004

4. . Wilson TJ, et al. J Immunol2012;188;4741–4745. doi:10.4049/jimmunol.1102651

Acknowledgements This work was supported by the Medical Research Council UK, Rheumatoid Arthritis Pathogenesis Centre of Excellence, and Karolinska Institutet Foundations for Rheumatology Research.

Disclosure of interest None declared