Background Plasma cells (PC) are considered key drivers of antibody mediated autoimmune diseases. Long-lived PC survive for years in their niches, preferentially in the bone marrow (BM) but also in inflamed tissues.^{1 2} A subset of PC lacking expression of CD19 has been identified.³ Better understanding of factors and pathways involved in survival and maintenance of long-lived PC is needed to find strategies to target PC in autoimmunity since targeting B cells or proliferating cells does not affect already existing PC.⁴ B cell receptor (BCR) signalling is a critical mediator of B cell survival and it was shown before that IgA⁺ and IgM⁺ PC in the BM express a functional BCR suggesting a potential role for the BCR signalling pathway.⁵

Objectives Expression of BCR associated molecules in BM PC and the response of CD19⁺ and CD19^- BM PC to BCR stimulation by anti-IgM/IgA/IgG was assessed to test whether these PC subsets are capable of responding to BCR mediated signals. We further investigated if the PC isotype has an impact on BCR signalling.

Methods BM samples from patients undergoing routine total hip arthroplasty without systemic immune manifestations were stained for baseline expression of spleen tyrosine kinase (Syk) and Bruton’s tyrosine kinase (Btk) as well as for the phosphosites pSyk (Y³⁵²) and pBtk (Y²²³). BM mononuclear cells have been isolated, stimulated with anti-IgM/IgA/IgG and the increase of fluorescence intensity of pSyk (Y³⁵²) and pBtk (Y²²³) was measured by intracellular flow-cytometric analyses. In some experiments, cells have been stimulated with anti-IgA alone and stained for the isotype additionally to the pPTK staining.

Results Whole BM stainings revealed that both CD19⁺ and CD19^- PC express the PTKs Syk and Btk at baseline. Both PC subsets showed the ability to respond to BCR stimulation with enhanced phosphorylation of the PTKs with a clear trend to reduced responsiveness among CD19^- PC. Ig staining revealed that IgA expression was identified on both membrane and intracellularly, whereas IgG was only expressed intracellularly. Co-staining of IgA with pPTKs showed that IgA⁺ PC in both subsets are responsible for enhanced PTK phosphorylation independently of CD19 expression.

Conclusions CD19⁺ and CD19^- BM PC express kinases involved in BCR signalling and respond by enhanced phosphorylation of PTKs upon BCR stimulation with IgA-expressing cells being exclusively responsible for this increase. Further functional consequences of IgA expression in BM PC and autoimmunity remain to be delineated.

References [1] Manz RA, et al. Lifetime of plasma cells in the bone marrow. Nature1997;388(6638):133–134.

[2] Tokoyoda K, et al. Organization and maintenance of immunological memory by stroma niches. Eur J Immunol2009;39(8):2095–2099.

[3] Mei HE, et al. A unique population of IgG-expressing plasma cells lacking CD19 is enriched in human bone marrow. Blood2015;125(11):1739–1748.

[4] Hoyer BF, et al. Short-lived plasmablasts and long-lived plasma cells contribute to chronic humoral autoimmunity in NZB/W mice. J Exp Med2004;199(11):1577–1584.

[5] Pinto D, et al. A functional BCR in human IgA and IgM plasma cells. Blood2013;121(20):4110–4114.

Disclosure of Interest None declared