Postactivated B cells in systemic lupus erythematosus: update on translational aspects and therapeutic considerations.

PURPOSE OF REVIEW This review summarizes recent insights and current understanding of the role of postactivated B cells in SLE and related pathogenic and potential therapeutic implications. RECENT FINDING B cells are considered key players in SLE and experience from various B-cell-targeted therapies underlines their clinical relevance. In the last years, new insights have been obtained on B-cell abnormalities within the complex pathophysiology of SLE. These insights involve a revised understanding of BCR signaling, that has been reported to be hyperresponsive in the past, but newer studies suggest a postactivation functiotype in terms of reduced BCR and TLR signaling. Despite comprehensive efforts to delineate B-cell abnormalities on assessing large-scale genomic, epigenomic and proteomic data, understanding functional impairments of cellular interactions and subcellular functions remains crucial. A recently identified enhanced protein tyrosine phosphatase (PTP) activity was found in relation to diminished BCR responses in SLE. This finding together with reduced cytokine production upon TLR9 activation appears to mark postactivated lupus B cells. Other studies identified increased PTP activity in line with a gain-of-function mutation of phosphatase PTPN22, one of the strongest SLE risk alleles. Improved understanding of these B cell abnormalities in SLE holds promise to gain further insights in mechanisms of autoimmunity and pave the way for selective therapies targeting key principles of chronic autoimmunity. SUMMARY SLE B cells (similar as previously described for lupus T cells) are characterized by a postactivation (exhausted) functiotype mandating consideration for innovative therapies

BTLA Expression and Function Are Impaired on SLE B Cells

B- and T-lymphocyte attenuator (BTLA/CD272) is an inhibitory checkpoint molecule expressed on T and B cells. Prior studies reported defective function of BTLA by T cells in patients with systemic lupus erythematosus (SLE), whereas nothing is known about its role on B cells in SLE, a disease with various B cell abnormalities. Peripheral blood mononuclear cells (PBMCs) from 23 healthy donors (HD) and 34 SLE patients were stained for BTLA and its expression on B cells was assessed. PBMCs or CD27(-)IgD(+) naive B cells were stimulated together with an activating anti-BTLA antibody or an inhibitor of spleen tyrosine kinase (SYK) and differentiation as well as the expression of activation markers CD71, PD-1 and CD86 were analyzed. Our phenotypic and functional studies revealed reduced BTLA expression on CD27(-)IgD(+) naive B cells from SLE patients (p=0.0017) related to anti-dsDNA antibody titers (p=0.0394) and SIGLEC-1/CD169 expression on monocytes (p=0.0196), a type I interferon marker related to disease activity. BTLA engagement was found to control CpG/TLR9 activation limiting plasmablast (p=0.0156) and B cell memory induction (p=0.0078) in normal B cells in contrast to other B cell activation pathways (CD40, BCR). These BTLA functions were impaired in SLE B cells. Inhibition of SYK was found to mimic the effects of BTLA activity in vitro. Thus, is it possible that reduced BTLA expression and function of CD27-IgD+ antigen- and T cell-inexperienced SLE B cells could be overcome by SYK inhibition which should be tested in future studies as potential therapeutic principle