Enhanced Tyrosine Phosphatase Activity Underlies Dysregulated B-cell Receptor Signaling and Promotes Survival of Human Lupus B-cells (original) (raw)

2015, Arthritis & Rheumatology

Objective. Systemic lupus erythematosus (SLE) is associated with hyperactivity of B cells and abnormalities of B cell receptor (BCR) signaling. To address the linkage between dysregulated BCR signaling and increased B cell function, we assessed immediate phosphorylation events in lupus B cells. Methods. B cells from SLE patients and healthy donors were analyzed by flow cytometry to assess phosphorylated CD22, Syk, and Akt as well as the basal expression of the BCR coreceptors CD22 and CD19. Confocal microscopy studies determined the recruitment of CD22 and the tyrosine phosphatase SH2 domain-containing phosphatase 1 to the activated BCR complex. Additionally, phosphatase activity in SLE versus healthy donor B cells was measured. Results. B cells from SLE patients showed diminished Syk phosphorylation and reduced intracellular calcium release after BCR activation as compared to B cells from healthy donors. This was related to an enhanced activity of tyrosine, but not serine/threonine, phosphatases and was corrected by inhibition of tyrosine phosphatase activity. In contrast to reduced Syk phosphorylation after BCR activation, phosphorylation of Akt was significantly increased in SLE B cells. The disturbed balance between Syk and Akt phosphorylation was significantly correlated with B cell survival following BCR engagement. Furthermore, CD272, but not CD271, B cells from SLE patients displayed increased expression and phosphorylation of the inhibitory BCR coreceptor CD22. Conclusion. These results indicate that an imbalance between serine and tyrosine phosphatases in SLE contributes to an intrinsically disturbed balance of BCR-initiated signaling pathways, resulting in enhanced survival of lupus B cells and differentiation into plasma cells. The development, maturation, and selection of B cells are tightly regulated by the affinity and activation threshold of the B cell receptor (BCR). Besides its important role during B cell differentiation and survival, the BCR plays a pivotal role in avoiding autoreactivity by initiating a variety of negative-selection events (1,2). After BCR crosslinking, conformation changes induced by cytoskeleton reorganization lead to a Lyn-dependent phosphorylation of the intracellular Iga/b immunoreceptor tyrosine-based activation motif (ITAM) of the BCR and its coreceptors (e.g., CD19 and CD22), followed by the recruitment and activation of the spleen tyrosine kinase (Syk). Subsequently, signaling molecules, such as Bruton's tyrosine kinase (BTK), phosphatidylinositol 3-kinase (PI3K) and phospholipase Cg2 (PLCg2) accumulate in a signalosome to initiate internalization, antigen processing, calcium release, and a specific gene expression profile (3). Even though Syk can directly activate PI3K (4), a Syk-independent PI3K/ Akt activation has been described (5), subdividing the BCR-associated signaling pathway into two axes: Syk activation linked to PLCg2 phosphorylation and calcium influx, and CD19 phosphorylation followed by activation Supported by the DFG (SFB 650 project TP12, SFB 650 project TP16, SFB 633 A14, SPP ImmunoBone [Do491/8-2], and project Do491/7-3).