Supplementary Material for: CD22 Regulates Adaptive and Innate Immune Responses of B Cells

B cells sense microenvironments through the B cell receptor (BCR) and Toll-like receptors (TLRs). While signals from BCR and TLRs synergize to distinguish self from nonself, inappropriate regulation can result in development of autoimmune disease. Here we show that CD22, an inhibitory co-receptor of BCR, also negatively regulates TLR signaling in B cells. CD22-deficient (Cd22–/–) B cells exhibit hyperactivation in response to ligands of TLRs 3, 4 and 9. Evidence suggests that this results from impaired induction of suppressors of cytokine signaling 1 and 3, well-known suppressors of TLR signaling. Antibody-mediated sequestration of CD22 on wild-type (WT) B cells augments proliferation by TLR ligands. Conversely, expression of CD22 in a Cd22–/– B cell line blunts responses to TLR ligands. We also show that lipopolysaccharide-induced transcription by nuclear factor-ĸB is inhibited by ectopic expression of CD22 in a TLR4 reporter cell line. Taken together, these results suggest that negative regulation of TLR signaling is an intrinsic property of CD22. Since TLRs and BCR activate B cells through different signaling pathways, and are differentially localized in B cells, CD22 exhibits a broader regulation of receptors that mediate adaptive and innate immune responses of B cells than previously recognized.

B细胞通过B细胞受体（B cell receptor, BCR）与Toll样受体（Toll-like receptor, TLR）感知微环境。BCR与TLR传来的信号可协同区分自身与非自身成分，但调控异常可引发自身免疫病。本研究证实，BCR的抑制性共受体CD22同样可负向调控B细胞内的TLR信号通路。CD22基因敲除（Cd22–/–）B细胞在受到TLR3、TLR4及TLR9配体刺激时会呈现过度活化状态。相关证据表明，这一现象源于细胞因子信号转导抑制因子1和3（suppressors of cytokine signaling 1 and 3）的诱导受损——二者是公认的TLR信号通路负调控因子。对野生型（wild-type, WT）B细胞表面的CD22进行抗体介导封阻，可增强TLR配体诱导的细胞增殖；反之，在Cd22–/– B细胞系中异位表达CD22，则会减弱其对TLR配体的应答反应。此外，本研究还发现，在TLR4报告细胞系中异位表达CD22，可抑制脂多糖（lipopolysaccharide, LPS）介导的核因子κB（nuclear factor-κB, NF-κB）转录激活。综上，上述结果表明负向调控TLR信号通路是CD22的固有特性。由于TLR与BCR通过不同的信号通路激活B细胞，且在B细胞内的定位存在差异，CD22对介导B细胞适应性与固有免疫应答的受体的调控范围，较此前认知更为广泛。