Higher‐order CpG‐DNA stimulation reveals distinct activation requirements for marginal zone and follicular B cells in lupus mice

Abstract

Mouse follicular B cells express TLR9 and respond vigorously to stimulation with singlestranded CpG-oligodeoxynucleotides (ODN). Surprisingly, follicular B cells do not respond to direct stimulation with other TLR9 ligands, such as bacterial DNA or class A(D) CpG-ODN capable of forming higher-order structures, unless other cell types are present. Here, we show that priming with interferons or with B cell-activating factor, or simultaneous co-engagement of the B cell receptor for antigen (BCR), can overcome this unresponsiveness. The effect of interferons occurs at the transcriptional level and is mediated through an autocrine/paracrine loop, which is dependent on IRF-1, IL-6 and IL-12 p40. We hypothesize that the lack of bystander activation of follicular B cells with more complex CpG ligands may be an important safety mechanism for avoiding autoimmunity. This will prevent resting B cells from responding to foreign or selfderived hypomethylated double-stranded CpG ligands unless these ligands are either delivered through the B cell receptor or under conditions where B cells are simultaneously co-engaged by activated plasmacytoid dendritic cells or TH1 cells. A corollary is that the heightened responsiveness of lupus B cells to TLR9-induced stimulation cannot be ascribed to unprimed follicular B cells, but is rather mediated by hypersensitive marginal zone B cells.

IRF5 haplotypes demonstrate diverse serological associations which predict serum interferon alpha activity and explain the majority of the genetic association with systemic lupus erythematosus

“…Plasmacytoid dendritic cells are one of the main IFNα producing cell types, and this may be this cell in which IRF5 variants exert their influence upon IFNα levels. In murine systems, plasmacytoid dendritic cells can activate B cells in multiple ways, including direct cell–cell contact,30 and IFNα produced by these cells can prime B cells to respond to TLR9 ligands 31. Thus, the feed-forward loop we suggest may manifest as an interaction between plasmacytoid dendritic cells and B cells in the presence of TLR ligands.…”

Differential Cytokine Production and Bystander Activation of Autoreactive B Cells in Response to CpG-A and CpG-B Oligonucleotides

“…This observation also has implications for B cell function in the context of autoimmune disease, where it is possible that B cells activated by BCR-recognition of a microbial antigen or autoantigen, can then more efficiently endocytose bystander self-antigens that are taken up nonspecifically and then engage a relocalized TLR. In accordance with this mechanism of activation, marginal zone (MZ) B cells isolated from lupus mice show enhanced responses to CpG-A when compared to MZ B cells from normal mice suggesting these cells may be in activated state in lupus-prone mice and more easily activated by bystander DNA (19, 20). …”

Innate immune recognition of, and regulation by, DNA

“…Although the precise mechanism involved in this species-specific TLR9 expression pattern remains unclear, it should be noted that distinct cis-and trans-regulation at the promoter region of the gene encoding TLR9 has been reported [51,52]. Moreover, growing recent evidence indicates that TLR9 expression and/or function can be upregulated in B cells [53] and other cell types, such as macrophages or cDCs stimulated with IFNb [52] or IFNg [54,55], monocytes infected with Yersinia pestis [56], granulocytes stimulated with granulocyte-macrophage colony-stimulating factor [57,58], and even epithelial cells cultured with transforming growth factor (TGF)a [59]. These observations might underscore the potential contribution of these various cell types, other than B cells and pDCs, to TLR9-mediated protective or pathogenic roles in infection and other immunological disorders.…”