MyD88‐dependent autoimmune disease in Lyn‐deficient mice

Abstract

Recent evidence suggests that systemic autoimmune disease depends on signals from TLR ligands, but little is known about how TLR-dependent pathways lead to the loss of self tolerance in vivo. To address this, we have examined the role of TLR signaling in Lyndeficient mice, which develop an autoimmune disease similar to SLE. We found that absence of the TLR signaling adaptor molecule MyD88 suppresses plasma cell differentiation of switched and unswitched B cells, and prevents the generation of antinuclear IgG antibodies and glomerulonephritis. In mixed chimeras the increased IgM and IgG antibody secretion in Lyn-deficient mice is at least partially due to B cellindependent effects of Lyn. We now show that MyD88 deficiency blocks the expansion and activation of DC in which Lyn is also normally expressed, and prevents the hypersecretion of proinflammatory cytokines IL-6 and IL-12 by Lyn-deficient DC. These findings further highlight the important role of TLR-dependent signals in both lymphocyte activation and autoimmune pathogenesis. IntroductionOur understanding of the pathways leading to autoantibody (autoAb) production in systemic lupus erythematosus (SLE) has been greatly enhanced by studies of murine SLE models, including mice with single targeted mutations to negative regulators of BCR signaling, e.g., the src-family protein tyrosine kinase Lyn [1][2][3], phosphatase SHP-1 [4] and coreceptor CD22 [5, 6]. Lyn deficiency manifests as the hypersecretion of IgM, IgA and IgG autoAb, which, by 17-20 weeks of age, develop specificity to nuclear components such as dsDNA, are deposited as immune complexes (IC) in the kidneys and lead to lethal proliferative glomerulonephritis [1][2][3]. Following BCR stimulation, Lyn-deficient B cells have greater and more prolonged intracellular calcium flux, more MAPK activation and hyperproliferate in comparison to WT B cells [1, 7, 8]. These findings, together with the fact that the expression of Lyn is limited to B, NK and myeloid cells [9], have led to the suggestion that B cell intrinsic effects are the main cause of SLE-like autoimmune disease. Growing support for the central role B cells play in the development of autoimmune disease (reviewed in [10]) and the discovery of lower levels of Lyn in B cells of human SLE patients have helped strengthen this idea [11].However, the development of high-affinity pathogenic autoAb, which are targeted at nuclear antigens and potentially damaging to organs including kidneys, depends on somatic hypermutation in the germinal center and collaboration between cells of the adaptive and innate immune systems. T cells play an important role in this process and autoantigen-specific T cells are required for secretion of IgG autoAb specific for nuclear components and end-organ damage in the spontaneous murine models, MRL-lpr/lpr and (NZBÂNZW)F 1 [18]. Disease in the gp96 transgenic mice is dependent on MyD88, an adaptor molecule required for signaling by all TLR except TLR3, which binds dsRNA, and TLR4, which also signals by a MyD88-independent pa...

Autoimmune Disease in Lyn-Deficient Mice Is Dependent on an Inflammatory Environment Established by IL-6

“…These earlier studies, however, used DCs that were derived from the bone marrow in the presence of GM-CSF (28,29). In the current study, we have found that splenic CD11c + DCs have an activated/mature phenotype, expressing elevated levels of costimulatory molecules on their surface when analyzed ex vivo from Lyn 2/2 mice, consistent with the recent findings of Silver et al (27). An explanation for these apparent contradictions may lie in the fact that Lyn 2/2 cells are hyper-responsive to GM-CSF (32).…”

“…It remains to determine why deficiency of Lyn results in hyperproduction of IL-6, but this finding suggests that Lyn is a negative regulator of the IL-6 pathway. The pathway leading to IL-6 production is likely to be related to TLR signaling, and this is supported by a recent study that has shown that loss of the important TLR signaling intermediate, MyD88, on a Lyn-deficient background prevents autoimmune disease development (27). Interestingly, this study showed that CD11c + DCs from Lyn 2/2 mice treated with TLR agonists were a source of IL-6; however, purified splenic DCs were used and these cultures, which were reported to be ∼ 83% pure (27), may have been contaminated with B cells, which we show are a potent source of IL-6.…”

“…Lyn-deficient mice lacking B cells fail to develop autoimmune disease (23), as do Lyn 2/2 mice with null mutations in molecules essential for early B cell activation such as Btk and CD19 (24)(25)(26). Although few studies have investigated the role of other hematopoietic cells and the contribution of cytokines, a recent report has shown that disruption of TLR signaling in Lyn 2/2 MyD88 2/2 mice prevented the production of pathogenic auto-Abs and associated glomerulonephritis, and has suggested a role for proinflammatory cytokine secretion by dendritic cells (DCs) in disease pathogenesis (27). Indeed, Lyn is highly expressed in DCs (28,29), and it has been suggested that increased maturation of Lyn 2/2 DCs is associated with autoimmunity in Lyn 2/2 mice (27 mice) were generated by crossing C57BL/6 Lyn 2/2 mice with C57BL/6 mice carrying a mutation in the membrane exon of the m-chain gene (31).…”

TLR-Activated B Cells Suppress T Cell-Mediated Autoimmunity

“…The contribution of TLR signaling to autoimmune diseases is also indicated by the requirement for MyD88 in spontaneous development of systemic lupus erythematosus (SLE) in lupus-prone mice (26,27). Unexpectedly, TLR stimulation can also suppress autoimmune pathogenesis, indicating a dual role for TLR in autoimmune diseases.…”

CTLA4Ig Alters the Course of Autoimmune Disease Development in Lyn−/− Mice

“…In this way, DNA and other self-Ags can be brought into contact with dendritic cell TLRs, activating them and allowing priming of specific T cells (14). Indeed, Lyn-deficient mice that are unable to signal in response to DNA ligation of TLR9 have a greatly diminished autoimmune phenotype (15). We hypothesized that T cells, activated by presentation of self-Ag, amplify and mature autoantibody production and drive inflammatory processes that compound the autoimmune phenotype of Lyn 2/2 mice.…”