Interleukin-18 receptor signaling is not required for autoantibody production and end-organ disease in murine lupus

Deletion of IL-18 Expression Ameliorates Spontaneous Kidney Failure in MRLlpr Mice

The role of IL-18 in the pathogenesis of systemic lupus erythematosus is still not definitively solved. In this study, we generated MRL^lpr mice, which develop a disease resembling systemic lupus erythematosus, genetically devoid of IL-18 expression. These mice in comparison to IL-18-competent MRL^lpr mice show reduced signs of renal pathogenesis, while other parameters such as mean survival time, lymphadenopathy, constitutive interferon-γ production, and frequency of CD3⁺B220⁺ abnormal T cells were without differences. We conclude that in the systemic lupus erythematosus syndrom IL-18 is involved specifically in the renal pathogenesis.

Autoantibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multifactorial disorder with multigenic inheritance and various environmental factors implicated in its aetiopathogenesis. Despite the multiple mechanisms involved in the aetiology of SLE being elusive, recent studies have made progress in our understanding of the pathogenic mechanisms via abnormal regulation of cell-mediated and humoral immunity that lead to tissue damage. The heterogeneity of the clinical manifestations probably reflects the complexity of the disease pathogenesis itself. The immune system in SLE is characterised by a complex interplay between overactive B cells, abnormally activated T cells and antigen-presenting cells. This interplay leads to the production of an array of inflammatory cytokines, apoptotic cells, diverse autoantibodies and immune complexes that in turn activate effector cells and the complement system, leading to tissue injury and damage which are the hallmarks of the clinical manifestations. SLE patients have dysregulation of inflammatory cytokines, chemokines and immune response-related genes, as well as of the genes involved in apoptosis, signal transduction and the cell cycle.

Definition of IFN-γ-related pathways critical for chemically-induced systemic autoimmunity

IFN-γ is essential for idiopathic and murine mercury-induced systemic autoimmunity (mHgIA), and heterozygous IFN-γ(+/-) mice also exhibit reduced disease. This suggests that blocking specific IFN-γ-related pathways that may only partially inhibit IFN-γ production or function will also suppress autoimmunity. To test this hypothesis, mice deficient in genes regulating IFN-γ expression (Casp1, Nlrp3, Il12a, Il12b, Stat4) or function (Ifngr1, Irf1) were examined for mHgIA susceptibility. Absence of either Ifngr1 or Irf1 resulted in a striking reduction of disease, while deficiency of genes promoting IFN-γ expression had modest to no effect. Furthermore, both Irf1- and Ifng-deficiency only modestly reduced the expansion of CD44(hi) and CD44(hi)CD55(lo) CD4(+) T cells, indicating that they are not absolutely required for T cell activation. Thus, there is substantial redundancy in genes that regulate IFN-γ expression in contrast to those that mediate later signaling events. These findings have implications for the therapeutic targeting of IFN-γ pathways in systemic autoimmunity.

Targeting transcription factor Stat4 uncovers a role for interleukin-18 in the pathogenesis of severe lupus nephritis in mice

Polymorphisms in the transcription factor Stat4 gene have been implicated as risk factors for systemic lupus erythematosus. Although some polymorphisms have a strong association with autoantibodies and nephritis, their impact on pathophysiology is still unknown. To explore this further we used signal transducers and activators of transcription 4 (Stat4) knockout MRL/MpJ-Fas(lpr)/Fas(lpr) (MRL-Fas(lpr)) mice and found that they did not differ in survival or renal function from Stat4-intact MRL-Fas(lpr) mice. Circulating interleukin (IL)-18 levels, however, were elevated in Stat4-deficient compared to Stat4-intact mice, suggesting that this interleukin might contribute to the progression of lupus nephritis independent of Stat4. In a second approach, Stat4 antisense or missense oligonucleotides or vehicle were given to MRL-Fas(lpr) mice with advanced nephritis. Each of these treatments temporarily ameliorated disease, although IL-18 was increased in each setting. Based on these findings, studies using gene transfer to overexpress IL-18 in MRL-Fas(lpr) and IL-12p40/IL-23 knockout MRL-Fas(lpr) mice reveal a critical role for IL-18 in mediating disease. Thus, the Stat4 and IL-12 (an activator of Stat4)-independent factor, IL-18, can drive autoimmune lupus nephritis in MRL-Fas(lpr) mice. Temporarily blocking Stat4 during advanced nephritis ameliorates disease, suggesting a time-dependent compensatory proinflammatory mechanism.

Toll-like receptor 7 (TLR7) modulates anti-nucleosomal autoantibody isotype and renal complement deposition in mice exposed to syngeneic late apoptotic cells

OBJECTIVES

The objectives of this study were to determine whether late apoptotic cell material directly induces autoantibodies characteristic of systemic lupus erythematosus (SLE) and to investigate the innate recognition pathways involved.

METHODS

B6, B6.MyD88(-/-), B6.TLR7(-/-) and B6.TLR9(-/-) mice were subcutaneously injected with B6 syngeneic late apoptotic thymocytes (SLATs) without adjuvant on days 0, 10, 24 and 37. Sera were tested for IgG antibodies to histones and double-stranded DNA (dsDNA) by ELISA and Crithidia luciliae indirect immunofluorescence. IgG and C3 deposition in kidney glomeruli was assessed by immunostaining and fluorescence microscopy.

RESULTS

SLAT injections induced anti-dsDNA and anti-histone antibodies of the IgG1 and IgG2b isotypes in B6 but not MyD88(-/-) mice. TLR7(-/-) and TLR9(-/-) mice injected with SLATs produced delayed or slightly more robust responses, respectively. SLAT injections induced IgG deposits in renal glomeruli of B6, TLR7(-/-) and TLR9(-/-) mice that were absent in MyD88(-/-) mice. Unlike B6 and TLR9(-/-) animals, TLR7(-/-) mice failed to exhibit IgG colocalised glomerular C3 deposits and demonstrated autoantibodies of primarily the IgG2a isotype.

CONCLUSIONS

Late apoptotic cell-induced anti-histone and anti-dsDNA antibodies require MyD88 but not Toll-like receptor (TLR)9. Moreover, TLR7 promotes glomerular C3 deposition, possibly through a mechanism of altered antibody isotype switching.

Increased cytokine production in interleukin-18 receptor alpha-deficient cells is associated with dysregulation of suppressors of cytokine signaling

Since interleukin (IL)-18 is a proinflammatory cytokine, mice lacking IL-18 or its ligand-binding receptor (IL-18R) should exhibit decreased cytokine and chemokine production. Indeed, production of IL-1alpha, IL-6, and MIP-1alpha was reduced in IL-18 knock-out (ko) mouse embryonic fibroblast (MEF)-like cells. Unexpectedly, we observed a paradoxical 10-fold increase in IL-1beta-induced IL-6 production in MEF cells from mice deficient in the IL-18R alpha-chain (IL-18Ralpha) compared with wild type MEF. Similar increases were observed for IL-1alpha, MIP-1alpha, and prostaglandin E2. Likewise, coincubation with a specific IL-18Ralpha-blocking antibody augmented IL-1beta-induced cytokines in wild type and IL-18 ko MEF. Stable lines of IL-18Ralpha-depleted human A549 cells were generated using shRNA, resulting in an increase of IL-1beta-induced IL-1alpha, IL-6, and IL-8 compared to scrambled small hairpin RNA. In addition, we silenced IL-18Ralpha with small interfering RNA in primary human blood cells and observed up to 4-fold increases in the secretion of lipopolysaccharide- and IL-12/IL-18-induced IL-1beta, IL-6, interferon-gamma, and CD40L. Mechanistically, despite increases in Stat1 and IL-6, induction of SOCS1 and -3 (suppressor of cytokine signaling 1 and 3) was markedly reduced in the absence of IL-18Ralpha. Consistent with these observations, activation of the p38alpha/beta and ERK1/2 MAPKs and of protein kinase B/Akt increased in IL-18Ralpha ko MEF, whereas the negative feedback kinase MSK2 was more active in IL-18 ko cells. These data reveal a role for SOCS1 and -3 in the seemingly paradoxical hyperresponsive state in cells deficient in IL-18Ralpha, supporting the concept that IL-18Ralpha participates in both pro- and anti-inflammatory responses and that an endogenous ligand engages IL-18Ralpha to deliver an inhibitory signal.

IL-18 is expressed in the intercalated cell of human kidney

We determined the cellular location of interleukin-18 (IL-18) and caspase-1 and the purinergic receptor P2X7, two proteins necessary for its activation and secretion. The mRNA and protein of IL-18 were detectable in normal human kidney by means of polymerase chain reaction (PCR), in situ hybridization, and Western blot. Immunohistochemistry located IL-18 to nephron segments containing calbinbin-D28k or aquaporin-2 that suggest location in the distal convoluted and the connecting tubule and to parts of the collecting duct. IL-18 was not detected in the thick ascending limb of Henle. Confocal microscopy showed that IL-18 was expressed in cells negative for calbindin-D28k and for aquaporin-2 but positive for the vacuolar H(+)-ATPase. This demonstrates that the intercalated cells produce IL-18. These segments were also positive for caspase-1 and P2X7 that are essential for IL-18 secretion. Our results show that IL-18 is constitutively expressed by intercalated cells of the late distal convoluted tubule, the connecting tubule, and the collecting duct of the healthy human kidney. Since IL-18 is an early component of the inflammatory cytokine cascade, its location suggests that renal intercalated cells may contribute to immediate immune response of the kidney.

Toll-like receptor 9 signaling protects against murine lupus

OBJECTIVE

Hypomethylated CpG-containing DNA, which is recognized by Toll-like receptor 9 (TLR-9), has been strongly implicated in the pathogenesis of autoantibody-mediated diseases such as systemic lupus erythematosus. This study was undertaken to determine the role of TLR-9 in the MRL/+ and MRL/lpr models of murine lupus.

METHODS

TLR-9-deficient MRL mice were generated by backcrossing a TLR-9-deficient allele against the MRL backgrounds by a speed congenic technique. Parameters of murine lupus were examined by routine methods. Regulatory T cell activity was assessed by autologous mixed lymphocyte reaction (AMLR), an in vitro assay for autoreactivity.

RESULTS

Surprisingly, TLR-9-deficient animals of both the MRL/+ and the MRL/lpr backgrounds developed more severe lupus, as judged by anti-DNA and rheumatoid factor autoantibodies, total serum Ig isotypes, lymphadenopathy, inflammatory infiltrates in the salivary gland and kidney, proteinuria, and mortality, in comparison with their TLR-9-sufficient littermates. In vitro, regulatory T cells from TLR-9-deficient animals were impaired in their ability to suppress the AMLR.

CONCLUSION

In the MRL model of murine lupus, TLR-9 signaling plays a protective role, perhaps by modulating the activity of regulatory T cells. These results contrast with findings of recent studies that implicate TLR-9 in the pathogenesis of anti-DNA responses, based in part on investigations in incompletely backcrossed TLR-9-deficient MRL/lpr mice in vivo or transgenic B cells in vitro. The present results highlight the need for caution in the assessment of disease paradigms based on the study of isolated cell populations in vitro, as well as in vivo studies of knockout animals involving non-ideal genetic models.

RNA-associated autoantigens activate B cells by combined B cell antigen receptor/Toll-like receptor 7 engagement

Previous studies (Leadbetter, E.A., I.R. Rifkin, A.H. Hohlbaum, B. Beaudette, M.J. Shlomchik, and A. Marshak-Rothstein. 2002. Nature. 416:603–607; Viglianti, G.A., C.M. Lau, T.M. Hanley, B.A. Miko, M.J. Shlomchik, and A. Marshak-Rothstein. 2003. Immunity. 19:837–847) established the unique capacity of DNA and DNA-associated autoantigens to activate autoreactive B cells via sequential engagement of the B cell antigen receptor (BCR) and Toll-like receptor (TLR) 9. We demonstrate that this two-receptor paradigm can be extended to the BCR/TLR7 activation of autoreactive B cells by RNA and RNA-associated autoantigens. These data implicate TLR recognition of endogenous ligands in the response to both DNA- and RNA-associated autoantigens. Importantly, the response to RNA-associated autoantigens was markedly enhanced by IFN-α, a cytokine strongly linked to disease progression in patients with systemic lupus erythematosus (SLE). As further evidence that TLRs play a key role in autoantibody responses in SLE, we found that autoimmune-prone mice, lacking the TLR adaptor protein MyD88, had markedly reduced chromatin, Sm, and rheumatoid factor autoantibody titers.