Spontaneous loss of tolerance of autoreactive B cells in Act1-deficient rheumatoid factor transgenic mice

Low Levels of Vitamin D Promote Memory B Cells in Lupus

Background: Vitamin D deficiency is a known risk factor for Systemic Lupus Erythematosus (SLE), yet clinical trials have not demonstrated efficacy and few studies have utilized lupus models to understand the mechanism underlying this relationship. The Act1-/- mouse is a spontaneous model of lupus and Sjögren's syndrome, characterized by increased Th17 cells and peripheral B cell expansion. Vitamin D3 has anti-inflammatory properties, reduces Th17 cells and impairs B cell differentiation/activation. Therefore, we assessed how varying amounts of vitamin D3 affected lupus-like disease in the Act1-/- mouse. Methods: Act1-/- mice were fed either low/restricted (0 IU/kg), normal (2 IU/kg), or high/supplemented (10 IU/kg) vitamin D3 chow for 9 weeks, after which lupus-like features were analyzed. Results: While we found no differences in Th17 cells between vitamin D3 groups, vitamin D3 restriction specifically promoted memory B cell development, accompanied by elevated levels of serum IgM, IgG1, IgG3, and anti-dsDNA IgG. A similar significant negative association between serum vitamin D and memory B cells was confirmed in a cohort of SLE patients. Conclusion: Low levels of vitamin D3 are associated with elevated levels of memory B cells in an animal model of lupus and well-controlled SLE patients.

Structure of a prokaryotic SEFIR domain reveals two novel SEFIR-SEFIR interaction modes

SEFIR domain-containing proteins are crucial for mammalian adaptive immunity. As a unique intracellular signaling domain, the SEFIR-SEFIR interactions mediate physical protein-protein interactions in the immune signaling network, especially the IL-17- and IL-25-mediated pathways. However, due to the lack of structural information, the detailed molecular mechanism for SEFIR-SEFIR assembly remains unclear. In the present study, we solved the crystal structures of a prokaryotic SEFIR domain from Bacillus cereus F65185 (BcSEFIR), where the SEFIR domain is located at the N terminus. The structure of BcSEFIR revealed two radically distinct SEFIR-SEFIR interaction modes. In the asymmetric form, the C-terminal tail of one SEFIR binds to the helix αA and βB-αB' segment of the other one, while in the symmetric form, the helices ηC and αE and the DE-segment compose the inter-protomer interface. The C-terminal tail of BcSEFIR, critical for asymmetric interaction, is highly conserved among the SEFIR domains of Act1 orthologs from different species, in particular three absolutely conserved residues that constitute an EXXXXPP motif. In the symmetric interaction mode, the most significant contacts made by residues on helix αE are highly conserved in Act1 SEFIR domains, constituted an RLI/LXE motif. The two novel SEFIR-SEFIR interaction modes might explain the structural basis for SEFIR domain-mediated complex assembly in signaling pathways.

The Act1 D10N missense variant impairs CD40 signaling in human B-cells

The TRAF3IP2 gene resides within one of at least 63 psoriasis susceptibility loci and encodes Act1, an adapter protein involved in IL-17 receptor and CD40 signaling pathways. TRAF3IP2 is distinctive (among <10% of candidate susceptibility genes) in that a strongly disease-associated variant encodes a missense SNP predicted to be functionally relevant (SNP rs33980500 C/T encoding Act1 pD10N). As assessed by flow cytometry, Act1 protein was expressed at the highest levels in monocytes, with lower levels in T-cells and B-cells. However, monocytes, T-cells and B-cells failed to respond to IL-17A stimulation of PBMC, as measured by flow cytometric determination of NF-κB phospho-p65. As an alternative stimulus, we treated PBMCs with trimerized recombinant human CD40L and assessed p65, p38 and Erk phosphorylation in CD19⁺ B-cells as a function of D10N genotype. The increase of phosphorylated p65, p38 and Erk was well-correlated across individuals, and CD40L-induced phosphorylation of p65, p38, and Erk was significantly attenuated in B-cells from Act1 D10N homozygotes, compared to heterozygotes and nullizygotes. Our results indicate that the Act1 D10N variant is a relevant genetic determinant of CD40L responsiveness in human B-cells, with the risk allele being associated with lower B-cell responses in an acute signaling context.

Dendritic Cells Regulate Extrafollicular Autoreactive B Cells via T Cells Expressing Fas and Fas Ligand

The extrafollicular (EF) plasmablast response to self-antigens that contain Toll-like receptor (TLR) ligands is prominent in murine lupus models and some bacterial infections, but the inhibitors and activators involved have not been fully delineated. Here, we used two conventional dendritic cell (cDC) depletion systems to investigate the role of cDCs on a classical TLR-dependent autoreactive EF response elicited in rheumatoid-factor B cells by DNA-containing immune complexes. Contrary to our hypothesis, cDC depletion amplified rather than dampened the EF response in Fas-intact but not Fas-deficient mice. Further, we demonstrated that cDC-dependent regulation requires Fas and Fas ligand (FasL) expression by T cells, but not Fas expression by B cells. Thus, cDCs activate FasL-expressing T cells that regulate Fas-expressing extrafollicular helper T (Tefh) cells. These studies reveal a regulatory role for cDCs in B cell plasmablast responses and provide a mechanistic explanation for the excess autoantibody production observed in Fas deficiency.

Requirement for Transcription Factor Ets1 in B Cell Tolerance to Self-Antigens

The differentiation and survival of autoreactive B cells is normally limited by a variety of self-tolerance mechanisms, including clonal deletion, anergy, and clonal ignorance. The transcription factor c-ets-1 (encoded by the Ets1 gene) has B cell-intrinsic roles in regulating formation of Ab-secreting cells by controlling the activity of Blimp1 and Pax5 and may be required for B cell tolerance to self-antigen. To test this, we crossed Ets1(-/-) mice to two different transgenic models of B cell self-reactivity, the anti-hen egg lysozyme BCR transgenic strain and the AM14 rheumatoid factor transgenic strain. BCR transgenic Ets1(-/-) mice were subsequently crossed to mice either carrying or lacking relevant autoantigens. We found that B cells lacking c-ets-1 are generally hyperresponsive in terms of Ab secretion and form large numbers of Ab-secreting cells even in the absence of cognate Ags. When in the presence of cognate Ag, different responses were noted depending on the physical characteristics of the Ag. We found that clonal deletion of highly autoreactive B cells in the bone marrow was intact in the absence of c-ets-1. However, peripheral B cells lacking c-ets-1 failed to become tolerant in response to stimuli that normally induce B cell anergy or B cell clonal ignorance. Interestingly, high-affinity soluble self-antigen did cause B cells to adopt many of the classical features of anergic B cells, although such cells still secreted Ab. Therefore, maintenance of appropriate c-ets-1 levels is essential to prevent loss of self-tolerance in the B cell compartment.

Genetic and cellular dissection of the activation of AM14 rheumatoid factor B cells in a mouse model of lupus

The RF-specific AM14 tg BCR has been used as a model to dissect the mechanisms of B cell tolerance to ICs containing nucleic acids. We have shown previously that AM14 RF B cells break tolerance in the TC mouse model of lupus through the dual engagement of the AM14 BCR and TLR9. In this study, we showed that neither the expression of Sle1 or Sle2 susceptibility loci alone was sufficient to activate AM14 RF B cells, suggesting that the production of antichromatin IgG2a(a) autoAg mediated by Sle1 and an intrinsically higher B cell activation mediated by Sle2 were required. We also showed that the B6 genetic background enhanced the selection of AM14 RF B cells to the MZB cell compartment regardless of the expression of the Sle loci and therefore, of their activation into AFCs. Furthermore, some AM14 RF B cells were selected into the B-1a compartment, where they did not differentiate into AFCs. Therefore, it is unlikely that the selection of AM14 RF B cells to the MZB or B-1a cell compartments in TC.AM14(a) mice is responsible for their breach of tolerance. Finally, we showed that the presence of expression of Sle1 in non-tg cells, most likely T cells, is necessary for the activation of AM14 RF B cells into AFCs. Overall, these results suggest a threshold model of activation of AM14 RF B cells on the B6 background with additive genetic and cellular contribution of multiple sources.

Decrease of Functional Activated T and B Cells and Treatment of Glomerulonephitis in Lupus-Prone Mice Using a Natural Flavonoid Astilbin

Treatment of systemic lupus erythematosus (SLE), a chronic inflammatory disease, involves the long-term use of immunosuppressive agents with significant side effects. New therapeutic approaches are being explored to find better treatment possibilities. In this study, age-matched female MRL/lpr mice were treated orally with a natural flavonoid astilbin. Astilbin administration started either at week 8 or week 12 of age though week 20. In the early treatment regimen, the treatment with astilbin reduced splenomegaly / lymphomegaly, autoantibody production and ameliorated lupus nephitis. Several serum cytokines were significantly decreased upon treatment including IFN-g, IL-17A, IL-1b, TNF-a and IL-6. Both spleen CD44^hiCD62L^lo activated T cells and CD138⁺B220^- plasma cells greatly declined. Furthermore, astilbin treatment resulted in decreased mitochondrial membrane potential in activated T cells and downregulated expression of the co-stimulatory molecules CD80 and CD86 on LPS stimulated B cells. Similar but less profound effectiveness was observed in the mice with established disease in the late treatment regimen. These results indicate that the natural product astilbin can mitigate disease development in lupus-prone mice by decreasing functional activated T and B cells.

The differential regulation of human ACT1 isoforms by Hsp90 in IL-17 signaling

IL-17 is a proinflammatory cytokine implicated in the pathogenesis of autoimmune diseases including psoriasis. ACT1 is an essential adaptor molecule in the IL-17 signaling pathway. A missense single nucleotide polymorphism (rs33980500; SNP-D10N) that resulted in the substitution of an asparagine for an aspartic acid at position 10 of ACT1 (ACT1-D10N) is associated with psoriasis susceptibility. Due to alternative splicing in humans, SNP-D10N encodes two mutated ACT1 proteins, ACT1-D10N and ACT1-D19N. Although both ACT1 isoforms are Hsp90 client proteins, the nine additional amino acids in ACT1-D19N provide an additional Hsp90 binding site that is absent in ACT1-D10N. Therefore, whereas ACT1-D10N is a dead protein that is unable to transduce IL-17 signals for gene expression, ACT1-D19N is fully responsive to IL-17. Intriguingly, the two ACT1 isoforms are differentially expressed in ACT1(D10N/D10N) fibroblasts and T cells. Fibroblasts express both isoforms equally, enabling ACT1-D19N to compensate for the loss of ACT1-D10N function. ACT1(D10N/D10N) T cells, however, express predominantly ACT1-D10N. Lacking this compensatory mechanism, ACT1(D10N/D10N) T cells behave like ACT1-deficient T cells, exhibiting a dysregulated and hyperactive Th17 phenotype with overproduction of IL-22 and IL-17. The hyperactive Th17 response combined with fully responsive fibroblasts likely synergized to contribute to psoriasis susceptibility in SNP-D10N patients.

An essential role of interleukin-17 receptor signaling in the development of autoimmune glomerulonephritis

In recent years, proinflammatory cytokines in the nephritic kidney appear to contribute to the pathogenesis of AGN. The complex inflammatory cytokine network that drives renal pathology is poorly understood. IL-17, the signature cytokine of Th17 cells, which promotes autoimmune pathology in a variety of settings, is beginning to be identified in acute and chronic kidney diseases as well. However, the role of IL-17-mediated renal damage in the nephritic kidney has not been elucidated. Here, with the use of a murine model of experimental AGN, we showed that IL-17RA signaling is critical for the development of renal pathology. Despite normal systemic autoantibody response and glomerular immune-complex deposition, IL-17RA(-/-) mice exhibit a diminished influx of inflammatory cells and kidney-specific expression of IL-17 target genes correlating with disease resistance in AGN. IL-17 enhanced the production of proinflammatory cytokines and chemokines from tECs. Finally, we were able to show that neutralization of IL-17A ameliorated renal pathology in WT mice following AGN. These results clearly demonstrated that IL-17RA signaling significantly contributes to renal tissue injury in experimental AGN and suggest that blocking IL-17RA may be a promising therapeutic strategy for the treatment of proliferative and crescentic glomerulonephritis.

Does antigen masking by ubiquitin chains protect from the development of autoimmune diseases?

Autoimmune diseases are characterized by the production of antibodies against self-antigens and generally arise from a failure of central or peripheral tolerance. However, these diseases may develop when newly appearing antigens are not recognized as self by the immune system. The mechanism by which some antigens are “invisible” to the immune system is not completely understood. Apoptotic and complement system defects or autophagy imbalance can generate this antigenic autoreactivity. Under particular circumstances, cellular debris containing autoreactive antigens can be recognized by innate immune receptors or other sensors and can eventually lead to autoimmunity. Ubiquitination may be one of the mechanisms protecting autoreactive antigens from the immune system that, if disrupted, can lead to autoimmunity. Ubiquitination is an essential post-translational modification used by cells to target proteins for degradation or to regulate other intracellular processes. The level of ubiquitination is regulated during T cell tolerance and apoptosis and E3 ligases have emerged as a crucial signaling pathway for the regulation of T cell tolerance toward self-antigens. I propose here that an unrecognized role of ubiquitin and ubiquitin-like proteins could be to render intracellular or foreign antigens (present in cellular debris resulting from apoptosis, complement system, or autophagy defects) invisible to the immune system in order to prevent the development of autoimmunity.