Overexpression of TLR7 promotes cell-intrinsic expansion and autoantibody production by transitional T1 B cells

Tolerogenic antigen-specific vaccine induces VISTA-enriched regulatory T cells and protects against arthritis in DRB1∗04:01 mice

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by joint inflammation, cartilage damage, and bone erosion. Despite improvements with the introduction of biological disease-modifying anti-rheumatic drugs (DMARDs), RA remains an incurable life-long disease. Advancements in peptide-based vaccination may open new avenues for treating autoimmune diseases, including RA, by inducing immune tolerance while maintaining normal immune function. We have already demonstrated the efficacy of a potent vaccine against RA, consisting of the mouse major histocompatibility complex class II (Aq) protein bound to the immunodominant type II collagen peptide COL2259-273, which needed to be galactosylated at position 264. To translate the vaccine to humans and to further enhance vaccine efficacy, we modified the glycine residue at position 265 and conjugated it with the human DRB1∗04:01 molecule. Remarkably, this modified vaccine (named DR4-AL179) provided robust effectiveness in suppressing arthritis in DRB1∗04:01-expressing mice without the need for galactosylation at position 264. DR4-AL179 vaccination induces tolerance involving multiple immunoregulatory pathways, including the activation of V-type immunoglobulin domain-containing suppressor of T cell activation (VISTA)-positive nonconventional regulatory T cells, which contribute to a potent suppressive response preventing arthritis development in mice. This modified RA vaccine offers a novel therapeutic potential for human autoimmune diseases.

Lymphopoiesis is attenuated upon hepatocyte-specific deletion of the cytochrome c oxidase assembly factor Sco1

Mutations that negatively impact mitochondrial function are highly prevalent in humans and lead to disorders with a wide spectrum of disease phenotypes, including deficiencies in immune cell development and/or function. Previous analyses of mice with a hepatocyte-specific cytochrome c oxidase (COX) deficiency revealed an unexpected peripheral blood leukopenia associated with splenic and thymic atrophy. Here, we use mice with a hepatocyte-specific deletion of the COX assembly factor Sco1 to show that metabolic defects extrinsic to the hematopoietic compartment lead to a pan-lymphopenia represented by severe losses in both B and T cells. We further demonstrate that immune defects in these mice are associated with the loss of bone marrow lymphoid progenitors common to both lineages and early signs of autoantibody-mediated autoimmunity. Our findings collectively identify hepatocyte dysfunction as a potential instigator of immunodeficiency in patients with congenital mitochondrial defects who suffer from chronic or recurrent infections.

Sustained NPSLE-like phenotype in the absence of systemic lupus-like disease in TLR7-deficient B6.Nba2 mice

OBJECTIVE

To investigate the role of Toll-like Receptor 7 (TLR7) in the development of neuropsychiatric lupus (NPSLE) in the B6.Nba2 murine model of SLE.

METHODS

TLR7-deficient B6.Nba2 mice were evaluated for the development of NPSLE symptoms through behavioral testing with comparison groups of wild-type NPSLE-prone B6.Nba2 and B6 controls. Behavioral testing results were evaluated in the context of biomarker data, including flow cytometry for immune cell activation, and enzyme-linked immunosorbent assays (ELISA) to measure serum cytokine and autoantibody levels, including autoantibodies against double stranded DNA (dsDNA) and DWEYS peptide. Brain and spleen tissues waere harvested, and immuno histochemical studies and inflammatory gene activation obtained via qPCR were further analyzed to characterize immune system activation and SLE and NPSLE development in the mice.

RESULTS

TLR7-deficient mice exhibited reduced signs of systemic SLE, including decreased splenomegaly, anti-dsDNA titers, and immune cell activation compared to wild-type mice. However, TLR7-deficient mice displayed a similar behavioral pattern to the NPSLE-prone B6.Nba2 mice, indicating NPSLE development was not influenced by TLR7. Knockout of TLR7 in B6.Nba2 mice also led to increased expression of TLR4 and TLR9, which suggests a possible role for these receptors in NPSLE pathogenesis.

CONCLUSION

While systemic lupus-like disease in the B6.Nba2 mouse model is dependent on TLR7, NPSLE development is not and may be influenced by TLR4 and TLR9 signaling. Thus, there may be separate mechanisms driving peripheral SLE compared to NPSLE with possible implications for pharmacologic management.

The immunological processes behind aquaporin 4-antibody seropositive neuromyelitis optica spectrum disorders

Ever since the discovery of pathogenic aquaporin 4-specific antibodies in the serum of patients with neuromyelitis optica spectrum disorders current knowledge about clinical observations and diagnosis, and about the underlying pathology and resulting therapies have been put forward in excellent reviews and primary publications. However, in order to further develop novel strategies for the treatment of this disease, there is an urgent need to understand the immunological processes associated with the formation of the pathogenic antibodies, and with aberrant immune responses observed in affected patients. In this review, we will highlight and evaluate important studies on these processes.

Cellular and molecular determinants mediating the dysregulated germinal center immune dynamics in systemic lupus erythematosus

Introduction

Systemic lupus erythematosus (SLE) is characterized by dysregulated humoral immunity, leading to the generation of autoreactive B cells that can differentiate both within and outside of lymph node (LN) follicles.

Methods

Here, we employed spatial transcriptomics and multiplex imaging to investigate the follicular immune landscaping and the in situ transcriptomic profile in LNs from SLE individuals.

Results

Our spatial transcriptomic analysis revealed robust type I IFN and plasma cell signatures in SLE compared to reactive, control follicles. Cell deconvolution revealed that follicular T cell subsets are mainly affected by the type I IFN fingerprint of SLE follicles. Dysregulation of TFH differentiation was documented by i) the significant reduction of Bcl6hi TFH cells, ii) the reduced cell density of potential IL-4 producing TFH cell subsets associated with the impaired transcriptomic signature of follicular IL-4 signaling and iii) the loss of their correlation with GC-B cells. This profile was accompanied by a marked reduction of Bcl6hi B cells and an enrichment of extrafollicular CD19hiCD11chiTbethi, age-associated B cells (ABCs), known for their autoreactive potential. The increased prevalence of follicular IL-21hi cells further reveals a hyperactive microenvironment in SLE compared to control.

Discussion

Taken together, our findings highlight the altered immunological landscape of SLE follicles, likely fueled by potent inflammatory signals such as sustained type I IFN and/or IL-21 signaling. Our work provides novel insights into the spatial molecular and cellular signatures of SLE follicular B and TFH cell dynamics, and points to druggable targets to restore immune tolerance and enhance vaccine responses in SLE patients.

The role of gut microbiota in different murine models of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by immune system dysfunction that can lead to serious health issues and mortality. Recent investigations highlight the role of gut microbiota alterations in modulating inflammation and disease severity in SLE. This review specifically summaries the variations in gut microbiota composition across various murine models of lupus. By focusing on these differences, we aim to elucidate the intricate relationship between gut microbiota dysbiosis and the development and progression of SLE in preclinical settings.

Cutting Edge: Low-dose Recombinant IL-2 Treatment Prevents Autoantibody Responses in Systemic Lupus Erythematosus via Regulatory T Cell-independent Depletion of T Follicular Helper Cells

The expansion of T follicular helper (Tfh) cells correlates with disease progression in human and murine systemic lupus erythematosus (SLE). Unfortunately, there are no therapies to deplete Tfh cells. Importantly, low-dose rIL-2-based immunotherapy shows potent immunosuppressive effects in SLE patients and lupus-prone mice, primarily attributed to the expansion of regulatory T cells (Tregs). However, IL-2 can also inhibit Tfh cell differentiation. In this study, we investigate the potential of low-dose rIL-2 to deplete Tfh cells and prevent autoantibody responses in SLE. Our data demonstrate that low-dose rIL-2 efficiently depletes autoreactive Tfh cells and prevents autoantibody responses in lupus-prone mice. Importantly, this immunosuppressive effect was independent of the presence of Tregs. The therapeutic potential of eliminating Tfh cells was confirmed by selectively deleting Tfh cells in lupus-prone mice. Our findings demonstrate the critical role of Tfh cells in promoting autoantibody responses and unveil, (to our knowledge), a novel Treg-independent immunosuppressive function of IL-2 in SLE.

Assessment of the Impact of RNase in Patients With Severe Fatigue Related to Post-Acute Sequelae of SARS-CoV-2 Infection: A Randomized Phase 2 Trial of RSLV-132

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA and RNA debris persist in viral reservoirs for weeks to months following infection, potentially triggering interferon production and chronic inflammation. RSLV-132 is a biologic drug composed of catalytically active human RNase1 fused to human IgG1 Fc and is designed to remain in circulation and digest extracellular RNA. We hypothesized that removal of SARS-CoV-2 viral RNA from latent reservoirs may improve inflammation, neuroinflammation, and fatigue associated with post-acute sequelae of SARS-CoV-2 infection (PASC).

METHODS

This was a phase 2, double-blind, placebo-controlled randomized clinical trial in participants with a 24-week history of PASC and severe fatigue. The primary endpoint of the trial assessed the impact of 6 intravenous doses of RSLV-132 on the mean change from baseline at day 71 in the Patient-Reported Outcomes Measurement Information System Fatigue Short Form 7a (PROMIS Fatigue SF 7a).

RESULTS

A statistically significant difference on day 71 was not observed with respect to the primary or secondary endpoints. This was likely due to a placebo response that increased during the trial. Statistically significant improvement in fatigue as measured by the PROMIS Fatigue SF 7a, Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-Fatigue), and Physicians Global Assessment (PGA) instruments were observed earlier in the trial, with women demonstrating greater responses to RSLV-132 than men.

CONCLUSION

While fatigue was not statistically significantly improved at Day 71, earlier timepoints revealed statistically significant improvement in fatigue and physician global assessment. The data suggest eliminating latent viral RNA by increasing serum RNase activity may improve fatigue in PASC patients. Women may respond better to this approach than men. Future studies will aim to confirm these findings.

Pharmacological activation of TLR7 exerts inhibition on the replication of EV-D68 in respiratory cells

The globally reemerging respiratory pathogen enterovirus D68 (EV-D68) is implicated in outbreaks of severe respiratory illness and associated with acute flaccid myelitis. However, there remains a lack of effective treatments for EV-D68 infection. In this work, we found that the host Toll-like receptor 7 (TLR7) proteins, which function as powerful innate immune sensors, were selectively elevated in expression in response to EV-D68 infection. Subsequently, we investigated the impact of Vesatolimod (GS-9620), a Toll-like receptor 7 agonist, on EV-D68 replication. Our findings revealed that EV-D68 infection resulted in increased mRNA levels of TLR7. Treatment with Vesatolimod significantly inhibited EV-D68 replication [half maximal effective concentration (EC50) = 0.1427 µM] without inducing significant cytotoxicity at virucidal concentrations. Although Vesatolimod exhibited limited impact on EV-D68 attachment, it suppressed RNA replication and viral protein synthesis after virus entry. Vesatolimod broadly inhibited the replication of circulating isolated strains of EV-D68. Furthermore, our findings demonstrated that treatment with Vesatolimod conferred resistance to both respiratory and neural cells against EV-D68 infection. Overall, these results present a promising strategy for drug development by pharmacologically activating TLR7 to initiate an antiviral state in EV-D68-infected cells selectively.IMPORTANCEConventional strategies for antiviral drug development primarily focus on directly targeting viral proteases or key components, as well as host proteins involved in viral replication. In this study, based on our intriguing discovery that enterovirus D68 (EV-D68) infection specifically upregulates the expression of immune sensor Toll-like receptor 7 (TLR7) protein, which is either absent or expressed at low levels in respiratory cells, we propose a potential antiviral approach utilizing TLR7 agonists to activate EV-D68-infected cells into an anti-viral defense state. Notably, our findings demonstrate that pharmacological activation of TLR7 effectively suppresses EV-D68 replication in respiratory tract cells through a TLR7/MyD88-dependent mechanism. This study not only presents a promising drug candidate and target against EV-D68 dissemination but also highlights the potential to exploit unique alterations in cellular innate immune responses induced by viral infections, selectively inducing a defensive state in infected cells while safeguarding uninfected normal cells from potential adverse effects associated with therapeutic interventions.

Integrin αvβ3 Limits Cytokine Production by Plasmacytoid Dendritic Cells and Restricts TLR-Driven Autoimmunity

Plasmacytoid dendritic cells (pDCs) are strongly implicated as a major source of IFN-I in systemic lupus erythematosus (SLE), triggered through TLR-mediated recognition of nucleic acids released from dying cells. However, relatively little is known about how TLR signaling and IFN-I production are regulated in pDCs. In this article, we describe a role for integrin αvβ3 in regulating TLR responses and IFN-I production by pDCs in mouse models. We show that αv and β3-knockout pDCs produce more IFN-I and inflammatory cytokines than controls when stimulated through TLR7 and TLR9 in vitro and in vivo. Increased cytokine production was associated with delayed acidification of endosomes containing TLR ligands, reduced LC3 conjugation, and increased TLR signaling. This dysregulated TLR signaling results in activation of B cells and promotes germinal center (GC) B cell and plasma cell expansion. Furthermore, in a mouse model of TLR7-driven lupus-like disease, deletion of αvβ3 from pDCs causes accelerated autoantibody production and pathology. We therefore identify a pDC-intrinsic role for αvβ3 in regulating TLR signaling and preventing activation of autoreactive B cells. Because αvβ3 serves as a receptor for apoptotic cells and cell debris, we hypothesize that this regulatory mechanism provides important contextual cues to pDCs and functions to limit responses to self-derived nucleic acids.

Genetic interrogation for sequence and copy number variants in systemic lupus erythematosus

Early-onset systemic lupus erythematosus presents with a more severe disease and is associated with a greater genetic burden, especially in patients from Black, Asian or Hispanic ancestries. Next-generation sequencing techniques, notably whole exome sequencing, have been extensively used in genomic interrogation studies to identify causal disease variants that are increasingly implicated in the development of autoimmunity. This Review discusses the known casual variants of polygenic and monogenic systemic lupus erythematosus and its implications under certain genetic disparities while suggesting an age-based sequencing strategy to aid in clinical diagnostics and patient management for improved patient care.

Heightened TLR7 signaling primes BCR-activated B cells in chronic graft-versus-host disease for effector functions

ABSTRACT

Chronic graft-versus-host disease (cGVHD) is a debilitating, autoimmune-like syndrome that can occur after allogeneic hematopoietic stem cell transplantation. Constitutively activated B cells contribute to ongoing alloreactivity and autoreactivity in patients with cGVHD. Excessive tissue damage that occurs after transplantation exposes B cells to nucleic acids in the extracellular environment. Recognition of endogenous nucleic acids within B cells can promote pathogenic B-cell activation. Therefore, we hypothesized that cGVHD B cells aberrantly signal through RNA and DNA sensors such as Toll-like receptor 7 (TLR7) and TLR9. We found that B cells from patients and mice with cGVHD had higher expression of TLR7 than non-cGVHD B cells. Using ex vivo assays, we found that B cells from patients with cGVHD also demonstrated increased interleukin-6 production after TLR7 stimulation with R848. Low-dose B-cell receptor (BCR) stimulation augmented B-cell responses to TLR7 activation. TLR7 hyperresponsiveness in cGVHD B cells correlated with increased expression and activation of the downstream transcription factor interferon regulatory factor 5. Because RNA-containing immune complexes can activate B cells through TLR7, we used a protein microarray to identify RNA-containing antigen targets of potential pathological relevance in cGVHD. We found that many of the unique targets of active cGVHD immunoglobulin G (IgG) were nucleic acid-binding proteins. This unbiased assay identified the autoantigen and known cGVHD target Ro-52, and we found that RNA was required for IgG binding to Ro-52. Herein, we find that BCR-activated B cells have aberrant TLR7 signaling responses that promote potential effector responses in cGVHD.

Evaluation of RNase therapy in systemic lupus erythematosus: a randomised phase 2a clinical trial of RSLV-132

BACKGROUND

Circulating, extracellular RNA is the primary trigger of type I interferon in systemic lupus erythematosus (SLE), and interferon is known to play a central pathogenic role in the disease. RSLV-132 is a catalytically active human RNase molecule fused to human IgG1 Fc designed to digest RNA and thereby decrease the chronic inflammation associated with SLE. The drug was evaluated in a cohort of patients with SLE with moderate-severe cutaneous disease activity and the presence of RNA immune complexes. The primary objective of the study was the assessment of the impact of 13 doses of 10 mg/kg RSLV-132 over 6 months on the mean Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI) score.

METHODS

Sixty-five patients meeting the entry criteria of a baseline CLASI score of 10 or greater and positivity of at least one of five autoantibodies to RNA-binding proteins (SM/RNP, SSA/Ro, SSB/La, Sm, RNP) were randomly assigned (2:1) to receive 13 doses of RSLV-132 10 mg/kg or placebo, respectively. Participants received study drug for 24 weeks on days 1, 8, 15, 29, 43, 57, 71, 85, 99, 113, 127, 141 and 155 with an end-of-treatment visit on day 169 and a follow-up visit at the end of the study on day 215. The primary objective was assessed on days 85 and 169. Secondary objectives included assessment of systemic disease activity using the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K), the British Isles Lupus Assessment Group 2004 Index and the Physician's Global Assessment. Data from these instruments were used to calculate the SLE Responder Index 4 (SRI-4) and the British Isles Lupus Assessment Group-based Composite Lupus Assessment (BICLA) scores.

RESULTS

The mean CLASI score change from baseline at day 169 was -5.7 (±7.0) in the placebo group and -6.2 (±8.5) in the RSLV-132 group. A subgroup of participants with moderate-severe systemic disease activity and high baseline SLEDAI scores (≥9) were analysed with respect to BICLA and SRI-4 responses. The RSLV-132 treated participants in the high SLEDAI subgroup had a greater percentage of BICLA responses (62% vs 44%) and SRI-4 responses (23% vs 11%) as compared with placebo. A second subgroup of participants with high baseline CLASI scores (≥21) were analysed with respect to BICLA and SRI-4 responses. The RSLV-132 treated participants in the high CLASI subgroup had a greater percentage of BICLA responses (28% vs 8%) and SRI-4 responses (39% vs 8%) as compared with placebo.

CONCLUSIONS

Six months of RSLV-132 therapy consisting of a weekly loading dose of RSLV-132 for 1 month, followed by 5 months of biweekly administrations did not significantly improve the mean CLASI score relative to placebo in this cohort of patients with SLE. The study entry criteria selected patients with moderate-severe cutaneous disease activity and no minimum SLEDAI score, which resulted in a wide range of systemic disease activity from inactive to severe as measured by SLEDAI. When the participants with higher SLEDAI and CLASI scores were analysed, a trend towards clinical improvement favouring RSLV-132 was observed. The results warrant further evaluation of RSLV-132 in SLE and suggest that patients with more active systemic disease are most likely to benefit from RNase therapy.

Lupus-associated innate receptors drive extrafollicular evolution of autoreactive B cells

In systemic lupus erythematosus, recent findings highlight the extrafollicular (EF) pathway as prominent origin of autoantibody-secreting cells (ASCs). CD21loCD11c+ B cells, associated with aging, infection, and autoimmunity, are contributors to autoreactive EF ASCs but have an obscure developmental trajectory. To study EF kinetics of autoreactive B cell in tissue, we adoptively transferred WT and gene knockout B cell populations into the 564Igi mice - an autoreactive host enriched with autoantigens and T cell help. Time-stamped analyses revealed TLR7 dependence in early escape of peripheral B cell tolerance and establishment of a pre-ASC division program. We propose CD21lo cells as precursors to EF ASCs due to their elevated TLR7 sensitivity and proliferative nature. Blocking receptor function reversed CD21 loss and reduced effector cell generation, portraying CD21 as a differentiation initiator and a possible target for autoreactive B cell suppression. Repertoire analysis further delineated proto-autoreactive B cell selection and receptor evolution toward self-reactivity. This work elucidates receptor and clonal dynamics in EF development of autoreactive B cells, and establishes modular, native systems to probe mechanisms of autoreactivity.

Genetics of SLE: mechanistic insights from monogenic disease and disease-associated variants

The past few years have provided important insights into the genetic architecture of systemic autoimmunity through aggregation of findings from genome-wide association studies (GWAS) and whole-exome or whole-genome sequencing studies. In the prototypic systemic autoimmune disease systemic lupus erythematosus (SLE), monogenic disease accounts for a small fraction of cases but has been instrumental in the elucidation of disease mechanisms. Defects in the clearance or digestion of extracellular or intracellular DNA or RNA lead to increased sensing of nucleic acids, which can break B cell tolerance and induce the production of type I interferons leading to tissue damage. Current data suggest that multiple GWAS SLE risk alleles act in concert with rare functional variants to promote SLE development. Moreover, introduction of orthologous variant alleles into mice has revealed that pathogenic X-linked dominant and recessive SLE can be caused by novel variants in TLR7 and SAT1, respectively. Such bespoke models of disease help to unravel pathogenic pathways and can be used to test targeted therapies. Cell type-specific expression data revealed that most GWAS SLE risk genes are highly expressed in age-associated B cells (ABCs), which supports the view that ABCs produce lupus autoantibodies and contribute to end-organ damage by persisting in inflamed tissues, including the kidneys. ABCs have thus emerged as key targets of promising precision therapeutics.

Impacts of plasma microbial lipopolysaccharide translocation on B cell perturbations and anti-CD4 autoantibody production in people with HIV on suppressive antiretroviral therapy

BACKGROUND

. Up to 20% of people with HIV (PWH) who undergo virologically suppressed antiretroviral therapy (ART) fail to experience complete immune restoration. We recently reported that plasma anti-CD4 IgG (antiCD4IgG) autoantibodies from immune non-responders specifically deplete CD4 + T cells via antibody-dependent cytotoxicity. However, the mechanism of antiCD4IgG production remains unclear.

METHODS

. Blood samples were collected from 16 healthy individuals and 25 PWH on suppressive ART. IgG subclass, plasma lipopolysaccharide (LPS), and antiCD4IgG levels were measured by ELISA. Gene profiles in B cells were analyzed by microarray and quantitative PCR. Furthermore, a patient-derived antiCD4IgG-producing B cell line was generated and stimulated with LPS in vitro. B cell IgG class switch recombination (CSR) was evaluated in response to LPS in splenic B cells from C57/B6 mice in vitro.

RESULTS

. Increased plasma anti-CD4 IgGs in PWH were predominantly IgG1 and associated with increased plasma LPS levels as well as B cell expression of TLR2, TLR4, and MyD88 mRNA in vivo. Furthermore, LPS stimulation induced antiCD4IgG production in the antiCD4IgG B cell line in vitro. Finally, LPS promoted CSR in vitro.

CONCLUSION

. Our findings suggest that persistent LPS translocation may promote anti-CD4 autoreactive B cell activation and antiCD4IgG production in PWH on ART, which may contribute to gradual CD4 + T cell depletion. This study suggests that reversing a compromised mucosal barrier could improve ART outcomes in PWH who fail to experience complete immune restoration.

B cell-activating factor (BAFF) from dendritic cells, monocytes and neutrophils is required for B cell maturation and autoantibody production in SLE-like autoimmune disease

Purpose and methods

B cell-activating factor (BAFF) contributes to the pathogenesis of autoimmune diseases including systemic lupus erythematosus (SLE). Although several anti-BAFF Abs and derivatives have been developed for the treatment of SLE, the specific sources of BAFF that sustain autoantibody (auto-Ab) producing cells have not been definitively identified. Using BAFF-RFP reporter mice, we identified major changes in BAFF-producing cells in two mouse spontaneous lupus models (Tlr7 Tg mice and Sle1), and in a pristane-induced lupus (PIL) model.

Results

First, we confirmed that similar to their wildtype Tlr7 Tg and Sle1 mice counterparts, BAFF-RFP Tlr7 Tg mice and BAFF-RFP Sle1 mice had increased BAFF serum levels, which correlated with increases in plasma cells and auto-Ab production. Next, using the RFP reporter, we defined which cells had dysregulated BAFF production. BAFF-producing neutrophils (Nphs), monocytes (MOs), cDCs, T cells and B cells were all expanded in the spleens of BAFF-RFP Tlr7 Tg mice and BAFF-RFP Sle1 mice compared to controls. Furthermore, Ly6C^hi inflammatory MOs and T cells had significantly increased BAFF expression per cell in both spontaneous lupus models, while CD8^- DCs up-regulated BAFF expression only in the Tlr7 Tg mice. Similarly, pristane injection of BAFF-RFP mice induced increases in serum BAFF levels, auto-Abs, and the expansion of BAFF-producing Nphs, MOs, and DCs in both the spleen and peritoneal cavity. BAFF expression in MOs and DCs, in contrast to BAFF from Nphs, was required to maintain homeostatic and pristane-induced systemic BAFF levels and to sustain mature B cell pools in spleens and BMs. Although acting through different mechanisms, Nph, MO and DC sources of BAFF were each required for the development of auto-Abs in PIL mice.

Conclusions

Our findings underscore the importance of considering the relative roles of specific myeloid BAFF sources and B cell niches when developing treatments for SLE and other BAFF-associated autoimmune diseases.

Toll-like receptors 7 and 9 regulate the proliferation and differentiation of B cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune illness marked by the loss of immune tolerance and the production of autoantibodies against nucleic acids and other nuclear antigens (Ags). B lymphocytes are important in the immunopathogenesis of SLE. Multiple receptors control abnormal B-cell activation in SLE patients, including intrinsic Toll-like receptors (TLRs), B-cell receptors (BCRs), and cytokine receptors. The role of TLRs, notably TLR7 and TLR9, in the pathophysiology of SLE has been extensively explored in recent years. When endogenous or exogenous nucleic acid ligands are recognized by BCRs and internalized into B cells, they bind TLR7 or TLR9 to activate related signalling pathways and thus govern the proliferation and differentiation of B cells. Surprisingly, TLR7 and TLR9 appear to play opposing roles in SLE B cells, and the interaction between them is still poorly understood. In addition, other cells can enhance TLR signalling in B cells of SLE patients by releasing cytokines that accelerate the differentiation of B cells into plasma cells. Therefore, the delineation of how TLR7 and TLR9 regulate the abnormal activation of B cells in SLE may aid the understanding of the mechanisms of SLE and provide directions for TLR-targeted therapies for SLE.

COVID-19 is associated with bystander polyclonal autoreactive B cell activation as reflected by a broad autoantibody production, but none is linked to disease severity

Coronavirus disease 2019 (COVID-19) is associated with autoimmune features and autoantibody production in a small subset of the population. Pre-existing neutralizing antitype I interferons (IFNs) autoantibodies are related to the severity of COVID-19. Plasma levels of IgG and IgM against 12 viral antigens and 103 self-antigens were evaluated using an antibody protein array in patients with severe/critical or mild/moderate COVID-19 disease and uninfected controls. Patients exhibited increased IgGs against Severe acute respiratory syndrome coronavirus-2 proteins compared to controls, but no difference was observed in the two patient groups. 78% autoreactive IgGs and 93% autoreactive IgMs were increased in patients versus controls. There was no difference in the plasma levels of anti-type I IFN autoantibodies or neutralizing anti-type I IFN activity of plasma samples from the two patient groups. Increased anti-type I IFN IgGs were correlated with higher lymphocyte accounts, suggesting a role of nonpathogenic autoantibodies. Notably, among the 115 antibodies tested, only plasma levels of IgGs against human coronavirus (HCOV)-229E and HCOV-NL63 spike proteins were associated with mild disease outcome. COVID-19 was associated with a bystander polyclonal autoreactive B cell activation, but none of the autoantibody levels were linked to disease severity. Long-term humoral immunity against HCOV-22E and HCOV-NL63 spike protein was associated with mild disease outcome. Understanding the mechanism of life-threatening COVID-19 is critical to reducing mortality and morbidity.

Partial RAG deficiency in humans induces dysregulated peripheral lymphocyte development and humoral tolerance defect with accumulation of T-bet+ B cells

The recombination-activating genes (RAG) 1 and 2 are indispensable for diversifying the primary B cell receptor repertoire and pruning self-reactive clones via receptor editing in the bone marrow; however, the impact of RAG1/RAG2 on peripheral tolerance is unknown. Partial RAG deficiency (pRD) manifesting with late-onset immune dysregulation represents an ‘experiment of nature’ to explore this conundrum. By studying B cell development and subset-specific repertoires in pRD, we demonstrate that reduced RAG activity impinges on peripheral tolerance through the generation of a restricted primary B cell repertoire, persistent antigenic stimulation and an inflammatory milieu with elevated B cell-activating factor. This unique environment gradually provokes profound B cell dysregulation with widespread activation, remarkable extrafollicular maturation and persistence, expansion and somatic diversification of self-reactive clones. Through the model of pRD, we reveal a RAG-dependent ‘domino effect’ that impacts stringency of tolerance and B cell fate in the periphery.

Patients with partial recombination-activating gene (RAG) deficiency (pRD) present variable late-onset autoimmune clinical phenotypes. Walter and colleagues identified a restricted primary B cell antigen receptor repertoire enriched for autoreactivity and clonal persistence in pRD. They described dysregulated B cell maturation with expansion of T-bet⁺ B cells revealing how RAG impacts stringency of tolerance and B cell fate in the periphery.

The NCF1 variant aggravates autoimmunity by facilitating the activation of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are a professional type I IFN producer that play critical roles in the pathogenesis of autoimmune diseases. However, both genetic regulation of the function of pDCs and their relationships with autoimmunity are largely undetermined. Here, we investigated the causality of the neutrophil cytosolic factor 1 (NCF1) missense variant, which is one of the most significant associated risk variants for lupus, and found that the substitution of arginine (R) for histidine (H) at position 90 in the NCF1 protein (NCF1 p.R90H) led to excessive activation of pDCs. A mechanism study demonstrated that p.R90H reduced the affinity of NCF1 for phospholipids, thereby impairing endosomal localization of NCF1. As NCF1 is a subunit of the NADPH oxidase 2 (NOX2) complex, this impairment led to an acidified endosomal pH and facilitated downstream TLR signaling. Consistently, the homozygous knockin mice manifested aggravated lupus progression in a pDC-dependent lupus model. More important, pharmaceutical intervention revealed that hydroxychloroquine (HCQ) could antagonize the detrimental function of NCF1 p.R90H in the lupus model and systemic lupus erythematosus samples, supporting the idea that NCF1 p.R90H could be identified as a genetic biomarker for HCQ application. Therefore, our study provides insights into the genetic control of pDC function and a paradigm for applying genetic variants to improve targeted therapy for autoimmune diseases.

Alphavirus Replicon Particle Vaccine Breaks B Cell Tolerance and Rapidly Induces IgG to Murine Hematolymphoid Tumor Associated Antigens

De novo immune responses to myeloid and other blood-borne tumors are notably limited and ineffective, making our ability to promote immune responses with vaccines a major challenge. While focus has been largely on cytotoxic cell-mediated tumor eradication, B-cells and the antibodies they produce also have roles in anti-tumor responses. Indeed, therapeutic antibody-mediated tumor cell killing is routinely employed in patients with hematolymphoid cancers, but whether endogenous antibody responses can be incited to blood-born tumors remains poorly studied. A major limitation of immunoglobulin therapies is that cell surface expression of tumor-associated antigen (TAA) targets is dynamic and varied, making promotion of polyclonal, endogenous B cell responses appealing. Since many TAAs are self-antigens, developing tumor vaccines that enable production of antibodies to non-polymorphic antigen targets remains a challenge. As B cell responses to RNA vaccines are known to occur, we employed the Viral Replicon Particles (VRP) which was constructed to encode mouse FLT3. The VRP-FLT3 vaccine provoked a rapid IgG B-cell response to this self-antigen in leukemia and lymphoma mouse models. In addition, IgGs to other TAAs were also produced. Our data suggest that vaccination with RNA viral particle vectors incites a loss of B-cell tolerance that enables production of anti-tumor antibodies. This proof of principle work provides impetus to employ such strategies that lead to a break in B-cell tolerance and enable production of broadly reactive anti-TAA antibodies as potential future therapeutic agents for patients with hematolymphoid cancers.

TLR7 gain-of-function genetic variation causes human lupus

Although circumstantial evidence supports enhanced Toll-like receptor 7 (TLR7) signalling as a mechanism of human systemic autoimmune disease1-7, evidence of lupus-causing TLR7 gene variants is lacking. Here we describe human systemic lupus erythematosus caused by a TLR7 gain-of-function variant. TLR7 is a sensor of viral RNA8,9 and binds to guanosine10-12. We identified a de novo, previously undescribed missense TLR7Y264H variant in a child with severe lupus and additional variants in other patients with lupus. The TLR7Y264H variant selectively increased sensing of guanosine and 2',3'-cGMP10-12, and was sufficient to cause lupus when introduced into mice. We show that enhanced TLR7 signalling drives aberrant survival of B cell receptor (BCR)-activated B cells, and in a cell-intrinsic manner, accumulation of CD11c+ age-associated B cells and germinal centre B cells. Follicular and extrafollicular helper T cells were also increased but these phenotypes were cell-extrinsic. Deficiency of MyD88 (an adaptor protein downstream of TLR7) rescued autoimmunity, aberrant B cell survival, and all cellular and serological phenotypes. Despite prominent spontaneous germinal-centre formation in Tlr7Y264H mice, autoimmunity was not ameliorated by germinal-centre deficiency, suggesting an extrafollicular origin of pathogenic B cells. We establish the importance of TLR7 and guanosine-containing self-ligands for human lupus pathogenesis, which paves the way for therapeutic TLR7 or MyD88 inhibition.

Defective Early B Cell Tolerance Checkpoints in Patients With Systemic Sclerosis Allow the Production of Self Antigen-Specific Clones

OBJECTIVE

Early selection steps preventing autoreactive naive B cell production are often impaired in patients with autoimmune diseases, but central and peripheral B cell tolerance checkpoints have not been assessed in patients with systemic sclerosis (SSc). This study was undertaken to characterize early B cell tolerance checkpoints in patients with SSc.

METHODS

Using an in vitro polymerase chain reaction-based approach that allows the expression of recombinant antibodies cloned from single B cells, we tested the reactivity of antibodies expressed by 212 CD19+CD21low CD10+IgMhigh CD27- new emigrant/transitional B cells and 190 CD19+CD21+CD10-IgM+CD27- mature naive B cells from 10 patients with SSc.

RESULTS

Compared to serum from healthy donors, serum from patients with SSc displayed elevated proportions of polyreactive and antinuclear-reactive new emigrant/transitional B cells that recognize topoisomerase I, suggesting that defective central B cell tolerance contributes to the production of serum autoantibodies characteristic of the disease. Frequencies of autoreactive mature naive B cells were also significantly increased in SSc patients compared to healthy donors, thus indicating that a peripheral B cell tolerance checkpoint may be impaired in SSc.

CONCLUSION

Defective counterselection of developing autoreactive naive B cells in SSc leads to the production of self antigen-specific B cells that may secrete autoantibodies and allow the formation of immune complexes, which promote fibrosis in SSc.

TLR7 Signaling in Lupus B Cells: New Insights into Synergizing Factors and Downstream Signals

PURPOSE OF THE REVIEW

Systemic lupus erythematosus (SLE) is driven by nucleic acid-containing antigens that stimulate endosomal TLRs. We review new advances in our understanding of how TLR7 signaling in B cells drives autoimmunity.

RECENT FINDINGS

Pathogenic B cell responses to TLR7 engagement are shaped by the disease-associated cytokine environment. TLR7, IFNγ, and IL-21 together promote the formation of autoreactive germinal centers and the ABC/DN2 B cell subset. BAFF and type 1 IFNs enhance autoantibody production from transitional B cells in concert with TLR7. TLR7 signaling components STAT1, BANK1, IRF5, SLC15A4, and CXorf21/TASL are associated genetically with SLE and important for lupus development in mice, while role of T-bet is controversial. Proper control of TLR7 trafficking by UNC93B1, syntenin-1, and αvβ3 integrin is critical for preventing autoimmunity. A better understanding of TLR7 signaling has revealed potential new therapeutic approaches for SLE, several of which are being tested in animal models or clinical trials.

Immunogenicity of In Vitro-Transcribed RNA

In vitro-transcribed RNAs are emerging as new biologics for therapeutic innovation, as exemplified by their application recently in SARS-CoV-2 vaccinations. RNAs prepared by in vitro transcription (IVT) allow transient expression of proteins of interest, conferring safety over DNA- or virus-mediated gene delivery systems. However, in vitro-transcribed RNAs should be used with caution because of their immunogenicity, which is in part triggered by double-stranded RNA (dsRNA) byproducts during IVT. Cellular innate immune response to dsRNA byproducts can lead to undesirable consequences, including suppression of protein synthesis and cell death, which in turn can detrimentally impact the efficacy of mRNA therapy. Thus, it is critical to understand the nature of IVT byproducts and the mechanisms by which they trigger innate immune responses.Our lab has been investigating the mechanisms by which the innate immune system discriminates between "self" and "nonself" RNA, with the focus on the cytoplasmic dsRNA receptors retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated 5 (MDA5). We have biochemically and structurally characterized critical events involving RNA discrimination and signal transduction by RIG-I or MDA5. We have used in vitro-transcribed RNAs as tools to investigate RNA specificity of RIG-I and MDA5, which required optimization of the IVT reaction and purification processes to eliminate the effect of IVT byproducts. In this Account, we summarize our current understanding of RIG-I and MDA5 and IVT reactions and propose future directions for improving IVT as a method to generate both research tools and therapeutics. Other critical proteins in cellular innate immune response to dsRNAs are also discussed. We arrange the contents in the following order: (i) innate immunity sensors for nonself RNA, including the RIG-I-like receptors (RLRs) in the cytosol and the toll-like receptors (TLRs) in the endosome, as well as cytoplasmic dsRNA-responding proteins, including protein kinase R (PKR) and 2',5'-oligoadenylate synthetases (OASes), illustrating the feature of protein-RNA binding and its consequences; (ii) the immunogenicity of IVT byproducts, specifically the generation of dsRNA molecules during IVT; and (iii) methods to reduce IVT RNA immunogenicity, including optimizations of RNA polymerases, reagents, and experimental conditions during IVT and subsequent purification.

Regulation of B Cell Responses in SLE by Three Classes of Interferons

There are three classes of interferons (type 1, 2, and 3) that can contribute to the development and maintenance of various autoimmune diseases, including systemic lupus erythematosus (SLE). Each class of interferons promotes the generation of autoreactive B cells and SLE-associated autoantibodies by distinct signaling mechanisms. SLE patients treated with various type 1 interferon-blocking biologics have diverse outcomes, suggesting that additional environmental and genetic factors may dictate how these cytokines contribute to the development of autoreactive B cells and SLE. Understanding how each class of interferons controls B cell responses in SLE is necessary for developing optimized B cell- and interferon-targeted therapeutics. In this review, we will discuss how each class of interferons differentially promotes the loss of peripheral B cell tolerance and leads to the development of autoreactive B cells, autoantibodies, and SLE.

X-linked recessive TLR7 deficiency in ~1% of men under 60 years old with life-threatening COVID-19

Autosomal inborn errors of type I IFN immunity and autoantibodies against these cytokines underlie at least 10% of critical COVID-19 pneumonia cases. We report very rare, biochemically deleterious X-linked TLR7 variants in 16 unrelated male individuals aged 7 to 71 years (mean: 36.7 years) from a cohort of 1,202 male patients aged 0.5 to 99 years (mean: 52.9 years) with unexplained critical COVID-19 pneumonia. None of the 331 asymptomatically or mildly infected male individuals aged 1.3 to 102 years (mean: 38.7 years) tested carry such TLR7 variants (p = 3.5 × 10-5). The phenotypes of five hemizygous relatives of index cases infected with SARS-CoV-2 include asymptomatic or mild infection (n=2, 5 and 38 years), or moderate (n=1, 5 years), severe (n=1, 27 years), or critical (n=1, 29 years) pneumonia. Two boys (aged 7 and 12 years) from a cohort of 262 male patients with severe COVID-19 pneumonia (mean: 51.0 years) are hemizygous for a deleterious TLR7 variant. The cumulative allele frequency for deleterious TLR7 variants in the male general population is < 6.5x10-4 We also show that blood B cell lines and myeloid cell subsets from the patients do not respond to TLR7 stimulation, a phenotype rescued by wild-type TLR7 The patients' blood plasmacytoid dendritic cells (pDCs) produce low levels of type I IFNs in response to SARS-CoV-2. Overall, X-linked recessive TLR7 deficiency is a highly penetrant genetic etiology of critical COVID-19 pneumonia, in about 1.8% of male patients below the age of 60 years. Human TLR7 and pDCs are essential for protective type I IFN immunity against SARS-CoV-2 in the respiratory tract.

Context-Dependent miR-21 Regulation of TLR7-Mediated Autoimmune and Foreign Antigen-Driven Antibody-Forming Cell and Germinal Center Responses

MicroRNAs (miRNAs) are involved in healthy B cell responses and the loss of tolerance in systemic lupus erythematosus (SLE), although the role of many miRNAs remains poorly understood. Dampening miR-21 activity was previously shown to reduce splenomegaly and blood urea nitrogen levels in SLE-prone mice, but the detailed cellular responses and mechanism of action remains unexplored. In this study, using the TLR7 agonist, imiquimod-induced SLE model, we observed that loss of miR-21 in Sle1b mice prevented the formation of plasma cells and autoantibody-producing Ab-forming cells (AFCs) without a significant effect on the magnitude of the germinal center (GC) response. We further observed reduced dendritic cell and monocyte numbers in the spleens of miR-21-deficient Sle1b mice that were associated with reduced IFN, proinflammatory cytokines, and effector CD4⁺ T cell responses. RNA sequencing analysis on B cells from miR-21-deficient Sle1b mice revealed reduced activation and response to IFN, and cytokine and target array analysis revealed modulation of numerous miR-21 target genes in response to TLR7 activation and type I IFN stimulation. Our findings in the B6.Sle1bYaa (Sle1b ^Yaa) spontaneous model recapitulated the miR-21 role in TLR7-induced responses with an additional role in autoimmune GC and T follicular helper responses. Finally, immunization with T-dependent Ag revealed a role for miR-21 in foreign Ag-driven GC and Ab, but not AFC, responses. Our data suggest a potential multifaceted, context-dependent role for miR-21 in autoimmune and foreign Ag-driven AFC and GC responses. Further study is warranted to delineate the cell-intrinsic requirements and mechanisms of miR-21 during infection and SLE development.

miRNA-Mediated Control of B Cell Responses in Immunity and SLE

Loss of B cell tolerance is central to autoimmune diseases such as systemic lupus erythematosus (SLE). As such, the mechanisms involved in B cell development, maturation, activation, and function that are aberrantly regulated in SLE are of interest in the design of targeted therapeutics. While many factors are involved in the generation and regulation of B cell responses, miRNAs have emerged as critical regulators of these responses within the last decade. To date, miRNA involvement in B cell responses has largely been studied in non-autoimmune, immunization-based systems. However, miRNA profiles have also been strongly associated with SLE in human patients and these molecules have proven critical in both the promotion and regulation of disease in mouse models and in the formation of autoreactive B cell responses. Functionally, miRNAs are small non-coding RNAs that bind to complementary sequences located in target mRNA transcripts to mediate transcript degradation or translational repression, invoking a post-transcriptional level of genetic regulation. Due to their capacity to target a diverse range of transcripts and pathways in different immune cell types and throughout the various stages of development and response, targeting miRNAs is an interesting potential therapeutic avenue. Herein, we focus on what is currently known about miRNA function in both normal and SLE B cell responses, primarily highlighting miRNAs with confirmed functions in mouse models. We also discuss areas that should be addressed in future studies and whether the development of miRNA-centric therapeutics may be a viable alternative for the treatment of SLE.

AID in Chronic Lymphocytic Leukemia: Induction and Action During Disease Progression

The enzyme activation-induced cytidine deaminase (AID) initiates somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin (Ig) genes, critical actions for an effective adaptive immune response. However, in addition to the benefits generated by its physiological roles, AID is an etiological factor for the development of human and murine leukemias and lymphomas. This review highlights the pathological role of AID and the consequences of its actions on the development, progression, and therapeutic refractoriness of chronic lymphocytic leukemia (CLL) as a model disease for mature lymphoid malignancies. First, we summarize pertinent aspects of the expression and function of AID in normal B lymphocytes. Then, we assess putative causes for AID expression in leukemic cells emphasizing the role of an activated microenvironment. Thirdly, we discuss the role of AID in lymphomagenesis, in light of recent data obtained by NGS analyses on the genomic landscape of leukemia and lymphomas, concentrating on the frequency of AID signatures in these cancers and correlating previously described tumor-gene drivers with the presence of AID off-target mutations. Finally, we discuss how these changes could affect tumor suppressor and proto-oncogene targets and how they could be associated with disease progression. Collectively, we hope that these sections will help to better understand the complex paradox between the physiological role of AID in adaptive immunity and its potential causative activity in B-cell malignancies.

Autoimmune B Cell Repertoire in a Mouse Model of Sjögren's Syndrome

In genetically prone individuals, chronic immune activation may lead to expansion of autoreactive lymphocyte clones that can induce organ damage developing autoimmune disorders. Sjögren’s Syndrome (SjS) is a systemic chronic autoimmune disease that primarily affects exocrine glands. Despite the accumulated evidences of profound B-cell alterations of humoral immunity, the repertoire and development of B-cell autoreactivity in SjS remains to be determined. We hypothesize that SjS mice will have an increased frequency of self-reactive B cells with a progressive evolution to antigen-driven oligoclonality. Here, we study the B cell repertoire of NOD.H-2^h4 mice, a mouse model of spontaneous autoimmunity mimicking SjS without developing diabetes. A library of 168 hybridomas from NOD.H-2^h4 mice and 186 C57BL/6J splenocytes at different ages was created. The presence of mono or polyreactive autoantibodies to several antigens was evaluated by ELISA, and their staining patterns and cellular reactivity were tested by IFA and FACS. We observed a higher frequency of autoreactivity among B-cell clones from NOD.H-2^h4 mice as compared to wild-type mice. The presence of polyreactive and autoreactive IgG clones increased with mice age. Strikingly, all anti-Ro52 autoantibodies were polyreactive. No loss of polyreactivity was observed upon antibody class switching to IgG. There was a progression to oligoclonality in IgG B cells with mice aging. Our results indicate that in the NOD.H-2^h4 mouse model of SjS, IgG+ B cells are mainly polyreactive and might expand following an unknown antigen-driven positive selection process.

B cell depletion therapies in autoimmune disease: advances and mechanistic insights

In the past 15 years, B cells have been rediscovered to be not merely bystanders but rather active participants in autoimmune aetiology. This has been fuelled in part by the clinical success of B cell depletion therapies (BCDTs). Originally conceived as a method of eliminating cancerous B cells, BCDTs such as those targeting CD20, CD19 and BAFF are now used to treat autoimmune diseases, including systemic lupus erythematosus and multiple sclerosis. The use of BCDTs in autoimmune disease has led to some surprises. For example, although antibody-secreting plasma cells are thought to have a negative pathogenic role in autoimmune disease, BCDT, even when it controls the disease, has limited impact on these cells and on antibody levels. In this Review, we update our understanding of B cell biology, review the results of clinical trials using BCDT in autoimmune indications, discuss hypotheses for the mechanism of action of BCDT and speculate on evolving strategies for targeting B cells beyond depletion.

Epstein-Barr Functional Mimicry: Pathogenicity of Oncogenic Latent Membrane Protein-1 in Systemic Lupus Erythematosus and Autoimmunity

Systemic lupus erythematosus (SLE) and other autoimmune diseases are propelled by immune dysregulation and pathogenic, disease-specific autoantibodies. Autoimmunity against the lupus autoantigen Sm is associated with cross-reactivity to Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1). Additionally, EBV latent membrane protein-1 (LMP1), initially noted for its oncogenic activity, is an aberrantly active functional mimic of the B cell co-stimulatory molecule CD40. Mice expressing a transgene (Tg) for the mCD40-LMP1 hybrid molecule (containing the cytoplasmic tail of LMP1) have mild autoantibody production and other features of immune dysregulation by 2-3 months of age, but no overt autoimmune disease. This study evaluates whether exposure to the EBV molecular mimic, EBNA-1, stimulates antigen-specific and concurrently-reactive humoral and cellular immunity, as well as lupus-like features. After immunization with EBNA-1, mCD40-LMP1 Tg mice exhibited enhanced, antigen-specific, cellular and humoral responses compared to immunized WT congenic mice. EBNA-1 specific proliferative and inflammatory cytokine responses, including IL-17 and IFN-γ, were significantly increased (p<0.0001) in mCD40-LMP1 Tg mice, as well as antibody responses to amino- and carboxy-domains of EBNA-1. Of particular interest was the ability of mCD40-LMP1 to drive EBNA-1 associated molecular mimicry with the lupus-associated autoantigen, Sm. EBNA-1 immunized mCD40-LMP1 Tg mice exhibited enhanced proliferative and cytokine cellular responses (p<0.0001) to the EBNA-1 homologous epitope PPPGRRP and the Sm B/B' cross-reactive sequence PPPGMRPP. When immunized with the SLE autoantigen Sm, mCD40-LMP1 Tg mice again exhibited enhanced cellular and humoral immune responses to both Sm and EBNA-1. Cellular immune dysregulation with EBNA-1 immunization in mCD40-LMP1 Tg mice was accompanied by enhanced splenomegaly, increased serum blood urea nitrogen (BUN) and creatinine levels, and elevated anti-dsDNA and antinuclear antibody (ANA) levels (p<0.0001 compared to mCD40 WT mice). However, no evidence of immune-complex glomerulonephritis pathology was noted, suggesting that a combination of EBV and genetic factors may be required to drive lupus-associated renal disease. These data support that the expression of LMP1 in the context of EBNA-1 may interact to increase immune dysregulation that leads to pathogenic, autoantigen-specific lupus inflammation.

Escape from X chromosome inactivation and female bias of autoimmune diseases

Generally, autoimmune diseases are more prevalent in females than males. Various predisposing factors, including female sex hormones, X chromosome genes, and the microbiome have been implicated in the female bias of autoimmune diseases. During embryogenesis, one of the X chromosomes in the females is transcriptionally inactivated, in a process called X chromosome inactivation (XCI). This equalizes the impact of two X chromosomes in the females. However, some genes escape from XCI, providing a basis for the dual expression dosage of the given gene in the females. In the present review, the contribution of the escape genes to the female bias of autoimmune diseases will be discussed.

Revisiting B cell tolerance and autoantibodies in seropositive and seronegative autoimmune rheumatic disease (AIRD)

Autoimmune rheumatic diseases (AIRD) are categorized seropositive or seronegative, dependent upon the presence or absence of specific autoreactive antibodies, including rheumatoid factor and anti-citrullinated protein antibodies. Autoantibody-based diagnostics have proved helpful in patient care, not only for diagnosis but also for monitoring of disease activity and prediction of therapy responsiveness. Recent work demonstrates that AIRD patients develop autoantibodies beyond those contained in the original categorization. In this study we discuss key mechanisms that underlie autoantibody development in AIRD: defects in early B cell development, genetic variants involved in regulating B cell and T cell tolerance, environmental triggers and antigen modification. We describe how autoantibodies can directly contribute to AIRD pathogenesis through innate and adaptive immune mechanisms, eventually culminating in systemic inflammation and localized tissue damage. We conclude by discussing recent insights that suggest distinct AIRD have incorrectly been denominated seronegative.

Taurine Metabolism Aggravates the Progression of Lupus by Promoting the Function of Plasmacytoid Dendritic Cells

OBJECTIVE

Type I interferons (IFNs) are critical in the development of systemic lupus erythematosus (SLE). Metabolic abnormalities cause dysregulation of multiple immune cells, but the metabolic regulation of type I IFN production is not well clarified in SLE. We undertook this study to define amino acid metabolism features in SLE and to explore the function of disease-relevant metabolites in the control of plasmacytoid dendritic cell (pDC)-mediated type I IFN production and the progression of SLE.

METHODS

Metabolomic profiling of the serum from SLE patients and healthy controls was performed by mass spectrometry. The effects of SLE-related metabolites on type I IFN production were explored in human and mouse pDCs. The reactive oxygen species (ROS) levels of pDCs from wild-type and Ncf1^-/- mice were measured by flow cytometry. Mechanisms were investigated by RNA sequencing and immunoblotting. In vivo effects of SLE-relevant metabolites were systemically analyzed in B6.Cg-Sle1^NZM2410/Aeg Yaa/DcrJ mice.

RESULTS

Taurine was higher in the serum from SLE patients compared to healthy controls (P < 0.001) and rheumatoid arthritis patients (P < 0.001). Taurine content was positively correlated with disease activity and the expression of IFN signature genes. The addition of taurine facilitated IFN regulatory factor 7 phosphorylation and enhanced type I IFN production by reducing the ROS levels in pDCs in a neutrophil cytosolic factor 1-dependent manner. Taurine supplementation promoted expression of type I IFN-induced genes, activated lymphocytes, and increased autoantibodies and proteinuria, leading to more serious nephritis.

CONCLUSION

Taurine metabolism is involved in the development of SLE by enhancing pDC-mediated type I IFN production. Targeted inhibition of taurine or implementation of a taurine-restricted diet has therapeutic potential in SLE.

B Cell αv Integrins Regulate TLR-Driven Autoimmunity

Systemic lupus erythematosus (SLE) is defined by loss of B cell tolerance, resulting in production of autoantibodies against nucleic acids and other cellular Ags. Aberrant activation of TLRs by self-derived RNA and DNA is strongly associated with SLE in patients and in mouse models, but the mechanism by which TLR signaling to self-ligands is regulated remains poorly understood. In this study, we show that αv integrin plays a critical role in regulating B cell TLR signaling to self-antigens in mice. We show that deletion of αv from B cells accelerates autoantibody production and autoimmune kidney disease in the Tlr7.1 transgenic mouse model of SLE. Increased autoimmunity was associated with specific expansion of transitional B cells, extrafollicular IgG2c-producing plasma cells, and activation of CD4 and CD8 T cells. Our data show that αv-mediated regulation of TLR signaling in B cells is critical for preventing autoimmunity and indicate that loss of αv promotes escape from tolerance. Thus, we identify a new regulatory pathway in autoimmunity and elucidate upstream signals that adjust B cell activation to prevent development of autoimmunity in a mouse model.

The Plasticity of Newly Formed B Cells

Newly formed B cells (NF-B cells) that emerge from the bone marrow to the periphery have often been referred to as immature or transitional B cells. However, NF-B cells have several striking characteristics, including a distinct BCR repertoire, high expression of AID, high sensitivity to PAMPs, and the ability to produce cytokines. A number of findings do not support their designation as immature because NF-B cells have the potential to become Ab-producing cells and to undergo class-switch recombination. In this review, we provide a fresh perspective on NF-B cell functions and describe some of the signals driving their activation. We summarize growing evidence supporting a role for NF-B cells in protection against infections and as a potential source of autoantibody-producing cells in autoimmune diseases such as systemic lupus erythematosus.

B-Cell-Intrinsic Type 1 Interferon Signaling Is Crucial for Loss of Tolerance and the Development of Autoreactive B Cells

Type 1 interferon (T1IFN) signaling promotes inflammation and lupus pathology, but its role in autoreactive B cell development in the antibody-forming cell (AFC) and germinal center (GC) pathways is unclear. Using a lupus model that allows for focused study of the AFC and GC responses, we show that T1IFN signaling is crucial for autoreactive B cell development in the AFC and GC pathways. Through bone marrow chimeras, DNA-reactive B cell transfer, and GC-specific Cre mice, we confirm that IFNαR signaling in B cells promotes autoreactive B cell development into both pathways. Transcriptomic analysis reveals gene expression alterations in multiple signaling pathways in non-GC and GC B cells in the absence of IFNαR. Finally, we find that T1IFN signaling promotes autoreactive B cell development in the AFC and GC pathways by regulating BCR signaling. These data suggest value for anti-IFNαR therapy in individuals with elevated T1IFN activity before clinical disease onset.

The B-cell-intrinsic mechanisms of type 1 interferon (T1IFN) signaling in regulating B cell tolerance is unclear. Domeier et al. show that T1IFN signaling in B cells causes loss of B cell tolerance, promoting autoreactive B cell development into the antibody-forming cell and germinal center pathways by regulating BCR signaling.

Targeting Antigens to CD180 but Not CD40 Programs Immature and Mature B Cell Subsets to Become Efficient APCs

Targeting Ags to the CD180 receptor activates both B cells and dendritic cells (DCs) to become potent APCs. After inoculating mice with Ag conjugated to an anti-CD180 Ab, B cell receptors were rapidly internalized. Remarkably, all B cell subsets, including even transitional 1 B cells, were programed to process, present Ag, and stimulate Ag-specific CD4⁺ T cells. Within 24-48 hours, Ag-specific B cells were detectable at T-B borders in the spleen; there, they proliferated in a T cell-dependent manner and induced the maturation of T follicular helper (T_FH) cells. Remarkably, immature B cells were sufficient for the maturation of T_FH cells after CD180 targeting: T_FH cells were induced in BAFFR^-/- mice (with only transitional 1 B cells) and not in μMT mice (lacking all B cells) following CD180 targeting. Unlike CD180 targeting, CD40 targeting only induced DCs but not B cells to become APCs and thus failed to efficiently induce T_FH cell maturation, resulting in slower and lower-affinity IgG Ab responses. CD180 targeting induces a unique program in Ag-specific B cells and to our knowledge, is a novel strategy to induce Ag presentation in both DCs and B cells, especially immature B cells and thus has the potential to produce a broad range of Ab specificities. This study highlights the ability of immature B cells to present Ag to and induce the maturation of cognate T_FH cells, providing insights toward vaccination of mature B cell-deficient individuals and implications in treating autoimmune disorders.

RIP1/RIP3-regulated necroptosis as a target for multifaceted disease therapy (Review)

Necroptosis is a type of programmed cell death with necrotic morphology, occurring in a variety of biological processes, including inflammation, immune response, embryonic development and metabolic abnormalities. The current nomenclature defines necroptosis as cell death mediated by signal transduction from receptor‑interacting serine/threonine kinase (RIP) 1 to RIP3 (hereafter called RIP1/RIP3). However, RIP3‑dependent cell death would be a more precise definition of necroptosis. RIP3 is indispensable for necroptosis, while RIP1 is not consistently involved in the signal transduction. Notably, deletion of RIP1 even promotes RIP3‑mediated necroptosis under certain conditions. Necroptosis was previously thought as an alternate process of cell death in case of apoptosis inhibition. Currently, necroptosis is recognized to serve a pivotal role in regulating various physiological processes. Of note, it mediates a variety of human diseases, such as ischemic brain injury, immune system disorders and cancer. Targeting and inhibiting necroptosis, therefore, has the potential to be used for therapeutic purposes. To date, research has elucidated the suppression of RIP1/RIP3 via effective inhibitors and highlighted their potential application in disease therapy. The present review focused on the molecular mechanisms of RIP1/RIP3‑mediated necroptosis, explored the functions of RIP1/RIP3 in necroptosis, discussed their potential as a novel therapeutic target for disease therapy, and provided valuable suggestions for further study in this field.

Individualizing Therapy in Lupus Nephritis

The ideal therapeutic approach for lupus nephritis (LN) is to quickly achieve a complete remission and maintain that response long-term while minimizing drug toxicity, and prevent tissue damage and death. The combination therapy consisting of multiple medications is aimed at incorporating drugs with complementary actions at reduced doses to achieve additive or synergistic therapeutic effects while minimizing toxicity. Here, we review the available evidence using combination therapies (triple therapy) and how such strategies can improve therapeutic efficacy in LN, which will mainly focus on the combination of high-dose corticosteroids with mycophenolate mofetil (MMF) and a calcineurin inhibitor (CNI) at low dose. We discuss the rationale, efficacy, and safety of the therapy, as well as its molecular mechanisms. We also discuss the questions raised from the trials and briefly describe emerging approaches developed on the basis of combination therapy, and these advances that promise to improve on the standard-of-care treatments and toward individual therapy in LN.

High TLR7 Expression Drives the Expansion of CD19+CD24hiCD38hi Transitional B Cells and Autoantibody Production in SLE Patients

Signaling through Toll-like receptor 7 (TLR7) drives the production of type I IFN and promotes the activation of autoreactive B cells and is implicated in the pathogenesis of systemic lupus erythematosus (SLE). While TLR7 has been extensively studied in murine lupus, much less is known about its role in the pathogenesis of human SLE. Genetic studies support a link between the TLR7 rs3853839 C/G polymorphism, which affects TLR7 mRNA turnover, and SLE susceptibility; however, the effects of this polymorphism on B cells have not been studied. Here we determined how changes in TLR7 expression affect peripheral B cells and auto-Ab production in SLE patients. High TLR7 expression in SLE patients driven by TLR7 rs3853839 C/G polymorphism was associated with more active disease and upregulation of IFN-responsive genes. TLR7^hi SLE patients showed an increase in peripheral B cells. Most notably, the percentage and numbers of CD19⁺CD24⁺⁺CD38⁺⁺ newly-formed transitional (TR) B cells were increased in TLR7^hi SLE patients as compared to HCs and TLR7^norm/lo SLE patients. Using auto-Ab arrays, we found an increase and enrichment of auto-Ab specificities in the TLR7^hi SLE group, including the production of anti-RNA/RNP-Abs. Upon in vitro TLR7 ligand stimulation, TR B cells isolated from TLR7^hi but not TLR7^norm/lo SLE patients produced anti-nuclear auto-Abs (ANA). Exposure of TR B cells isolated from cord blood to IFNα induced the expression of TLR7 and enabled their activation in response to TLR7 ligation in vitro. Our study shows that overexpression of TLR7 in SLE patients drives the expansion of TR B cells. High TLR7 signaling in TR B cells promotes auto-Ab production, supporting a possible pathogenic role of TR B cells in human SLE.

Transitional B cells in quiescent SLE: An early checkpoint imprinted by IFN

Systemic lupus (SLE) is characterized by a break of B cell tolerance that plays a central role in disease pathophysiology. An early checkpoint defect occurs at the transitional stage leading to the survival of autoreactive B cells and consequently the production of pathogenic autoantibodies. The main purpose of our work was to determine whether transitional B cells, as the most immature naïve B cell subset upstream of pathogenic B cells, display specific features compared to healthy non SLE subjects. Through extensive analysis of transitional B cells from untreated or low treated, mostly Caucasian, SLE patients, we demonstrated that transitional (T1 and T2) B cell frequencies were increased in SLE and positively correlated with disease activity. SLE transitional B cells displayed defects in two closely inter-related molecules (i.e. TLR9 defective responses and CD19 downregulation). RNA sequencing of sorted transitional B cells from untreated patients revealed a predominant overexpression of interferon stimulated genes (ISGs) even out of flares. In addition, early transitional B cells from the bone marrow displayed the highest interferon score, reflecting a B cell interferon burden of central origin. Hence, the IFN signature in transitional B cells is not confined to African American SLE patients and exists in quiescent disease since the medullary stage. These results suggest that in SLE these 3 factors (i.e. IFN imprintment, CD19 downregulation and TLR9 responses impairment) could take part at the early transitional B cell stage in B cell tolerance by-pass, ultimately leading in periphery to the expansion of autoantibodies-secreting cells.

Sources of Pathogenic Nucleic Acids in Systemic Lupus Erythematosus

A hallmark of systemic lupus erythematosus (SLE), and several related autoimmune diseases, is the presence of autoantibodies against nucleic acids and nucleic acid-binding proteins, as well as elevated type I interferons (IFNs), which appear to be instrumental in disease pathogenesis. Here we discuss the sources and proposed mechanisms by which a range of cellular RNA and DNA species can become pathogenic and trigger the nucleic acid sensors that drive type I interferon production. Potentially SLE-promoting DNA may originate from pieces of chromatin, from mitochondria, or from reverse-transcribed cellular RNA, while pathogenic RNA may arise from mis-localized, mis-processed, ancient retroviral, or transposable element-derived transcripts. These nucleic acids may leak out from dying cells to be internalized and reacted to by immune cells or they may be generated and remain to be sensed intracellularly in immune or non-immune cells. The presence of aberrant DNA or RNA is normally counteracted by effective counter-mechanisms, the loss of which result in a serious type I IFN-driven disease called Aicardi-Goutières Syndrome. However, in SLE it remains unclear which mechanisms are most critical in precipitating disease: aberrant RNA or DNA, overly sensitive sensor mechanisms, or faulty counter-acting defenses. We propose that the clinical heterogeneity of SLE may be reflected, in part, by heterogeneity in which pathogenic nucleic acid molecules are present and which sensors and pathways they trigger in individual patients. Elucidation of these events may result in the recognition of distinct "endotypes" of SLE, each with its distinct therapeutic choices.

Cutting Edge: BCAP Promotes Lupus-like Disease and TLR-Mediated Type I IFN Induction in Plasmacytoid Dendritic Cells

Systemic lupus erythematosus severity correlates with elevated serum levels of type I IFNs, cytokines produced in large quantities by plasmacytoid dendritic cells (pDC) in response to engagement of TLR7 and TLR9 with endocytosed nucleic acids. B cell adaptor for PI3K (BCAP) promoted many aspects of TLR7-driven lupus-like disease, including Isg15 and Ifit1 expression in blood and an immature pDC phenotype associated with higher IFN production. BCAP^-/- mice produced significantly less serum IFN-α than wild-type mice after injection of TLR9 agonist, and BCAP promoted TLR7 and TLR9-induced IFN-α production specifically in pDC. TLR-induced IFN-α production in pDC requires DOCK2-mediated activation of Rac1 leading to activation of IKKα, a mechanism we show was dependent on BCAP. BCAP^-/- pDC had decreased actin polymerization and Rac1 activation and reduced IKKα phosphorylation upon TLR9 stimulation. We show a novel role for BCAP in promoting TLR-induced IFN-α production in pDC and in systemic lupus erythematosus pathogenesis.

Extrafollicular responses in humans and SLE

Chronic autoimmune diseases, and in particular Systemic Lupus Erythematosus (SLE), are endowed with a long-standing autoreactive B-cell compartment that is presumed to reactivate periodically leading to the generation of new bursts of pathogenic antibody-secreting cells (ASC). Moreover, pathogenic autoantibodies are typically characterized by a high load of somatic hypermutation and in some cases are highly stable even in the context of prolonged B-cell depletion. Long-lived, highly mutated antibodies are typically generated through T-cell-dependent germinal center (GC) reactions. Accordingly, an important role for GC reactions in the generation of pathogenic autoreactivity has been postulated in SLE. Nevertheless, pathogenic autoantibodies and autoimmune disease can be generated through B-cell extrafollicular (EF) reactions in multiple mouse models and human SLE flares are characterized by the expansion of naive-derived activated effector B cells of extrafollicular phenotype. In this review, we will discuss the properties of the EF B-cell pathway, its relationship to other effector B-cell populations, its role in autoimmune diseases, and its contribution to human SLE. Furthermore, we discuss the relationship of EF B cells with Age-Associated B cells (ABCs), a TLR-7-driven B-cell population that mediates murine autoimmune and antiviral responses.

RNA is taking its Toll: Impact of RNA-specific Toll-like receptors on health and disease

RNA-sensing Toll-like receptors (TLRs) are often described as antiviral receptors of the innate immune system. However, the past decade has shown that the function and relevance of these receptors are far more complex. They were found to be essential for the detection of various bacterial, archaeal, and eukaryotic microorganisms and facilitate the discrimination between dead and living microbes. The cytokine and interferon response profile that is triggered has the potential to improve the efficacy of next-generation vaccines and may prevent the development of asthma and allergy. Nevertheless, the ability to recognize foreign RNA comes with a cost as also damaged host cells can release nucleic acids that might induce an inappropriate immune response. Thus, it is not surprising that RNA-sensing TLRs play a key role in various autoimmune diseases. However, promising new inhibitors and antagonists are on the horizon to improve their treatment.