The 52 000 MW Ro/SS-A autoantigen in Sjögren's syndrome/systemic lupus erythematosus (Ro52) is an interferon-gamma inducible tripartite motif protein associated with membrane proximal structures

Viral infection and spread are inhibited by the polyubiquitination and downregulation of TRPV2 channel by the interferon-stimulated gene TRIM21

Interferon (IFN) contributes to the host's antiviral response by inducing IFN-stimulated genes (ISGs). However, their functional targets and the mechanism of action remain elusive. Here, we report that one such ISG, TRIM21, interacts with and degrades the TRPV2 channel in myeloid cells, reducing its expression and providing host protection against viral infections. Moreover, viral infection upregulates TRIM21 in paracrine and autocrine manners, downregulating TRPV2 in neighboring cells to prevent viral spread to uninfected cells. Consistently, the Trim21-/- mice are more susceptible to HSV-1 and VSV infection than the Trim21+/+ littermates, in which viral susceptibility is rescued by inhibition or deletion of TRPV2. Mechanistically, TRIM21 catalyzes the K48-linked ubiquitination of TRPV2 at Lys295. TRPV2K295R is resistant to viral-infection-induced TRIM21-dependent ubiquitination and degradation, promoting viral infection more profoundly than wild-type TRPV2 when reconstituted into Lyz2-Cre;Trpv2fl/fl myeloid cells. These findings characterize targeting the TRIM21-TRPV2 axis as a conducive strategy to control viral spread to bystander cells.

Machine Learning for the Identification of a Common Signature for Anti-SSA/Ro 60 Antibody Expression Across Autoimmune Diseases

OBJECTIVE

Anti-Ro autoantibodies are among the most frequently detected extractable nuclear antigen autoantibodies, mainly associated with primary Sjögren's syndrome (SS), systemic lupus erythematosus (SLE), and undifferentiated connective tissue disease (UCTD). This study was undertaken to determine if there is a common signature for all patients expressing anti-Ro 60 autoantibodies regardless of their disease phenotype.

METHODS

Using high-throughput multiomics data collected from the cross-sectional cohort in the PRECISE Systemic Autoimmune Diseases (PRECISESADS) study Innovative Medicines Initiative (IMI) project (genetic, epigenomic, and transcriptomic data, combined with flow cytometry data, multiplexed cytokines, classic serology, and clinical data), we used machine learning to assess the integrated molecular profiling of 520 anti-Ro 60+ patients compared to 511 anti-Ro 60- patients with primary SS, patients with SLE, and patients with UCTD, and 279 healthy controls.

RESULTS

The selected clinical features for RNA-Seq, DNA methylation, and genome-wide association study data allowed for a clear distinction between anti-Ro 60+ and anti-Ro 60- patients. The different features selected using machine learning from the anti-Ro 60+ patients constituted specific signatures when compared to anti-Ro 60- patients and healthy controls. Remarkably, the transcript Z score of 3 genes (ATP10A, MX1, and PARP14), presenting with overexpression associated with hypomethylation and genetic variation and independently identified using the Boruta algorithm, was clearly higher in anti-Ro 60+ patients compared to anti-Ro 60- patients regardless of disease type. Our findings demonstrated that these signatures, enriched in interferon-stimulated genes, were also found in anti-Ro 60+ patients with rheumatoid arthritis and those with systemic sclerosis and remained stable over time and were not affected by treatment.

CONCLUSION

Anti-Ro 60+ patients present with a specific inflammatory signature regardless of their disease type, suggesting that a dual therapeutic approach targeting both Ro-associated RNAs and anti-Ro 60 autoantibodies should be considered.

The emerging roles of TRIM21 in coordinating cancer metabolism, immunity and cancer treatment

Tripartite motif containing-21 (TRIM21), an E3 ubiquitin ligase, was initially found to be involved in antiviral responses and autoimmune diseases. Recently studies have reported that TRIM21 plays a dual role in cancer promoting and suppressing in the occurrence and development of various cancers. Despite the fact that TRIM21 has effects on multiple metabolic processes, inflammatory responses and the efficacy of tumor therapy, there has been no systematic review of these topics. Herein, we discuss the emerging role and function of TRIM21 in cancer metabolism, immunity, especially the immune response to inflammation associated with tumorigenesis, and also the cancer treatment, hoping to shine a light on the great potential of targeting TRIM21 as a therapeutic target.

The role of virus infections in Sjögren’s syndrome

Primary Sjögren’s syndrome (pSS) is an autoimmune disease with a clinical picture of not only mainly exocrine gland involvement, with dryness symptoms, but also internal organ and systems involvement. The epithelial damage and releasing of antigens, which, in some circumstances, become autoantigens, underlay the pathogenesis of pSS. The activation of autoimmune processes in pSS leads to the hyperactivation of B cells with autoantibody production and other immunological phenomena such as hypergammaglobulinemia, production of cryoglobulins, or formation of extra-nodal lymphoid tissue. Among the risk factors for the development of this disease are viral infections, which themselves can activate autoimmune reactions and influence the host’s immune response. It is known that viruses, through various mechanisms, can influence the immune system and initiate autoimmune reactions. These mechanisms include molecular mimicry, bystander activation, production of superantigens—proteins encoded by viruses—or a programming to produce viral cytokines similar to host cytokines such as, e.g., interleukin-10. Of particular importance for pSS are viruses which not only, as expected, activate the interferon pathway but also play a particular role, directly or indirectly, in B cell activation or present tropism to organs also targeted in the course of pSS. This article is an attempt to present the current knowledge of the influence specific viruses have on the development and course of pSS.

The molecular mechanisms that drive intracellular neutralization by the antibody-receptor and RING E3 ligase TRIM21

The cytosolic antibody receptor and RING E3 ligase TRIM21 targets intracellular, antibody-coated immune complexes for degradation and activates the immune system. Here we review how TRIM21 degrades diverse targets and how this activity can be exploited in molecular biology and for the development of new therapeutics. In addition, we compare what is known about TRIM21's mechanism to other TRIM proteins and RING E3 ligases.

The Emerging Roles of Tripartite Motif Proteins (TRIMs) in Acute Lung Injury

Acute lung injury (ALI) is an inflammatory disorder of the lung that causes high mortality and lacks any pharmacological intervention. Ubiquitination plays a critical role in the pathogenesis of ALI as it regulates the alveolocapillary barrier and the inflammatory response. Tripartite motif (TRIM) proteins are one of the subfamilies of the RING-type E3 ubiquitin ligases, which contains more than 80 distinct members in humans involved in a broad range of biological processes including antivirus innate immunity, development, and tumorigenesis. Recently, some studies have shown that several members of TRIM family proteins play important regulatory roles in inflammation and ALI. Herein, we integrate emerging evidence regarding the roles of TRIMs in ALI. Articles were selected from the searches of PubMed database that had the terms "acute lung injury," "ubiquitin ligases," "tripartite motif protein," "inflammation," and "ubiquitination" using both MeSH terms and keywords. Better understanding of these mechanisms may ultimately lead to novel therapeutic approaches by targeting TRIMs for ALI treatment.

Targeted protein degradation using intracellular antibodies and its application to neurodegenerative disease

Antibodies mediate the majority of their effects in the extracellular domain, or in intracellular compartments isolated from the cytosol. Under a growing list of circumstances, however, antibodies are found to gain access to the cytoplasm. Cytosolic immune complexes are bound by the atypical antibody receptor TRIM21, which mediates the rapid degradation of the immune complexes at the proteasome. These discoveries have informed the development of TRIM-Away, a technique to selectively deplete proteins using delivery of antibodies into cells. A range of related approaches that elicit selective protein degradation using intracellular constructs linking antibody fragments to degradative effector functions have also been developed. These methods hold promise for inducing the degradation of proteins as both research tools and as a novel therapeutic approach. Protein aggregates are a pathophysiological feature of neurodegenerative diseases and are considered to have a causal role in pathology. Immunotherapy is emerging as a promising route towards their selective targeting, and a role of antibodies in the cytosol has been demonstrated in cell-based assays. This review will explore the mechanisms by which therapeutic antibodies engage and eliminate intracellularly aggregated proteins. We will discuss how future developments in intracellular antibody technology may enhance the therapeutic potential of such antibody-derived therapies.

Anti-TRIM21 antibody is associated with aberrant B-cell function and type I interferon production in systemic lupus erythematosus

BACKGROUND

TRIM21 is a member of the tripartite motif family proteins and is one of the autoantigens which react with anti-SS-A antibody (Ab) present in sera of patients with systemic lupus erythematosus (SLE) and Sjögren's syndrome. Previous studies have shown that TRIM21 dysfunction promotes aberrant B-cell differentiation and Ab production in SLE, and anti-TRIM21 Ab may be related to the TRIM21 dysfunction in human SLE pathogenesis. Here, we examined the relationship between anti-TRIM21 Ab and clinical and immunological characteristics in SLE patients.

METHODS

Twenty-seven patients with SLE (23 women and four men) before immunosuppressive therapies, who fulfilled the revised 1997 American College of Rheumatology criteria for SLE, and four healthy controls (3 women and one man) were enrolled in the study. SLE patients were divided into two groups according to the seropositivity for anti-TRIM21 Ab. Serum anti-TRIM21 Ab levels were measured using enzyme-linked immunosorbent assays. The serum levels of cytokines and immunoglobulins were measured by cytometer beads arrays. The expression levels of TRIM21 protein in peripheral mononuclear cells (PBMCs) from SLE patients were evaluated by Western blotting.

RESULTS

Sixteen and 9 patients showed seronegativity and seropositivity for anti-TRIM21 Ab, respectively. There were no significant differences in the background parameters, including female ratio, age, disease duration, SLE activity, and laboratory data between the two groups. The serum levels of interferon (IFN)-β were significantly higher in patients with anti-TRIM21 Ab as compared with those without anti-TRIM21 Ab (P = .043). The levels of IgG1 and IgA were significantly higher in SLE patients with anti-TRIM21 Ab as compared with those without anti-TRIM21 Ab (P = .0022 and .032, respectively). The PBMCs of patients with anti-TRIM21 Ab showed a significantly lower expression of TRIM21 protein as compared with those of patients without anti-TRIM21 Ab (P = .014).

CONCLUSIONS

Anti-TRIM21 Ab seropositivity was related to B-cell abnormalities and type I IFN overproduction in SLE patients. These findings suggest that anti-TRIM21 Ab may have an inhibitory effect on TRIM21 functions and be a novel biomarker for the level of dependence on type I IFN overproduction and B-cell abnormalities.

TRIM21/Ro52 - Roles in Innate Immunity and Autoimmune Disease

TRIM21 (Ro52/SSA1) is an E3 ubiquitin ligase with key roles in immune host defence, signal transduction, and possibly cell cycle regulation. It is also an autoantibody target in Sjögren's syndrome, systemic lupus erythematosus, and other rheumatic autoimmune diseases. Here, we summarise the structure and function of this enzyme, its roles in innate immunity, adaptive immunity and cellular homeostasis, the pathogenesis of autoimmunity against TRIM21, and the potential impacts of autoantibodies to this intracellular protein.

Toxoplasma gondii Type-I ROP18 Targeting Human E3 Ligase TRIM21 for Immune Escape

Toxoplasma gondii is an intracellular pathogen that exerts its virulence through inhibiting host’s innate immune responses, which is mainly related to the type II interferon (IFN-γ) response. IFN-γ inducible tripartite motif 21 (TRIM21), an E3 ligase, plays an important role in anti-infection responses against the intracellular pathogens including bacteria, virus, and parasite. We found that T. gondii virulence factor ROP18 of the type I RH strain (TgROP18_I) interacted with human TRIM21, and promoted the latter’s phosphorylation, which subsequently accelerated TRIM21 degradation through lysosomal pathway. Furthermore, TRIM21 protein level was found to be upregulated during RH and CEP strains of T. gondii infection. TRIM21 knocking down reduced the ubiquitin labeling on the parasitophorous vacuole membrane (PVM) [which led to parasitophorous vacuole (PV) acidification and death of CEP tachyzoites], and relieved the inhibition of CEP proliferation induced by IFN-γ in human foreskin fibroblast (HFF) cells which was consistent with the result of TRIM21 overexpression. On the other hand, TRIM21 overexpression enhanced the inhibition of CEP proliferation, and inhibited the binding of IκB-α with p65 to activate the IFN-γ-inducible NF-κB pathway, which might be resulted by TRIM21-IκB-α interaction. In brief, our research identified that in human cells, IFN-γ-inducible TRIM21 functioned in the innate immune responses against type III T. gondii infection; however, TgROP18_I promoted TRIM21 phosphorylation, leading to TRIM21 degradation for immune escape in type I strain infection.

Ubiquitination modification: critical regulation of IRF family stability and activity

Interferon regulatory factors (IRFs) play pivotal and critical roles in innate and adaptive immune responses; thus, precise and stringent regulation of the stability and activation of IRFs in physiological processes is necessary. The stability and activities of IRFs are directly or indirectly targeted by endogenous and exogenous proteins in an ubiquitin-dependent manner. However, few reviews have summarized how host E3 ligases/DUBs or viral proteins regulate IRF stability and activity. Additionally, with recent technological developments, details about the ubiquitination of IRFs have been continuously revealed. As knowledge of how these proteins function and interact with IRFs may facilitate a better understanding of the regulation of IRFs in immune responses or other biological processes, we summarized current studies on the direct ubiquitination of IRFs, with an emphasis on how these proteins interact with IRFs and affect their activities, which may provide exciting targets for drug development by regulating the functions of specific E3 ligases.

Anti-Ro52 Autoantibodies Are Related to Chronic Graft-vs.-Host Disease After Allogeneic Hematopoietic Stem Cell Transplantation

Chronic graft-vs.-host disease (cGVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Previous studies have shown that autoantibodies play an important role in the development of cGVHD. Anti-nuclear autoantibodies (ANA) is the most frequently detected autoantibodies in patients with cGVHD, but the role of anti-Ro52 autoantibodies (anti-Ro52) in cGVHD remains largely unknown. In this study, we analyzed autoantibodies from 84 patients after allo-HSCT, including 42 with active cGVHD and 42 without cGVHD. Autoantibodies were found in 36 (42.9%) patients. Among these autoantibody-positive patients, 28 (77.8%) patients had active cGVHD. The most frequent autoantibodies in patients with active cGVHD were ANA (50.0%), anti-Ro52 (28.6%) and anti-mitochondrial autoantibodies type 2 (4.8%). We further explored the association between anti-Ro52 and cGVHD. Patients with active cGVHD had higher anti-Ro52 levels than patients without cGVHD (P < 0.05). The increases of anti-Ro52 levels were more significant in patients with moderate/severe cGVHD compared to those of patients without cGVHD (P < 0.05). Stratified and multivariable logistic regression analysis demonstrated that moderate/severe cGVHD was an independent risk factor for the levels of anti-Ro52 (P < 0.01). ROC analysis confirmed anti-Ro52 as a risk factor for progression of skin cGVHD. Moreover, the anti-Ro52 levels were highly correlated with the levels of B cell-activating factor (BAFF) and IgG1 antibodies. Our study demonstrates that anti-Ro52 is associated with cGVHD. The increased levels of anti-Ro52 were associated with higher levels of BAFF and IgG1 antibodies, suggesting a mechanistic link between elevated anti-Ro52 levels and aberrant B cell homeostasis.

TRIM21 Dysfunction Enhances Aberrant B-Cell Differentiation in Autoimmune Pathogenesis

TRIM21 is one of the autoantigens that reacts with an anti-SS-A antibody (Ab) present in patients with systemic lupus erythematosus (SLE) and Sjögren's syndrome. TRIM21 is thought to play a role in B-cell proliferation and apoptosis, among other activities. Here we examined a pathological role of TRIM21 in SLE. Trim21-deficient MRL/lpr mice were generated by backcrossing Trim21-deficient C57BL/6 mice to MRL/lpr mice. The levels of serum anti-dsDNA Ab and urine protein at 28 weeks of age were significantly higher in Trim21-deficient MRL/lpr mice as compared to wild-type MRL/lpr mice (p = 0.029 and 0.003, respectively). Resting B cells from Trim21-deficient mice showed significantly higher abilities to differentiate into plasmablasts and to produce Ab as compared with control mice. Due to the reduction of TRIM21-mediated ubiquitylation, IRF5 protein expression was increased in Trim21-deficient MRL/lpr mice (p = 0.021), which correlated with increased plasmablast generation and immunoglobulin production. B cells from SLE patients with anti-TRIM21 Ab seropositivity also showed a significantly higher ability to differentiate into plasmablasts as compared with those without anti-TRIM21 Ab or healthy controls. These results suggest that TRIM21 dysfunction contributes to SLE pathogenesis by promoting B-cell differentiation, for which anti-TRIM21 Ab may be partly responsible.

TRIM21-mediated proteasomal degradation of SAMHD1 regulates its antiviral activity

SAMHD1 possesses multiple functions, but whether cellular factors regulate SAMHD1 expression or its function remains not well characterized. Here, by investigating why cultured RD and HEK293T cells show different sensitivity to enterovirus 71 (EV71) infection, we demonstrate that SAMHD1 is a restriction factor for EV71. Importantly, we identify TRIM21, an E3 ubiquitin ligase, as a key regulator of SAMHD1, which specifically interacts and degrades SAMHD1 through the proteasomal pathway. However, TRIM21 has no effect on EV71 replication itself. Moreover, we prove that interferon production stimulated by EV71 infection induces increased TRIM21 and SAMHD1 expression, whereas increasing TRIM21 overrides SAMHD1 inhibition of EV71 in cells and in a neonatal mouse model. TRIM21-mediated degradation of SAMHD1 also affects SAMHD1-dependent restriction of HIV-1 and the regulation of interferon production. We further identify the functional domains in TRIM21 required for SAMHD1 binding and the ubiquitination site K622 in SAMHD1 and show that phosphorylation of SAMHD1 at T592 also blocks EV71 restriction. Our findings illuminate how EV71 overcomes SAMHD1 inhibition via the upregulation of TRIM21.

TRIM21 Mitigates Human Lung Microvascular Endothelial Cells' Inflammatory Responses to LPS

Endothelial cell (EC) inflammation is regarded as an important pathogenic feature of many inflammatory diseases, including acute lung injury and sepsis. An increase in EC inflammation results in neutrophil infiltration from the blood to the site of inflammation, further promoting EC permeability. The ubiquitin E3 ligase TRIM21 has been implicated in human disorders; however, the roles of TRIM21 in endothelial dysfunction and acute lung injury have not been reported. Here, we reveal an antiinflammatory property of TRIM21 in a mouse model of acute lung injury and human lung microvascular ECs. Overexpression of TRIM21 by lentiviral vector infection effectively dampened LPS-induced neutrophil infiltration, cytokine release, and edema in mice. TRIM21 inhibited human lung microvascular endothelial cell inflammatory responses as evidenced by attenuation of the NF-κB pathway, release of IL-8, expression of intercellular adhesion molecules, and adhesion of monocytes to ECs. Furthermore, we demonstrated that TRIM21 was predominantly degraded by an increase in its monoubiquitination and lysosomal degradation after inflammatory stimuli. Thus, inhibition of vascular endothelial inflammation by TRIM21 provides a novel therapeutic target to lessen pulmonary inflammation.

Rare missense variants in the human cytosolic antibody receptor preserve antiviral function

The genetic basis of most human disease cannot be explained by common variants. One solution to this ‘missing heritability problem’ may be rare missense variants, which are individually scarce but collectively abundant. However, the phenotypic impact of rare variants is under-appreciated as gene function is normally studied in the context of a single ‘wild-type’ sequence. Here, we explore the impact of naturally occurring missense variants in the human population on the cytosolic antibody receptor TRIM21, using volunteer cells with variant haplotypes, CRISPR gene editing and functional reconstitution. In combination with data from a panel of computational predictors, the results suggest that protein robustness and purifying selection ensure that function is remarkably well-maintained despite coding variation.

TRIM21-From Intracellular Immunity to Therapy

Tripartite motif containing-21 (TRIM21) is a cytosolic ubiquitin ligase and antibody receptor that provides a last line of defense against invading viruses. It does so by acting as a sensor that intercepts antibody-coated viruses that have evaded extracellular neutralization and breached the cell membrane. Upon engagement of the Fc of antibodies bound to viruses, TRIM21 triggers a coordinated effector and signaling response that prevents viral replication while at the same time inducing an anti-viral cellular state. This dual effector function is tightly regulated by auto-ubiquitination and phosphorylation. Therapeutically, TRIM21 has been shown to be detrimental in adenovirus based gene therapy, while it may be favorably utilized to prevent tau aggregation in neurodegenerative disorders. In addition, TRIM21 may synergize with the complement system to block viral replication as well as transgene expression. TRIM21 can also be utilized as a research tool to deplete specific proteins in cells and zebrafish embryos. Here, we review our current biological understanding of TRIM21 in light of its versatile functions.

E3 Ligase Trim21 Ubiquitylates and Stabilizes Keratin 17 to Induce STAT3 Activation in Psoriasis

Keratin 17 (K17), a marker of keratinocyte hyperproliferation, is a type I intermediate filament that is overexpressed in psoriatic epidermis and plays a critical pathogenic role by stimulating T cells. However, the posttranslational modification of K17, which is reversible and targetable, has not been elucidated. Herein, we reported that K17 could be modified through ubiquitination that controlled its stability and led to the phosphorylation and nuclear translocation of its interactor signal transducers and activators of transcription 3 (STAT3), which is a key regulator of cell proliferation in psoriasis. First, we stimulated human keratinocyte cell line HaCaT cells with psoriasis (pso)-mix, which is a cytokine pool (IL-17, IL-22, tumor necrosis factor-α, and IFN-γ) mimicking the in vitro "psoriasis-like" status and found that the ubiquitination of K17 was essential to stabilize its protein expression in pso-mix-treated HaCaT cells. Subsequently, tripartite motif-containing protein 21 was identified as the E3 ligase of K17, which ubiquitylated K17 via K63 linkage to maintain K17 stabilization. More importantly, we uncovered that K17 was a direct interactor of STAT3, and K17 ubiquitination could promote STAT3 activation in pso-mix-treated HaCaT cells. Our study demonstrated that targeting K17 ubiquitination may be a potential therapeutic approach by attenuating STAT3 signaling in psoriasis.

Dysfunction of TRIM21 in interferon signature of systemic lupus erythematosus

OBJECTIVES

TRIM21 is an E3 ubiquitin ligase for interferon regulatory factors (IRFs) that are involved in innate and acquired immunity. Here, we evaluated the role of TRIM21 in the interferon (IFN) signature of systemic lupus erythematosus (SLE).

METHODS

Twenty SLE patients and 24 healthy controls were enrolled in this study. We analyzed mRNA expression of TRIM21, type I IFN, and IFN-inducible genes in peripheral blood mononuclear cell (PBMC). The protein levels of IRFs were assessed by Western blotting in PBMCs cultured with or without MG-132.

RESULTS

The expression of TRIM21 mRNA and protein was significantly higher in SLE PBMCs as compared to healthy controls. There was a correlation between TRIM21 mRNA expression and SLE activities. In contrast to a negative correlation between mRNA expression level of TRIM21 and those of type I IFNs in healthy controls, we found a positive correlation between them in anti-TRIM21 antibody-positive SLE patients. Neither positive nor negative correlation was observed in the autoantibody-negative SLE patients. Western-blotting analysis revealed impaired ubiquitin-dependent proteasomal degradation of IRFs in SLE PBMCs.

CONCLUSION

Our study showed ubiquitin-dependent proteasomal degradation of IRFs was impaired in anti-TRIM21 antibody-dependent and -independent fashions, leading to amplification of IFN signature in SLE.

TRIM21 and the Function of Antibodies inside Cells

Therapeutic antibodies targeting disease-associated antigens are key tools in the treatment of cancer and autoimmunity. So far, therapeutic antibodies have targeted antigens that are, or are presumed to be, extracellular. A largely overlooked property of antibodies is their functional activity inside cells. The diverse literature dealing with intracellular antibodies emerged historically from studies of the properties of some autoantibodies. The identification of tripartite motif (TRIM) 21 as an intracellular Fc receptor linking cytosolic antibody recognition to the ubiquitin proteasome system brings this research into sharper focus. We review critically the research related to intracellular antibodies, link this to the TRIM21 effector mechanism, and highlight how this work is exposing the previously restricted intracellular space to the potential of therapeutic antibodies.

IgV peptide mapping of native Ro60 autoantibody proteomes in primary Sjögren's syndrome reveals molecular markers of Ro/La diversification

We have used high-resolution mass spectrometry to sequence precipitating anti-Ro60 proteomes from sera of patients with primary Sjögren's syndrome and compare immunoglobulin variable-region (IgV) peptide signatures in Ro/La autoantibody subsets. Anti-Ro60 were purified by elution from native Ro60-coated ELISA plates and subjected to combined de novo amino acid sequencing and database matching. Monospecific anti-Ro60 Igs comprised dominant public and minor private sets of IgG1 kappa and lambda restricted heavy and light chains. Specific IgV amino acid substitutions stratified anti-Ro60 from anti-Ro60/La responses, providing a molecular fingerprint of Ro60/La determinant spreading and suggesting that different forms of Ro60 antigen drive these responses. Sequencing of linked anti-Ro52 proteomes from individual patients and comparison with their anti-Ro60 partners revealed sharing of a dominant IGHV3-23/IGKV3-20 paired clonotype but with divergent IgV mutational signatures. In summary, anti-Ro60 IgV peptide mapping provides insights into Ro/La autoantibody diversification and reveals serum-based molecular markers of humoral Ro60 autoimmunity.

TRIM21: a cytosolic Fc receptor with broad antibody isotype specificity

Antibodies are key molecules in the fight against infections. Although previously thought to mediate protection solely in the extracellular environment, recent research has revealed that antibody-mediated protection extends to the cytosolic compartment of cells. This postentry viral defense mechanism requires binding of the antibody to a cytosolic Fc receptor named tripartite motif containing 21 (TRIM21). In contrast to other Fc receptors, TRIM21 shows remarkably broad isotype specificity as it does not only bind IgG but also IgM and IgA. When viral pathogens coated with these antibody isotypes enter the cytosol, TRIM21 is rapidly recruited and efficient neutralization occurs before the virus has had the time to replicate. In addition, inflammatory signaling is induced. As such, TRIM21 acts as a cytosolic sensor that engages antibodies that have failed to protect against infection in the extracellular environment. Here, we summarize our current understanding of how TRIM21 orchestrates humoral immunity in the cytosolic environment.

Brief Report: Patients With Primary Sjögren's Syndrome Who Are Positive for Autoantibodies to Tripartite Motif-Containing Protein 38 Show Greater Disease Severity

OBJECTIVE

Autoantibodies reactive with Ro52 (tripartite motif-containing protein 21 [TRIM21]) are detected in 70% of patients with primary Sjögren's syndrome (SS). TRIM21 belongs to a 34-member C-IV family of TRIM proteins. Although autoantibodies against other TRIM proteins within the C-IV family have been detected in the sera of patients with primary SS, their clinical relevance remains unclear. This study was undertaken to investigate the frequency of anti-TRIM38 in patients with primary SS and evaluate its association with various clinical measures of the disease.

METHODS

Serum samples from patients with primary SS (n = 235) and controls (n = 50) were analyzed for reactivity with in vitro-transcribed and -translated (35) S-methionine-labeled TRIM38 protein. The associations of anti-TRIM38 with various laboratory and clinical measures of primary SS were evaluated. Reactivity of anti-TRIM38 with different structural domains of TRIM38 was analyzed. Affinity-purified anti-TRIM38 antibodies were used to immunoprecipitate TRIM21.

RESULTS

TRIM38-reactive autoantibodies were detected in the sera of 24 of the 235 patients with primary SS and 2 of the 50 controls. Anti-TRIM38 positivity was significantly associated with the presence of anti-Ro60, anti-Ro52, anti-La, rheumatoid factor, and hypergammaglobulinemia. Clinically, anti-TRIM38 was associated with significantly higher ocular surface staining scores, lower Schirmer's test scores, and minor labial salivary gland biopsy focus scores of ≥3.0. Anti-TRIM38 antibodies mainly recognized the cortactin-binding protein 2 (CortBP-2; amino acids 128-238) and the B30.2/SPRY (amino acids 268-465) domains on TRIM38. Affinity-purified antibodies to TRIM38-CortBP-2 and TRIM38-B30.2/SPRY domains reacted with TRIM21.

CONCLUSION

Our data demonstrate that anti-TRIM38 specificity arising in a subset of patients with primary SS is associated with increased severity of the disease.

Rituximab in the Treatment of Jo1 Antibody–associated Antisynthetase Syndrome: Anti-Ro52 Positivity as a Marker for Severity and Treatment Response

Objective.

Rituximab (RTX) has been used successfully for the treatment of severe Jo1 antibody-associated antisynthetase syndrome. The aim of this retrospective study was to evaluate the effect of RTX in severe Jo1 antisynthetase syndrome and determine predictive factors for response.

Methods.

There were 61 patients with Jo1 antisynthetase syndrome identified; 18 of these received RTX. One patient was lost to followup. The remaining 17 patients and 30 out of 43 patients who were treated with conventional immunosuppressive (IS) drugs were followed for a mean of 35 months and 84 months, respectively.

Results.

Polymyositis/dermatomyositis (95%) and interstitial lung disease (ILD; 66%) were the dominant clinical manifestations. Detection of anti-Ro52 antibodies (43%) was significantly associated with acute-onset ILD (p = 0.016) with O2 dependency, and patients with high concentrations of anti-Ro52 (20%) had the highest risk (p = 0.0005). Sixteen out of 18 patients (89%) showed a fast and marked response to RTX. Among those patients who were highly positive for anti-Ro52, response to RTX was seen in 7 out of 7 cases (100%), but no response to cyclophosphamide (n = 4), cyclosporine A (n = 3), azathioprine (n = 9), methotrexate (n = 5), or leflunomide (n = 2) was observed. One patient treated with RTX died of pneumonia.

Conclusion.

RTX is effective in the treatment of severe forms of Jo1 antisynthetase syndrome. In our retrospective study, the presence of high anti-Ro52 antibody concentrations predicts severe acute-onset ILD and nonresponse to IS drugs. In contrast to conventional IS, RTX is equally effective in patients with Jo1 antisynthetase syndrome, independent of their anti-Ro52 antibody status.

Lysine11-Linked Polyubiquitination of the AnkB F-Box Effector of Legionella pneumophila

The fate of the polyubiquitinated protein is determined by the lysine linkages involved in the polymerization of the ubiquitin monomers, which has seven lysine residues (K(6), K(11), K(27), K(29), K(33), K(48), and K(63)). The translocated AnkB effector of the intravacuolar pathogen Legionella pneumophila is a bona fide F-box protein, which is localized to the cytosolic side of the Legionella-containing vacuole (LCV) and is essential for intravacuolar proliferation within macrophages and amoebae. The F-box domain of AnkB interacts with the host SCF1 E3 ubiquitin ligase that triggers the decoration of the LCV with K(48)-linked polyubiquitinated proteins that are targeted for proteasomal degradation. Here we report that AnkB becomes rapidly polyubiquitinated within the host cell, and this modification is independent of the F-box domain of AnkB, indicating host-mediated polyubiquitination. We show that the AnkB effector interacts specifically with the host E3 ubiquitin ligase Trim21. Mass spectrometry analyses have shown that AnkB is modified by K(11)-linked polyubiquitination, which has no effect on its stability. This work shows the first example of K(11)-linked polyubiquitination of a bacterial effector and its interaction with the host Trim21 ubiquitin ligase.

Toll-like receptor 3 stimulation promotes Ro52/TRIM21 synthesis and nuclear redistribution in salivary gland epithelial cells, partially via type I interferon pathway

Up-regulated expression of Ro52/tripartite motif-containing protein 21 (TRIM21), Ro60/TROVE domain family, member 2 (TROVE2) and lupus LA protein/Sjögren's syndrome antigen B (La/SSB) autoantigens has been described in the salivary gland epithelial cells (SGEC) of patients with Sjögren's syndrome (SS). SGECs, the key regulators of autoimmune SS responses, express high levels of surface functional Toll-like receptor (TLR)-3, whereas Ro52/TRIM21 negatively regulates TLR-3-mediated inflammation. Herein, we investigated the effect of TLR-3-signalling on the expression of Ro52/TRIM21, as well as Ro60/TROVE2 and La/SSB autoantigens, by SGECs. The effect of TLR-3 or TLR-4 stimulation on autoantigen expression was evaluated by polyI:C or lipopolysaccharide (LPS) treatment, respectively, of SGEC lines (10 from SS patients, 12 from non-SS controls) or HeLa cells, followed by analysis of mRNA and protein expression. PolyI:C, but not LPS, resulted in a two-step induction of Ro52/TRIM21 mRNA expression by SGECs, a 12-fold increment at 6 h followed by a 2.5-fold increment at 24-48 h, whereas it induced a late two-fold up-regulation of Ro60/TROVE2 and La/SSB mRNAs at 48 h. Although protein expression levels were not affected significantly, the late up-regulation of Ro52/TRIM21 mRNA was accompanied by protein redistribution, from nucleolar-like pattern to multiple coarse dots spanning throughout the nucleus. These late phenomena were mediated significantly by interferon (IFN)-β production, as attested by cognate secretion and specific inhibition experiments and associated with IFN regulatory factor (IRF)3 degradation. TLR-3-signalling had similar effects on SGECs obtained from SS patients and controls, whereas it did not affect the expression of these autoantigens in HeLa cells. TLR-3 signalling regulates the expression of autoantigens by SGECs, implicating innate immunity pathways in their over-expression in inflamed tissues and possibly in their exposure to the immune system.

Intracellular antibody immunity and the cytosolic Fc receptor TRIM21

Until recently, it was thought that antibody effector mechanisms were mediated purely by Fc receptors expressed on professional cells, following capture of immune complexes in the extracellular space. Recently a new Fc receptor, TRIM21, was discovered that is expressed by cells of all histogenetic lineages and which mediates immune responses intracellularly. This new receptor possesses many unique structural and functional properties. TRIM21 binds both IgG and IgM, interacts primarily with the CH3 rather than CH2 domain and engages two heavy chains simultaneously. This latter property allows TRIM21 to bind antibodies with a higher affinity than any other Fc receptor. TRIM21 is cytosolic, has both effector and signalling functions and is exquisitely conserved in mammals. The discovery of this missing part of humoral immunity has important implications for where and how antibodies work.

Common and specific associations of anti-SSA/Ro60 and anti-Ro52/TRIM21 antibodies in systemic lupus erythematosus

Little information exists about the association of anti-SSA/Ro60 and anti-Ro52/TRIM21 with systemic lupus erytematosus (SLE) features. In this work, we analysed the associations of both anti-Ro reactivities with clinical and immunological manifestations in 141 SLE patients. Photosensitivity and xerophtalmia/xerostomia were found to be positively associated with both anti-SSA/Ro60 (P = 0.024 and P = 0.019, resp.) and anti-Ro52/TRIM21 (P = 0.026 and P = 0.022, resp.). In contrast, a negative association was detected regarding anti-phospholipid antibodies, anti-SSA/Ro60 having a stronger effect (P = 0.014) than anti-Ro52/TRIM21. Anti-SSA/Ro60 showed a specific positive association with hypocomplementemia (P = 0.041), mainly with low C4 levels (P = 0.008), whereas anti-Ro52/TRIM21 was found to be positively associated with Raynaud's phenomenon (P = 0.026) and cytopenia (P = 0.048) and negatively associated with anti-dsDNA (P = 0.013). Lymphocytes are involved in the relationship between anti-Ro52/TRIM21 and cytopenia since positive patients showed lower cell levels than negative patients (P = 0.036). In conclusion, anti-SSA/Ro60 and anti-Ro52/TRIM21 showed both common and specific associations in SLE. These data thus increase evidence of the different associations of the two anti-Ro specificities even in a particular disease.

Cutaneous lupus erythematosus: clinical aspects and molecular pathogenesis

Lupus erythematosus (LE) is an autoimmune disease with diverse clinical manifestations ranging from limited cutaneous (CLE) to potentially life-threatening systemic disease (SLE). Susceptibility to LE arises from genetic variation in multiple loci, and disease activity is provoked by exogenous or endogenous trigger(s), the best characterized of which is exposure to ultraviolet radiation (UVR). Amongst patients with LE, a cluster of photosensitive subjects with cutaneous lesions and positivity for anti-Ro/SSA autoantibodies have been described. The Ro52 antigen belongs to the tripartite motif protein family and has E3 ligase activity. New data reveal that Ro52 ubiquitinates interferon regulatory factors and modulates their transcriptional activity, indicating an important role for Ro52 in inflammation as a negative feedback regulator. Our findings indicate that UVR exposure induces upregulation of Ro52 in the CLE target cell, the keratinocyte, and that Ro52 is upregulated in spontaneous and UVR-induced CLE lesions. Recently described functional analysis of Ro52-deficient mice revealed that loss of Ro52 results in uncontrolled inflammation in response to minor skin injury leading to an LE-like condition. In summary, emerging data suggest that abnormal function or regulation of Ro52 contributes to the pathogenesis of UVR-induced CLE in genetically susceptible individuals. Ro52 may thus be an interesting therapeutic target, as its activation could contribute to downregulation of the chronic inflammatory process in LE. Here, we review the available data on the pathogenesis of CLE and, in particular, the role of the Ro52 autoantigen.

Clinical and pathological roles of Ro/SSA autoantibody system

Anti-Ro/SSA antibodies are among the most frequently detected autoantibodies against extractable nuclear antigens and have been associated with systemic lupus erythematosus (SLE) and Sjögren's syndrome (SS). Although the presence of these autoantibodies is one of the criteria for the diagnosis and classification of SS, they are also sometimes seen in other systemic autoimmune diseases. In the last few decades, the knowledge of the prevalence of anti-Ro/SSA antibodies in various autoimmune diseases and symptoms has been expanded, and the clinical importance of these antibodies is increasing. Nonetheless, the pathological role of the antibodies is still poorly understood. In this paper, we summarize the milestones of the anti-Ro/SSA autoantibody system and provide new insights into the association between the autoantibodies and the pathogenesis of autoimmune diseases.

Ro52/SSA sensitizes cells to death receptor-induced apoptosis by down-regulating c-FLIP(L)

Ro52/SSA is an autoantigen that presents in patients with SS (Sjögren's syndrome) and SLE (systemic lupus erythematosus). It increases cell death and redistributes itself to apoptotic blebs, but its pro-apoptotic function has not been completely identified. Overexpression of Ro52/SSA promoted cell apoptosis induced by DR (death receptor) in caspase-8-dependent manner. Ro52/SSA expression down-regulated c-FLIP(L) [cellular (Fas-associated death domain)-like interleukin 1β-converting enzyme-inhibitory protein long form] expression, and Ro52/SSA siRNAs (small interfering RNAs) increased c-FLIP(L) production, indicating that Ro52/SSA plays a role in c-FLIP(L) regulation. Ro52/SSA negatively regulated c-FLIP(L) transcriptional level probably by suppressing NF-κB (nuclear factor κB) signalling. The data suggest that Ro52/SSA is involved in DR-mediated apoptosis by regulating c-FLIP(L).

Secreted human Ro52 autoantibody proteomes express a restricted set of public clonotypes

Long-lived secreted autoantibody responses in systemic autoimmunity are generally regarded to be polyclonal and to express a diverse B-cell repertoire. Here, we have used a proteomic approach based on de novo sequencing to determine the clonality and V region structures of human autoantibodies directed against a prototypic systemic autoantigen, Ro52 (TRIM21). Remarkably, anti-Ro52 autoantibodies from patients with primary Sjögren's syndrome, systemic lupus erythematosus, systemic sclerosis or polymyositis were restricted to two IgG1 kappa clonotypes that migrated as a single species on isoelectric focusing; shared a common light chain paired with one of two closely-related heavy chains; and were public in unrelated patients. Targeted mass spectrometry using these uniquely mutated V region peptides as surrogates detected anti-Ro52 autoantibodies in human sera with high sensitivity and specificity compared with traditional ELISA. Mass spectrometry-based detection of specific autoantibody motifs provides a powerful new tool for analysis of humoral autoimmunity.

Autoantigen TRIM21/Ro52 as a Possible Target for Treatment of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic, systemic, and autoimmune disease, whose etiology is still unknown. Although there has been progress in the treatment of SLE through the use of glucocorticoid and immunosuppressive drugs, these drugs have limited efficacy and pose significant risks of toxicity. Moreover, prognosis of patients with SLE has remained difficult to assess. TRIM21/Ro52/SS-A1, a 52-kDa protein, is an autoantigen recognized by antibodies in sera of patients with SLE and Sjögren's syndrome (SS), another systemic autoimmune disease, and anti-TRIM21 antibodies have been used as a diagnostic marker for decades. TRIM21 belongs to the tripartite motif-containing (TRIM) super family, which has been found to play important roles in innate and acquired immunity. Recently, TRIM21 has been shown to be involved in both physiological immune responses and pathological autoimmune processes. For example, TRIM21 ubiquitylates proteins of the interferon-regulatory factor (IRF) family and regulates type I interferon and proinflammatory cytokines. In this paper, we summarize molecular features of TRIM21 revealed so far and discuss its potential as an attractive therapeutic target for SLE.

Anti-ro52 antibodies and interstitial lung disease in connective tissue diseases excluding scleroderma

Introduction. The presence of anti-Ro52 antibodies has been reported in a wide variety of autoimmune diseases, particularly in myositis, scleroderma, and autoimmune liver diseases. Clinical significance of anti-Ro52 antibodies remains controversial, and studies are lacking for clarifying the association of anti-Ro52 with interstitial lung disease (ILD) in connective tissue diseases (CTD). Objectives. To determine if anti-Ro52 antibodies are associated with ILD in CTD other than scleroderma. Methods. Single-center, retrospective study based on immunoblotting panel analysis and patients clinical records. Results. In our connective tissue disease cohort, 162 patients had immunoblotting panels with anti-Ro52 reactivity analysis, 41 (25,3%) had inclusion criteria. Among the 41 selected sera, 85.4% (n = 35) had anti-Ro52 reactivity. The prevalence of ILD in the positive anti-Ro52 antibodies was 71.4% (n = 25), and 16.7% (n = 1) in the negative anti-Ro52 group (P = 0.018). Overall sensitivity (96.2%), specificity (83.3%), positive (71.4%) and negative (83.3%) predictive values of anti-Ro52 antibodies to determine ILD in CTD is detailed in this study. Conclusion. Ro52 autoantibodies are associated with ILD in CTD excluding scleroderma. We suggest that the presence of anti-Ro52 reactivity in CTD should increase the clinician curiosity for the search of ILD.

A conserved host and pathogen recognition site on immunoglobulins: structural and functional aspects

A common site in the constant region (Fc) of immunoglobulins is recognized by host receptors and is a frequent target of proteins expressed by pathogens. This site is located at the junction of two constant domains in the antibody heavy chains and produces a large shallow cavity formed by loops of the CH2 and CH3 domains in IgG and IgA (CH3 and CH4 domains in IgM). Crystal structures have been determined for complexes of IgG-Fc and IgA-Fc with a structurally diverse set of host, pathogen and in vitro selected ligands. While pathogen proteins may directly block interactions with the immunoglobulins thereby evading host immunity, it is likely that the same pathogen molecules also interact with other host factors to carry out their primary biological function. Herein we review the structural and functional aspects of host and pathogen molecular recognition of the common site on the Fc of immunoglobulins. We also propose that some pathogen proteins may promote virulence by affecting the bridging between innate and adaptive immunity.

The complexity of Sjögren's syndrome: novel aspects on pathogenesis

In Sjögren's syndrome, like in most other autoimmune diseases, the enigma leading to a pathogenic attack against self has not yet been solved. By definition, the disease must be mediated by specific immune reactions against endogenous tissues to qualify as an autoimmune disease. In Sjögren's syndrome the autoimmune response is directed against the exocrine glands, which, as histopathological hallmark of the disease, display persistent and progressive focal mononuclear cell infiltrates. Clinically, the disease in most patients is manifested by two severe symptoms: dryness of the mouth (xerostomia) and the eyes (keratoconjunctivitis sicca). A number of systemic features have also been described and the presence of autoantibodies against the ubiquitously expressed ribonucleoprotein particles Ro (Sjögren's-syndrome-related antigen A - SSA) and La (SSB) further underline the systemic nature of Sjögren's syndrome. The original explanatory concept for the pathogenesis of Sjögren's syndrome proposed a specific, self-perpetuating, immune mediated loss of acinar and ductal cells as the principal cause of salivary gland hypofunction. Although straightforward and plausible, the hypothesis, however, falls short of accommodating several Sjögren's syndrome-related phenomena and experimental findings. Consequently, researchers considered immune-mediated salivary gland dysfunction prior to glandular destruction and atrophy as potential molecular mechanisms underlying the symptoms of dryness in Sjögren's syndrome. Accordingly, apoptosis, fibrosis and atrophy of the salivary glands would represent consequences of salivary gland hypofunction. The emergence of advanced bio-analytical platforms further enabled the identification of potential biomarkers with the intent to improve Sjögren's syndrome diagnosis, promote the development of prognostic tools for Sjögren's syndrome and the long-term goal to identify possible processes for therapeutic treatment interventions. In addition, such approaches allowed us to glimpse at the apparent complexity of Sjögren's syndrome.

Clinical significance of anti-Ro52 (TRIM21) antibodies non-associated with anti-SSA 60kDa antibodies: results of a multicentric study

Ro52 antigen has recently been identified as TRIM21 protein, but the clinical significance of anti-Ro52/TRIM21 antibodies remains controversial. The aim of this multicentric study was to investigate the significance of anti-Ro52 antibodies without anti-SSA/Ro60 antibodies in various connective diseases. Sera were selected by each laboratory using its own method (ELISA, immunodot or Luminex technology), and then performed with ANA Screen BioPlex™ reagent (BIO-RAD). Among the 247 screened sera, 155/247 (63%) were confirmed as anti-Ro52 positive and anti-SSA/Ro60 negative. These sera were analyzed for the detection of other antibodies in relation with clinical settings. Isolated anti-Ro52 antibodies were detected in 89/155 (57%) sera. For the remaining sera (66/155), the main antibodies associations were Sm/SmRNP or Chromatin (n=38; 57%), Jo1 (n=17; 26%) and CenpB (n=9; 14%). Clinical data from the 155 patients showed high prevalence in autoimmune diseases (73%) including myositis or dermatomyositis (n=30), lupus (n=23); Sjögren and/or sicca syndrome (n=27); CREST or Systemic sclerosis (n=11) and autoimmune hepatitis (n=11). We found that pulmonary manifestations were often associated with the presence of anti-Ro52 antibodies (n=34, 22%), in addition with anti-tRNA synthetases, anti-SRP or anti-Ku antibodies (18/34) or isolated in half of cases (16/34). Separate detection of anti-Ro52 antibodies might be useful in related antisynthetase syndrome diagnosis. The presence of anti-Ro52 antibodies should probably precede development of autoimmune disease and must induce sequential follow-up of positive patients, particularly in interstitial lung disease progression.

Location of immunization and interferon-γ are central to induction of salivary gland dysfunction in Ro60 peptide immunized model of Sjögren's syndrome

INTRODUCTION

Anti-Ro antibodies can be found in the serum of the majority of patients with Sjögren's syndrome (SS). Immunization with a 60-kDa Ro peptide has been shown to induce SS-like symptoms in mice. The aim of this study was to investigate factors involved in salivary gland (SG) dysfunction after immunization and to test whether the induction of SS could be improved.

METHODS

Ro60 peptide immunization was tested in Balb/c mice, multiple antigenic peptide (MAP)-Ro60 and Pertussis toxin (PTX) were tested in SJL/J mice. In addition, two injection sites were compared in these two strains: the abdominal area and the tailbase. Each group of mice was tested for a loss of SG function, SG lymphocytic infiltration, anti-Ro and anti-La antibody formation, and cytokine production in cultured cells or homogenized SG extracts.

RESULTS

Ro60 peptide immunization in the abdominal area of female Balb/c mice led to impaired SG function, which corresponded with increased Th1 cytokines (IFN-γ and IL-12) systemically and locally in the SG. Moreover, changing the immunization conditions to MAP-Ro60 in the abdominal area, and to lesser extend in the tailbase, also led to impaired SG function in SJL/J mice. As was seen in the Balb/c mice, increased IFN-γ in the SG draining lymph nodes accompanied the SG dysfunction. However, no correlation was observed with anti-MAP-Ro60 antibody titers, and there was no additional effect on disease onset or severity.

CONCLUSIONS

Effective induction of salivary gland dysfunction after Ro60 peptide immunization depended on the site of injection. Disease induction was not affected by changing the immunization conditions. However, of interest is that the mechanism of action of Ro60 peptide immunization appears to involve an increase in Th1 cytokines, resulting in the induction of SG dysfunction.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.

Tripartite-motif proteins and innate immune regulation

The tripartite motif containing (TRIM) proteins are a family of proteins that have been implicated in many biological processes including cell differentiation, apoptosis, transcriptional regulation and signaling pathways. Many TRIM proteins are upregulated by the immunologically important Type I and Type II interferons and several, including TRIM5α and TRIM19/PML, restrict viral replication. There is growing evidence that TRIMs also play an important role in the broader immune response through regulating signaling pathways such as the RIG-I pathway. In this review we discuss recent research elucidating TRIM regulation of a number of pathways important in immunity and review the latest findings relating to viral restriction by TRIMs.

Autoimmune response and target autoantigens in Sjogren's syndrome

BACKGROUND

Primary Sjogren's syndrome (pSS) is characterized by the presence of autoantibodies targeting mainly the Ro/La ribonucleoprotein complex. It is now appreciated that the production of autoantibodies is an antigen-driven immune response.

DESIGN

In this review, candidate mechanisms for autoantigen presentation and perpetuation of the autoimmune response within the autoimmune tissue lesion of pSS are discussed.

RESULTS

Several studies have shown that the epithelial cell in labial salivary glands of patients with Sjogren's syndrome is activated, bearing characteristics of an antigen-presenting cell, as suggested by inappropriate expression of class II HLA and co-stimulatory molecules. Other studies have confirmed that in salivary glands, there is an increased autoantigen presentation via apoptotic blebs and bodies, exosomes and heat shock protein-mediated cross-priming. There is also an increased expression of interferon (IFN)-induced genes, such as the autoantigen Ro52, which provide negative feedback regulation in inflammation. Ro60 and La autoantigens also appear to play a major role in the local autoimmune response in Sjogren's syndrome. In this regard, La and Ro60 the messenger RNA (mRNA) expression is upregulated in the affected salivary glands with different isoforms of La autoantigen mRNA to be expressed in patients with pSS. At the protein level, La/SSB in pSS salivary glands is found to be post-translationally modified.

CONCLUSIONS

 Autoantigen alterations in a microenvironment of local inflammation with increased in situ apoptosis, Toll-like receptor (TLR) signalling and antigen presentation may drive the autoimmune response and local autoantibody production in pSS.

Cytoplasmic relocation of Daxx induced by Ro52 and FLASH

The RING-finger protein Ro52/TRIM21 is known to be an autoantigen and is recognized by anti-Ro/SSA antibodies, which are commonly found in patients with Sjögren's syndrome and systemic lupus erythematosus. We recently showed that Ro52 is an E3 ubiquitin ligase and localizes to cytoplasmic bodies that are highly motile along the microtubule network. To expand our knowledge of Ro52, we searched partners co-operating with Ro52. We performed a yeast two-hybrid screening of a human brain cDNA library with Ro52 as bait. This screening identified several genes encoding Ro52-interacting proteins, including the apoptosis-related proteins, Daxx and FLASH. Further yeast two-hybrid assays revealed that Daxx binds to the B30.2 domain of Ro52 and that FLASH binds to coiled-coil domains of Ro52 through its death-effector domain-recruiting domain. These results suggest that Ro52, Daxx, and FLASH form heteromeric protein complexes. Indeed, this was supported by results of immunoprecipitation experiments in which we found that Daxx is co-immunoprecipitated with Ro52 in the presence of overexpressed FLASH. Importantly, our fluorescence microscopy revealed that, although Daxx is predominantly located in the nucleus, overexpression of both Ro52 and FLASH leads to relocation of Daxx into the cytoplasm. Thus, Ro52 seems to co-operate with FLASH to induce cytoplasmic localization of Daxx in cells.

Downregulation of active IKK beta by Ro52-mediated autophagy

Upon activation, NF-kappaB translocates into the nucleus and initiates many biological events. This NF-kappaB signaling is mainly induced by the protein kinase IKK beta. Early in this signaling pathway, IKK beta is phosphorylated for activation by several factors, such as pro-inflammatory cytokines and the Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1). In cells expressing Tax protein, IKK beta is persistently phosphorylated, which chronically activates NF-kappaB signaling. But the active IKK beta is conjugated with a monoubiquitin by the E3 ubiquitin ligase Ro52, and the IKK beta-induced NF-kappaB signaling is downregulated. However, the mechanism of the downregulation has been unknown. Here, we show that Ro52-mediated monoubiquitination is involved in the subcellular translocation of active IKK beta to autophagosomes. Furthermore, using reporter assays, we show that Ro52 suppresses IKK beta-induced NF-kappaB signaling and that this suppression is blocked by an autophagy inhibitor. These results suggest that Ro52-mediated monoubiquitination plays a critical role in the downregulation of active IKK beta through autophagy.

[How can we diagnose and better understand inflammatory myopathies? The usefulness of auto-antibodies]

The inflammatory myopathies are a group of quite proteiform, systemic auto-immune diseases which include polymyositis, dermatomyositis and inclusion body myopathies. To facilitate the diagnosis, classification criteria (Bohan and Peter, 1975) have been proposed, based essentially on clinical criteria. In addition, over the past fifteen years, auto-antibodies characterizing certain forms of inflammatory myopathy have been identified. One distinguishes schematically: auto-antibodies specific for myositis and auto-antibodies sometimes associated with myositis. Concerning the myositis specific auto-antibodies (MSA), schematically there are a dozen specificities which are classed according to the cellular distribution of the auto-antigen. The most characteristic are certainly the auto-antibodies directed against cytoplasmic antigens: the anti-tRNA synthetases (anti-Jo-1 (PL-1), anti-PL-7, PL-12, EJ, OJ, JS, KS, ZO, YRS), anti-SRP (signal recognition particle), anti-Mas and anti-KJ, anti-Fer (eEF1), anti-Wa and anti-CADM p140. Other auto-antibodies are directed against nuclear auto-antigens: the anti-Mi-2, anti-PMS (PMS1, PMS2) and related antibodies (MLH1, DNA PKcs…), anti-56 kDa, anti-MJ (NXP-2), anti-SAE and anti-p155/p140 (TIF-1γ). Concerning the auto-antibodies sometimes associated with myositis (myositis associated auto-antibodies or MAA), they can also be observed in other auto-immune diseases. These auto-antibodies are directed against nuclear or nucleolar auto-antigens: the anti-PM-Scl, anti-Ku, anti-RNP (U1 RNP and U2 RNP, U4/U6 RNP and U5 RNP), anti-Ro 52 kDa and more rarely, anti-Ro 60 kDa and anti-La. The auto-antibodies related to myositis are biological tools which are of interest in two main ways. They have allowed us to sort out the nosology of these inflammatory myopathies, in particular by defining anti-tRNA synthetase syndrome. It now remains to determine how they might be employed to complement the classical clinico-biological diagnostic criteria. In this perspective, it will be indispensable first of all to diffuse and standardize the methods of detection. The latter are at the moment very heterogeneous as they use techniques and above all antigenic preparations which are extremely diverse. These antibodies are also very interesting "physiopathological" tools to try to better understand myositis. The example of anti-tRNA synthetases is a particularly original model of auto-immunization, which allows one to establish a link between an initial, probably poorly specific muscular lesion and the appearance of auto-antibodies which maintain and aggravate the muscular disease.

Extraordinary antigenicity of the human Ro52 autoantigen

Autoantibody levels to the SSA complex, composed of Ro52 and Ro60 proteins, are commonly measured in the diagnoses of Sjögren's Syndrome (SjS), as well as other rheumatological diseases. One of these proteins, Ro52, is an interferon-inducible member of the tripartite motif family bearing a RING motif functioning as an E3 ligase that ubiquitinates interferon regulatory factor 8 and other proteins. Using Luciferase Immunoprecipitation Systems (LIPS) we explored the antigenicity of Ro52 in detail. Analysis of antibody responses against Ro52 and 20 other established antigens revealed that Ro52 had the highest antibody titers and most likely represents one of the most immunogenic human proteins. While the antibody titers in many of the SjS patients were significantly and substantially higher than the controls, all healthy individuals had anti-Ro52 autoantibodies. N- and C-terminal fragments of Ro52 showed immunoreactivity in these serum samples, but the sums of these antibody titers were significantly lower than the antibody titers directed against the full-length Ro52. Antibody profiling of controls and SjS patients with three different N-terminal fragments of Ro52 revealed that the coiled-coil region was the most useful diagnostic (66% sensitivity), followed by the B-box (31% sensitivity), and then the RING-finger (24% sensitivity). The C-terminal region of Ro52, containing the B30.2 domain, showed higher antibody titers in SjS patients compared to controls and this region was responsible for the high level of Ro52 immunoreactivity in healthy individuals. Analysis of immunoreactivity to TRIM5, a Ro52-related protein, and the B30.2 domain from BTN1 and pyrin, failed to show significant antibody titers with the control or SjS patient serum. These results highlight the unusually high level of Ro52 antigenicity and demonstrate that autoantibodies are directed at both linear and conformational epitopes spanning the entire molecule.