Antibody- and TRIM21-dependent intracellular restriction of Salmonella enterica

Target-induced clustering activates Trim-Away of pathogens and proteins

Trim-Away is a recently developed technology that exploits off-the-shelf antibodies and the RING E3 ligase and cytosolic antibody receptor TRIM21 to carry out rapid protein depletion. How TRIM21 is catalytically activated upon target engagement, either during its normal immune function or when repurposed for targeted protein degradation, is unknown. Here we show that a mechanism of target-induced clustering triggers intermolecular dimerization of the RING domain to switch on the ubiquitination activity of TRIM21 and induce virus neutralization or drive Trim-Away. We harness this mechanism for selective degradation of disease-causing huntingtin protein containing long polyglutamine tracts and expand the Trim-Away toolbox with highly active TRIM21-nanobody chimeras that can also be controlled optogenetically. This work provides a mechanism for cellular activation of TRIM RING ligases and has implications for targeted protein degradation technologies.

B-Cells and Antibodies as Contributors to Effector Immune Responses in Tuberculosis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb), is still a major threat to mankind, urgently requiring improved vaccination and therapeutic strategies to reduce TB-disease burden. Most present vaccination strategies mainly aim to induce cell-mediated immunity (CMI), yet a series of independent studies has shown that B-cells and antibodies (Abs) may contribute significantly to reduce the mycobacterial burden. Although early studies using B-cell knock out animals did not support a major role for B-cells, more recent studies have provided new evidence that B-cells and Abs can contribute significantly to host defense against Mtb. B-cells and Abs exist in many different functional subsets, each equipped with unique functional properties. In this review, we will summarize current evidence on the contribution of B-cells and Abs to immunity toward Mtb, their potential utility as biomarkers, and their functional contribution to Mtb control.

LAMP3 induces apoptosis and autoantigen release in Sjögren's syndrome patients

Primary Sjögren's syndrome (pSS) is a complex autoimmune disease characterized by dysfunction of secretory epithelia with only palliative therapy. Patients present with a constellation of symptoms, and the diversity of symptomatic presentation has made it difficult to understand the underlying disease mechanisms. In this study, aggregation of unbiased transcriptome profiling data sets of minor salivary gland biopsies from controls and Sjögren's syndrome patients identified increased expression of lysosome-associated membrane protein 3 (LAMP3/CD208/DC-LAMP) in a subset of Sjögren's syndrome cases. Stratification of patients based on their clinical characteristics suggested an association between increased LAMP3 expression and the presence of serum autoantibodies including anti-Ro/SSA, anti-La/SSB, anti-nuclear antibodies. In vitro studies demonstrated that LAMP3 expression induces epithelial cell dysfunction leading to apoptosis. Interestingly, LAMP3 expression resulted in the accumulation and release of intracellular TRIM21 (one component of SSA), La (SSB), and α-fodrin protein, common autoantigens in Sjögren's syndrome, via extracellular vesicles in an apoptosis-independent mechanism. This study defines a clear role for LAMP3 in the initiation of apoptosis and an independent pathway for the extracellular release of known autoantigens leading to the formation of autoantibodies associated with this disease.ClinicalTrials.gov Identifier: NCT00001196, NCT00001390, NCT02327884.

HDAC6 regulates antibody-dependent intracellular neutralization of viruses via deacetylation of TRIM21

Tripartite motif-containing protein 21 (TRIM21) is a cytosolic antibody receptor that targets the internalized virus-antibody complex to the proteasome for degradation. However, the precise mechanism regulating TRIM21 activity is unknown. Here we show that TRIM21 is a substrate of histone deacetylase 6 (HDAC6) and that its function is regulated by acetylation. HDAC6 interacts with TRIM21 through its PRYSPRY motif and deacetylates TRIM21 at lysine 385 and lysine 387, thus promoting its homodimerization. Inhibiting HDAC6 activity increases TRIM21 acetylation, and hyperacetylation blocks TRIM21 dimerization and ubiquitination, preventing its binding to the virus-antibody complex and its degradation via the ubiquitin-proteasome pathway. HDAC6 depletion or inhibition increases virus accumulation in cells, indicative of an impaired capacity for antibody-dependent intracellular neutralization of viruses, whereas TRIM21 acetylation-deficient K385/387R mutant rescues HDAC6 depletion-caused ADIN impairment. These findings provide evidence for HDAC6 as a novel regulator of TRIM21-mediated intracellular innate immunity.

Accessibility of O Antigens Shared between Salmonella Serovars Determines Antibody-Mediated Cross-Protection

Pathogenic Salmonella serovars produce clinical manifestations ranging from systemic infection typhoid to invasive nontyphoidal Salmonella disease in humans. These serovars share a high degree of homology at the genome and the proteome level. However, whether infection or immunization with one serovar provides protection against other serovars has not been well studied. We show in this study that immunization of mice with live typhoidal serovar, Salmonella Typhi, generates cross-reactive immune responses, which provide far greater resistance against challenge with nontyphoidal serovar Salmonella Enteritidis than with another nontyphoidal serovar, Salmonella Typhimurium. Splenic T cells from these immunized mice produced similar levels of IL-2 and IFN-γ upon ex vivo stimulation with Ags prepared from S Enteritidis and S Typhimurium. In contrast, Abs against S Typhi interacted with live intact S Enteritidis but did not bind intact S Typhimurium. These pathogen-reactive Abs were largely directed against oligosaccharide (O)-antigenic determinant of LPS that S Typhi shares with S Enteritidis. Abs against the O determinant, which S Typhi shares with S Typhimurium, were present in the sera of immunized mice but did not bind live intact Salmonella because of surface inaccessibility of this determinant. Similar accessibility-regulated interaction was seen with Abs generated against S Typhimurium and S Enteritidis. Our results suggest that the ability of protective Abs elicited with one Salmonella serovar to engage with and consequently provide protection against another Salmonella serovar is determined by the accessibility of shared O Ags. These findings have significant and broader implications for immunity and vaccine development against pathogenic Salmonellae.

TRIM proteins in autophagy: selective sensors in cell damage and innate immune responses

Autophagy, a main intracellular catabolic process, is induced in response to a variety of cellular stresses to promptly degrade harmful agents and to coordinate the activity of prosurvival and prodeath processes in order to determine the fate of the injured cells. While the main components of the autophagy machinery are well characterized, the molecular mechanisms that confer selectivity to this process both in terms of stress detection and cargo engulfment have only been partly elucidated. Here, we discuss the emerging role played by the E3 ubiquitin ligases of the TRIM family in regulating autophagy in physiological and pathological conditions, such as inflammation, infection, tumorigenesis, and muscle atrophy. TRIM proteins employ different strategies to regulate the activity of the core autophagy machinery, acting either as scaffold proteins or via ubiquitin-mediated mechanisms. Moreover, they confer high selectivity to the autophagy-mediated degradation as described for the innate immune response, where TRIM proteins mediate both the engulfment of pathogens within autophagosomes and modulate the immune response by controlling the stability of signaling regulators. Importantly, the elucidation of the molecular mechanisms underlying the regulation of autophagy by TRIMs is providing important insights into how selective types of autophagy are altered under pathological conditions, as recently shown in cancer and muscular dystrophy.

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.

Ehrlichia chaffeensis Outer Membrane Protein 1-Specific Human Antibody-Mediated Immunity Is Defined by Intracellular TRIM21-Dependent Innate Immune Activation and Extracellular Neutralization

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism.

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism. Addition of OMP-1-specific huMAb EHRL-15 (IgG1) prevented infection by blocking attachment/entry, a mechanism previously reported; conversely, OMP-1-specific huMAb EHRL-4 (IgG3) engaged intracellular TRIM21 and initiated an immediate innate immune response and rapid intracellular degradation of ehrlichiae. EHRL-4-TRIM21-mediated inhibition was significantly impaired in TRIM21 knockout THP-1 cells. EHRL-4 interacted with cytosolic Fc receptor TRIM21, observed by confocal microscopy and confirmed by co-immunoprecipitation. E. chaffeensis-EHRL-4-TRIM21 complexes caused significant upregulation of proinflammatory cytokine/chemokine transcripts and resulted in rapid (<30 min) nuclear accumulation of NF-κB and TRIM21 and ehrlichial destruction. We investigated the role of TRIM21 in the autophagic clearance of ehrlichiae in the presence of EHRL-4. Colocalization between EHRL-4-opsonized ehrlichiae, polyubiquitinated TRIM21, autophagy regulators (ULK1 and beclin 1) and effectors (LC3 and p62), and lysosome-associated membrane protein 2 (LAMP2) was observed. Moreover, autophagic flux defined by conversion of LC3I to LC3II and accumulation and degradation of p62 was detected, and EHRL-4-mediated degradation of E. chaffeensis was abrogated by the autophagy inhibitor 3-methyladenine. Our results demonstrate that huMAbs are capable of inhibiting E. chaffeensis infection by distinct effector mechanisms: extracellularly by neutralization and intracellularly by engaging TRIM21, which mediates a rapid innate immune response that mobilizes the core autophagy components, triggering localized selective autophagic degradation of ehrlichiae.

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.

Enhancing Antigen Cross-Presentation in Human Monocyte-Derived Dendritic Cells by Recruiting the Intracellular Fc Receptor TRIM21

Suboptimal immune responses to pathogens contribute to chronic infections. One way to improve immune responses is to boost Ag presentation. In this study, we investigate the potential of the tripartite motif-containing 21 (TRIM21) pathway. TRIM21 is a ubiquitously expressed cytosolic protein that recognizes the Fc region of Abs. When Abs that are bound to pathogens enter the cell as immune complexes, binding of TRIM21 to Fc initiates downstream inflammatory signaling and targets the immune complexes for proteasomal degradation. In APCs, peptides generated by proteasomes are loaded onto MHC class I molecules to stimulate CD8 T cell responses, which are crucial for effective immunity to pathogens. We hypothesized that increasing the affinity between immune complexes and TRIM21 might markedly improve CD8 T cell responses to Ags processed by the TRIM21 pathway. Using phage display technology, we engineered the human IgG1 Fc to increase its affinity for TRIM21 by 100-fold. Adenovirus immune complexes with the engineered Fc induced greater maturation of human dendritic cells (DC) than immune complexes with unmodified Fc and stimulated increased Ag-specific CD8 T cell proliferation and IFN-γ release in cocultures of DC-PBMC. Thus, by increasing the affinity between Fc and TRIM21, Ags from immune complexes undergo enhanced cross-presentation on DC, leading to greater CD8 T cell responses. Our study reveals an approach that could potentially be used in vaccines to increase cytotoxic T cell responses against Ags that are targeted or delivered by Fc-modified Abs.

TRIM21 Restricts Coxsackievirus B3 Replication, Cardiac and Pancreatic Injury via Interacting With MAVS and Positively Regulating IRF3-Mediated Type-I Interferon Production

Tripartite motif-containing 21 (TRIM21) is a regulator of tissue inflammation and pro-inflammatory cytokine production, and has been implicated in negative regulation of IRF3-dependent type I interferon signaling. However, the antiviral activity of TRIM21 varies among diverse viruses and its role on regulation of type I interferon remains inconsistent in different microbial infections. Here, we investigate the potential role for TRIM21 in controlling Coxsackievirus B3 (CVB3) replication and susceptible organ pathology. We found that CVB3 infection up-regulated the expression of TRIM21 in hearts of mice and cardiomyocytes at early phase of infection. Knock-down of TRIM21 resulted in increased viral replication, while overexpression led to increased phosphorylation and dimerization of IRF3, increased IFN-β transcription and reduced viral replication in vitro. We demonstrate that TRIM21 promotes the activation of IRF3 in CVB3-infected cells via interacting with MAVS and catalyzing the K27-linked polyubiquitination of MAVS, thereby enhancing type I interferon signaling. The RING domain of ubiquitin ligase activity and PRY-SPRY domain of TRIM21 are critical for its anti-viral effect. In vivo overexpression of TRIM21 significantly protected mice against viral myocarditis by suppressing CVB3 replication and reducing cardiac inflammatory cytokine production. While TRIM21 deficient mice exhibited a decreased IFN-β production, an increased cardiac and pancreatic CVB3 replication, and aggravated pancreatic injury as well as myocarditis during acute infection. Thus, our results demonstrate TRIM21 as a positive regulator of IFN-β signaling by targeting MAVS during CVB3 infection and suggest it as a potent host defense against CVB3 infection and viral-induced injury in hearts and pancreas.

Autoantibodies against the Immunoglobulin-Binding Region of Ro52 Link its Autoantigenicity with Pathogen Neutralization

Ro52/TRIM21 plays a key role in antibody-dependent pathogen neutralization and is a major autoantigen in systemic lupus erythematosus, Sjögren's syndrome (SS), and other autoimmune diseases. Here we evaluated immunoreactivity against Ro52-related molecules in SS and healthy volunteers. Although most proteins examined were not antigenic, several TRIM paralogs, including TRIM22, and TRIM38, showed sporadic immunoreactivity in SS. In contrast, the murine Ro52 ortholog with limited linear homology demonstrated high levels of autoantibodies implicating the importance of shared conformational epitopes. To further explore the autoantigencity of Ro52, deletion and point mutant analyses were employed revealing previously hidden, robust autoantibodies directed against its C-terminal immunoglobulin-binding domain. Another autoantibody, rheumatoid factor, targeting the Fc region of IgG, strongly overlapped with Ro52 seropositivity (odds ratio 14; P < 0.0001). These convergent mechanistic findings support a model whereby intracellular Ro52-bound antibody-coated pathogen complexes, released or misprocessed from infected cells, drive autoantigenicity against Ro52 and the Fc region of IgG.

RNF166 Determines Recruitment of Adaptor Proteins during Antibacterial Autophagy

Xenophagy is a form of selective autophagy that involves the targeting and elimination of intracellular pathogens through several recognition, recruitment, and ubiquitination events. E3 ubiquitin ligases control substrate selectivity in the ubiquitination cascade; however, systematic approaches to map the role of E3 ligases in antibacterial autophagy have been lacking. We screened more than 600 putative human E3 ligases, identifying E3 ligases that are required for adaptor protein recruitment and LC3-bacteria colocalization, critical steps in antibacterial autophagy. An unbiased informatics approach pinpointed RNF166 as a key gene that interacts with the autophagy network and controls the recruitment of ubiquitin as well as the autophagy adaptors p62 and NDP52 to bacteria. Mechanistic studies demonstrated that RNF166 catalyzes K29- and K33-linked polyubiquitination of p62 at residues K91 and K189. Thus, our study expands the catalog of E3 ligases that mediate antibacterial autophagy and identifies a critical role for RNF166 in this process.

A novel whole-bacterial enzyme linked-immunosorbant assay to quantify Chlamydia trachomatis specific antibodies reveals distinct differences between systemic and genital compartments

Chlamydia trachomatis (CT) is the leading sexually transmitted bacterial infection. The continued global burden of CT infection strongly predicates the need for a vaccine to supplement current chlamydial control programs. The correlates of protection against CT are currently unknown, but they must be carefully defined to guide vaccine design. The localized nature of chlamydial infection in columnar epithelial cells of the genital tract necessitates investigation of immunity at the site of infection. The purpose of this study was to develop a sensitive whole bacterial enzyme-linked immunosorbent assay (ELISA) to quantify and compare CT-specific IgG and IgA in sera and genital secretions from CT-infected women. To achieve this, elementary bodies (EBs) from two of the most common genital serovars (D and E) were attached to poly-L-lysine-coated microtiter plates with glutaraldehyde. EB attachment and integrity were verified by the presence of outer membrane antigens and the absence of bacterial cytoplasmic antigens. EB-specific IgG and IgA standards were developed by pooling sera with high titers of CT-specific antibodies from infected women. Serum, endocervical and vaginal secretions, and endocervical cytobrush specimens from CT-infected women were used to quantify CT-specific IgG and IgA which were then normalized to total IgG and IgA, respectively. Analyses of paired serum and genital samples revealed significantly higher proportions of EB-specific antibodies in genital secretions compared to sera. Cervical and vaginal secretions and cytobrush specimens had similar proportions of EB-specific antibodies, suggesting any one of these genital sampling techniques could be used to quantify CT-specific antibodies when appropriate normalization methodologies are implemented. Overall, these results illustrate the need to investigate genital tract CT antibody responses, and our assay provides a useful quantitative tool to assess natural immunity in defined clinical groups and CT vaccine trials.

Ubiquitin enzymes in the regulation of immune responses

Ubiquitination plays a central role in the regulation of various biological functions including immune responses. Ubiquitination is induced by a cascade of enzymatic reactions by E1 ubiquitin activating enzyme, E2 ubiquitin conjugating enzyme, and E3 ubiquitin ligase, and reversed by deubiquitinases. Depending on the enzymes, specific linkage types of ubiquitin chains are generated or hydrolyzed. Because different linkage types of ubiquitin chains control the fate of the substrate, understanding the regulatory mechanisms of ubiquitin enzymes is central. In this review, we highlight the most recent knowledge of ubiquitination in the immune signaling cascades including the T cell and B cell signaling cascades as well as the TNF signaling cascade regulated by various ubiquitin enzymes. Furthermore, we highlight the TRIM ubiquitin ligase family as one of the examples of critical E3 ubiquitin ligases in the regulation of immune responses.

TRIM21 is critical for survival of Toxoplasma gondii infection and localises to GBP-positive parasite vacuoles

Interferon gamma (IFNγ) is the major proinflammatory cytokine conferring resistance to the intracellular vacuolar pathogen Toxoplasma gondii by inducing the destruction of the parasitophorous vacuole (PV). We previously identified TRIM21 as an IFNγ-driven E3 ubiquitin ligase mediating the deposition of ubiquitin around pathogen inclusions. Here, we show that TRIM21 knockout mice were highly susceptible to Toxoplasma infection, exhibiting decreased levels of serum inflammatory cytokines and higher parasite burden in the peritoneum and brain. We demonstrate that IFNγ drives recruitment of TRIM21 to GBP1-positive Toxoplasma vacuoles, leading to Lys63-linked ubiquitination of the vacuole and restriction of parasite early replication without interfering with vacuolar disruption. As seen in vivo, TRIM21 impacted the secretion of inflammatory cytokines. This study identifies TRIM21 as a previously unknown modulator of Toxoplasma gondii resistance in vivo thereby extending host innate immune recognition of eukaryotic pathogens to include E3 ubiquitin ligases.

Ro52/TRIM21-deficient expression and function in different subsets of peripheral blood mononuclear cells is associated with a proinflammatory cytokine response in patients with idiopathic inflammatory myopathies

The presence of anti-Ro52/tripartite motif 21 (Trim21) autoantibodies has been associated with a distinctive clinical profile and has gained value as a prognostic marker in idiopathic inflammatory myopathies (IIM). The aim of the present work was to analyse Ro52/Trim21 expression in different subsets of peripheral blood mononuclear cells (PBMCs) of patients with IIM, as well as the ubiquitination profile and its association with proinflammatory cytokine production. We included 18 patients with recent-onset IIM and 18 age- and gender-matched healthy donors. PBMCs were isolated and different subsets (CD4⁺ , CD8⁺ , CD14⁺ ) were purified by magnetic selection. The expression of Ro52/Trim21 in different PBMC subsets of patients with IIM and healthy donors was analysed by Western blot. We assessed the presence of myositis-specific and associated autoantibodies by enzyme-linked immunosorbent assay (ELISA). Cytokine levels were measured by cytometric bead array. Patients with IIM showed decreased protein expression of Ro52/Trim21 in comparison to healthy controls in PBMC (0·97 ± 0·60 versus 1·84 ± 0·92, P = 0·016), CD4⁺ lymphocytes (0·79 ± 0·54 versus 2·41 ± 0·78, P = 0·017), and monocytes (0·87 ± 0·35 versus 1·89 ± 0·20, P < 0·001). There were no significant differences among IIM groups. Also, a lower K48-mediated ubiquitination profile was found, predominantly in CD4⁺ lymphocytes. Furthermore, after mitogenic stimulation, there was a higher synthesis of proinflammatory cytokines by T cells [interleukin (IL)-17A and tumour necrosis factor (TNF)-α] and monocytes [IL-6 and interferon (IFN)-α] from IIM patients compared with healthy controls. Our data suggest that patients with IIM, mainly DM, are characterized by a deficient expression of Ro52/TRIM21 in different PBMC subsets (CD4⁺ lymphocytes and monocytes), along with lower K48-mediated ubiquitination, which is associated with a proinflammatory cytokine response.

Antibody-antigen kinetics constrain intracellular humoral immunity

During infection with non-enveloped viruses, antibodies stimulate immunity from inside cells by activating the cytosolic Fc receptor TRIM21. This intracellular humoral response relies on opsonized viral particles reaching the cytosol intact but the antigenic and kinetic constraints involved are unknown. We have solved the structure of a potent TRIM21-dependent neutralizing antibody in complex with human adenovirus 5 hexon and show how these properties influence immune activity. Structure-guided mutagenesis was used to generate antibodies with 20,000-fold variation in affinity, on-rates that differ by ~50-fold and off-rates by >175-fold. Characterization of these variants during infection revealed that TRIM21-dependent neutralization and NFκB activation was largely unaffected by on-rate kinetics. In contrast, TRIM21 antiviral activity was exquisitely dependent upon off-rate, with sub-μM affinity antibodies nevertheless unable to stimulate signaling because of fast dissociation kinetics. These results define the antibody properties required to elicit an efficient intracellular immune response during viral infection.

Surveillance for Intracellular Antibody by Cytosolic Fc Receptor TRIM21

TRIM21 has emerged as an atypical Fc receptor that is broadly conserved and widely expressed in the cytoplasm of mammalian cells. Viruses that traffic surface-bound antibodies into the cell during infection recruit TRIM21 via a high affinity interaction between Fc and TRIM21 PRYSPRY domain. Following binding of intracellular antibody, TRIM21 acts as both antiviral effector and sensor for innate immune signalling. These activities serve to reduce viral replication by orders of magnitude in vitro and contribute to host survival during in vivo infection. Neutralization occurs rapidly after detection and requires the activity of the ubiquitin-proteasome system. The microbial targets of this arm of intracellular immunity are still being identified: TRIM21 activity has been reported following infection by several non-enveloped viruses and intracellular bacteria. These findings extend the sphere of influence of antibodies to the intracellular domain and have broad implications for immunity. TRIM21 has been implicated in the chronic auto-immune condition systemic lupus erythematosus and is itself an auto-antigen in Sjögren’s syndrome. This review summarises our current understanding of TRIM21’s role as a cytosolic Fc receptor and briefly discusses pathological circumstances where intracellular antibodies have been described, or are hypothesized to occur, and may benefit from further investigations of the role of TRIM21.

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.

Precision autophagy directed by receptor regulators - emerging examples within the TRIM family

Selective autophagy entails cooperation between target recognition and assembly of the autophagic apparatus. Target recognition is conducted by receptors that often recognize tags, such as ubiquitin and galectins, although examples of selective autophagy independent of these tags are emerging. It is less known how receptors cooperate with the upstream autophagic regulators, beyond the well-characterized association of receptors with Atg8 or its homologs, such as LC3B (encoded by MAP1LC3B), on autophagic membranes. The molecular details of the emerging role in autophagy of the family of proteins called TRIMs shed light on the coordination between cargo recognition and the assembly and activation of the principal autophagy regulators. In their autophagy roles, TRIMs act both as receptors and as platforms ('receptor regulators') for the assembly of the core autophagy regulators, such as ULK1 and Beclin 1 in their activated state. As autophagic receptors, TRIMs can directly recognize endogenous or exogenous targets, obviating a need for intermediary autophagic tags, such as ubiquitin and galectins. The receptor and regulatory features embodied within the same entity allow TRIMs to govern cargo degradation in a highly exact process termed 'precision autophagy'.

A re-evaluation of the role of B cells in protective immunity to Chlamydia infection

Chlamydia trachomatis is the etiological agent of the most commonly reported bacterial sexual transmitted infection (STI) in North America and Europe. The control of Chlamydia infection is hindered by the asymptomatic nature of initial infection but the consequence of untreated infection seriously threatens the reproductive health of young women. Unfortunately, there is no licensed vaccine for Chlamydia vaccine, in part due to our incomplete understanding of the immune response to Chlamydia urogenital infection. It has been well established that T cell-mediated immunity plays a dominant role in protective immunity against Chlamydia and thus the importance of B cells is somewhat underappreciated. Here, we summarize recent progress on understanding the role of B cells during Chlamydia genital tract infections and discuss how B cells and humoral immunity make an effective contribution to host defense against important intracellular pathogens, including Chlamydia.

IgG subclasses and allotypes: from structure to effector functions

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (FcγR) and C1q. As a result, the different subclasses have different effector functions, both in terms of triggering FcγR-expressing cells, resulting in phagocytosis or antibody-dependent cell-mediated cytotoxicity, and activating complement. The Fc-regions also contain a binding epitope for the neonatal Fc receptor (FcRn), responsible for the extended half-life, placental transport, and bidirectional transport of IgG to mucosal surfaces. However, FcRn is also expressed in myeloid cells, where it participates in both phagocytosis and antigen presentation together with classical FcγR and complement. How these properties, IgG-polymorphisms and post-translational modification of the antibodies in the form of glycosylation, affect IgG-function will be the focus of the current review.