Antibody-antigen kinetics constrain intracellular humoral immunity

TRIM21 and Fc-engineered antibodies: decoding its complex antibody binding mode with implications for viral neutralization

TRIM21 is a pivotal effector in the immune system, orchestrating antibody-mediated responses and modulating immune signaling. In this comprehensive study, we focus on the interaction of TRIM21 with Fc engineered antibodies and subsequent implications for viral neutralization. Through a series of analytical techniques, including biosensor assays, mass photometry, and electron microscopy, along with structure predictions, we unravel the intricate mechanisms governing the interplay between TRIM21 and antibodies. Our investigations reveal that the TRIM21 capacity to recognize, bind, and facilitate the proteasomal degradation of antibody-coated viruses is critically dependent on the affinity and avidity interplay of its interactions with antibody Fc regions. We suggest a novel binding mechanism, where TRIM21 binding to one Fc site results in the detachment of PRYSPRY from the coiled-coil domain, enhancing mobility due to its flexible linker, thereby facilitating the engagement of the second site, resulting in avidity due to bivalent engagement. These findings shed light on the dual role of TRIM21 in antiviral immunity, both in recognizing and directing viruses for intracellular degradation, and demonstrate its potential for therapeutic exploitation. The study advances our understanding of intracellular immune responses and opens new avenues for the development of antiviral strategies and innovation in tailored effector functions designed to leverage TRIM21s unique binding mode.

A novel fully human anti-NT-ANGPTL3 antibody from phage display library exhibits potent ApoB, TG, and LDL-C lowering activities in hyperlipidemia mice

Dyslipidemia is characterized by elevated plasma levels of low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and TG-rich lipoprotein (TGRLs) in circulation, and is closely associated with the incidence and development of cardiovascular disease. Angiopoietin-like protein 3 (ANGPTL3) deficiency has been identified as a cause of familial combined hypolipidemia in humans, which allows it to be an important therapeutic target for reducing plasma lipids. Here, we report the discovery and characterization of a novel fully human antibody F1519-D95aA against N-terminal ANGPTL3 (NT-ANGPTL3), which potently inhibits NT-ANGPTL3 with a KD as low as 9.21 nM. In hyperlipidemic mice, F1519-D95aA shows higher apolipoprotein B (ApoB) and TG-lowering, and similar LDL-C reducing activity as compared to positive control Evinacumab (56.50% vs 26.01% decrease in serum ApoB levels, 30.84% vs 25.28% decrease in serum TG levels, 23.32% vs 22.52% decrease in serum LDLC levels, relative to vehicle group). Molecular docking and binding energy calculations reveal that the F1519-D95aA-ANGPTL3 complex (10 hydrogen bonds, -65.51 kcal/mol) is more stable than the Evinacumab-ANGPTL3 complex (4 hydrogen bonds, -63.76 kcal/mol). Importantly, F1519-D95aA binds to ANGPTL3 with different residues in ANGPTL3 from Evinacumab, suggesting that F1519-D95aA may be useful for the treatment of patients resistant to Evinacumab. In conclusion, F1519-D95aA is a novel fully human anti-NT-ANGPTL3 antibody with potent plasma ApoB, TG, and LDL-C lowering activities, which can potentially serve as a therapeutic agent for hyperlipidemia and relevant cardiovascular diseases.

Trim-Away ubiquitinates and degrades lysine-less and N-terminally acetylated substrates

TRIM proteins are the largest family of E3 ligases in mammals. They include the intracellular antibody receptor TRIM21, which is responsible for mediating targeted protein degradation during Trim-Away. Despite their importance, the ubiquitination mechanism of TRIM ligases has remained elusive. Here we show that while Trim-Away activation results in ubiquitination of both ligase and substrate, ligase ubiquitination is not required for substrate degradation. N-terminal TRIM21 RING ubiquitination by the E2 Ube2W can be inhibited by N-terminal acetylation, but this doesn't prevent substrate ubiquitination nor degradation. Instead, uncoupling ligase and substrate degradation prevents ligase recycling and extends functional persistence in cells. Further, Trim-Away degrades substrates irrespective of whether they contain lysines or are N-terminally acetylated, which may explain the ability of TRIM21 to counteract fast-evolving pathogens and degrade diverse substrates.

Potent TRIM21 and complement-dependent intracellular antiviral immunity requires the IgG3 hinge

Humans have four IgG antibody subclasses that selectively or differentially engage immune effector molecules to protect against infections. Although IgG1 has been studied in detail and is the subclass of most approved antibody therapeutics, increasing evidence indicates that IgG3 is associated with enhanced protection against pathogens. Here, we report that IgG3 has superior capacity to mediate intracellular antiviral immunity compared with the other subclasses due to its uniquely extended and flexible hinge region, which facilitates improved recruitment of the cytosolic Fc receptor TRIM21, independently of Fc binding affinity. TRIM21 may also synergize with complement C1/C4-mediated lysosomal degradation via capsid inactivation. We demonstrate that this process is potentiated by IgG3 in a hinge-dependent manner. Our findings reveal differences in how the four IgG subclasses mediate intracellular immunity, knowledge that may guide IgG subclass selection and engineering of antiviral antibodies for prophylaxis and therapy.

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.

Virus neutralisation by intracellular antibodies

For decades antibodies were largely thought to provide protection in extracellular spaces alone, mediating their effector functions by mechanisms such as entry-blocking, complement activation and phagocyte recruitment. However, a wealth of research has shown that antibodies are also capable of neutralising numerous viruses inside cells. Efficacy has now been demonstrated at virtually all intracellular stages of the viral life cycle. Antibodies can neutralise viruses in endosomes by blocking uncoating, fusion mechanisms, or new particle egress. Neutralisation can also occur in the cytosol via recruitment of the intracellular antibody receptor TRIM21. In addition to these direct neutralisation effects, recent research has shown that antibodies can mediate virus control indirectly by promoting MHC class I presentation and thereby increasing the CD8 T cell response. This provides valuable new insight into how non-neutralising antibodies can mediate potent protection in vivo. Overall, the importance of understanding the mechanisms of intracellular neutralisation by antibodies is highlighted by the ongoing need to develop new methods to control viruses. Using or inducing antibodies to block virus replication inside cells is now an innovative approach used by several vaccination and therapeutic strategies.

Structural engineering of chimeric antigen receptors targeting HLA-restricted neoantigens

Chimeric antigen receptor (CAR) T cells have emerged as a promising class of therapeutic agents, generating remarkable responses in the clinic for a subset of human cancers. One major challenge precluding the wider implementation of CAR therapy is the paucity of tumor-specific antigens. Here, we describe the development of a CAR targeting the tumor-specific isocitrate dehydrogenase 2 (IDH2) with R140Q mutation presented on the cell surface in complex with a common human leukocyte antigen allele, HLA-B*07:02. Engineering of the hinge domain of the CAR, as well as crystal structure-guided optimization of the IDH2^R140Q-HLA-B*07:02-targeting moiety, enhances the sensitivity and specificity of CARs to enable targeting of this HLA-restricted neoantigen. This approach thus holds promise for the development and optimization of immunotherapies specific to other cancer driver mutations that are difficult to target by conventional means.

Chimeric antigen receptor T cells in the clinic currently target cell-type-specific extracellular antigens on malignant cells. Here, authors engineer tumor-specific chimeric antigen receptor T cells that target human leukocyte antigen-presented neoantigens derived from mutant intracellular proteins.

A functional assay for serum detection of antibodies against SARS-CoV-2 nucleoprotein

The humoral immune response to SARS-CoV-2 results in antibodies against spike (S) and nucleoprotein (N). However, whilst there are widely available neutralization assays for S antibodies, there is no assay for N-antibody activity. Here, we present a simple in vitro method called EDNA (electroporated-antibody-dependent neutralization assay) that provides a quantitative measure of N-antibody activity in unpurified serum from SARS-CoV-2 convalescents. We show that N antibodies neutralize SARS-CoV-2 intracellularly and cell-autonomously but require the cytosolic Fc receptor TRIM21. Using EDNA, we show that low N-antibody titres can be neutralizing, whilst some convalescents possess serum with high titres but weak activity. N-antibody and N-specific T-cell activity correlates within individuals, suggesting N antibodies may protect against SARS-CoV-2 by promoting antigen presentation. This work highlights the potential benefits of N-based vaccines and provides an in vitro assay to allow the antibodies they induce to be tested.

Computational modeling of microfluidic data provides high-throughput affinity estimates for monoclonal antibodies

Affinity measurement is a fundamental step in the discovery of monoclonal antibodies (mAbs) and of antigens suitable for vaccine development. Innovative affinity assays are needed due to the low throughput and/or limited dynamic range of available technologies. We combined microfluidic technology with quantum-mechanical scattering theory, in order to develop a high-throughput, broad-range methodology to measure affinity. Fluorescence intensity profiles were generated for out-of-equilibrium solutions of labelled mAbs and their antigen-binding fragments migrating along micro-columns with immobilized cognate antigen. Affinity quantification was performed by computational data analysis based on the Landau probability distribution. Experiments using a wide array of human or murine antibodies against bacterial or viral, protein or polysaccharide antigens, showed that all the antibody-antigen capture profiles (n = 841) generated at different concentrations were accurately described by the Landau distribution. A scale parameter W, proportional to the full-width-at-half-maximum of the capture profile, was shown to be independent of the antibody concentration. The W parameter correlated significantly (Pearson's r [p-value]: 0.89 [3 × 10-8]) with the equilibrium dissociation constant KD, a gold-standard affinity measure. Our method showed good intermediate precision (median coefficient of variation: 5%) and a dynamic range corresponding to KD values spanning from ~10-7 to ~10-11 Molar. Relative to assays relying on antibody-antigen equilibrium in solution, even when they are microfluidic-based, the method's turnaround times were decreased from 2 days to 2 h. The described computational modelling of antibody capture profiles represents a fast, reproducible, high-throughput methodology to accurately measure a broad range of antibody affinities in very low volumes of solution.

Structure-Based Modeling of Complement C4 Mediated Neutralization of Adenovirus

Adenovirus (AdV) infection elicits a strong immune response with the production of neutralizing antibodies and opsonization by complement and coagulation factors. One anti-hexon neutralizing antibody, called 9C12, is known to activate the complement cascade, resulting in the deposition of complement component C4b on the capsid, and the neutralization of the virus. The mechanism of AdV neutralization by C4b is independent of downstream complement proteins and involves the blockage of the release of protein VI, which is required for viral escape from the endosome. To investigate the structural basis underlying how C4b blocks the uncoating of AdV, we built a model for the complex of human adenovirus type-5 (HAdV5) with 9C12, together with complement components C1 and C4b. This model positions C4b near the Arg-Gly-Asp (RGD) loops of the penton base. There are multiple amino acids in the RGD loop that might serve as covalent binding sites for the reactive thioester of C4b. Molecular dynamics simulations with a multimeric penton base and C4b indicated that stabilizing interactions may form between C4b and multiple RGD loops. We propose that C4b deposition on one RGD loop leads to the entanglement of C4b with additional RGD loops on the same penton base multimer and that this entanglement blocks AdV uncoating.

The structure of enteric human adenovirus 41-A leading cause of diarrhea in children

Human adenovirus (HAdV) types F40 and F41 are a prominent cause of diarrhea and diarrhea-associated mortality in young children worldwide. These enteric HAdVs differ notably in tissue tropism and pathogenicity from respiratory and ocular adenoviruses, but the structural basis for this divergence has been unknown. Here, we present the first structure of an enteric HAdV-HAdV-F41-determined by cryo-electron microscopy to a resolution of 3.8 Å. The structure reveals extensive alterations to the virion exterior as compared to nonenteric HAdVs, including a unique arrangement of capsid protein IX. The structure also provides new insights into conserved aspects of HAdV architecture such as a proposed location of core protein V, which links the viral DNA to the capsid, and assembly-induced conformational changes in the penton base protein. Our findings provide the structural basis for adaptation of enteric HAdVs to a fundamentally different tissue tropism.

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.

Antibody and DNA sensing pathways converge to activate the inflammasome during primary human macrophage infection

Inflammasomes are potent innate immune signalling complexes that couple cytokine release with pro-inflammatory cell death. However, pathogens have evolved strategies to evade this cell autonomous system. Here, we show how antibodies combine with innate sensors in primary human macrophages to detect viral infection and activate the inflammasome. Our data demonstrate that antibody opsonisation of virions can activate macrophages in multiple ways. In the first, antibody binding of adenovirus causes lysosomal damage, activating NLRP3 to drive inflammasome formation and IL-1β release. Importantly, this mechanism enhances virion capture but not infection and is accompanied by cell death, denying the opportunity for viral replication. Unexpectedly, we also find that antibody-coated viruses, which successfully escape into the cytosol, trigger a second system of inflammasome activation. These viruses are intercepted by the cytosolic antibody receptor TRIM21 and the DNA sensor cGAS. Together, these sensors stimulate both NLRP3 inflammasome formation and NFκB activation, driving dose-dependent IL-1β and TNF secretion, without inducing cell death. Our data highlight the importance of cooperativity between multiple sensing networks to expose viruses to the inflammasome pathway, which is particularly important for how our innate immune system responds to infection in the presence of pre-existing immunity.

A tri-ionic anchor mechanism drives Ube2N-specific recruitment and K63-chain ubiquitination in TRIM ligases

The cytosolic antibody receptor TRIM21 possesses unique ubiquitination activity that drives broad-spectrum anti-pathogen targeting and underpins the protein depletion technology Trim-Away. This activity is dependent on formation of self-anchored, K63-linked ubiquitin chains by the heterodimeric E2 enzyme Ube2N/Ube2V2. Here we reveal how TRIM21 facilitates ubiquitin transfer and differentiates this E2 from other closely related enzymes. A tri-ionic motif provides optimally distributed anchor points that allow TRIM21 to wrap an Ube2N~Ub complex around its RING domain, locking the closed conformation and promoting ubiquitin discharge. Mutation of these anchor points inhibits ubiquitination with Ube2N/Ube2V2, viral neutralization and immune signalling. We show that the same mechanism is employed by the anti-HIV restriction factor TRIM5 and identify spatially conserved ionic anchor points in other Ube2N-recruiting RING E3s. The tri-ionic motif is exclusively required for Ube2N but not Ube2D1 activity and provides a generic E2-specific catalysis mechanism for RING E3s.

Improvement in affinity and thermostability of a fully human antibody against interleukin-17A by yeast-display technology and CDR grafting

Monoclonal antibodies (mAbs) are widely used in many fields due to their high specificity and ability to recognize a broad range of antigens. IL-17A can induce a rapid inflammatory response both alone and synergistically with other proinflammatory cytokines. Accumulating evidence suggests that therapeutic intervention of IL-17A signaling offers an attractive treatment option for autoimmune diseases and cancer. Here, we present a combinatorial approach for optimizing the affinity and thermostability of a novel anti-hIL-17A antibody. From a large naïve phage-displayed library, we isolated the anti-IL-17A mAb 7H9 that can neutralize the effects of recombinant human IL-17A. However, the modest neutralization potency and poor thermostability limit its therapeutic applications. In vitro affinity optimization was then used to generate 8D3 by using yeast-displayed random mutagenesis libraries. This resulted in four key amino acid changes and provided an approximately 15-fold potency increase in a cell-based neutralization assay. Complementarity-determining regions (CDRs) of 8D3 were further grafted onto the stable framework of the huFv 4D5 to improve thermostability. The resulting hybrid antibody 9NT/S has superior stabilization and affinities beyond its original antibody. Human fibrosarcoma cell-based assays and in vivo analyses in mice indicated that the anti-IL-17A antibody 9NT/S efficiently inhibited the secretion of IL-17A-induced proinflammatory cytokines. Therefore, this lead anti-IL-17A mAb might be used as a potential best-in-class candidate for treating IL-17A related diseases.

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.

Epitope mapping of antibodies induced with a conserved rhinovirus protein generating protective anti-rhinovirus immunity

Human rhinovirus (RV) infections are the principle cause of common colds and precipitate asthma and chronic obstructive pulmonary disease (COPD) exacerbations. Currently there is no vaccine for RV which is largely due to the existence of ∼160 serotypes/strains. We demonstrated previously that immunising mice with highly conserved VP4 and VP2 regions of the RV polyprotein (RV-A16 VP0) generated cross-reactive immunity to RV in vivo. The current study investigated and mapped the epitopes of RV-A16 VP0 that are targets for antibodies in serum samples from VP0 immunisation and RV challenge studies in mice. Recombinant capsid proteins, peptide pools and individual peptides spanning the immunogen sequence (RV-A16 VP0) were assessed for IgG binding sites to identify epitopes. We found that peptide pools covering the C-terminus of VP4, the N-terminus of VP2 and the neutralising NIm-II site within VP2 were bound by serum IgG from immunised mice. The NIm-II site peptide pool blocked IgG binding to the immunogen RV-A16 VP0 and individual peptides within the pool binding IgG were further mapped. Thus, we have identified immunodominant epitopes of RV vaccine candidate RV-A16 VP0, noting that strong IgG binding antibodies were observed that target a key neutralising epitope that is highly variable amongst RV serotypes.

Complement C4 Prevents Viral Infection through Capsid Inactivation

The complement system is vital for anti-microbial defense. In the classical pathway, pathogen-bound antibody recruits the C1 complex (C1qC1r₂C1s₂) that initiates a cleavage cascade involving C2, C3, C4, and C5 and triggering microbial clearance. We demonstrate a C4-dependent antiviral mechanism that is independent of downstream complement components. C4 inhibits human adenovirus infection by directly inactivating the virus capsid. Rapid C4 activation and capsid deposition of cleaved C4b are catalyzed by antibodies via the classical pathway. Capsid-deposited C4b neutralizes infection independent of C2 and C3 but requires C1q antibody engagement. C4b inhibits capsid disassembly, preventing endosomal escape and cytosolic access. C4-deficient mice exhibit heightened viral burdens. Additionally, complement synergizes with the Fc receptor TRIM21 to block transduction by an adenovirus gene therapy vector but is partially restored by Fab virus shielding. These results suggest that the complement system could be altered to prevent virus infection and enhance virus gene therapy efficacy.

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Complement components C1 and C4 mediate potent neutralization of adenovirus

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Deposition of C4b on the viral capsid inactivates capsid disassembly

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C4 exerts direct antiviral functions independent from its role as a C3-convertase

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C4 antiviral functions synergize with TRIM21-mediated virus neutralization

Gold Binding Peptide Identified from Microfluidic Biopanning: An Experimental and Molecular Dynamics Study

Biopanning refers to the processes of screening peptides with a high affinity to a target material. Microfluidic biopanning has advantages compared to conventional biopanning which requires large amounts of the target material and involves inefficient multiple pipetting steps to remove nonspecific or low-affinity peptides. Here, we fabricate a microfluidic biopanning system to identify a new gold-binding peptide (GBP). A polydimethylsiloxane microfluidic device is fabricated and bonded to a glass slide with a gold pattern that is deposited by electron-beam evaporation. The microfluidic biopanning system can provide high adjustability in the washing step during the biopanning process because the liquid flow rate and the resulting shear stress can be precisely controlled. The surface plasmon resonance analysis shows that the binding affinity of the identified GBP is comparable to previously reported GBPs. Moreover, molecular dynamics simulations are performed to understand its binding affinity against the gold surface in detail. Theoretical calculations suggest that the association and dissociation rates of the GBPs depend on their sequence-dependent conformations and interactions with the gold surface. These findings provide insight into designing efficient biopanning tools and peptides with a high affinity for various target materials.

Characterising antibody avidity in individuals of varied Mycobacterium tuberculosis infection status using surface plasmon resonance

There is increasing evidence supporting a role for antibodies in protection against tuberculosis (TB), with functional antibodies being described in the latent state of TB infection. Antibody avidity is an important determinant of antibody-mediated protection. This study characterised the avidity of antibodies against Ag85A, an immunodominant Mycobacterium tuberculosis (M.tb) antigen and constituent of several anti-TB vaccine candidates, in individuals of varied M.tb infection status. Avidity of Ag85A specific antibodies was measured in 30 uninfected controls, 34 individuals with latent TB infection (LTBI) and 75 active pulmonary TB (APTB) cases, employing the more commonly used chaotrope-based dissociation assays, and surface plasmon resonance (SPR). Chaotrope-based assays indicated that APTB was associated with a higher antibody avidity index compared to uninfected controls [adjusted geometric mean ratio (GMR): 1.641, 95% confidence interval (CI): 1.153, 2.337, p = 0.006, q = 0.018] and to individuals with LTBI [adjusted GMR: 1.604, 95% CI: 1.282, 2.006, p < 0.001, q <0.001]. SPR assays showed that APTB was associated with slower dissociation rates, an indication of higher avidity, compared to uninfected controls (adjusted GMR: 0.796, 95% CI: 0.681, 0.932, p = 0.004, q = 0.012) and there was also weak evidence of more avid antibodies in the LTBI compared to the uninfected controls (adjusted GMR: 0.871, 95% CI: 0.763, 0.994, p = 0.041, q = 0.123). We found no statistically significant differences in anti-Ag85A antibody avidity between the APTB and LTBI groups. This study shows that antibodies of increased avidity are generated against a principle vaccine antigen in M.tb infected individuals. It would be important to determine whether TB vaccines are able to elicit a similar response. Additionally, more research is needed to determine whether antibody avidity is important in protection against infection and disease.

TRIM21 mediates antibody inhibition of adenovirus-based gene delivery and vaccination

Viral-based delivery vectors have huge potential in the treatment of human disease. Adenoviral vectors specifically have proven highly efficacious in delivering corrected genes, as part of gene therapy, and vaccine epitopes for treating cancer and infectious disease. A principal obstacle to their widespread use is that antibodies potently neutralize them, limiting treatment to naïve patients. How antibodies block adenovirus-based transduction has long remained a mystery because, even though they prevent transgene expression, they do not prevent transgene delivery into target tissue. Here we show that the cytosolic antibody receptor TRIM21 is responsible for intercepting adenoviral gene therapy and vaccine vectors and neutralizing them. Gene KO of TRIM21 or a single-antibody mutation that prevents interaction is sufficient to restore transgene expression.

Adenovirus has enormous potential as a gene-therapy vector, but preexisting immunity limits its widespread application. What is responsible for this immune block is unclear because antibodies potently inhibit transgene expression without impeding gene transfer into target cells. Here we show that antibody prevention of adenoviral gene delivery in vivo is mediated by the cytosolic antibody receptor TRIM21. Genetic KO of TRIM21 or a single-antibody point mutation is sufficient to restore transgene expression to near-naïve immune levels. TRIM21 is also responsible for blocking cytotoxic T cell induction by vaccine vectors, preventing a protective response against subsequent influenza infection and an engrafted tumor. Furthermore, adenoviral preexisting immunity can lead to an augmented immune response upon i.v. administration of the vector. Transcriptomic analysis of vector-transduced tissue reveals that TRIM21 is responsible for the specific up-regulation of hundreds of immune genes, the majority of which are components of the intrinsic or innate response. Together, these data define a major mechanism underlying the preimmune block to adenovirus gene therapy and demonstrate that TRIM21 efficiently blocks gene delivery in vivo while simultaneously inducing a rapid program of immune transcription.

Intracellular antibody signalling is regulated by phosphorylation of the Fc receptor TRIM21

Cell surface Fc receptors activate inflammation and are tightly controlled to prevent autoimmunity. Antibodies also simulate potent immune signalling from inside the cell via the cytosolic antibody receptor TRIM21, but how this is regulated is unknown. Here we show that TRIM21 signalling is constitutively repressed by its B-Box domain and activated by phosphorylation. The B-Box occupies an E2 binding site on the catalytic RING domain by mimicking E2-E3 interactions, inhibiting TRIM21 ubiquitination and preventing immune activation. TRIM21 is derepressed by IKKβ and TBK1 phosphorylation of an LxxIS motif in the RING domain, at the interface with the B-Box. Incorporation of phosphoserine or a phosphomimetic within this motif relieves B-Box inhibition, promoting E2 binding, RING catalysis, NF-κB activation and cytokine transcription upon infection with DNA or RNA viruses. These data explain how intracellular antibody signalling is regulated and reveal that the B-Box is a critical regulator of RING E3 ligase activity.

Antibodies are molecules made by the immune system that protect us from infections. They were discovered over 100 years ago, and for most of that time scientists thought they only worked in the bloodstream. Yet recent research showed that when a virus infects our cells it also carries antibodies in with it. Once inside the cell, a protein called TRIM21 recognises the antibody-covered virus and – together with other proteins called ubiquitin enzymes – targets it for destruction via the cell’s waste disposal system. At the same time TRIM21 sends a signal to the cell’s nucleus to activate certain genes that protect cells across the body from subsequent infection.

The genes activated by TRIM21 have potent antiviral activity. Yet they can also damage the body’s own tissues if they are switched on at the wrong time, which may lead to autoimmune diseases like rheumatoid arthritis and multiple sclerosis. It is thus critical that TRIM21 is carefully regulated and only activated during an infection, but it was not clear how this is achieved.

Dickson, Fletcher et al. now show that although TRIM21 is made all the time and is always ready to detect an incoming virus it is made in an inactive state. This is because part of TRIM21, called a B-Box, inhibits the protein’s own activity. This was an unexpected discovery because, although the B-Box domain is found in around 100 other human proteins, it was unclear what it did. Dickson, Fletcher et al. then combined structural biology and biochemical approaches to show that the B-Box mimics specific enzymes that work with TRIM21, and blocks them from binding to it. This keeps TRIM21 in an inactive state.

Next, Dickson, Fletcher et al. discovered that TRIM21 becomes active when a chemical tag, specifically a phosphate group, is added to the protein. This modification displaces the B-Box, allowing ubiquitin enzymes to bind to TRIM21 and switch on its activity. Further experiments then showed that this process helps regulate TRIM21 during infections with different viruses, including rhinovirus – the virus behind the common cold – and adenovirus – a common cause of respiratory infection.

Antibodies are now used to treat many medical conditions, but present technologies are based on our understanding of how antibodies work outside cells. By revealing the basis of antibody immunity inside cells, these new findings may lead to new treatments for a range of conditions. Future studies could also explore how failures in the TRIM21 system contribute to autoimmune diseases.

Intracellular Antiviral Immunity

Innate immunity is traditionally thought of as the first line of defense against pathogens that enter the body. It is typically characterized as a rather weak defense mechanism, designed to restrict pathogen replication until the adaptive immune response generates a tailored response and eliminates the infectious agent. However, intensive research in recent years has resulted in better understanding of innate immunity as well as the discovery of many effector proteins, revealing its numerous powerful mechanisms to defend the host. Furthermore, this research has demonstrated that it is simplistic to strictly separate adaptive and innate immune functions since these two systems often work synergistically rather than sequentially. Here, we provide a broad overview of innate pattern recognition receptors in antiviral defense, with a focus on the TRIM family, and discuss their signaling pathways and mechanisms of action with special emphasis on the intracellular antibody receptor TRIM21.

Sequence analysis of feline immunoglobulin mRNAs and the development of a felinized monoclonal antibody specific to feline panleukopenia virus

In response to immunization, B-cells generate a repertoire of antigen-specific antibodies. Antibody-based immunotherapies hold great promise for treating a variety of diseases in humans. Application of antibody-based immunotherapy in cats is limited by the lack of species-specific complete sequences for mRNAs encoding rearranged heavy and light chain immunoglobulins in B cells. To address this barrier, we isolated mRNAs from feline peripheral blood mononuclear cells (PBMCs), and used available immunoglobulin sequences and 5′ and 3′ RACE to clone and sequence heavy and light chain immunoglobulin mRNAs. We recovered mRNA from PBMCs from two cats, cloned and sequenced the variable and constant domains of the feline heavy chains of IgG1a (IGHG1a), IgG2 (IGHG2), and IgA (IGHA), and the light chains (lambda and kappa). Using these sequences, we prepared two bicistronic vectors for mammalian expression of a representative feline heavy (IGHG1a) together with a light (lambda or kappa) chain. Here we report novel feline Ig sequences, a technique to express antigen-specific felinized monoclonal antibodies, and the initial characterization of a functional felinized monoclonal antibody against feline panleukopenia virus.

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.