CD72 is an inhibitory pattern recognition receptor that recognizes ribosomes and suppresses production of anti-ribosome autoantibody.

B cell responses to nucleic acid-containing self-antigens that involve intracellular nucleic acid sensors play a crucial role in autoantibody production in SLE. CD72 is an inhibitory B cell co-receptor that down-regulates BCR signaling, and prevents the development of SLE. We previously showed that CD72 recognizes the RNA-containing self-antigen Sm/RNP, a target of SLE-specific autoantibodies, and induces B cell tolerance to Sm/RNP by specifically inhibiting B cell response to this self-antigen. Here, we address whether CD72 inhibits B cell response to ribosomes because the ribosome is an RNA-containing self-antigen and is a target of SLE-specific autoantibodies as well as Sm/RNP. We demonstrate that CD72 recognizes ribosomes as a ligand, and specifically inhibits BCR signaling induced by ribosomes. Although conventional protein antigens by themselves do not induce proliferation of specific B cells, ribosomes induce proliferation of B cells reactive to ribosomes in a manner dependent on RNA. This proliferative response is down-regulated by CD72. These results suggest that ribosomes activate B cells by inducing dual signaling through BCR and intracellular RNA sensors and that CD72 inhibits B cell response to ribosomes. Moreover, CD72-/- but not CD72+/+ mice spontaneously produce anti-ribosome autoantibodies. Taken together, CD72 induces B cell self-tolerance to ribosomes by recognizing ribosomes and inhibiting RNA-dependent B cell response to this self-antigen. CD72 appears to prevent development of SLE by inhibiting autoimmune B cell responses to multiple RNA-containing self-antigens. Because these self-antigens but not protein self-antigens induce RNA-dependent B cell activation, self-tolerance to RNA-containing self-antigens may require a distinct tolerance mechanism mediated by CD72.

Translation-A tug of war during viral infection.

Viral reproduction is contingent on viral protein synthesis that relies on the host ribosomes. As such, viruses have evolved remarkable strategies to hijack the host translational apparatus in order to favor viral protein production and to interfere with cellular innate defenses. Here, we describe the approaches viruses use to exploit the translation machinery, focusing on commonalities across diverse viral families, and discuss the functional relevance of this process. We illustrate the complementary strategies host cells utilize to block viral protein production and consider how cells ensure an efficient antiviral response that relies on translation during this tug of war over the ribosome. Finally, we highlight potential roles mRNA modifications and ribosome quality control play in translational regulation and innate immunity. We address these topics in the context of the COVID-19 pandemic and focus on the gaps in our current knowledge of these mechanisms, specifically in viruses with pandemic potential.

ABC-F translation factors: from antibiotic resistance to immune response.

Energy-dependent translational throttle A (EttA) from Escherichia coli is a paradigmatic ABC-F protein that controls the first step in polypeptide elongation on the ribosome according to the cellular energy status. Biochemical and structural studies have established that ABC-F proteins generally function as translation factors that modulate the conformation of the peptidyl transferase center upon binding to the ribosomal tRNA exit site. These factors, present in both prokaryotes and eukaryotes but not in archaea, use related molecular mechanisms to modulate protein synthesis for heterogenous purposes, ranging from antibiotic resistance and rescue of stalled ribosomes to modulation of the mammalian immune response. Here, we review the canonical studies characterizing the phylogeny, regulation, ribosome interactions, and mechanisms of action of the bacterial ABC-F proteins, and discuss the implications of these studies for the molecular function of eukaryotic ABC-F proteins, including the three human family members.

An emerging ribosomopathy affecting the skeleton due to biallelic variations in NEPRO.

Cartilage hair hypoplasia (CHH), anauxetic dysplasia 1, and anauxetic dysplasia 2 are rare metaphyseal dysplasias caused by biallelic pathogenic variants in RMRP and POP1, which encode the components of RNAse-MRP endoribonuclease complex (RMRP) in ribosomal biogenesis pathway. Nucleolus and neural progenitor protein (NEPRO), encoded by NEPRO (C3orf17), is known to interact with multiple protein subunits of RMRP. We ascertained a 6-year-old girl with skeletal dysplasia and some features of CHH. RMRP and POP1 did not harbor any causative variant in the proband. Parents-child trio exomes revealed a candidate biallelic variant, c.435G>C, p.(Leu145Phe) in NEPRO. Two families with four affected individuals with skeletal dysplasia and a homozygous missense variant, c.280C>T, p.(Arg94Cys) in NEPRO, were identified from literature and their published phenotype was compared in detail to the phenotype of the child we described. All the five affected individuals have severe short stature, brachydactyly, skin laxity, joint hypermobility, and joint dislocations. They also have short metacarpals, broad middle phalanges, and metaphyseal irregularities. Protein modeling and stability prediction showed that the mutant protein has decreased stability. Both the reported variants are in the same domain of the protein. Our report delineates the clinical and radiological characteristics of an emerging ribosomopathy caused by biallelic variants in NEPRO.

Immunoribosomes: Where's there's fire, there's fire.

The MHC class I antigen presentation pathway enables T cell immunosurveillance of cancer cells, viruses and other intracellular pathogens. Rapidly degraded newly synthesized proteins (DRiPs) are a major source of self-, and particularly, viral antigenic peptides. A number of findings support the idea that a substantial fraction of antigenic peptides are synthesized by "immunoribosomes", a subset of translating ribosomes that generate class I peptides with enhanced efficiency. Here, we review the evidence for the immunoribosome hypothesis.

Ribosome display for the rapid generation of high-affinity Zika-neutralizing single-chain antibodies.

BACKGROUND: Zika virus (ZIKV) is an emerging pathogen with no approved therapeutics and only limited diagnostics available. To address this gap, six mouse single-chain antibodies (scFvs) to ZIKV envelope (E) protein were isolated rapidly and efficiently from a ribosome-displayed antibody library constructed from the spleens of five immunized mice. METHODOLOGY/RESULTS: In this report, we have generated a panel of mouse scFvs to ZIKV E protein using ribosome display. The six scFvs demonstrated no cross-reactivity with DENV2 NGC envelope protein, suggesting specificity for ZIKV E protein. These scFvs showed differences in their affinity: two (scFv45-3, scFv63-1) of them were dominant after four rounds of panning, and showed higher affinity (an apparent Kd values from 19 to 27 nM) than the other four (scFv5-1, scFv7-2, scFv38-1, and scFv51-2). All six scFvs showed ZIKV-neutralizing activity in the plaque reduction neutralization test (PRNT) assay and their neutralizing activity was positively correlated with their affinities. CONCLUSIONS/SIGNIFICANCE: The scFvs (45-3 and 63-1) with highest affinity may have dual utility as diagnostics capable of recognizing ZIKV E subtypes and may be further developed to treat ZIKV infection. Our approach has the added advantage of generating Fc receptor-deficient antibodies, minimizing concern of antibody-dependent enhancement (ADE) of infection.

Ribosome display and selection of single-chain variable fragments effectively inhibit growth and progression of microspheres in vitro and in vivo.

Distinguishing the surface markers of cancer stem cells (CSCs) is a useful method for early diagnosis and treatment of tumors, as CSCs may participate in tumorigenesis and metastasis by migrating into the circulatory system. However, the potential targets of CSCs are expressed at low levels in the natural state and are always changing. Thus, dynamic screening has been reported to be an effective measure for exploring CSC markers. In recent years, diverse single-chain variable fragments (scFvs) have been widely used in immunotherapy. In this study, we determined that the scFvs, screened using RD, had a high affinity to microspheres and could inhibit their progression. We also observed that the selected scFvs underwent evolution in vitro, and antitumor-associated proteins were successfully expressed. Combined with chemotherapy, the scFvs had a synergistic effect on the inhibition of the microspheres' progression in vitro and in vivo, which could be ascribed to their high affinity for stem-like cells and the inhibition of the microspheres' collective behaviors. In addition, proteins inhibiting CD44+ /CD24+ and MAPK were involved. Our data indicated that dynamic screening of the scFvs in a natural state was of great significance in the inhibition of the microspheres in vitro and in vivo.

Hepatitis C virus: Enslavement of host factors.

Hepatitis C virus (HCV) has infected over 170 million people world-wide. This infection causes severe liver damage that can progress to hepatocellular carcinoma leading to death of the infected patients. Development of a cell culture model system for the study of HCV infection in the recent past has helped the researchers world-wide to understand the biology of this virus. Studies over the past decade have revealed the tricks played by the virus to sustain itself, for as long as 40 years, in the host setup without being eliminated by the immune system. Today we understand that the host organelles and different cellular proteins are affected during HCV infection. This cytoplasmic virus has all the cellular organelles at its disposal to successfully replicate, from ribosomes and intracellular membranous structures to the nucleus. It modulates these organelles at both the structural and the functional levels. The vast knowledge about the viral genome and viral proteins has also helped in the development of drugs against the virus. Despite the achieved success rate to cure the infected patients, we struggle to eliminate the cases of recurrence and the non-responders. Such cases might emerge owing to the property of the viral genome to accumulate mutations during its succeeding replication cycles which favours its survival. The current situation calls an urgent need for alternate therapeutic strategies to counter this major problem of human health. © 2017 IUBMB Life, 70(1):41-49, 2018.

Diverging mRNA and Protein Networks in Activated Microglia Reveal SRSF3 Suppresses Translation of Highly Upregulated Innate Immune Transcripts.

Uncontrolled microglial activation may lead to the development of inflammation-induced brain damage. Here, we uncover a ribosome-based mechanism/checkpoint involved in control of the innate immune response and microglial activation. Using an in vivo model system for analysis of the dynamic translational state of microglial ribosomes, with mRNAs as input and newly synthesized peptides as an output, we find a marked dissociation of microglia mRNA and protein networks following innate immune challenge. Highly upregulated and ribosome-associated mRNAs were not translated, resulting in two distinct microglial molecular signatures, a highly specialized pro-inflammatory mRNA signature and an immunomodulatory/homeostatic protein signature. We find that this is due to specific translational suppression of highly expressed mRNAs through a 3' UTR-mediated mechanism involving the RNA-binding protein SRSF3. This discovery suggests avenues for therapeutic modulation of innate immune response in resident microglia.

Ubiquitinated Proteins Isolated From Tumor Cells Are Efficient Substrates for Antigen Cross-Presentation.

We have previously shown that inhibition of the proteasome causes defective ribosomal products to be shunted into autophagosomes and subsequently released from tumor cells as defective ribosomal products in Blebs (DRibbles). These DRibbles serve as an excellent source of antigens for cross-priming of tumor-specific T cells. Here, we examine the role of ubiquitinated proteins (Ub-proteins) in this pathway. Using purified Ub-proteins from tumor cells that express endogenous tumor-associated antigen or exogenous viral antigen, we tested the ability of these proteins to stimulate antigen-specific T-cell responses, by activation of monocyte-derived dendritic cells generated from human peripheral blood mononuclear cells. Compared with total cell lysates, we found that purified Ub-proteins from both a gp100-specific melanoma cell line and from a lung cancer cell line expressing cytomegalovirus pp65 antigen produced a significantly higher level of IFN-γ in gp100- or pp65-specific T cells, respectively. In addition, Ub-proteins from an allogeneic tumor cell line could be used to stimulate tumor-infiltrating lymphocytes isolated and expanded from non-small cell lung cancer patients. These results establish that Ub-proteins provide a relevant source of antigens for cross-priming of antitumor immune responses in a variety of settings, including endogenous melanoma and exogenous viral antigen presentation, as well as antigen-specific tumor-infiltrating lymphocytes. Thus, ubiquitin can be used as an affinity tag to enrich for unknown tumor-specific antigens from tumor cell lysates to stimulate tumor-specific T cells ex vivo or to be used as vaccines to target short-lived proteins.

'Z-DNA like' fragments in RNA: a recurring structural motif with implications for folding, RNA/protein recognition and immune response.

Since the work of Alexander Rich, who solved the first Z-DNA crystal structure, we have known that d(CpG) steps can adopt a particular structure that leads to forming left-handed helices. However, it is still largely unrecognized that other sequences can adopt 'left-handed' conformations in DNA and RNA, in double as well as single stranded contexts. These 'Z-like' steps involve the coexistence of several rare structural features: a C2'-endo puckering, a syn nucleotide and a lone pair-π stacking between a ribose O4' atom and a nucleobase. This particular arrangement induces a conformational stress in the RNA backbone, which limits the occurrence of Z-like steps to ≈0.1% of all dinucleotide steps in the PDB. Here, we report over 600 instances of Z-like steps, which are located within r(UNCG) tetraloops but also in small and large RNAs including riboswitches, ribozymes and ribosomes. Given their complexity, Z-like steps are probably associated with slow folding kinetics and once formed could lock a fold through the formation of unique long-range contacts. Proteins involved in immunologic response also specifically recognize/induce these peculiar folds. Thus, characterizing the conformational features of these motifs could be a key to understanding the immune response at a structural level.

Secreted autoantibody repertoires in Sjögren's syndrome and systemic lupus erythematosus: A proteomic approach.

The structures of epitopes bound by autoantibodies against RNA-protein complexes have been well-defined over several decades, but little is known of the clonality, immunoglobulin (Ig) variable (V) gene usage and mutational status of the autoantibodies themselves at the level of the secreted (serum) proteome. A novel proteomic workflow is presented based on affinity purification of specific Igs from serum, high-resolution two-dimensional gel electrophoresis, and de novo and database-driven sequencing of V-region proteins by mass spectrometry. Analysis of anti-Ro52/Ro60/La proteomes in primary Sjögren's syndrome (SS) and anti-Sm and anti-ribosomal P proteomes in systemic lupus erythematosus (SLE) has revealed that these antibody responses are dominated by restricted sets of public (shared) clonotypes, consistent with common pathways of production across unrelated individuals. The discovery of shared sets of specific V-region peptides can be exploited for diagnostic biomarkers in targeted mass spectrometry platforms and for tracking and removal of pathogenic clones.

Lupus anti-ribosomal P autoantibody proteomes express convergent biclonal signatures.

Lupus-specific anti-ribosomal P (anti-Rib-P) autoantibodies have been implicated in the pathogenesis of neurological complications in systemic lupus erythematosus (SLE). The aim of the present study was to determine variable (V)-region signatures of secreted autoantibody proteomes specific for the Rib-P heterocomplex and investigate the molecular basis of the reported cross-reactivity with Sm autoantigen. Anti-Rib-P immunoglobulins (IgGs) were purified from six anti-Rib-P-positive sera by elution from enzyme-linked immunosorbent assay (ELISA) plates coated with either native Rib-P proteins or an 11-amino acid peptide (11-C peptide) representing the conserved COOH-terminal P epitope. Rib-P- and 11-C peptide-specific IgGs were analysed for heavy (H) and light (L) chain clonality and V-region expression using an electrophoretic and de-novo and database-driven mass spectrometric sequencing workflow. Purified anti-Rib-P and anti-SmD IgGs were tested for cross-reactivity on ELISA and their proteome data sets analysed for shared clonotypes. Anti-Rib-P autoantibody proteomes were IgG1 kappa-restricted and comprised two public clonotypes defined by unique H/L chain pairings. The major clonotypic population was specific for the common COOH-terminal epitope, while the second shared the same pairing signature as a recently reported anti-SmD clonotype, accounting for two-way immunoassay cross-reactivity between these lupus autoantibodies. Sequence convergence of anti-Rib-P proteomes suggests common molecular pathways of autoantibody production and identifies stereotyped clonal populations that are thought to play a pathogenic role in neuropsychiatric lupus. Shared clonotypic structures for anti-Rib-P and anti-Sm responses suggest a common B cell clonal origin for subsets of these lupus-specific autoantibodies.

Isolation of high-affinity, neutralizing anti-idiotype antibodies by phage and ribosome display for application in immunogenicity and pharmacokinetic analyses.

Anti-idiotype antibodies against a therapeutic antibody are key reagents for the development of immunogenicity and pharmacokinetic (PK) assays during pre-clinical and clinical development. Here we have used a combination of phage and ribosome display to isolate a panel of monoclonal anti-idiotype antibodies with sub-nanomolar affinity and high specificity to a human anti-IgE monoclonal antibody. Anti-idiotype antibodies were enriched from scFv libraries using phage display, and a biochemical epitope competition assay was used to identify anti-idiotypes which neutralized IgE binding, which was essential for the intended use of the anti-idiotypes as positive controls in neutralizing anti-drug antibody (Nab) assays. The phage display-derived anti-idiotype antibodies were rapidly affinity-matured using a random point mutagenesis approach in ribosome display. Ten anti-idiotype antibodies with improved neutralizing activity relative to the parent antibodies displayed sub-nanomolar affinity for the anti-IgE antibody, representing up to 20-fold improvements in affinity from just two rounds of affinity-based selection. The optimized anti-idiotype antibodies retained the specificity of the parent antibodies, and importantly, were fit for purpose for use in PK and anti-drug antibody (ADA) assays. The approach we describe here for generation of anti-idiotype antibodies to an anti-IgE antibody is generically applicable for the rapid isolation and affinity maturation of anti-idiotype antibodies to any antibody-based drug candidate.

The clinical utility of anti-ribosomal P autoantibodies in systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a chronic inflammatory disease that can affect multiple organs and thus has a large spectrum of clinical presentations. Assessment of the autoantibody profile is fundamental for the clinical management of SLE patients, providing important data for diagnosis, clinical characterization and disease activity evaluation. Anti-ribosomal P protein (anti-Rib-P, anti-P) antibody, described in the 1980s, is a serological marker for SLE that is present in 13-20% of cases. This reactivity was initially thought to be associated with neuropsychiatric involvement in SLE, with certain conflicting results. Subsequently, associations of anti-Rib-P with liver and renal involvement in lupus were reported. Recently, anti-Rib-P was detected in autoimmune hepatitis patients. Anti-Rib-P reactivity to Trypanosoma cruzi ribosomal target antigens in patients with Chagas heart disease has also been described. This review focuses on the usefulness of the determination of anti-Rib-P in SLE and in other autoimmune and non-autoimmune disorders in clinical practice.

Re-examining class-I presentation and the DRiP hypothesis.

MHC class I molecules present peptides derived from intracellular proteins, enabling immune surveillance by CD8(+) T cells and the elimination of virus-infected and cancerous cells. It has been argued that the dominant source of MHC class I-presented peptides is through proteasomal degradation of newly synthesized defective proteins, termed defective ribosomal products (DRiPs). Here, we critically examine the DRiP hypothesis and discuss recent studies indicating that antigenic peptides are generated from the entire proteome and not just from failures in protein synthesis or folding.

Translating DRiPs: MHC class I immunosurveillance of pathogens and tumors.

MHC class I molecules display oligopeptides on the cell surface to enable T cell immunosurveillance of intracellular pathogens and tumors. Speed is of the essence in detecting viruses, which can complete a full replication cycle in just hours, whereas tumor detection is typically a finding-the-needle-in-the-haystack exercise. We review current evidence supporting a nonrandom, compartmentalized selection of peptidogenic substrates that focuses on rapidly degraded translation products as a main source of peptide precursors to optimize immunosurveillance of pathogens and tumors.

Translation of pre-spliced RNAs in the nuclear compartment generates peptides for the MHC class I pathway.

The scanning of maturing mRNAs by ribosomes plays a key role in the mRNA quality control process. When ribosomes first engage with the newly synthesized mRNA, and if peptides are produced, is unclear, however. Here we show that ribosomal scanning of prespliced mRNAs occurs in the nuclear compartment, and that this event produces peptide substrates for the MHC class I pathway. Inserting antigenic peptide sequences in introns that are spliced out before the mRNAs exit the nuclear compartment results in an equal amount of antigenic peptide products as when the peptides are encoded from the main open reading frame (ORF). Taken together with the detection of intron-encoded nascent peptides and RPS6/RPL7-carrying complexes in the perinucleolar compartment, these results show that peptides are produced by a translation event occurring before mRNA splicing. This suggests that ribosomes occupy and scan mRNAs early in the mRNA maturation process, and suggests a physiological role for nuclear mRNA translation, and also helps explain how the immune system tolerates peptides derived from tissue-specific mRNA splice variants.

MHC class I antigen presentation of DRiP-derived peptides from a model antigen is not dependent on the AAA ATPase p97.

CD8(+) T cells are responsible for killing cells of the body that have become infected or oncogenically transformed. In order to do so, effector CD8(+) T cells must recognize their cognate antigenic peptide bound to a MHC class I molecule that has been directly presented by the target cell. Due to the rapid nature of antigen presentation, it is believed that antigenic peptides are derived from a subset of newly synthesized proteins which are degraded almost immediately following synthesis and termed Defective Ribosomal Products or DRiPs. We have recently reported on a bioassay which can distinguish antigen presentation of DRiP substrates from other forms of rapidly degraded proteins and found that poly-ubiquitin chain disassembly may be necessary for efficient DRiP presentation. The AAA ATPase p97 protein is necessary for efficient cross-presentation of antigens on MHC class I molecules and plays an important role in extracting mis-folded proteins from the endoplasmic reticulum. Here, we find that genetic ablation or chemical inhibition of p97 does not diminish DRiP antigen presentation to any great extent nor does it alter the levels of MHC class I molecules on the cell surface, despite our observations that p97 inhibition increased the levels of poly-ubiquitinated proteins in the cell. These data demonstrate that inhibiting poly-ubiquitin chain disassembly alone is insufficient to abolish DRiP presentation.