Human cytomegalovirus evades antibody-mediated immunity through endoplasmic reticulum-associated degradation of the FcRn receptor

The role of immunoglobulin transport receptor, neonatal Fc receptor in mucosal infection and immunity and therapeutic intervention

The neonatal Fc receptor (FcRn) can transport IgG and antigen-antibody complexes participating in mucosal immune responses that protect the host from most pathogens' invasion via the respiratory, digestive, and urogenital tracts. FcRn expression can be triggered upon stimulation with pathogenic invasion on mucosal surfaces, which may significantly modulate the innate immune response of the host. As an immunoglobulin transport receptor, FcRn is implicated in the pathophysiology of immune-related diseases such as infection and autoimmune disorders. In this review, we thoroughly summarize the recent advancement of FcRn in mucosal immunity and its therapeutic strategy. This includes insights into its regulation mechanisms of FcRn expression influenced by pathogens, its emerging role in mucosal immunity and its potential probability as a therapeutic target in infection and autoimmune diseases.

Mechanisms of substrate processing during ER-associated protein degradation

Maintaining proteome integrity is essential for long-term viability of all organisms and is overseen by intrinsic quality control mechanisms. The secretory pathway of eukaryotes poses a challenge for such quality assurance as proteins destined for secretion enter the endoplasmic reticulum (ER) and become spatially segregated from the cytosolic machinery responsible for disposal of aberrant (misfolded or otherwise damaged) or superfluous polypeptides. The elegant solution provided by evolution is ER-membrane-bound ubiquitylation machinery that recognizes misfolded or surplus proteins or by-products of protein biosynthesis in the ER and delivers them to 26S proteasomes for degradation. ER-associated protein degradation (ERAD) collectively describes this specialized arm of protein quality control via the ubiquitin-proteasome system. But, instead of providing a single strategy to remove defective or unwanted proteins, ERAD represents a collection of independent processes that exhibit distinct yet overlapping selectivity for a wide range of substrates. Not surprisingly, ER-membrane-embedded ubiquitin ligases (ER-E3s) act as central hubs for each of these separate ERAD disposal routes. In these processes, ER-E3s cooperate with a plethora of specialized factors, coordinating recognition, transport and ubiquitylation of undesirable secretory, membrane and cytoplasmic proteins. In this Review, we focus on substrate processing during ERAD, highlighting common threads as well as differences between the many routes via ERAD.

Nanoluciferase Reporter Zika Viruses as Tools for Assessing Infection Kinetics and Antibody Potency

Zika virus (ZIKV) has become endemic in multiple tropical and subtropical regions and has the potential to become widespread in countries with limited prior exposure to this infection. One of the most concerning sequelae of ZIKV infection is the teratogenic effect on the developing fetus, with the mechanisms of viral spread to and across the placenta remaining largely unknown. Although vaccine trials and prophylactic or therapeutic treatments are being studied, there are no approved treatments or vaccines for ZIKV. Appropriate tests, including potency and in vivo assays to assess the safety and efficacy of these modalities, can greatly aid both the research of the pathophysiology of the infection and the development of anti-ZIKV therapeutics. Building on previous work, we tested reporter ZIKV variants that express nanoluciferase in cell culture and in vivo assays. We found that these variants can propagate in cells shown to be susceptible to the widely used clinical isolate PRVABC59, including Vero and human placenta cell lines. When used in neutralization assays with bioluminescence as readout, these variants gave rise to neutralization curves similar to those produced by PRVABC59, while being better suited for performing high-throughput assays. In addition, the engineered reporter variants can be useful research tools when used in other in vitro and in vivo assays, as we illustrated in transcytosis experiments and a pilot study in guinea pigs.

Fc receptors act as innate immune receptors during infection?

Innate immunity constitutes the first nonspecific immunological line of defense against infection. In this response, a variety of mechanisms are activated: the complement system, phagocytosis, and the inflammatory response. Then, adaptive immunity is activated. Major opsonization mediators during infections are immunoglobulins (Igs), the function of which is mediated through Fc receptors (FcRs). However, in addition to their role in adaptive immunity, FcRs have been shown to play a role in innate immunity by interacting directly with bacteria in the absence of their natural ligands (Igs). Additionally, it has been hypothesized that during the early phase of bacterial infection, FcRs play a protective role via innate immune functions mediated through direct recognition of bacteria, and as the infection progresses to later phases, FcRs exhibit their established function as receptors in adaptive immunity. This review provides detailed insight into the potential role of FcRs as innate immune mediators of the host defense against bacterial infection independent of opsonins.

Porcine Epidemic Diarrhea Virus nsp13 Protein Downregulates Neonatal Fc Receptor Expression by Causing Promoter Hypermethylation through the NF-κB Signaling Pathway

Porcine epidemic diarrhea virus (PEDV) is a highly pathogenic porcine enteric coronavirus that causes severe watery diarrhea and even death in piglets. The neonatal Fc receptor (FcRn) is the only transport receptor for IgG. FcRn expressed by intestinal epithelial cells can transport IgG from breast milk to piglets to provide immune protection. Previous studies have shown that viral infection affects FcRn expression. In this study, we showed for the first time, to our knowledge, that FcRn expression can be influenced by methyltransferases. In addition, we found that PEDV inhibited FcRn protein synthesis in porcine small intestinal epithelial cells postinfection. Then, we found that PEDV interfered with the transcription of genes through aberrant methylation modification of the FcRn promoter. DNA methyltransferase 3b (DNMT3b) has been implicated in this process. Using a series of PEDV structural and nonstructural protein (nsp) expression plasmids, we showed that nsp13 plays an important role in this aberrant methylation modification. PEDV nsp13 can affect the NF-κB canonical pathway and promote DNMT3b protein expression by facilitating p65 protein binding to chromatin. PEDV caused aberrant methylation of the FcRn promoter via DNMT3b. The same phenomenon was found in animal experiments with large white piglets. IgG transcytosis demonstrated that PEDV nsp13 can inhibit bidirectional IgG transport by FcRn. In addition, the core region of nsp13 (230-597 aa) is critical for FcRn inhibition. Taken together, to our knowledge, our findings revealed a novel immune escape mechanism of PEDV and shed new light on the design and development of vaccines and drugs.

Selenoprotein S Interacts with the Replication and Transcription Complex of SARS-CoV-2 by Binding nsp7

The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) replicates and evades detection using ER membranes and their associated protein machinery. Among these hijacked human proteins is selenoprotein S (selenos). This selenoprotein takes part in the protein quality control, signaling, and the regulation of cytokine secretion. While the role of selenos in the viral life cycle is not yet known, it has been reported to interact with SARS-CoV-2 nonstructural protein 7 (nsp7), a viral protein essential for the replication of the virus. We set to study whether selenos and nsp7 interact directly and if they can still bind when nsp7 is bound to the replication and transcription complex of the virus. Using biochemical assays, we show that selenos binds directly to nsp7. In addition, we found that selenos can bind to nsp7 when it is in a complex with the coronavirus's minimal replication and transcription complex, comprised of nsp7, nsp8, and the RNA-dependent RNA polymerase nsp12. In addition, through crosslinking experiments, we mapped the interaction sites of selenos and nsp7 in the replication complex and showed that the hydrophobic segment of selenos is essential for binding to nsp7. This arrangement leaves an extended helix and the intrinsically disordered segment of selenos-including the reactive selenocysteine-exposed and free to potentially recruit additional proteins to the replication and transcription complex.

ELISPOT assays with pp65 peptides or whole HCMV antigen are reliable predictors of immune control of HCMV infection in seropositive kidney transplant recipients

Human cytomegalovirus (HCMV) infection represents a major complication for solid organ transplant recipients. The aim of this study was to verify if the measurement of HCMV-specific T-cells could help to identify patients protected against HCMV disease cytokine flow cytometry using infected dendritic cells as stimulus (CFC-iDC, which discriminates between CD4⁺ and CD8⁺ T cells), and ELISPOT, using infected cell lysate (ELISPOT-iCL) or pp65 (ELISPOT-pp65) as stimulus, were adopted. Among the 47 kidney transplant recipients (KTR) enrolled, 29 had a self-resolving HCMV infection (Controllers) and 18 required antiviral treatment (Non-Controllers). HCMV-specific T-cell frequency at the peak of HCMV infection identified Controllers and Non-Controllers, although the diagnostic performance of CD8⁺ CFC-iDC (area under the curve [AUC] of the receiver-operator characteristic curve: 0.65) was lower than that of CD4⁺ CFC-iDC (AUC: 0.83), ELISPOT-iCL (AUC: 0.83) and ELISPOT-pp65 (AUC: 0.80). CFC-iDC detected a protective immune reconstitution significantly earlier (median time: 38 days) than ELISPOT-iCL and ELISPOT-pp65 (median time: 126 and 133 days, respectively). Time to protective immune reconstitution in Non-Controllers was significantly longer than in Controllers with the ELISPOT and the CD4⁺ CFC-iDC assays, but not with CD8⁺ CFC-iDC. The majority of patients did not require antiviral treatment after protective immune reconstitution, with the exception of five patients according to CFC-iDC assay, one patient according to ELISPOT-iCL assay and three patients according to ELISPOT-pp65 assay. Monitoring the HCMV-specific immunological reconstitution with is effective in discriminating KTR at risk of or protected from HCMV disease and the ELISPOT assays are suitable for implementation in the clinical setting.

Zika Virus Infection and Antibody Neutralization in FcRn Expressing Placenta and Engineered Cell Lines

As a developmental toxicant, Zika virus (ZIKV) attacks both the growing nervous system, causing congenital Zika syndrome, and the placenta, resulting in pathological changes and associated adverse fetal outcomes. There are no vaccines, antibodies, or other treatments for ZIKV, despite the potential for its re-emergence. Multiple studies have highlighted the risk of antibodies for enhancing ZIKV infection, including during pregnancy, but the mechanisms for such effects are not fully understood. We have focused on the ability of the neonatal Fc receptor (FcRn) to interact with ZIKV in the presence and absence of relevant antibodies. We found that ZIKV replication was higher in Marvin Darby Canine Kidney (MDCK) cells that overexpress FcRn compared to those that do not, and knocking down FcRn decreased ZIKV RNA production. In the placenta trophoblast BeWo cell line, ZIKV infection itself downregulated FcRn at the mRNA and protein levels. Addition of anti-ZIKV antibodies to MDCK/FcRn cells resulted in non-monotonous neutralization curves with neutralization attenuation and even enhancement of infection at higher concentrations. Non-monotonous neutralization was also seen in BeWo cells at intermediate antibody concentrations. Our studies highlight the underappreciated role FcRn plays in ZIKV infection and may have implications for anti-ZIKV prophylaxis and therapy in pregnant women.

The neonatal Fc receptor expression during macrophage differentiation is related to autophagy

The neonatal Fc receptor (FcRn) plays a central role in recycling and biodistributing immunoglobulin G. FcRn is also involved in many physiological immune functions as well as pathological immune responses in cancer or autoimmune diseases. Low levels of FcRn in tumor cells and the microenvironment is associated with poor prognosis in non-small cell lung cancers. Among cells that are present in the tumor microenvironment, macrophages express high levels of FcRn. Macrophages are involved in these pathophysiological contexts by their dual differentiation states of pro- or anti-inflammatory macrophages. However, variations in FcRn protein expression have not been described in macrophage subtypes. In this work, we studied FcRn expression in an in vitro model of pro- and anti-inflammatory macrophage differentiation. We demonstrated an inverse relation between FcRn protein and mRNA expression in macrophage populations. Autophagy, which is involved in protein degradation and acquisition of phagocytic function in macrophages, participated in regulating FcRn levels. Intravenous immunoglobulin protected FcRn against autophagosome degradation in anti-inflammatory macrophages. Our data demonstrate that autophagy participates in regulating FcRn expression in pro- and anti-inflammatory macrophages. This finding raises new questions concerning the regulation of FcRn in immune functions.

Lymph node fibroblastic reticular cells preserve a tolerogenic niche in allograft transplantation through laminin α4

Lymph node (LN) fibroblastic reticular cells (FRCs) define LN niches and regulate lymphocyte homeostasis through producing diverse extracellular matrix (ECM) components. We examined the role of ECM laminin α4 (Lama4) using FRC-Lama4 conditional KO Pdgfrb-Cre-/- × Lama4fl/fl mice. Single-cell RNA-sequencing (scRNA-Seq) data showed the promoter gene Pdgfrb was exclusively expressed in FRCs. Depleting FRC-Lama4 reduced Tregs and dendritic cells, decreased high endothelial venules, impaired the conduit system, and downregulated T cell survival factors in LNs. FRC-Lama4 depletion impaired the homing of lymphocytes to LNs in homeostasis and after allografting. Alloantigen-specific T cells proliferated, were activated to greater degrees in LNs lacking FRC-Lama4, and were more prone to differentiate into effector phenotypes relative to the Treg phenotype. In murine cardiac transplantation, tolerogenic immunosuppression was not effective in FRC-Lama4 recipients, which produced more alloantibodies than WT. After lung transplantation, FRC-Lama4-KO mice had more severe graft rejection with fewer Tregs in their LNs. Overall, FRC-Lama4 critically contributes to a tolerogenic LN niche by supporting T cell migration, constraining T cell activation and proliferation, and promoting Treg differentiation. Hence, it serves as a therapeutic target for immunoengineering.

Congenital Human Cytomegalovirus Infection Is Associated With Decreased Transplacental IgG Transfer Efficiency Due to Maternal Hypergammaglobulinemia

BACKGROUND

Placentally transferred maternal immunoglobulin G (IgG) protects against pathogens in early life, yet vertically transmitted infections can interfere with transplacental IgG transfer. Although human cytomegalovirus (HCMV) is the most common placentally-transmitted viral infection worldwide, the impact of congenital HCMV (cCMV) infection on transplacental IgG transfer has been underexplored.

METHODS

We evaluated total and antigen-specific maternal and cord blood IgG levels and transplacental IgG transfer efficiency in a US-based cohort of 93 mother-infant pairs including 27 cCMV-infected and 66 cCMV-uninfected pairs, of which 29 infants were born to HCMV-seropositive nontransmitting mothers and 37 to HCMV-seronegative mothers. Controls were matched on sex, race/ethnicity, maternal age, and delivery year.

RESULTS

Transplacental IgG transfer efficiency was decreased by 23% (95% confidence interval [CI] 10-36%, P = .0079) in cCMV-infected pairs and 75% of this effect (95% CI 28-174%, P = .0085) was mediated by elevated maternal IgG levels (ie, hypergammaglobulinemia) in HCMV-transmitting women. Despite reduced transfer efficiency, IgG levels were similar in cord blood from infants with and without cCMV infection.

CONCLUSIONS

Our results indicate that cCMV infection moderately reduces transplacental IgG transfer efficiency due to maternal hypergammaglobulinemia; however, infants with and without cCMV infection had similar antigen-specific IgG levels, suggesting comparable protection from maternal IgG acquired via transplacental transfer.

In Utero Development and Immunosurveillance of B Cell Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most frequent type of pediatric cancer with a peak incidence at 2–5 years of age. ALL frequently begins in utero with the emergence of clinically silent, preleukemic cells. Underlying leukemia-predisposing germline and acquired somatic mutations define distinct ALL subtypes that vary dramatically in treatment outcomes. In addition to genetic predisposition, a second hit, which usually occurs postnatally, is required for development of overt leukemia in most ALL subtypes. An untrained, dysregulated immune response, possibly due to an abnormal response to infection, may be an important co-factor triggering the onset of leukemia. Furthermore, the involvement of natural killer (NK) cells and T helper (Th) cells in controlling the preleukemic cells has been discussed. Identifying the cell of origin of the preleukemia-initiating event might give additional insights into potential options for prevention. Modulation of the immune system to achieve prolonged immunosurveillance of the preleukemic clone that eventually dies out in later years might present a future directive. Herein, we review the concepts of prenatal origin as well as potential preventive approaches to pediatric B cell precursor (BCP) ALL.

Insight for Immunotherapy of HCMV Infection

Human cytomegalovirus (HCMV), a ubiquitous in humans, has a high prevalence rate. Young people are susceptible to HCMV infection in developing countries, while older individuals are more susceptible in developed countries. Most patients have no obvious symptoms from the primary infection. Studies have indicated that the virus has gradually adapted to the host immune system. Therefore, the control of HCMV infection requires strong immune modulation. With the recent advances in immunotherapy, its application to HCMV infections is receiving increasing attention. Here, we discuss the immune response to HCMV infection, the immune escape mechanism, and the different roles that HCMV plays in various types of immunotherapy, including vaccines, adoptive cell therapy, checkpoint blockade therapy, and targeted antibodies.

Cytomegalovirus Disease in Renal Transplanted Patients: Prevalence, Determining Factors, and Influence on Graft and Patients Outcomes

The prevalence and the factors related to cytomegalovirus (CMV) disease (CMVd) during the 1st year of renal transplantation (RTx) and the relationship between CMVd and early and long-term graft and RTx-patient (RTx-p) survival were evaluated. In 505 RTx-p, followed up for 8(5-11) years, data were recorded after 1-(T1) and 12-(T12) months of RTx. CMVd was defined either by CMV replication without clinical signs of disease (CMVr, 43%), or CMV replication with signs of disease (CMVs, 57%). During the 1st year of RTx, 45% of RTx-p had CMVd (CMVd+). CMVd+ patients were older than CMVd- patients. Female gender and Donor CMV-IgG+ (CMV IgG-D+)/recipient IgG- (CMV IgG-R-) status were more prevalent in CMVd+. At T1, CMVd+ had lower albumin, haemoglobin, and higher uric-acid and reactive C-protein than CMVd- and, at T1 and T12, received more steroids. Albumin-T1 was the unique factor in determining CMVd+, maintaining its significance also after the inclusion of IgG-D+/IgG-R- status to the model. CMVs had higher prevalence of CMV IgG-D+/IgG-R- than CMVr. CMVd, CMVr, and CMVs had no impact on graft loss (11% of RTx-p) and RTx-p death (8% of RTx-p). CMVd is highly prevalent during the 1st year of RTx. Albumin-T1 influences CMVd insurgence. CMVd did not impact on RTx and RTx-p loss.

Morphology Remodeling and Selective Autophagy of Intracellular Organelles during Viral Infections

Viruses have evolved different strategies to hijack subcellular organelles during their life cycle to produce robust infectious progeny. Successful viral reproduction requires the precise assembly of progeny virions from viral genomes, structural proteins, and membrane components. Such spatial and temporal separation of assembly reactions depends on accurate coordination among intracellular compartmentalization in multiple organelles. Here, we overview the rearrangement and morphology remodeling of virus-triggered intracellular organelles. Focus is given to the quality control of intracellular organelles, the hijacking of the modified organelle membranes by viruses, morphology remodeling for viral replication, and degradation of intracellular organelles by virus-triggered selective autophagy. Understanding the functional reprogram and morphological remodeling in the virus-organelle interplay can provide new insights into the development of broad-spectrum antiviral strategies.

Ub to no good: How cytomegaloviruses exploit the ubiquitin proteasome system

Human cytomegalovirus (HCMV) is a ubiquitous member of the Betaherpesvirinae subfamily, causing life-threatening diseases in individuals with impaired, immature, or senescent immunity. Accordingly, HIV-infected AIDS patients, transplant recipients, and congenitally infected neonates frequently suffer from symptomatic episodes of HCMV replication. Like all viruses, HCMV has a split relationship with the host proteome. Efficient virus replication can only be achieved if proteins involved in intrinsic, innate, and adaptive immune responses are sufficiently antagonized. Simultaneously, the abundance and function of proteins involved in the synthesis of chemical building blocks required for virus production, such as nucleotides, amino acids, and fatty acids, must be preserved or even enriched. The ubiquitin (Ub) proteasome system (UPS) constitutes one of the most relevant protein decay systems of eukaryotic cells. In addition to the regulation of the turn-over and abundance of thousands of proteins, the UPS also generates the majority of peptides presented by major histocompatibility complex (MHC) molecules to allow surveillance by T lymphocytes. Cytomegaloviruses exploit the UPS to regulate the abundance of viral proteins and to manipulate the host proteome in favour of viral replication and immune evasion. After summarizing the current knowledge of CMV-mediated misuse of the UPS, we discuss the evolution of viral proteins utilizing the UPS for the degradation of defined target proteins. We propose two alternative routes of adapter protein development and their mechanistic consequences.

Isolation and Identification of Porcine Deltacoronavirus and Alteration of Immunoglobulin Transport Receptors in the Intestinal Mucosa of PDCoV-Infected Piglets

Porcine deltacoronavirus (PDCoV) is a porcine enteropathogenic coronavirus that causes watery diarrhea, vomiting, and frequently death in piglets, causing serious economic losses to the pig industry. The strain CHN-JS-2017 was isolated and identified by cytopathology, immunofluorescence assays, transmission electron microscopy, and sequence analysis. A nucleotide sequence alignment showed that the whole genome of CHN-JS-2017 is 97.4%-99.6% identical to other PDCoV strains. The pathogenicity of the CHN-JS-2017 strain was investigated in orally inoculated five-day-old piglets; the piglets developed acute, watery diarrhea, but all recovered and survived. CHN-JS-2017 infection-induced microscopic lesions were observed, and viral antigens were detected mainly by immunohistochemical staining in the small intestine. The neonatal Fc receptor (FcRn) and polymeric immunoglobulin receptor (pIgR) are crucial immunoglobulin (Ig) receptors for the transcytosis ofimmunoglobulin G (IgG), IgA, or IgM. Importantly, CHN-JS-2017 infected five-day-old piglets could significantly down-regulate the expression of FcRn, pIgR, and nuclear factor-kappa B (NF-κB)in the intestinal mucosa. Note that the level of FcRn mRNA in the intestinal mucosa of normal piglets is positively correlated with pIgR and NF-κB. At the same time, the expressions of FcRn, pIgR, and NF-κB mRNA are also positively correlated in infected piglets. These results may help explain the immunological and pathological changes associated with porcine deltacorononirus infection.