An infectivity-enhancing site on the SARS-CoV-2 spike protein targeted by antibodies.

Antibodies against the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein prevent SARS-CoV-2 infection. However, the effects of antibodies against other spike protein domains are largely unknown. Here, we screened a series of anti-spike monoclonal antibodies from coronavirus disease 2019 (COVID-19) patients and found that some of antibodies against the N-terminal domain (NTD) induced the open conformation of RBD and thus enhanced the binding capacity of the spike protein to ACE2 and infectivity of SARS-CoV-2. Mutational analysis revealed that all of the infectivity-enhancing antibodies recognized a specific site on the NTD. Structural analysis demonstrated that all infectivity-enhancing antibodies bound to NTD in a similar manner. The antibodies against this infectivity-enhancing site were detected at high levels in severe patients. Moreover, we identified antibodies against the infectivity-enhancing site in uninfected donors, albeit at a lower frequency. These findings demonstrate that not only neutralizing antibodies but also enhancing antibodies are produced during SARS-CoV-2 infection.

Heme-mediated SPI-C induction promotes monocyte differentiation into iron-recycling macrophages.

Splenic red pulp macrophages (RPM) degrade senescent erythrocytes and recycle heme-associated iron. The transcription factor SPI-C is selectively expressed by RPM and is required for their development, but the physiologic stimulus inducing Spic is unknown. Here, we report that Spic also regulated the development of F4/80(+)VCAM1(+) bone marrow macrophages (BMM) and that Spic expression in BMM and RPM development was induced by heme, a metabolite of erythrocyte degradation. Pathologic hemolysis induced loss of RPM and BMM due to excess heme but induced Spic in monocytes to generate new RPM and BMM. Spic expression in monocytes was constitutively inhibited by the transcriptional repressor BACH1. Heme induced proteasome-dependent BACH1 degradation and rapid Spic derepression. Furthermore, cysteine-proline dipeptide motifs in BACH1 that mediate heme-dependent degradation were necessary for Spic induction by heme. These findings are the first example of metabolite-driven differentiation of a tissue-resident macrophage subset and provide new insights into iron homeostasis.

The Fc Domain of Immunoglobulin Is Sufficient to Bridge NK Cells with Virally Infected Cells.

Clearance of pathogens or tumor cells by antibodies traditionally requires both Fab and Fc domains of IgG. Here, we show the Fc domain of IgG alone mediates recognition and clearance of herpes simplex virus (HSV1)-infected cells. The human natural killer (NK) cell surface is naturally coated with IgG bound by its Fc domain to the Fcγ receptor CD16a. NK cells utilize the Fc domain of bound IgG to recognize gE, an HSV1-encoded glycoprotein that also binds the Fc domain of IgG but at a site distinct from CD16a. The bridge formed by the Fc domain between the HSV1-infected cell and the NK cell results in NK cell activation and lysis of the HSV1-infected cell in the absence of HSV1-specific antibody in vitro and prevents fatal HSV1 infection in vivo. This mechanism also explains how bacterial IgG-binding proteins regulate NK cell function and may be broadly applicable to Fcγ-receptor-bearing cells.

Structural basis for RIFIN-mediated activation of LILRB1 in malaria.

The Plasmodium species that cause malaria are obligate intracellular parasites, and disease symptoms occur when these parasites replicate in human blood. Despite the risk of immune detection, the parasite delivers proteins that bind to host receptors on the cell surfaces of infected erythrocytes. In the causative parasite of the most deadly form of malaria in humans, Plasmodium falciparum, RIFINs form the largest family of surface proteins displayed by erythrocytes1. Some RIFINs can bind to inhibitory immune receptors, and these RIFINs act as targets for unusual antibodies that contain a LAIR1 ectodomain2-4 or as ligands for LILRB15. RIFINs stimulate the activation of and signalling by LILRB15, which could potentially lead to the dampening of human immune responses. Here, to understand how RIFINs activate LILRB1-mediated signalling, we determine the structure of a RIFIN bound to LILRB1. We show that this RIFIN mimics the natural activating ligand of LILRB1, MHC class I, in its LILRB1-binding mode. A single mutation in the RIFIN disrupts the complex, blocks LILRB1 binding of all tested RIFINs and abolishes signalling in a reporter assay. In a supported lipid bilayer system, which mimics the activation of natural killer (NK) cells by antibody-dependent cell-mediated cytotoxicity, both RIFIN and MHC are recruited to the immunological synapse of NK cells and reduce the activation of NK cells, as measured by the mobilization of perforin. Therefore, LILRB1-binding RIFINs mimic the binding mode of the natural ligand of LILRB1 and suppress the function of NK cells.

Neutrophil infiltration during inflammation is regulated by PILRα via modulation of integrin activation.

Acute inflammatory responses are important in host defense, whereas dysregulated inflammation results in life-threatening complications. Here we found that paired immunoglobulin-like type 2 receptor alpha (PILRα), an inhibitory receptor containing immunoreceptor tyrosine-based inhibitory motifs (ITIMs), negatively regulated neutrophil infiltration during inflammation. Pilra(-/-) mice had increased neutrophil recruitment to inflammatory sites and were highly susceptible to endotoxin shock. Pilra(-/-) neutrophils showed enhanced transmigration ability and increased adhesion to the ß(2) integrin ligand ICAM-1. PILRα expressed on neutrophils constitutively associated in cis with its ligands, resulting in clustering of PILRα during stimulation with a chemoattractant. Clustering of PILRα enhanced ITIM-mediated signaling, thus modulating ß(2) integrin inside-out activation. These data demonstrate that neutrophil recruitment in inflammatory responses is regulated by PILRα via modulation of integrin activation.

SARS-CoV-2 ORF8 is a viral cytokine regulating immune responses.

Many patients with severe COVID-19 suffer from pneumonia and the elucidation of the mechanisms underlying the development of this severe condition is important. The in vivo function of the ORF8 protein secreted by SARS-CoV-2 is not well understood. Here, we analyzed the function of ORF8 protein by generating ORF8-knockout SARS-CoV-2 and found that the lung inflammation observed in wild-type SARS-CoV-2-infected hamsters was decreased in ORF8-knockout SARS-CoV-2-infected hamsters. Administration of recombinant ORF8 protein to hamsters also induced lymphocyte infiltration into the lungs. Similar pro-inflammatory cytokine production was observed in primary human monocytes treated with recombinant ORF8 protein. Furthermore, we demonstrated that the serum ORF8 protein levels are well-correlated with clinical markers of inflammation. These results demonstrated that the ORF8 protein is a SARS-CoV-2 viral cytokine involved in the immune dysregulation observed in COVID-19 patients, and that the ORF8 protein could be a novel therapeutic target in severe COVID-19 patients.

Contributions of the N-terminal flanking residues of an antigenic peptide from the Japanese cedar pollen allergen Cry j 1 to the T-cell activation by HLA-DP5.

Cry j 1 is a major allergen present in Japanese cedar (Cryptomeria japonica) pollens. Peptides with the core sequence of KVTVAFNQF from Cry j 1 ('pCj1') bind to HLA-DP5 and activate Th2 cells. In this study, we noticed that Ser and Lys at positions -2 and -3, respectively, in the N-terminal flanking (NF) region to pCj1 are conserved well in HLA-DP5-binding allergen peptides. A competitive binding assay showed that the double mutation of Ser(-2) and Lys(-3) to Glu [S(P-2)E/K(P-3)E] in a 13-residue Cry j 1 peptide (NF-pCj1) decreased its affinity for HLA-DP5 by about 2-fold. Similarly, this double mutation reduced, by about 2-fold, the amount of NF-pCj1 presented on the surface of mouse antigen-presenting dendritic cell line 1 (mDC1) cells stably expressing HLA-DP5. We established NF-pCj1-specific and HLA-DP5-restricted CD4+ T-cell clones from HLA-DP5 positive cedar pollinosis (CP) patients, and analyzed their IL-2 production due to the activation of mouse TG40 cells expressing the cloned T-cell receptor by the NF-pCj1-presenting mDC1 cells. The T-cell activation was actually decreased by the S(P-2)E/K(P-3)E mutation, corresponding to the reduction in the peptide presentation by this mutation. In contrast, the affinity of NF-pCj1·HLA-DP5 for the T-cell receptor was not affected by the S(P-2)E/K(P-3)E mutation, as analyzed by surface plasmon resonance. Considering the positional and side-chain differences of these NF residues from previously reported T-cell activating sequences, the mechanisms of enhanced T-cell activation by Ser(-2) and Lys(-3) of NF-pCj1 may be novel.

Corrigendum: Immune evasion of Plasmodium falciparum by RIFIN via inhibitory receptors.

This corrects the article DOI: 10.1038/nature24994.

LILRB4 signalling in leukaemia cells mediates T cell suppression and tumour infiltration.

Immune checkpoint blockade therapy has been successful in treating some types of cancer but has not shown clinical benefits for treating leukaemia1. This result suggests that leukaemia uses unique mechanisms to evade this therapy. Certain immune inhibitory receptors that are expressed by normal immune cells are also present on leukaemia cells. Whether these receptors can initiate immune-related primary signalling in tumour cells remains unknown. Here we use mouse models and human cells to show that LILRB4, an immunoreceptor tyrosine-based inhibition motif-containing receptor and a marker of monocytic leukaemia, supports tumour cell infiltration into tissues and suppresses T cell activity via a signalling pathway that involves APOE, LILRB4, SHP-2, uPAR and ARG1 in acute myeloid leukaemia (AML) cells. Deletion of LILRB4 or the use of antibodies to block LILRB4 signalling impeded AML development. Thus, LILRB4 orchestrates tumour invasion pathways in monocytic leukaemia cells by creating an immunosuppressive microenvironment. LILRB4 represents a compelling target for the treatment of monocytic AML.

Neoself Antigens Presented on MHC Class II Molecules in Autoimmune Diseases.

Major histocompatibility complex (MHC) class II molecules play a crucial role in immunity by presenting peptide antigens to helper T cells. Immune cells are generally tolerant to self-antigens. However, when self-tolerance is broken, immune cells attack normal tissues or cells, leading to the development of autoimmune diseases. Genome-wide association studies have shown that MHC class II is the gene most strongly associated with the risk of most autoimmune diseases. When misfolded self-antigens, called neoself antigens, are associated with MHC class II molecules in the endoplasmic reticulum, they are transported by the MHC class II molecules to the cell surface without being processed into peptides. Moreover, neoself antigens that are complexed with MHC class II molecules of autoimmune disease risk alleles exhibit distinct antigenicities compared to normal self-antigens, making them the primary targets of autoantibodies in various autoimmune diseases. Elucidation of the immunological functions of neoself antigens presented on MHC class II molecules is crucial for understanding the mechanism of autoimmune diseases.

Extracellular transportation of α-synuclein by HLA class II molecules.

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive accumulation of α-synuclein aggregates in form of Lewy bodies. Genome-wide association studies have revealed that human leukocyte antigen (HLA) class II is a PD-associated gene, although the mechanisms linking HLA class II and PD remain elusive. Here, we identified a novel function of HLA class II in the transport of intracellular α-synuclein to the outside of cells. HLA class II molecules and α-synuclein formed complexes and moved to the cell surface at various degrees among HLA-DR alleles. HLA-DR with a DRB5∗01:01 allele, a putative PD-risk allele, substantially translocated normal and conformationally abnormal α-synuclein to the cell surface and extracellular vesicles. α-Synuclein/HLA class II complexes were found in A2058 melanoma cells, which express intrinsic α-synuclein and HLA-DR with DRB5∗01:01. Our findings will expand our knowledge of unconventional HLA class II function from autoimmune diseases to neurodegenerative disorders, shedding light on the association between the GWAS-prioritized PD-risk gene HLA-DR and α-synuclein.

Rheumatoid factor recognizes specific domains of the IgG heavy chain complexed with HLA class II molecules.

OBJECTIVE: We previously reported that RF recognized the IgG heavy chain (IgGH)/RA-susceptible HLA class II molecule complex. In the present study, we investigated the molecular mechanisms underlying HLA binding to and the RF recognition of IgGH. METHODS: We synthesized various types of IgGH segments, including VH, CH1, CH2 and CH3, and transfected them with or without HLA class II molecules into the Human Embryonic Kidney 293T cell line. IgGH single domains linked with the HLA-Cw3 peptide, which binds to the binding groove of the HLA class II molecule, were also synthesized. The expression of IgGH domains on the cell surface and their recognition by RF were examined using flow cytometry. RESULTS: Flag-tagged IgGH segments containing CH1 (CH1, VH-CH1, CH1-CH2, VH-CH1-CH2, CH1-CH2-CH3 and VH-CH1-CH2-CH3) were clearly presented on the cell surface by HLA-DR4, while segments without the CH1 domain were expressed at a low level, and the CH3 single domain was only weakly detected on the cell surface, even with HLA-DR4. We then transfected IgGH single domains linked to the Cw3 peptide together with HLA-DR4 and showed that RF-containing sera from RA patients only recognized the CH3 domain and none of the other single domains. When various segments without the Cw3 peptide were transfected with HLA-DR4, only the CH1-CH2-CH3 segment and full-length IgGH were detected by the sera of RA patients. CONCLUSION: The CH1 domain of IgGH binds to the RA-susceptible HLA-DR molecule and is expressed on the cell surface. RF specifically recognizes the CH3 domain of the IgGH/HLA-DR4 complex.

TRIM28 Expression on Dendritic Cells Prevents Excessive T Cell Priming by Silencing Endogenous Retrovirus.

Acquired immune reaction is initiated by dendritic cells (DCs), which present Ags to a few naive Ag-specific T cells. Deregulation of gene expression in DCs may alter the outcome of the immune response toward immunodeficiency and/or autoimmune diseases. Expression of TRIM28, a nuclear protein that mediates gene silencing through heterochromatin, decreased in DCs from old mice, suggesting alteration of gene regulation. Mice specifically lacking TRIM28 in DCs show increased DC population in the spleen and enhanced T cell priming toward inflammatory effector T cells, leading to acceleration and exacerbation in experimental autoimmune encephalomyelitis. TRIM28-deficient DCs were found to ectopically transcribe endogenous retrovirus (ERV) elements. Combined genome-wide analysis revealed a strong colocalization among the decreased repressive histone mark H3K9me3-transcribed ERV elements and the derepressed host genes that were related to inflammation in TRIM28-deficient DCs. This suggests that TRIM28 occupancy of ERV elements critically represses expression of proximal inflammatory genes on the genome. We propose that gene silencing through repressive histone modification by TRIM28 plays a role in maintaining the integrity of precise gene regulation in DCs, which prevents aberrant T cell priming to inflammatory effector T cells.

PILRalpha is a herpes simplex virus-1 entry coreceptor that associates with glycoprotein B.

Glycoprotein B (gB) is one of the essential components for infection by herpes simplex virus-1 (HSV-1). Although several cellular receptors that associate with glycoprotein D (gD), such as herpes virus entry mediator (HVEM) and Nectin-1, have been identified, specific molecules that mediate HSV-1 infection by associating with gB have not been elucidated. Here, we found that paired immunoglobulin-like type 2 receptor (PILR) alpha associates with gB, and cells transduced with PILRalpha become susceptible to HSV-1 infection. Furthermore, HSV-1 infection of human primary cells expressing both HVEM and PILRalpha was blocked by either anti-PILRalpha or anti-HVEM antibody. Our results demonstrate that cellular receptors for both gB and gD are required for HSV-1 infection and that PILRalpha plays an important role in HSV-1 infection as a coreceptor that associates with gB. These findings uncover a crucial aspect of the mechanism underlying HSV-1 infection.

Immune evasion of Plasmodium falciparum by RIFIN via inhibitory receptors.

Malaria is among the most serious infectious diseases affecting humans, accounting for approximately half a million deaths each year. Plasmodium falciparum causes most life-threatening cases of malaria. Acquired immunity to malaria is inefficient, even after repeated exposure to P. falciparum, but the immune regulatory mechanisms used by P. falciparum remain largely unknown. Here we show that P. falciparum uses immune inhibitory receptors to achieve immune evasion. RIFIN proteins are products of a polymorphic multigene family comprising approximately 150-200 genes per parasite genome that are expressed on the surface of infected erythrocytes. We found that a subset of RIFINs binds to either leucocyte immunoglobulin-like receptor B1 (LILRB1) or leucocyte-associated immunoglobulin-like receptor 1 (LAIR1). LILRB1-binding RIFINs inhibit activation of LILRB1-expressing B cells and natural killer (NK) cells. Furthermore, P. falciparum-infected erythrocytes isolated from patients with severe malaria were more likely to interact with LILRB1 than erythrocytes from patients with non-severe malaria, although an extended study with larger sample sizes is required to confirm this finding. Our results suggest that P. falciparum has acquired multiple RIFINs to evade the host immune system by targeting immune inhibitory receptors.

Anti-ß2-glycoprotein I/HLA-DR Antibody and Adverse Obstetric Outcomes.

Anti-ß2-glycoprotein I/HLA-DR (anti-ß2GPI/HLA-DR) antibody has been reported to be associated with antiphospholipid syndrome and recurrent pregnancy loss (RPL). We conducted a prospective multicenter cross-sectional study aimed at evaluating whether the anti-ß2GPI/HLA-DR antibody is associated with adverse obstetric outcomes and RPL. From 2019 to 2021, serum anti-ß2GPI/HLA-DR antibody levels (normal, <73.3 U) were measured in 462 women with RPL, 124 with fetal growth restriction (FGR), 138 with hypertensive disorders of pregnancy (HDP), 71 with preterm delivery before 34 gestational weeks (preterm delivery (PD) ≤ 34 GWs), and 488 control women who experienced normal delivery, by flow cytometry analysis. The adjusted odds ratios (aORs) of anti-ß2GPI/HLA-DR antibody positivity for adverse obstetric outcomes and RPL were evaluated on the basis of comparisons between the control and each patient group, using multivariable logistic regression analysis. The following were the positivity rates for the anti-ß2GPI/HLA-DR antibody in the patient and control groups: RPL, 16.9%; FGR, 15.3%; HDP, 17.4%; PD ≤ 34 GWs, 11.3%; and the control, 5.5%. It was demonstrated that anti-ß2GPI/HLA-DR antibody positivity was a significant risk factor for RPL (aOR, 3.3 [95% confidence interval {CI} 1.9-5.6], p < 0.001), FGR (2.7 [1.3-5.3], p < 0.01), and HDP (2.7 [1.4-5.3], p < 0.01) although not for PD ≤ 34 GWs. For the first time, our study demonstrated that the anti-ß2GPI/HLA-DR antibody is involved in the pathophysiology underlying FGR and HDP, as well as RPL.

Regulation of Siglec-7-mediated varicella-zoster virus infection of primary monocytes by cis-ligands.

Varicella-zoster virus (VZV) first infects hematopoietic cells, with the infected cells then acting to distribute the virus throughout the body. Sialic acid-binding immunoglobulin-like lectin (Siglec) family molecules recognize sialic acid-containing molecules on the same cell surface, called cis-ligands, or molecules on other cells or soluble agents, called trans-ligands. Among the Siglec family molecules, Siglec-4 and Siglec-7 mediate VZV infection through association with glycoprotein B (gB). As Siglec-7, but not Siglec-4, is expressed on hematopoietic cells such as monocytes, the regulatory mechanism by which Siglec-7 associates with gB is important to our understanding of VZV infection of blood cells. Here, we found that Siglec-7 is required for VZV to infect human primary monocytes. Furthermore, treatment of primary monocytes with sialidase enhanced both VZV gB binding to monocytes and VZV infectivity. Calcium influx in primary monocytes decreased the expression of Siglec-7 cis-ligands and increased VZV infectivity. These results demonstrate that the Siglec-7 cis-ligands present on primary monocytes play an important role in VZV infection through regulation of the interaction between gB and Siglec-7.

Siglec-7 mediates varicella-zoster virus infection by associating with glycoprotein B.

Sialic acid immunoglobulin-like lectin (Siglec) family molecules are immune regulatory receptors that bind to specific molecules containing sialic acids. Varicella-zoster virus (VZV), a member of the herpesvirus family, infects hematopoietic cells and spreads throughout the body, causing chickenpox, shingles, and, sometimes fatal encephalomyelitis. However, the cellular entry receptors that are required for VZV to infect hematopoietic cells have remained unclear. Here, we found that Siglec-7, mainly expressed on hematopoietic cells, binds to VZV envelope glycoprotein B in a sialic acid-dependent manner. Furthermore, Siglec-7 mediated VZV infection by inducing membrane fusion. Our findings provide the first evidence for a molecular mechanism by which VZV infects hematopoietic cells.

Cell surface-expressed Ro52/IgG/HLA-DR complex is targeted by autoantibodies in patients with inflammatory myopathies.

Intracellular proteins are often targeted by autoantibodies in autoimmune diseases; however, the mechanism through which intracellular molecules are targeted remains unknown. We previously found that several intracellular misfolded proteins are transported to the cell surface by HLA class II molecules and are recognized by autoantibodies in some autoimmune diseases, such as rheumatoid arthritis, antiphospholipid syndrome, and microscopic polyangiitis. Ro52 is an intracellular Fc receptor that is a target antigen for myositis-associated autoantibodies. We analyzed the role of HLA class II molecules in the autoantibody recognition of Ro52. Ro52 alone was not transported to the cell surface by HLA class II molecules; however, it was transported to the cell surface in the presence of both IgG heavy chain and HLA class II molecules to form a Ro52/IgG/HLA-DR complex. The Ro52/IgG/HLA-DR complex was specifically recognized by autoantibodies from some patients with inflammatory myopathies. We then evaluated 120 patients with inflammatory myopathies with four types of myositis-specific antibodies and analyzed the autoantibodies against the Ro52/IgG/HLA-DR complex. The specific antibodies against the Ro52/IgG/HLA-DR complex were detected in 90% and 93% of patients who were positive for anti-MDA5 and anti-ARS antibodies, respectively. In individual patients with these two inflammatory myopathies, changes in serum titers of anti-Ro52/IgG/HLA-DR-specific antibodies were correlated with the levels of KL-6 (R = 0.51 in anti-MDA5 antibody-positive DM patients, R = 0.67 in anti-ARS antibody-positive PM/DM patients with respiratory symptoms) and CK (R = 0.63 in anti-ARS antibody-positive PM/DM patients with muscle symptoms) over time. These results suggest that antibodies against Ro52/IgG/HLA-DR expressed on the cell surface could be involved in the pathogenesis of inflammatory myopathy subgroups.

The major histocompatibility complex: new insights from old molecules into the pathogenesis of autoimmunity.

The major histocompatibility complex (MHC) is a central molecule in the immune system. The MHC is also a highly polymorphic gene that is the most strongly involved in susceptibility to many autoimmune diseases. Therefore, the elucidation of the mechanism by which specific MHC alleles are involved in autoimmune disease susceptibility is important to our understanding of the pathogenesis of autoimmune diseases. Specific autoantibody production is observed in many autoimmune diseases, but the mechanism underlying their production remains unclear. We have found that MHC class II molecules exhibit a chaperone-like function to transport misfolded proteins in the endoplasmic reticulum to the outside of cells. Furthermore, misfolded proteins transported to the cell surface by MHC class II molecules of autoimmune disease risk alleles are major autoantibody targets. In this article, I propose a novel mechanism underlying autoimmune diseases mediated by misfolded proteins complexed with MHC class II molecules.