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To date, structural analysis has demonstrated a highly consistent binding pattern of the TCR to the MHC molecule. Rossjohn and colleagues reveal the first structures of two human T reg cell TCRs and show that they bind with a reversed polarity to the MHC. Central to adaptive immunity is the interaction between the αβ T cell receptor (TCR) and peptide presented by the major histocompatibility complex (MHC) molecule. Presumably reflecting TCR-MHC bias and T cell signaling constraints, the TCR universally adopts a canonical polarity atop the MHC. We report the structures of two TCRs, derived from human induced T regulatory (iT reg ) cells, complexed to an MHC class II molecule presenting a proinsulin-derived peptide. The ternary complexes revealed a 180° polarity reversal compared to all other TCR-peptide-MHC complex structures. Namely, the iT reg TCR α-chain and β-chain are overlaid with the α-chain and β-chain of MHC class II, respectively. Nevertheless, this TCR interaction elicited a peptide-reactive, MHC-restricted T cell signal. Thus TCRs are not 'hardwired' to interact with MHC molecules in a stereotypic manner to elicit a T cell signal, a finding that fundamentally challenges our understanding of TCR recognition.

Background The mucosal origins hypothesis of rheumatoid arthritis (RA) posits that inhalant exposures, such as cigarette smoke and crystalline silica (c-silica), trigger immune responses in the lungs that contribute to joint disease onset. However, the relationship between inhalants, lung inflammation, and inflammatory arthritis remains poorly understood. Results This study compared the development of inflammatory arthritis in two genetically susceptible mouse strains, the BXD2/TyJ (BXD2) and chimeric HLA-DR4-IE transgenic (DR4-Tg), following delivery of c-silica to the lungs via oropharyngeal aspiration. In BXD2 mice, c-silica exposure was associated with rapid arthritis development, marked by synovial cell hyperplasia, pannus formation, and severe erosion of cartilage and bone. These features were preceded by pulmonary inflammation characterized by lymphoid-like cell clusters lining vessels and bronchi which cell-specific immunofluorescent microscopy identified as organized lymphoid structures consistent with inducible bronchus-associated lymphoid tissue (iBALT). Inflammatory arthritis was also preceded by autoantibodies associated with RA and other systemic autoimmune diseases including anti-citrullinated protein autoantibodies (ACPA) and rheumatoid factor (RF) in bronchoalveolar lavage fluid (BALF). However, the most predominant autoantibodies in BALF were against extractable nuclear antigens (ENA). Anti-ENA were also prominent in serum and microarray autoantigen analysis confirmed the response as targeting components of Sm and RNP small nuclear ribonucleoproteins (snRNPs). In contrast, DR4-Tg mice had no signs of arthritis, milder lung inflammation lacking iBALT, and negligible autoantibody responses. Conclusion Genetic predisposition beyond HLA-DR4 alone is required for the immunological manifestations that lead to c-silica mediated inflammatory arthritis. These findings provide novel insights into the relationship between mucosal exposure and RA pathogenesis.

Peptides were designed to induce immune tolerance to the major antigen associated with house dust mite (HDM) allergy, Der p 1. HDM is commonly associated with allergic responses in allergic rhinitis and asthma, with Der p 1 specific T-cells implicated in ongoing disease. Tolerogenic peptide immunotherapy can induce tolerance in pathogenic T-cells, bypass mast cell activation and hence reduce the risk of anaphylaxis. A pan-DR binding epitope of Der p 1, covering the broad population, was tested for efficacy in HLA-DR transgenic mice. Potential pan-HLA-DR binding tolerogenic T-cell epitopes from Der p 1 were predicted and manufactured (synthetic peptides A-E). Participants included HDM sensitised (allergic rhinitis/asthma, n=25), non-HDM sensitised (atopic controls sensitised to ≥1 other aero-allergens, n=10) and non-atopic healthy controls, n=10). Peripheral blood mononuclear cells (PBMC) were collected and screened for immune responses to Der p 1 or test peptides A-E. Mapping of minimal T-cell epitopes, apitope (antigen-processing independent epitope) validation and tolerance induction were tested in HLA-DR transgenic mice. HDM-sensitised subjects have an elevated response to pan-DR binding peptide D 30mer. Peptide analogue D121B, containing the minimal epitope and optimised for solubility, was verified as a tolerogenic apitope and induced tolerance against Der p 1 antigens in HLA-DR4 transgenic mice . A tolerogenic peptide, apitope D121B, reduces T-cell immune responses to Der p 1 and is a promising candidate for further development as an immunotherapy for HDM-associated allergic rhinitis and asthma.

CD4+ T cells orchestrate adaptive immune responses via binding of antigens to their receptors through specific peptide/MHC-II complexes. To study these responses, it is essential to identify protein-derived MHC-II peptide ligands that constitute epitopes for T cell recognition. However, generating cells expressing single MHC-II alleles and isolating these proteins for use in peptide elution or binding studies is time consuming. Here, we express human MHC alleles (HLA-DR4 and HLA-DQ6) as native, noncovalent αβ dimers on yeast cells for direct flow cytometry-based screening of peptide ligands from selected antigens. We demonstrate rapid, accurate identification of DQ6 ligands from pre-pro-hypocretin, a narcolepsy-related immunogenic target. We also identify 20 DR4-binding SARS-CoV-2 spike peptides homologous to SARS-CoV-1 epitopes, and one spike peptide overlapping with the reported SARS-CoV-2 epitope recognized by CD4+ T cells from unexposed individuals carrying DR4 subtypes. Our method is optimized for immediate application upon the emergence of novel pathogens.

Aims/hypothesis Dietary sugar intake may increase insulin production, stress the beta cells and increase the risk for islet autoimmunity (IA) and subsequent type 1 diabetes. Methods Since 1993, the Diabetes Autoimmunity Study in the Young (DAISY) has followed children at increased genetic risk for type 1 diabetes for the development of IA (autoantibodies to insulin, GAD or protein tyrosine phosphatase-like protein [IA2] twice or more in succession) and progression to type 1 diabetes. Information on intake of fructose, sucrose, total sugars, sugar-sweetened beverages, beverages with non-nutritive sweetener and juice was collected prospectively throughout childhood via food frequency questionnaires (FFQs). We examined diet records for 1,893 children (mean age at last follow-up 10.2 years); 142 developed IA and 42 progressed to type 1 diabetes. HLA genotype was dichotomised as high risk ( HLA-DR3/4,DQB1*0302 ) or not. All Cox regression models were adjusted for total energy, FFQ type, type 1 diabetes family history, HLA genotype and ethnicity. Results In children with IA, progression to type 1 diabetes was significantly associated with intake of total sugars (HR 1.75, 95% CI 1.07–2.85). Progression to type 1 diabetes was also associated with increased intake of sugar-sweetened beverages in those with the high-risk HLA genotype (HR 1.84, 95% CI 1.25–2.71), but not in children without it (interaction p value = 0.02). No sugar variables were associated with IA risk. Conclusions/interpretation Sugar intake may exacerbate the later stage of type 1 diabetes development; sugar-sweetened beverages may be especially detrimental to children with the highest genetic risk of developing type 1 diabetes.

Immunogenetic as well as environmental and occupational exposures have been linked to the development of rheumatoid arthritis (RA), RA-associated lung disease, and other primary lung disorders. Importantly, various inhalants can trigger post-translational protein modifications, resulting in lung autoantigen expression capable of stimulating pro-inflammatory and/or pro-fibrotic immune responses. To further elucidate gene-environment interactions contributing to pathologic lung inflammation, we exploited an established model of organic dust extract (ODE) exposure with and without collagen-induced arthritis (CIA) in C57BL/6 wild type (WT) versus HLA-DR4 transgenic mice. ODE-induced airway infiltration driven by neutrophils was significantly increased in DR4 versus WT mice, with corresponding increases in bronchoalveolar lavage fluid (BALF) levels of TNF-⍺, IL-6, and IL-33. Lung histopathology demonstrated increased number of ectopic lymphoid aggregates comprised of T and B cells following ODE exposure in DR4 mice. ODE also induced citrullination, malondialdehyde acetaldehyde (MAA) modification, and vimentin expression that co-localized with MAA and was enhanced in DR4 mice. Serum and BALF anti-MAA antibodies were strikingly increased in ODE-treated DR4 mice. Coupling ODE exposure with Type II collagen immunization (CIA) resulted in similarly augmented pro-inflammatory lung profiles in DR4 mice (relative to WT mice) that was accompanied by a profound increase in infiltrating lung CD4 + and CD8 + T cells as well as CD19 + CD11b + autoimmune B cells. Neither modeling strategy induced significant arthritis. These findings support a model in which environmental insults trigger enhanced post-translational protein modification and lung inflammation sharing immunopathological features with RA-associated lung disease in the selected immunogenetic background of HLA-DR4 mice.

The failure to eliminate self‐reactive T cells during negative selection is a prerequisite for autoimmunity. To escape deletion, autoreactive T‐cell receptors (TCRs) may form unstable complexes with self‐peptide–MHC by adopting suboptimal binding topologies compared with anti‐microbial TCRs. Alternatively, escape can occur by weak binding between self‐peptides and MHC. We determined the structure of a human autoimmune TCR (MS2‐3C8) bound to a self‐peptide from myelin basic protein (MBP) and the multiple sclerosis‐associated MHC molecule HLA‐DR4. MBP is loosely accommodated in the HLA‐DR4‐binding groove, accounting for its low affinity. Conversely, MS2‐3C8 binds MBP–DR4 as tightly as the most avid anti‐microbial TCRs. MS2‐3C8 engages self‐antigen via a docking mode that resembles the optimal topology of anti‐foreign TCRs, but is distinct from that of other autoreactive TCRs. Combined with a unique CDR3β conformation, this docking mode compensates for the weak binding of MBP to HLA‐DR4 by maximizing interactions between MS2‐3C8 and MBP. Thus, the MS2‐3C8–MBP–DR4 complex reveals the basis for an alternative strategy whereby autoreactive T cells escape negative selection, yet retain the ability to initiate autoimmunity. This structural study shows how T‐cell receptors engage self‐peptides that trigger autoimmune reactions, providing insight into how T cells bearing such TCRs can escape negative selection.

Homozygous HLA-DR4/I-Ed transgenic mice (tgm) spontaneously developed colitis similar to human ulcerative colitis. We explored whether endoplasmic reticulum stress in colonic epithelial cells due to overexpression of HLA-DR4/I-Ed was involved in the pathogenesis of colitis.Methods:Major histocompatibility complex class II transactivator-knockout (CIITAKO) background tgm were established to test the involvement of HLA-DR4/I-Ed expression in the pathogenesis of colitis. Histological and cellular analyses were performed and the effect of oral administration of the molecular chaperone tauroursodeoxycholic acid (TUDCA) and antibiotics were investigated. IgA content of feces and serum and presence of IgA-coated fecal bacteria were also investigated.Results:Aberrantly accumulated HLA-DR4/I-Ed molecules in colonic epithelial cells were observed only in the colitic homozygous tgm, which was accompanied by upregulation of the endoplasmic reticulum stress marker Binding immunoglobulin protein (BiP) and reduced mucus. Homozygous tgm with CIITAKO, and thus absent of HLA-DR4/I-Ed expression, did not develop colitis. Oral administration of TUDCA to homozygotes reduced HLA-DR4/I-Ed and BiP expression in colonic epithelial cells and restored the barrier function of the intestinal tract. The IgA content of feces and serum, and numbers of IgA-coated fecal bacteria were higher in the colitic tgm, and antibiotic administration suppressed the expression of HLA-DR4/I-Ed and colitis.Conclusions:The pathogenesis of the colitis observed in the homozygous tgm was likely due to endoplasmic reticulum stress, resulting in goblet cell damage and compromised mucus production in the colonic epithelial cells in which HLA-DR4/I-Ed molecules were heavily accumulated. Commensal bacteria seemed to be involved in the accumulation of HLA-DR4/I-Ed, leading to development of the colitis.

According to the quality of response they mediate, autoreactive T cells recognizing islet beta cell peptides could represent both disease effectors in the development of type 1 diabetes (T1DM) and directors of tolerance in nondiabetic individuals or those undergoing preventative immunotherapy. A combination of the rarity of these cells, inadequate technology, and poorly defined epitopes, however, has hampered examination of this paradigm. We have identified a panel of naturally processed islet epitopes by direct elution from APCs bearing HLA-DR4. Employing these epitopes in a sensitive, novel cytokine enzyme-linked immunosorbent spot assay, we show that the quality of autoreactive T cells in patients with T1DM exhibits extreme polarization toward a proinflammatory Th1 phenotype. Furthermore, we demonstrate that rather than being unresponsive, the majority of nondiabetic, HLA-matched control subjects also manifest a response against islet peptides, but one that shows extreme T regulatory cell (Treg, IL-10-secreting) bias. We conclude that development of T1DM depends on the balance of autoreactive Th1 and Treg cells, which may be open to favorable manipulation by immune intervention.

Invariant NKT (iNKT) cells are infrequent but important immunomodulatory lymphocytes that exhibit CD1d‐restricted reactivity with glycolipid Ags. iNKT cells express a unique T‐cell receptor (TCR) composed of an invariant α‐chain, paired with a limited range of β‐chains. Superantigens (SAgs) are microbial toxins defined by their ability to activate conventional T cells in a TCR β‐chain variable domain (Vβ)‐specific manner. However, whether iNKT cells are directly activated by bacterial SAgs remains an open question. Herein, we explored the responsiveness of mouse and human iNKT cells to a panel of staphylococcal and streptococcal SAgs and examined the contribution of major histocompatibility complex (MHC) class II and CD1d to these responses. Bacterial SAgs that target mouse Vβ8, such as staphylococcal enterotoxin B (SEB), were able to activate mouse hybridoma and primary hepatic iNKT cells in the presence of mouse APCs expressing human leukocyte antigen (HLA)‐DR4. iNKT cell‐mediated cytokine secretion in SEB‐challenged HLA‐DR4‐transgenic mice was CD1d‐independent and accompanied by a high interferon‐γ:interleukin‐4 ratio consistent with an in vivo Th1 bias. Furthermore, iNKT cells from SEB‐injected HLA‐DR4‐transgenic mice, and iNKT cells from SEB‐treated human PBMCs, showed early activation by intracellular cytokine staining and CD69 expression. Unlike iNKT cell stimulation by α‐galactosylceramide, stimulation by SEB did not induce TCR downregulation of either mouse or human iNKT cells. We conclude that Vβ8‐targeting bacterial SAgs can activate iNKT cells by utilizing a novel pathway that requires MHC class II interactions, but not CD1d. Therefore, iNKT cells fulfill important effector functions in response to bacterial SAgs and may provide attractive targets in the management of SAg‐induced illnesses.