Exosomes are nanometer-sized vesicles released by a number of cell types including those of the immune system, and often contain numerous immune recognition molecules including MHC molecules. We demonstrate in this study that exosomes can display a significant proportion of their MHC class I (MHC I) content in the form of disulfide-linked MHC I dimers. These MHC I dimers can be detected after release from various cell lines, human monocyte-derived dendritic cells, and can also be found in human plasma. Exosome-associated dimers exhibit novel characteristics which include 1) being composed of folded MHC I, as detected by conformational-dependent Abs, and 2) dimers forming between two different MHC I alleles. We show that dimer formation is mediated through cysteine residues located in the cytoplasmic tail domains of many MHC I molecules, and is associated with a low level of glutathione in exosomes when compared with whole cell lysates. We propose these exosomal MHC I dimers as novel structures for recognition by immune receptors.

Autism spectrum condition (ASC) is recognized as having an inflammatory component. Post-mortem brain samples from patients with ASC display neuroglial activation and inflammatory markers in cerebrospinal fluid, although little is known about the underlying molecular mechanisms. Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a protein found in almost all cell types and mediates regulation of immune response by inducing the expression of inflammatory cytokines and chemokines, establishing a feedback mechanism that can produce chronic or excessive inflammation. This article describes immunodetection and immunofluorescence measurements of NF-κB in human post-mortem samples of orbitofrontal cortex tissue donated to two independent centers: London Brain Bank, Kings College London, UK (ASC: n = 3, controls: n = 4) and Autism Tissue Program, Harvard Brain Bank, USA (ASC: n = 6, controls: n = 5). The hypothesis was that concentrations of NF-κB would be elevated, especially in activated microglia in ASC, and pH would be concomitantly reduced (i.e., acidification). Neurons, astrocytes, and microglia all demonstrated increased extranuclear and nuclear translocated NF-κB p65 expression in brain tissue from ASC donors relative to samples from matched controls. These between-groups differences were increased in astrocytes and microglia relative to neurons, but particularly pronounced for highly mature microglia. Measurement of pH in homogenized samples demonstrated a 0.98-unit difference in means and a strong (F = 98.3; p = 0.00018) linear relationship to the expression of nuclear translocated NF-κB in mature microglia. Acridine orange staining localized pH reductions to lysosomal compartments. In summary, NF-κB is aberrantly expressed in orbitofrontal cortex in patients with ASC, as part of a putative molecular cascade leading to inflammation, especially of resident immune cells in brain regions associated with the behavioral and clinical symptoms of ASC.

The endoplasmic reticulum-located multimolecular peptideloading complex functions to load optimal peptides onto major histocompatibility complex (MHC) class I molecules for presentation to CD8 ؉ T lymphocytes. Two oxidoreductases, ERp57 and protein-disulfide isomerase, are known to be components of the peptide-loading complex. Within the peptide-loading complex ERp57 is normally found disulfide-linked to tapasin, through one of its two thioredoxin-like redox motifs. We describe here a novel trimeric complex that disulfide links together MHC class I heavy chain, ERp57 and tapasin, and that is found in association with the transporter associated with antigen processing peptide transporter. The trimeric complex normally represents a small subset of the total ERp57-tapasin pool but can be significantly increased by altering intracellular oxidizing conditions. Direct mutation of a conserved structural cysteine residue implicates an interaction between ERp57 and the MHC class I peptide-binding groove. Taken together, our studies demonstrate for the first time that ERp57 directly interacts with MHC class I molecules within the peptide-loading complex and suggest that ERp57 and protein-disulfide isomerase act in concert to regulate the redox status of MHC class I during antigen presentation.To bind a high affinity, optimized pool of peptides, MHC 6 class I molecules undergo a series of chaperone-mediated interactions within the endoplasmic reticulum (ER) (1, 2). For most MHC class I molecules, this process occurs in the multicomponent MHC class I peptide-loading complex (PLC), which in addition to ␤ 2 -microglobulin (␤2m)-associated MHC class I molecules includes the chaperone calreticulin, the oxidoreductases ERp57 and protein-disulfide isomerase (PDI), the class I-specific accessory molecule tapasin, and the peptide transporter TAP (3-5). A series of experimental reports have indicated that the PLC acts cooperatively, and that the absence of any of the components results in the disruption of MHC class I assembly, and a reduction in the efficiency of antigen presentation to T cells (6 -9).Both ERp57 and PDI are integral components of the PLC (10 -12). These oxidoreductases are related to thioredoxin (TR) and share two functional CXXC motifs (denoted TR1 and TR2) (13-15). Possession of this motif permits family members to catalyze reduction, oxidation, and isomerization of disulfide bonds within substrate polypeptides (16). ERp57 normally appears to be in a GSH-dependent reduced state within the ER (17, 18), and is thus likely to be involved in reduction or isomerization reactions. However, a striking feature of ERp57 is that, in the presence of tapasin, it preferentially forms an interaction through its TR1 motif with an unpaired cysteine in tapasin at position 95 (19). The normal escape pathway that exists to release ERp57 from substrate polypeptides is inhibited by this interaction (20), leading to the whole cellular pool of tapasin being disulfide-linked to ERp57, and a large pool of cellular ERp57 being likewise oc...

Introduction Ankylosing spondylitis (AS) is a severe, chronic inflammatory arthritis, with a strong association to the human major histocompatibilty complex (MHC) class I allele human leucocyte antigen (HLA) B27. Disulfide-linked HLA-B27 heavychain homodimers have been implicated as novel structures involved in the aetiology of AS. We have studied the formation of HLA-B27 heavy-chain homodimers in human dendritic cells, which are key antigen-presenting cells and regulators of mammalian immune responses.

Objective. The class I major histocompatibility complex (MHC) molecule HLA-B27 exhibits a strong association with the autoimmune inflammatory arthritis disorder ankylosing spondylitis (AS) and with other related spondylarthropathies. In the absence of both a defined autoimmune response and a target autoantigen(s), the propensity of HLA-B27 to misfold has been hypothesized to be a major parameter in disease pathogenesis. We undertook this study to test the hypothesis that HLA-B27 misfolding is due to exposure of cysteine residues within the heavy chain to the oxidizing environment of the endoplasmic reticulum.Methods. A rapid acidification and alkylation modification method was used to examine cysteine residue exposure and accessibility within AS-associated and non-AS-associated HLA-B27 subtypes.Results. This novel approach to probing in vivo class I MHC structure revealed that the HLA-B27 heavy chain adopts conformations not previously described. Furthermore, amino acid residues specific to subtypes HLA-B*2706, B*2709, and B*2704 can have an impact on these novel conformations and on cysteine residue exposure.Conclusion. HLA-B27 can adopt novel conformations, resulting in differential accessibility of cysteine residues, which can explain the propensity to misfold. Cysteine exposure in the HLA-B27 heavy chain is also affected by residues within the 114 and 116 regions, thereby providing a potential biochemical basis for the association of HLA-B27 subtypes with AS.