Transport of misfolded endoplasmic reticulum proteins to the cell surface by MHC class II molecules

Laboratory of Immunochemistry, WPI Immunology Frontier Research Center, Osaka University, Suita, Osaka 565-0871, Japan.
International Immunology (Impact Factor: 2.54). 01/2013; 25(4):235-246. DOI: 10.1093/intimm/dxs155
Source: PubMed


Nascent MHC class II molecules are associated with the invariant chain and are transported to the endolysosomal pathway, where MHC class II molecules acquire peptide antigens. On the other hand, misfolded endoplasmic reticulum (ER) proteins are generally degraded in the cells and are neither expressed on the cell surface nor secreted. Here, we found that MHC class II molecules associate with some misfolded ER proteins via the peptide-binding groove in competition with invariant chain. The misfolded proteins associated with MHC class II molecules are transported intact to the cell surface without processing to peptides. Furthermore, these complexes efficiently stimulate antigen-specific B cells. These findings reveal that MHC class II molecules function as a chaperone for the cell surface expression of misfolded ER proteins. In addition, we suggest that MHC class II molecules present not only peptides but also intact host-cell-derived proteins on the cell surface. These findings provide new insights into the function of MHC class II molecules.

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    • "In the absence of Ii, both mouse and human MHCIIs bind a collection of long polypeptides, most likely originating from misfolded ER proteins (24–26). Interestingly, even in the presence of Ii, it was recently reported that MHCIIs displayed some ER polypeptides at the plasma membrane, the latter competing with Ii for the class II binding groove (27). Despite the fact that MHCIIs can associate with ER polypeptides, there are numerous functional examples of endogenous and exogenous CD4 T cell epitopes that are presented in the absence of Ii expression [reviewed in Ref. (28)]. "
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    ABSTRACT: The peptide repertoire (peptidome) associated with MHC class II molecules (MHCIIs) is influenced by the polymorphic nature of the peptide binding groove but also by cell-intrinsic factors. The invariant chain (Ii) chaperones MHCIIs, affecting their folding and trafficking. Recent discoveries relating to Ii functions have provided insights as to how it edits the MHCII peptidome. In humans, the Ii gene encodes four different isoforms for which structure-function analyses have highlighted common properties but also some non-redundant roles. Another layer of complexity arises from the fact that Ii heterotrimerizes, a characteristic that has the potential to affect the maturation of associated MHCIIs in many different ways, depending on the isoform combinations. Here, we emphasize the peptide editing properties of Ii and discuss the impact of the various isoforms on the MHCII peptidome.
    Frontiers in Immunology 12/2013; 4:443. DOI:10.3389/fimmu.2013.00443
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    ABSTRACT: Recently a new pathogenic mechanism for autoantibody production was proposed. Misfolded proteins bind to MHC class II in the endoplasmic reticulum and are processed to be presented at the cell surface. Misfolded proteins are not trimmed to peptides, but are presented as they are together with MHC class II molecules. Such misfolded protein/MHC class II complex can stimulate B cells, but not T cells, and will induce autoantibody production. One of such examples is the case of IgG heavy chain (IgGH). HLA class II can bind IgGH and presents it to the cell surface. Such IgGH/HLA class II complex can be recognized by rheumatoid factor. Surprisingly, RA susceptible HLA class II alleles can present IgGH efficiently, but RA resistant HLA class II alleles cannot. Therefore susceptibility to certain autoimmune diseases may be determined by the affinity of misfolded autoantigens to certain HLA class II alleles. Such new autoimmune mechanisms may explain the unexplained autoantibody production mechanisms.
    Japanese Journal of Clinical Immunology 01/2014; 37(6):462-7. DOI:10.2177/jsci.37.462
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    ABSTRACT: Specific HLA class II alleles are strongly associated with susceptibility to rheumatoid arthritis (RA); however, how HLA class II regulates susceptibility to RA has remained unclear. Recently, we found a unique function of HLA class II molecules: their ability to aberrantly transport cellular misfolded proteins to the cell surface without processing to peptides. Rheumatoid factor (RF) is an autoantibody that binds to denatured IgG or Fc fragments of IgG and is detected in 70-80% of RA patients but also in patients with other diseases. Here, we report that intact IgG heavy chain (IgGH) is transported to the cell surface by HLA class II via association with the peptide-binding groove and that IgGH/HLA class II complexes are specifically recognized by autoantibodies in RF-positive sera from RA patients. In contrast, autoantibodies in RF-positive sera from non-RA individuals did not bind to IgGH/HLA class II complexes. Of note, a strong correlation between autoantibody binding to IgG complexed with certain HLA-DR alleles and the odds ratio for that allele's association with RA was observed (r = 0.81; P = 4.6 × 10(-5)). Our findings suggest that IgGH complexed with certain HLA class II alleles is a target for autoantibodies in RA, which might explain why these HLA class II alleles confer susceptibility to RA.
    Proceedings of the National Academy of Sciences 02/2014; 111(10). DOI:10.1073/pnas.1401105111 · 9.67 Impact Factor
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