Peptide binding and editing: generation of TCR ligands

A role for protein disulfide isomerase in the early folding and assembly of MHC class I molecules

Proper folding and assembly of major histocompatibility complex (MHC) class I complexes are essential for optimal peptide loading and subsequent antigen presentation. MHC class I folding involves the coordinated formation of multiple disulfide bonds within MHC class I molecules. However, the regulation of disulfide bond formation during the early process of MHC class I folding is uncharacterized. Here, we show that protein disulfide isomerase (PDI) catalyzes the disulfide bond formation of MHC class I molecules and thereby facilitates the assembly of MHC class I heavy chain with beta(2)-microglobulin (beta(2)m). Depletion of PDI but not ERp57 by RNAi interfered with the disulfide bond formation in the MHC class I molecules. In the absence of PDI, the association of free class I heavy chain with calnexin increased, whereas the assembly of MHC class I heavy chain-beta(2)m heterodimers was delayed. These observations suggest that PDI-catalyzed disulfide bond formation of MHC class I molecules is an event downstream of the interaction of class I molecules with calnexin and upstream of their interaction with beta(2)m. Thus, our data establish a critical function for PDI in the early assembly of MHC class I molecules.

Redox-regulated export of the major histocompatibility complex class I-peptide complexes from the endoplasmic reticulum

In contrast to the fairly well-characterized mechanism of assembly of MHC class I-peptide complexes, the disassembly mechanism by which peptide-loaded MHC class I molecules are released from the peptide-loading complex and exit the endoplasmic reticulum (ER) is poorly understood. Optimal peptide binding by MHC class I molecules is assumed to be sufficient for triggering exit of peptide-filled MHC class I molecules from the ER. We now show that protein disulfide isomerase (PDI) controls MHC class I disassembly by regulating dissociation of the tapasin-ERp57 disulfide conjugate. PDI acts as a peptide-dependent molecular switch; in the peptide-bound state, it binds to tapasin and ERp57 and induces dissociation of the tapasin-ERp57 conjugate. In the peptide-free state, PDI is incompetent to bind to tapasin or ERp57 and fails to dissociate the tapasin-ERp57 conjugates, resulting in ER retention of MHC class I molecules. Thus, our results indicate that even after optimal peptide loading, MHC class I disassembly does not occur by default but, rather, is a regulated process involving PDI-mediated interactions within the peptide-loading complex.

Molecular mechanisms of MHC class I-antigen processing: redox considerations

Major histocompatibility complex (MHC) class I molecules present antigenic peptides to the cell surface for screening by CD8(+) T cells. A number of ER-resident chaperones assist the assembly of peptides onto MHC class I molecules, a process that can be divided into several steps. Early folding of the MHC class I heavy chain is followed by its association with beta(2)-microglobulin (beta(2)m). The MHC class I heavy chain-beta(2)m heterodimer is incorporated into the peptide-loading complex, leading to peptide loading, release of the peptide-filled MHC class I molecules from the peptide-loading complex, and exit of the complete MHC class I complex from the ER. Because proper antigen presentation is vital for normal immune responses, the assembly of MHC class I molecules requires tight regulation. Emerging evidence indicates that thiol-based redox regulation plays critical roles in MHC class I-restricted antigen processing and presentation, establishing an unexpected link between redox biology and antigen processing. We review the influences of redox regulation on antigen processing and presentation. Because redox signaling pathways are a rich source of validated drug targets, newly discovered redox biology-mediated mechanisms of antigen processing may facilitate the development of more selective and therapeutic drugs or vaccines against immune diseases.

Impaired assembly of the major histocompatibility complex class I peptide-loading complex in mice deficient in the oxidoreductase ERp57

The thiol-oxidoreductase ERp57 is an integral component of the peptide-loading complex of the major histocompatibility complex (MHC) class I pathway, but its function is unknown. To investigate its function in antigen presentation, we generated ERp57-deficient mice. Death in utero caused by ubiquitous ERp57 deletion was prevented by specific deletion in the B cell compartment. We demonstrate that ERp57 was central for recruitment of MHC class I molecules into the loading complex. In ERp57-deficient cells, we found short-lived interaction of MHC class I molecules with the loading complex. Thus, in the steady state, very few MHC class I molecules were present in the loading complex. Surface H-2K(b)-peptide expression and stability were reduced, and presentation of a model antigen was decreased. Our results indicate that ERp57 does not influence the redox state of MHC class I molecules but is an essential structural component required for stable assembly of the peptide-loading complex.

I-Aq and I-Ap bind and present similar antigenic peptides despite differing in their ability to mediate susceptibility to autoimmune arthritis

Susceptibility to collagen induced arthritis (CIA) in the murine model is linked to expression of the MHC class II alleles, I-Aq and I-Ar. We have examined the molecular basis for this MHC-linked susceptibility by studying the antigen presentation function of two class II molecules, I-Aq and I-Ap, that are closely related yet differ in mediating susceptibility to CIA. These class II molecules differ by only 4 amino acids, yet only mice expressing I-Aq develop CIA. Although the I-Ap molecule can bind the same immunodominant determinant from type II collagen as I-Aq, H-2p APC have difficulty generating I-Ap:CII peptide complexes when processing of CII is required. Immunization of H-2p mice with type II collagen (CII) generated only a weak T cell response when compared to H-2q mice, whereas immunization with the a CII peptide containing the dominant determinant induced a strong T cell response in both strains. In antigen presentation assays, H-2p APC were very inefficient in stimulating T cells when native CII was used as antigen, however they presented CII synthetic peptides with similar efficiency as H-2q APC. Processing and presentation of other antigens by H-2p APC was not affected. Using soluble class II binding assays, the affinity of I-Ap for the CII dominant peptide was 10 to 50 fold lower than I-Aq, however, this reduced affinity was not a general defect in I-Ap function. I-Aq and I-Ap had virtually identical affinities for binding other antigenic peptides. These data indicate that MHC-based susceptibility to autoimmunity may involve more than simple determinant selection and that the successful generation of an antigenic peptide by processing may be related to the overall affinity of the peptide for the MHC molecule.