HLA-DR molecules from an antigen-processing mutant cell line are associated with invariant chain peptides

Abstract

The invariant chain, which associates with the major histocompatibility complex (MHC) class II molecules in the endoplasmic reticulum, serves two functions important in antigen processing. First, it prevents class II molecules from binding peptides in the early stages of intracellular transport. Second, it contains a cytoplasmic signal that targets the class II-invariant chain complex to an acidic endosomal compartment. Proteolytic cleavage and subsequent dissociation of the invariant chain then occurs, allowing peptides derived from endocytosed proteins to bind to released class II molecules before their expression at the cell surface. Certain human cell lines that are mutant in one or more MHC-linked genes are defective in class II-restricted antigen processing. Here we show that in transfectants of one of these cell lines, T2, this deficiency results in the association of a large proportion of class II molecules with a nested set of invariant-chain-derived peptides (class II-associated invariant chain peptides, or CLIP). HLA-DR3 molecules isolated from T2 transfectants can be efficiently loaded with antigenic peptides by exposure to a low pH in vitro, perhaps reflecting the in vivo conditions in which peptides associate with class II molecules. Addition of synthetic CLIP inhibits the loading process, indicating that CLIP may define the region of the invariant chain responsible for obstructing the class II binding site.

β57-Asp Plays an Essential Role in the Unique SDS Stability of HLA-DQA1*0102/DQB1*0602 αβ Protein Dimer, the Class II MHC Allele Associated with Protection from Insulin-Dependent Diabetes Mellitus

“…This is thought to be the case because CLIP is the predominant peptide found associated with HLA class II molecules in HLA-DM-deficient cells, and in these same cells HLA class II molecules are not SDS stable (10,11). However, because of the unique SDS stability of DQ0602 in the HLA-DM-deficient cell line BLS-1, we hypothesized that DQ0602 was uniquely interacting with CLIP, resulting in the SDS stability of DQ0602.…”

“…Removal of CLIP and the addition of antigenic peptides are important for SDS stability of HLA class II proteins (7)(8)(9)(10)(11)(12)(13)(14)(15). The unique stability of DQ0602 in the HLA-DM-deficient cell line BLS-1 suggested the possibility that DQ0602 interacted with CLIP in a unique way that resulted in SDS stability.…”

“…Previous studies have indicated that HLA class II proteins in HLA-DM-deficient cells are loaded with CLIP (10,11). Removal of CLIP and the addition of antigenic peptides are important for SDS stability of HLA class II proteins (7)(8)(9)(10)(11)(12)(13)(14)(15).…”

“…This process is dependent on HLA-DM such that in HLA-DM-deficient cells, the SDS-resistant property of various HLA-DR, -DQ, and -DP molecules is generally lost (5,(7)(8)(9). Peptides eluted from DR3 on HLA-DM-deficient cells are predominantly derived from invariant chain (10,11). These peptides are called class II-associated invariant chain peptides (CLIPs).…”

“…These peptides are called class II-associated invariant chain peptides (CLIPs). The SDS stability of the MHC class II ␣␤ dimers in HLA-DM-deficient cells can be induced by the addition of antigenic peptides (10,12). In assays with purified proteins, HLA-DM has been shown to replace CLIP with peptides that have slower intrinsic rates of dissociation to HLA-DR (13)(14)(15).…”