Impaired immune responses and altered peptide repertoire in tapasin-deficient mice

Tapasin: an ER chaperone that controls MHC class I assembly with peptide

The stable assembly of MHC class I molecules with peptides in the endoplasmic reticulum (ER) involves several accessory molecules. One of these accessory molecules is tapasin, a transmembrane protein that tethers empty class I molecules to the peptide transporter associated with antigen processing (TAP). Here, evidence is presented that tapasin retains class I molecules in the ER until they acquire high-affinity peptides.

Quantitative and qualitative influences of tapasin on the class I peptide repertoire

Tapasin is critical for efficient loading and surface expression of most HLA class I molecules. The high level surface expression of HLA-B*2705 on tapasin-deficient 721.220 cells allowed the influence of this chaperone on peptide repertoire to be examined. Comparison of peptides bound to HLA-B*2705 expressed on tapasin-deficient and -proficient cells by mass spectrometry revealed an overall reduction in the recovery of B*2705-bound peptides isolated from tapasin-deficient cells despite similar yields of B27 heavy chain and beta(2)-microglobulin. This indicated that a proportion of suboptimal ligands were associated with B27, and they were lost during the purification process. Notwithstanding this failure to recover these suboptimal peptides, there was substantial overlap in the repertoire and biochemical properties of peptides recovered from B27 complexes derived from tapasin-positive and -negative cells. Although many peptides were preferentially or uniquely isolated from B*2705 in tapasin-positive cells, a number of species were preferentially recovered in the absence of tapasin, and some of these peptide ligands have been sequenced. In general, these ligands did not exhibit exceptional binding affinity, and we invoke an argument based on lumenal availability and affinity to explain their tapasin independence. The differential display of peptides in tapasin-negative and -positive cells was also apparent in the reactivity of peptide-sensitive alloreactive CTL raised against tapasin-positive and -negative targets, demonstrating the functional relevance of the biochemical observation of changes in peptide repertoire in the tapasin-deficient APC. Overall, the data reveal that tapasin quantitatively and qualitatively influences ligand selection by class I molecules.

HLA-DM recognizes the flexible conformation of major histocompatibility complex class II

DM facilitates formation of high affinity complexes of peptide-major histocompatibility complex (MHC) by release of class II MHC-associated invariant chain peptide (CLIP). This has been proposed to occur through discrimination of complex stability. By probing kinetic and conformational intermediates of the wild-type and mutant human histocompatibility leukocyte antigen (HLA)-DR1-peptide complexes, and examining their reactivities with DM, we propose that DM interacts with the flexible hydrophobic pocket 1 of DR1 and converts the molecule into a conformation that is highly peptide receptive. A more rigid conformation, generated upon filling of pocket 1, is less susceptible to DM effects. Thus, DM edits peptide-MHC by recognition of the flexibility rather than stability of the complex.