Peptides derived from cellular proteins are continuously exposed at the cell surface, in association with MHC class I molecules. Recognition of these MHC–peptide complexes by circulating lymphocytes is critical to the immune system’s tolerance of self-derived antigenic peptides, and to the onset of immune reactions against exogenous peptide antigens, such as viral antigens. Antigenic peptides were thought to be derived from the proteolytic degradation of a fraction of total cellular proteins. However, the results of two studies recently published in Nature (Reits et al. Nature 404, 774–778, 2000; Schubert et al. Nature 404, 770–774, 2000) indicate that the generation of antigenic peptides may instead depend upon continuous protein synthesis, the antigens themselves being created through the degradation of up to 30% of these newly synthesized proteins. Although apparently wasteful, this system would ensure rapid, efficient presentation of antigens to the immune system, regardless of the half lives of individual proteins.

The transporters associated with antigen processing (TAPs) allow antigenic peptides that have been generated in the cytosol to be transported into the lumen of the endoplasmic reticulum (ER), where they can form complexes with MHC class I molecules. These complexes are, in turn, transported to the plasma membrane. Reits et al. investigated the relationship between the motility of a TAP tagged with green fluorescent protein (TAP–GFP) and its activity, and found that its motility decreases in situations in which peptide translocation is blocked (such as in the presence of peptides with very large side chains). The picture shows the subcellular location of TAP–GFP in living Mel JuSo cells, melanoma cells used as antigen presenters.

Thus, measurements of diffusion can be used to visualize conformational changes, a finding that has potentially broad applications. Reits et al. used the motility of TAP–GFP as a readout for intracellular peptides in vivo, and found that one-third of all cellular TAP proteins are normally busy translocating peptides; this proportion increases to 100% during an acute influenza infection. Continuous protein translation seems to be the source of substrates for TAP, as TAP–GFP motility was blocked by inhibitors of protein synthesis, such as cycloheximide. This finding supports the idea that antigenic peptides are derived from ‘freshly made’ proteins. TAP–GFP motility was also correlated with the transport of MHC class I molecules from the ER to the plasma membrane, a result that was anticipated, as MHC class I peptides can only translocate to the plasma membrane once they are complexed with an antigenic peptide.

Defective ribosomal products (DRiPs) are polypeptides that fail to fold properly as a result of errors in translation or of defective post-translational modifications. They have previously been suggested to be a source of antigenic peptides. Schubert et al. monitored the recovery of protein contents after transiently labelling newly synthesized proteins, in the presence or absence of protease inhibitors and of inhibitors of protein synthesis. They were thus able to show that up to 30% of newly synthesized proteins are DRiPs, that some of them have indeed been targeted for proteolytic degradation (by conjugation with ubiquitin), and that some of them are viral antigens (as shown by experiments using virus-transfected cells).

Overall, the results from these two studies form a body of evidence indicating that protein synthesis may be the principal means by which antigenic peptides are generated.