MHC class II loading of high or low affinity peptides directed by Ii/peptide fusion constructs: implications for T cell activation.

CD4(+) T cells recognize peptides presented on the cell surface of antigen presenting cells in the MHC class II context. The biosynthesis and transport of MHC class II molecules depend on the type II transmembrane invariant chain (Ii) and are tightly regulated processes. Ii is known to bind to the MHC class II peptide-binding groove via its class II-associated Ii peptide (CLIP) region early in the biosynthetic pathway to prevent premature peptide binding. In this study we have genetically exchanged CLIP with peptides of either high or low affinity for the class II peptide binding groove and utilized the properties of Ii to manipulate MHC class II loading. An inducible promoter controlled expression of the Ii/peptide fusion constructs, and presentation at different expression levels was studied. Both peptides were excised from Ii and presented on MHC class II molecules as shown by liquid chromatography-tandem mass spectrometry, but the high affinity peptide was presented more efficiently than the low affinity peptide. Both peptides were efficient in eliciting T cell responses at high Ii/peptide concentration independent of the duration of T cell stimulation. The peptides were also able to elicit an IL-2 response at low expression levels; however, the kinetic differed as the T cells required longer duration of T cell contact to reach a significant T cell response. This probably reflects the number of class II/peptide complexes at the cell surface and is discussed.