Major histocompatibility complex class I-binding peptides are recycled to the cell surface after internalization

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (β₂-microglobuhn; β₂ m).The heavy chain, which comprise the actual peptide binding α-1 and α-2 domains, can exist at the cell surface in different forms, either free, bound to β₂m or as a ternary complex with β₂m and peptides. MHC-I chains are also known to internalize, and recycle to the cell surface, and this has been suggested to be important in peptide presentation. Whether MHC-I-bound peptides also can recycle is not known. We have investigated this by using both peptide transporter mutant RMA-S cells and EL4 cells loaded with D^b-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Galα4Gal) at a position which did not interfere with D^b binding or immunogenicity, and peptide recycling tested with Gal₂-specific monoclonal antibodies. By flow cytometry, a return of Gal₂ epitopes to the cell surface was found, after cellular internalization and cell surface clearance by pronase treatment. This peptide recycling could be discriminated from free fluid-phase uptake and was inhibited by methylamine, chloroquine and low temperature (18°C) but not by leupeptin. Second, specific CTL were reacted with peptide-loaded target cells after complete removal of surface D^b molecules by pronase, and after different times of incubation at 37C to allow reexpression. By this procedure, reappearance of target cell susceptibility was confirmed. The results are in agreement with a model for optimizing peptide presentation by recycling through an intracellular compartment similar to early endosomes in certain antigen-presenting cells.