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

1993, European Journal of Immunology

Cytotoxic T lymphocytes (CTL) recognize target antigens as short, processed peptides bound to major histocompatibility complex class I (MHC-I) heavy and light chains (p2-microglobulin; P2-m). The heavy chain, which comprise the actual peptide binding a-1 and a-2 domains, can exist at the cell surface in different forms, either free, bound to PZ-m or as a ternary complex with 62-m 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 Db-binding peptides, by two different approaches. First, peptides were covalently linked with galabiose (Gala4Gal) at a position which did not interfere with Db binding or immunogenicity, and peptide recycling tested with Gal*-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 Db molecules by pronase, and after different times of incubation at 37°C 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.