HLA-DO IS A NEGATIVE MODULATOR OF HLA-DM-MEDIATED MHC CLASS-II PEPTIDE LOADING

Background: Class II molecules of the major histocompatibility complex become loaded with antigenic peptides after dissociation of invariant chain-derived peptides (CLIP) from the peptide-binding groove. The hu man leukocyte antigen (HLA)-DM is a prerequisite for this process, whi ch takes place in specialised intracellular compartments, HLA-DM catal yses the peptide-exchange process, simultaneously functioning as a pep tide 'editor', favouring the presentation of stably binding peptides, Recently, HLA-DO, an unconventional class II molecule, has been found associated with HLA-DM in B cells, yet its function has remained elusi ve, Results: The function of the HLA-DO complex was investigated by ex pression of both chains of the HLA-DO heterodimer (either alone or fus ed to green fluorescent protein) in human Mel JuSo cells. Expression o f HLA-DO resulted in greatly enhanced surface expression of CLIP via H LA-DR3, the conversion of class II complexes to the SDS-unstable pheno type and reduced antigen presentation to T-cell clones. Analysis of pe ptides eluted from HLA-DR3 demonstrated that CLIP was the major peptid e bound to class II in the HLA-DO transfectants. Peptide exchange assa ys in vitro revealed that HLA-DO functions directly at the level of cl ass II peptide loading by inhibiting the catalytic action or HLA-DM, C onclusions: HLA-DO is a negative modulator of HLA-DM. By stably associ ating with HLA-DMI the catalytic action of HLA-DM on class II peptide loading is inhibited, HLA-DO thus affects the peptide repertoire that is eventually presented to the immune system by MHC class II molecules .