Recent studies have revealed that the conserved major histocompatibility complex class II molecule, HLA-DO, inhibits the class II antigen-processing pathway. HLA-DO, expressed in only a subset of antigen-presenting cells, binds HLA-DM and blocks HLA-DM-catalyzed peptide loading of class II molecules.
Jensen, P. E. (2004). Antigen processing: HLA-DO – a hitchhiking inhibitor of HLA-DM. Current Biology, 8(4), R128–R131. https://doi.org/10.1016/s0960-9822(98)70988-1