van Ham S M, Tjin E P, Lillemeier B F, Grüneberg U, van Meijgaarden K E, Pastoors L, Verwoerd D, Tulp A, Canas B, Rahman D, Ottenhoff T H, Pappin D J, Trowsdale J, Neefjes J
Department of Cellular Biochemistry, The Netherlands Cancer Institute, Plesmanlaan 121, 1066, CX Amsterdam, The Netherlands.
Curr Biol. 1997 Dec 1;7(12):950-7. doi: 10.1016/s0960-9822(06)00414-3.
Class II molecules of the major histocompatibility complex become loaded with antigenic peptides after dissociation of invariant chainderived peptides (CLIP) from the peptide-binding groove. The human leukocyte antigen (HLA)-DM is a prerequisite for this process, which takes place in specialised intracellular compartments. HLA-DM catalyses the peptide-exchange process, simultaneously functioning as a peptide '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 elusive.
The function of the HLA-DO complex was investigated by expression of both chains of the HLA-DO heterodimer (either alone or fused to green fluorescent protein) in human Mel JuSo cells. Expression of HLA-DO resulted in greatly enhanced surface expression of CLIP via HLA-DR3, the conversion of class II complexes to the SDS-unstable phenotype and reduced antigen presentation to T-cell clones. Analysis of peptides eluted from HLA-DR3 demonstrated that CLIP was the major peptide bound to class II in the HLA-DO transfectants. Peptide exchange assays in vitro revealed that HLA-DO functions directly at the level of class II peptide loading by inhibiting the catalytic action of HLA-DM.
HLA-DO is a negative modulator of HLA-DM. By stably associating with HLA-DM, 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.
主要组织相容性复合体的II类分子在不变链衍生肽（CLIP）从肽结合槽解离后会加载抗原肽。人类白细胞抗原（HLA）-DM是这一过程的先决条件，该过程发生在特殊的细胞内区室中。HLA-DM催化肽交换过程，同时作为肽“编辑者”发挥作用，有利于稳定结合肽的呈递。最近，一种非常规的II类分子HLA-DO已被发现在B细胞中与HLA-DM相关联，但其功能仍不清楚。
通过在人Mel JuSo细胞中表达HLA-DO异二聚体的两条链（单独或与绿色荧光蛋白融合）来研究HLA-DO复合体的功能。HLA-DO的表达导致CLIP通过HLA-DR3在表面的表达大大增强，II类复合体转变为SDS不稳定表型，并减少了向T细胞克隆的抗原呈递。对从HLA-DR3洗脱的肽的分析表明，CLIP是HLA-DO转染细胞中与II类结合的主要肽。体外肽交换试验表明，HLA-DO通过抑制HLA-DM的催化作用直接在II类肽加载水平发挥作用。
HLA-DO是HLA-DM的负调节因子。通过与HLA-DM稳定结合，HLA-DM对II类肽加载的催化作用受到抑制。因此，HLA-DO影响最终由MHC II类分子呈递给免疫系统的肽库。
