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 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.


RESULTS
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.


CONCLUSIONS
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.
van Ham
S M
SM
Department of Cellular Biochemistry, The Netherlands Cancer Institute, Plesmanlaan 121, 1066, CX Amsterdam, The Netherlands. vanham@nki.nl
Tjin
E P
EP
Lillemeier
B F
BF
Grüneberg
U
U
van Meijgaarden
K E
KE
Pastoors
L
L
Verwoerd
D
D
Tulp
A
A
Canas
B
B
Rahman
D
D
Ottenhoff
T H
TH
Pappin
D J
DJ
Trowsdale
J
J
Neefjes
J
J
D000595:Amino Acid Sequence; D017951:Antigen Presentation; D000944:Antigens, Differentiation, B-Lymphocyte / Q000378:metabolism; D002460:Cell Line; D006681:HLA-D Antigens / Q000235:genetics / Q000378:metabolism*; D015803:HLA-DR3 Antigen / Q000378:metabolism; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D000944:Antigens, Differentiation, B-Lymphocyte; C416186:H-2O antigen; C416189:H2-M antigens; D006681:HLA-D Antigens; C084245:HLA-DM antigens; C046148:HLA-DO antigens; D015803:HLA-DR3 Antigen; D000949:Histocompatibility Antigens Class II; D011993:Recombinant Fusion Proteins; C045084:invariant chain