16
Introduction
1.7
HLA-DO: a negative regulator of HLA-DM
There are at least three factors that determine peptide editing activity of DM, which
in turn influences the peptide repertoire presented on MHC II positive cells: the overall
expression levels of DM, the extent of co-localization of DM with MHC II molecules in
endosomic compartments, and the expression of another non-classical MHC II
molecule, HLA-DO (DO, H-2O in mice), which seems to play an important role in
modulating DM activity (Leddon
and Sant, 2010).
DO is expressed in B cells, thymic epithelial cells and certain subsets of dendritic
cells (Hornell et al., 2006) and its expression level is further regulated by cell
maturation. The gene products of DO are structurally similar to classical MHC II
molecules and even display more similarity to MHC II molecules than does DM with
approximately 60% similarity compared to approximately 28% (Cho et al., 1991; Kelly
et al., 1991; Servenius et al., 1987; Tonnelle et al., 1985; Trowsdale and Kelly, 1985).
Similar to DM, DO genes show limited polymorphism (Inoko et al., 1985; Jonsson and
Rask, 1989; Naruse et al., 1999; Naruse et al., 2002; Servenius et al., 1987; van Lith et
al., 2002). But the expression pattern of DO is different from that of DM and classical
MHC II molecules which are controlled by the class II transactivator (CIITA) and
upregulated by the cytokine interferon-γ. Whereas the DOα chain transcription is
dependent on CIITA, the DOß chain seems to be subject of tight and differential
regulation independent of CIITA even though CIITA expression can still increase DOß
chain expression (Nagarajan et al., 2002).
It has been shown that DO requires DM association to efficiently exit the ER and
associates with DM during and after transport to the endocomal/lysosomal compartment
(Liljedahl et al., 1996). Mutagenesis studies mapped the DM binding site on the DOα
chain (Deshaies et al., 2005).
Ex vivo experiments with human T cells (Denzin et al.,
1997) and melanoma cell lines (van Ham et al., 1997) revealed an inhibitory role for
DO illustrated by increased levels of MHC class II/CLIP complexes on the cell surface
of transfectants expressing DO.
In vitro experiments with purified DO and DM showed
decreased DM activity during class II peptide loading reactions (Denzin et al., 1997;
Liljedahl et al., 1998; van Ham et al., 1997). Further studies have suggested that DO
inhibition of DM-catalyzed peptide exchange is dependent on pH with lower inhibitory
function at pH lower than pH 5.0 (Kropshofer et al., 1998; Liljedahl et al., 1996; van
18
Introduction
di-peptides
Instead of altering the strength of hydrogen bonds between peptide and MHC II
molecules, the strategy of Falk, Roetzschke and colleagues was to target the P1 pocket
of DR1 which anchors the N-terminal part of the peptide. The di-peptide Tyr-Arg was
found to accelerate peptide exchange in DR1 molecules (Gupta et al., 2008). Assuming
the tyrosine binds in the P1 pocket, the N- and C-termini of the di-peptide were
chemically modified with the intent to optimize putative hydrogen bonds which resulted
in increased activity with effective concentrations of 10
-4
to 10
-3
M. Furthermore,
mutations in the P1 pocket affected the activity of the di-peptides, supporting the
hypothesis that the di-peptide targeted the area of the P1 pocket.
adamantyl compounds
The adamantyl compounds were found by Roetzschke, Falk and colleagues using an
ELISA-based assay immobilizing DR1 molecules and measuring peptide loading by
detection of biotinylated HA peptide (Hopner et al., 2006). Enhanced peptide loading
was observed in the absence of DM and most of the activity could be attributed to the
adamantane group. Similar to the di-peptides described above, the adamantyl
compounds are selective for MHC II alleles with a large P1 pocket versus a smaller P1
pocket, indicating that the adamantly compounds may act by binding directly to that
site.
inorganic metal complexes
Another group of MLE was found by a high-throughput approach, screening for
small molecules that remove peptides from MHC II molecules. Two metal complexes
were found, cis-platin and carboplatin, which are typically used as chemotherapy
agents, and also related metal complexes with palladium and gold showed activity (De
Wall et al., 2006). The different metal complexes have a square-planar configuration
with the metal ion acting as a weak Lewis acid which might suggest interactions with
sulphur atoms of cysteines or methionines (De Wall et al., 2006). The most potent metal
complex,
cis-platin, is active in the range of 10
-6
– 10
-5
M.
small molecule enhancers of HLA-DM
In another high-throughput approach around 100,000 compounds of drug-like small
molecule libraries were screened by Wucherpfennig and colleagues (Call et al., 2009;