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4.2.2
Surface plasmon resonance experiments
SPR experiments were prepared and carried out as described in 3.2.5. Chips were
regenerated by injection of either HA peptide (50 µM) or MBP peptide (5 µM). For
experiments at different temperatures than 25 ºC the chip was again normalized after
temperature change. Biotinylated DM wt and DM mut were prepared as described in
3.2.1. Binding of the reference flow cell (DM mut) was subtracted from binding in the
experimental flow cell (DM wt).
4.2.3
Fluorescence polarization experiments
Fluorescence polarization (FP) experiments were conducted on a Victor
3
V
multilabel plate reader (PerkinElmer) using black polystyrene 384-well flat-bottomed
plates (Corning). DR/peptide complexes were incubated with MBP(85-99) peptide
which was labeled at position P5 (lysine-to-cysteine substitution) with a maleimide
derivative of Alexa Fluor 488 (Molecular Probes). Measurements were performed in
triplicates in a volume of 40 µL in 50 mM citrate phosphate buffer (pH 5.3) containing
150 mM NaCl.
4.3
Results and discussion
4.3.1
Preparing the high-affinity complexes HLA-DR2/MBP and HLA-DR1/HA
The high-affinity complexes DR1/HA and DR2/MBP were prepared to be tested for
DM binding in SPR experiments at different temperatures. HA peptide is a high-affinity
influenza hemagglutinin peptide of amino acid 306-318. MBP peptide is part of myelin
basic protein (residues 85-99) and has a high-affinity to DR2. DR2/MBP was produced
in
Sf9 insect cells and purified using affinity, ion exchange and gel filtration
chromatography. The linker between MBP peptide and DR was proteolytically cleaved.
To prepare the high-affinity DR1/HA complex, DR1/CLIP was produced in CHO cells
and the linker between peptide and DR proteolytically cleaved after affinity
chromatography. Following the peptide exchange protocol described in 4.2.1, HA
peptide was loaded onto DR1 molecules and DR1/HA further purified by gel filtration
and affinity chromatography.
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Chapter III
4.3.2
Measuring HLA-DM binding to HLA-DR2/MBP and HLA-DR1/HA using
surface plasmon resonance
Surface plasmon resonance experiments at 25 ºC showed only little DM binding to
the high-affinity DR/peptide complexes DR2/MBP and DR1/HA (see figure 4.1, A, B).
To test whether DM binding to the high-affinity complexes is enhanced with higher
temperature SPR experiments were carried out at 37 ºC. As can be seen in figure 4.1
(D) definitive DR2/MBP binding to DM was measured at 37 ºC with 69, 155 and 273
response units for 10 μM, 20 μM and 40 μM DR2/MBP, respectively. DR1/HA was
injected at 10 μM, 20 μM and 40 μM, as well, at 37 ºC and DM binding of 13, 22 and
39
response units was measured, respectively, (figure 4.1, C).
Figure 4.1: DM binding to high-affinity MHC II/peptide complexes at 25 ºC and 37 ºC. (A, B)
DR1/HA (10 µM) was injected for 7 minutes (A) and DR2/MBP (2 µM) for 5 minutes (B) at a flow rate
of 15 µL/min at 25 ºC, followed by buffer injection (black arrow) and peptide injection (green arrow), 50
µM HA peptide (A) and 10 µM CLIP peptide (A), respectively. (C, D) At 37 ºC, DR1/HA (C) and
DR2/MBP (D), respectively, were injected for 10 minutes at a flow rate of 15 μL/min at 10 μM (blue),
20 μM (red) and 40 μM (green), followed by injection of buffer and injection of 50 μM HA peptide (C) or
5 μM MBP peptide (D). (A-D) SPR assays were carried out in consecutive flow cells with 500 RU of
immobilized DM wt and DM mut, respectively, in citrate phosphate buffer (pH 5.3). Binding in the DM
mut flow cell was subtracted from measurements in the DM wt flow cell. (experiments in A and B were
carried out by Anne-Kathrin Anders)