Small oligomers efficiently trigger C1 activation. A, size-exclusion chromatography elution profile of mouse recombinant PrP after incubation for 18 h at 40 °C (dashed line) or 70 °C (plain line) in the presence of 100 mm NaCl. Three types of oligomers can be purified (type I, I′, and II oligomers). mAu, milliabsorbance units. B, C1 activation assay under different conditions. The C1s-C1r-C1r-C1s tetramer was incubated for 90 min at 37 °C in the presence of C1q alone (lane 1), C1q + C1 inhibitor (lane 2), C1q + C1 inhibitor + PrP monomer (lane 3), C1q + C1 inhibitor + oligomer I (lane 4), C1q + C1 inhibitor + oligomer I′ (lane 5), C1q + C1 inhibitor + oligomer II (lane 6), or C1q + C1 inhibitor + PrP fibrils (lane 7). Each sample was subjected to SDS-PAGE under reducing conditions and C1s was revealed by Western blot. The presence of the C1s A chain indicates complement activation. The gel shown is representative of three independent measurements.