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Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

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Science  16 Feb 2018:
Vol. 359, Issue 6377, pp. 794-797
DOI: 10.1126/science.aao4988
  • Fig. 1 C1-mAb complexes observed on liposomes with tomography.

    (A) A 10-nm-thick slice through a dual-axis tomogram showing C1 complexes (arrows) bound to surface-associated antibody complexes. Scale bar, 20 nm. (B) Reconstruction of a single C1-IgG1 complex shown from the top (left) and side (right) at 25 Å resolution. (C) Focused alignment and classification of the complexes, excluding the membrane and Fab regions (masks used in focused reconstructions are provided in fig. S2E), revealed density from the C1r2s2 platform extending out either side of the C1q stalk. (D) Neighboring C1-mAb complexes from larger subvolumes showing a common spacing of 23 nm between complexes, as measured from centers of IgG1 platforms. All volumes were filtered to 25-nm resolution and masked, and disconnected densities with volumes less than 5 nm3 were removed for clarity.

  • Fig. 2 Soluble C1-IgG16 complexes display heterogeneous structures.

    (A) Representative two-dimensional (2D) class averages. Colored boxes indicate three classes corresponding to main 3D classes, as shown below. Scale bar, 25 nm. (B) Main 3D classes after focused 3D classification and 3D refinement, showing the “bottom platform” segment of the reconstructions indicating heterogeneities (highlighted by arrows). Percentage of particles in each class are indicated. Particle colors correspond to the color of the boxes in (A). (C) 3D reconstructions after post-processing of the major class, showing two side views (left and middle). Densities have been colored to indicate density for C1q (yellow; with collagen arms and gC1q units numbered 1 through 6), C1r and C1s (blue and magenta, respectively) and IgG1-Fc regions (pink). (D) Top and side views of the bottom platform after sixfold averaging (right, top and bottom, respectively).

  • Fig. 3 Fc-gC1q interactions in C1-IgG16.

    (A) Structural models of Fc regions (magenta) and gC1q headpieces (orange) fitted into the density, top and side view of the Fc-gC1q hexamer (left), and zoom in of a gC1q trimeric with C1q-A, -B, and -C domains (red, blue, and orange, respectively) and an Fc dimer with CH2-CH3 and CH2′-CH3′ (right), with Fc glycans shown in green. (B) (Left) gC1q-Fc interaction site 1 and site 2 are shown indicated by boxes, with interacting loops FG (site 1) and BC and DE (site 2) labeled. (Right) Zoom in of interaction sites 1 and 2, with key interacting residues shown in stick representation and labeled. (C) Complement-dependent cytotoxicity assays of Raji cells opsonized with wild-type (WT) and mutated CD20 mAb IgG1-7D8 (n = 3 independent experiments) exposed to C1q-deficient serum to which a titration of 1 ng/mL to 60 μg/mL C1q was added. Cell lysis was assessed with flow cytometry by using propidium iodide staining. Bars show the average area under the curve (AUC) for this dose response normalized against the AUC obtained with the unmutated WT IgG1-7D8 set to 100% NO AB:control reactions without IgG1 added.

  • Fig. 4 Structural model of C1 fitted into C1-IgG16 density.

    (A) Model for C1q-A, -B, and -C hexamer indicating collagen-like segments forming a N-terminal stalk region, six collagen-like triple helices, and C-terminal trimeric gC1q modules. Shown are top and side views (left and middle) of C1q and side, sliced through top and bottom view (third column, left to right) of the C1q stalk region. Numbering of each C1q arm is as in Fig. 2. (B) Model for C1r and C1s heterotetramer showing C1r CUB1-EGF-CUB2 (blue) and C1s CUB1-EGF-CUB2-CCP1 domains (cyan). Shown are (left) top view and (top right) side view at lower contour level, with the latter revealing density for the CCP1 domain of C1r. An illustration of the domain arrangement is shown for clarity (bottom right). (C) Overall C1-IgG16 models in density. CCP2-SP domains lacking density have been added by using orientations derived from crystal structures.

Supplementary Materials

  • Structures of C1-IgG1 provide insights into how danger pattern recognition activates complement

    Deniz Ugurlar, Stuart C. Howes, Bart-Jan de Kreuk, Roman I. Koning, Rob N. de Jong, Frank J. Beurskens, Janine Schuurman, Abraham J. Koster, Thomas H. Sharp, Paul W. H. I. Parren, Piet Gros

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • Figs. S1 to S7 
    • Table S1
    • References 

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