1. Supplementary Figure S1: The secondary structure of C1q is irreversibly altered after heat-denaturation. The far UV spectrum was measured by circular dichroism from 190 to 260 nm. Far UV CD spectra of C1q before (black line) and after heat-denaturation and cooling down to 20°C (dotted line) are displayed.

2. a. b.
Supplementary Figure S2: C1q has no inhibitory effect on either human MoDCs or mDCs activation. Human MoDCs and mDCs were stimulated in vitro for 24 hours by either LPS (1 µg/mL) + IFNg (25 ng/mL) or poly I:C (10 µg/mL), respectively, under serum-free conditions. Cytokine production (a) by MoDCs and (b) mDCs were analyzed by using a multiplex cytokine quantification assay. Data are shown as means ± SEMs (n = 9 for MoDCs and n = 3 for mDCs).

3. a. b.
Supplementary Figure S3: C1q has no inhibitory effect on either murine BM-DCs or cDCs activation. Murine BM-DCs and cDCs were stimulated in vitro for 24 hours by LPS (1 µg/mL), under serum-free conditions. Cytokine production (a) by BM-DCs and (b) cDCs were analyzed by using a multiplex cytokine quantification assay. Data are shown as means ± SEMs (n = 4 for BM-DCs and cDCs).

4. Supplementary Figure S4: Analysis of C1q receptors on murine dendritic cells. Cell surface expression of CD35, CD93 and LAIR-1 in cDCs (CD11c+ F4/80+) and pDCs (mPDCA G8+) was analyzed by flow cytometry. Results are expressed as percentages of positive cells detected with specific antibodies after subtracting the values obtained with a control isotype. Data are shown as means ± SEMs (n = 3).

5. a.
Control isotype
Control siRNA
LAIR-1 siRNA b.
Supplementary Figure S5: C1q treatment still decreases CpGA mediated activation of pDCs even when LAIR-1 is down-regulated.
(a) LAIR-1 expression detected on murine pDCs incubated with either control or LAIR-1 siRNAs. (b) cytokine production by siRNA-treated pDCs subsequently incubated for 24 hours with either C1q (25 µg/mL), CpGA (5 µg/mL) or a combination of CpGA and C1q, was analyzed by using a multiplex cytokine quantification assay. Data are shown as means ± SEMs (n = 4).