DC-SIGN binds to C1q at the IgG-binding site. (A) IgG reduced the binding of C1q to DC-SIGN significantly. ELISA experiments were performed using C1q premixed with human IgG in a 1:10 molar ratio for 15 minutes at 37°C. C1q/IgG was added to DC-SIGN– or BSA-coated plates and binding was detected using C1q pAb. One representative experiment is shown (n = 3). *P < .05. (B) Synthetic peptide corresponding to C1q-A chain residues 155-164 bound to DC-SIGN, and the Arg residues (162-163) were not required for binding. ELISA binding studies were performed using 2 synthetic peptides, the C1q-A-chain peptide containing 2 adjacent Arg residues (RR) and another peptide that had 2 glutamines (QQ) substituting for the 2 Arg residues at positions 162-163 (n = 3). *P < .05. (C) Competition of a synthetic peptide corresponding to C1q-A 155-164 did not diminish binding of C1q to DC-SIGN. Competition ELISA experiments were performed using purified, soluble C1q and the synthetic C1q peptide RR. Biotinylated C1q and increasing concentrations of RR (0-80 μg/mL) were added to DC-SIGN–coated plates and binding was detected using anti-C1q oAb. One representative experiment is shown (n = 3).