Synergy-inducing chemokines form heteromeric complexes with SLC, do not interfere with the binding of CCR7 ligands, but can bind CCR7 in the presence of ELC. SLC, BCA-1, or SLC plus BCA-1 were subjected to immunoprecipitation (Immppt) with anti-BCA-1 or anti-SLC antibodies. The samples were then analyzed by Western blotting and the presence of chemokines was detected with anti-BCA-1 (A) or anti-SLC (B) antibodies. (C-D) MIP-1α or SLC plus MIP-1α were subjected to immunoprecipitation with anti-SLC or anti-MIP-1α antibodies. The samples were then analyzed by Western blotting, and the presence of MIP-1α or SLC detected with a specific antibody. (E-F) CCR7-transfected cells were incubated with 0.15 nM ¹²⁵I-ELC and increasing concentrations of unlabeled SLC (○), BCA-1 (▪), or MDC (▴). The curves (means of duplicate determinations) are representative for 1 of 3 experiments performed. (G) CCR7-transfected cells were incubated with 0.15 nM ¹²⁵I-MDC or 0.15 nM ¹²⁵I-MIP-1α, as indicated, and increasing concentrations of unlabeled ELC. Data (mean ± SD) are shown as percentage of the increase of ¹²⁵I-chemokine binding obtained in 3 independent experiments. The difference between the enhancements observed with ¹²⁵I-MDC and ¹²⁵I-MIP-1α in the presence of ELC is statistically significant (P < .005). (H) Schematic representation of the binding events corresponding to panels D-F. Please refer to text (“Synergy-inducing chemokine binds to CCR7 in the presence of ELC”) for details.