Recognition Versus Adaptive Up-regulation and Degradation of CC Chemokines by the Chemokine Decoy Receptor D6 Are Determined by Their N-terminal Sequence

The chemokine decoy receptor D6 controls inflammatory responses by selective recognition and degradation of most CCR1 to CCR5 agonistic ligands. CCL14 is a homeostatic chemokine present at high concentrations in the serum with a weak agonist activity on CCR1. Under inflammatory conditions, plasmin and UPA-mediated truncation of 8 amino acids generates the potent CCR1/CCR3/CCR5 isoform CCL14(9-74), which is further processed and inactivated by dipeptidyl peptidase IV/CD26 that generates CCL14(11-74). Here we report that D6 efficiently binds both CCL14 and its truncated isoforms. Like other D6 ligands, the biologically active CCL14(9-74) induces adaptive up-regulation of D6 expression on the cell membrane and is rapidly and efficiently degraded. In contrast, the D6-mediated degradation of the biologically inactive isoforms CCL14(1-74) and CCL14(11-74) is very inefficient. Thus, D6 cooperates with CD26 in the negative regulation of CCL14 by the selective degradation of its biologically active isoform. Analysis of a panel of CC chemokines and their truncated isoforms revealed that D6-mediated chemokine degradation does not correlate with binding affinity. Conversely, degradation efficiency is positively correlated with D6 adaptive up-regulation. Sequence analysis indicated that a proline residue in position 2 of D6 ligands is dispensable for binding but crucial for D6 adaptive up-regulation and efficient degradation.