Interactions between chemokines and enzymes are vital in immunoregulation. Structural protein citrullination by peptidylarginine deiminase (PAD) has been associated with autoimmunity. In this report, we identified a novel naturally occurring posttranslational modification of chemokines, that is, the deimination of arginine at position 5 into citrulline of CXC chemokine ligand 10 (CXCL10) by rabbit PAD and human PAD2. Citrullination reduced (> 10-fold) the chemoattracting and signaling capacity of CXCL10 for CXC chemokine receptor 3 (CXCR3) transfectants; however, it did not affect CXCR3 binding. On T lymphocytes, though, citrullinated CXCL10 remained active but was again weaker than authentic CXCL10. PAD was also able to convert CXCL11, causing an impairment of CXCR3 signaling and T-cell activation, though less pronounced than for CXCL10. Similarly, receptor binding properties of CXCL11 were not altered by citrullination. However, deimination decreased heparin binding properties of both CXCL10 and CXCL11. Overall, chemokines are the first immune modulators reported of being functionally modified by citrullination. These data provide new structure-function dimensions for chemokines in leukocyte mobilization, disclosing an anti-inflammatory role for PAD. Additionally because citrullination has severe consequences for chemokine biology, this invites to reassess the involvement and impact of PAD and citrullinated peptides in inflammation, autoimmunity, and hematologic disorders.
IntroductionChemokines are a family of chemoattractive cytokines that regulate the recruitment of leukocytes toward inflammatory sites. Chemokines also exert homeostatic properties in cell migration during development and immunosurveillance and affect angiogenesis as well as tumor growth. 1,2 Based on the pattern of conserved cysteine residues in their amino acid structure, chemokines are classified into CXC, CC, CX 3 C, and C subfamilies. 1,3,4 Chemokines exhibit redundancy in their binding and signaling capacities in that the ligands can interact with more than one chemokine receptor and vice versa. 5 CXC chemokine ligand 10 (CXCL10) or interferon-␥-inducible protein-10 (IP-10) is a potent attractant of lymphocytes and natural killer cells, recognized by CXC chemokine receptor 3 (CXCR3), a 7-transmembrane spanning G protein-coupled receptor (GPCR). 6 Interestingly, CXCL10 also exerts angiostatic properties depending on CXCR3; however, the exact mechanism is still unclarified. 7 It was also suggested that CXCL10 binds to another unidentified receptor that does not interact with other CXCR3 ligands, that is, interferon T cell ␣-chemoattractant (I-TAC/CXCL11) and monokine induced by interferon-␥ (Mig/CXCL9). 8 CXCL11, on the other hand, was found to bind a second receptor, that is, CXCR7. 9 CXCR7 was reported to be expressed in various transformed cells and tumor development of human lymphoma or carcinoma in mice was diminished by treatment with a CXCR7 antagonist. [9][10][11] Recently, a function in the coordination of cell migration was al...