Analysis of polymorphism in human cytomegalovirus (HCMV) chemokine, vCXCL-1, and its role in cellular activation

The human cytomegalovirus (HCMV) viral chemokine gene, UL146, shows a high degree of variability in clinical isolates. The UL146-produced viral chemokine, vCXCL-1, has homology to CXC chemokines and is predicted to be an immune modulator that may contribute to the pathogenesis of HCMV infections. In the analysis of clinical isolates from congenitally infected infants, we found 11 distinct vCXCL-1 clades. Although the four cysteine residues that create two disulfide bonds providing the essential structure for CXC chemokines,are conserved, the N-loop region, which is important for receptor binding and activation, was hypervariable. One clade also contained a modified glutamic acid-leucine-arginine (ELR) motif (asparagine-glycine-arginine / NGR), which regulates binding to CXCR1 and CXCR2 receptors. Based on this sequence information, we hypothesize that these proteins differentially activate neutrophils, which may have a role in HCMV pathogenesis. To address these functional differences, we produced representative vCXCL-1 proteins from each of the 11 clades using a baculovirus protein expression system. Using competition binding assays, we have examined their binding affinities to either CXCR1 or CXCR2 expressed on HEK293 cells. All vCXCL-1s bound to CXCR2 with different binding affinities. Interestingly, only three vCXCL-1s bound to CXCR1 while the others demonstrated did not. We analyzed functional differences between the vCXCL-1s in calcium mobilization, adhesion molecule induction, and chemotaxis on human peripheral blood neutrophils (PBNs). Although the binding affinities to CXCR2 and/or CXCR1 were variable, all vCXCL-1s were capable of inducing intracellular calcium mobilization in PBNs and upregulating adhesion molecules on the surface of PBNs to similar levels as human CXCL1. However, the potency of the vCXCL-1s in the chemotaxis of neutrophils varied and was affinity independent. We also examined secondary chemokine production upon vCXCL-1 treatment on neutrophil-like HL60 T2 cells using real-time PCR. The results showed CCL22 induction was affinity dependent. Taken together, these results provide insights into the potential role of vCXCL-1 in HCMV pathogenesis and how the variability in these chemokines can affect neutrophil function.