The chemokine dose and the time period during which the chemotactic gradient is established determine the number of leukocytes that infiltrate inflamed tissues. At suboptimal chemokine concentrations, neutrophils may require a priming agent or a second stimulus for full activation. An interesting mode of cooperative action to reach maximal migration is synergy between chemokines. This was first observed between the plasma CC chemokine regakine-1 and the tissue CXC chemokine ligand interleukin-8 (IL-8/CXCL8) in neutrophil chemotaxis. Addition of antibodies against IL-8 or regakine-1 in the Boyden microchamber assay abrogated this synergy. Other CC chemokines, such as CC chemokine ligand-2 monocyte chemotactic protein-1 (MCP-1/CCL2), MCP-2 (CCL8), and MCP-3 (CCL7) as well as the CXC chemokine receptor-4 (CXCR4) agonist stromal cell-derived factor-1alpha (SDF-1alpha/CXCL12), also dose-dependently enhanced neutrophil chemotaxis toward a suboptimal concentration of IL-8. These chemokines synergized equally well with the anaphylatoxin C5a in neutrophil chemotaxis. Alternatively, IL-8 and C5a did not synergize with an inactive precursor form of CXCL7, connective tissue-activating peptide-III/CXCL7, or the chemoattractant neutrophil-activating peptide-2/CXCL7. In the chemotaxis assay under agarose, MCP-3 dose-dependently increased the migration distance of neutrophils toward IL-8. In addition, the combination of IL-8 and MCP-3 resulted in enhanced neutrophil shape change. AMD3100, a specific CXCR4 inhibitor, reduced the synergistic effect between SDF-1alpha and IL-8 significantly. SDF-1alpha, but not MCP-1, synergized with IL-8 in chemotaxis with CXCR1-transfected, CXCR4-positive Jurkat cells. Thus, proinflammatory chemokines (IL-8, MCP-1), coinduced during infection in the tissue, synergize with each other or with constitutive chemokines (regakine-1, SDF-1alpha) to enhance the inflammatory response.
