Interleukin 8 dimerization as a mechanism for regulation of neutrophil adherence-dependent oxidant production

Interleukin 8 (IL-8), a member of the CXC subfamily of chemoattractant cytokines, induces a range of functional responses in human neutrophils via its interactions with two high-affinity cell-surface receptors, CXCR1 and CXCR2. Like other CXC chemokines, IL-8 forms homodimers at physiologic concentrations. Monomers and dimers bind to CXC receptors with high affinity and induce various functions. Binding to glycosaminoglycans decreases the dimerization constant, enhancing surface-bound dimer formation. However, a specific role for IL-8 dimerization has not been identified. We explored the hypothesis that certain neutrophil responses to IL-8 were induced primarily by the IL-8 dimers. To this end, two dimerization-deficient IL-8 mutant proteins, M3 and M4, were used in various functional assays. In contrast to native IL-8, these proteins existed primarily as monomers at micromolar concentrations. The mutants retained high-affinity binding to both CXC receptors and potently induced neutrophil calcium flux, chemotaxis, and elastase release. In contrast to native IL-8, neither mutant inhibited tumor necrosis factor alpha-induced oxidant production. Additionally, M4 was less effective than native IL-8 at desensitizing neutrophil migration. These data suggest that although IL-8 dimers or monomers are sufficient for several neutrophil functions, dimers may participate in suppression of specific surface-dependent neutrophil responses.