The affinities of purified recombinant human IL-18 binding protein (BP) and ectromelia and cowpox virus homologs for human and murine IL-18 were compared by plasmon resonance. The dissociation constants of human IL-18BP were similar for murine and human IL-18. By contrast, the dissociation constants of the viral proteins for murine IL-18 were 12- to 50-fold lower than that for human IL-18. The ectromelia and cowpox virus proteins were biologically active, as judged by their ability to inhibit induction of interferon-γ by murine and human IL-18. The relative affinities of the orthopoxvirus IL-18BPs are consistent with the rodent host range of the viruses.
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