The myxoma virus-soluble interferon-γ receptor homolog, M-T7, inhibits interferon-γ in a species-specific manner

The myxoma virus M-T7 protein contains significant sequence similarity to the ligand binding domain of the mammalian interferon-γ receptors, and functions as a soluble homolog which can bind and inhibit the biological activities of rabbit interferon-γ (Upton, C., Mossman, K., and McFadden, G. (1992) Science 258: 1369-1372). M-T7, the most abundantly secreted protein from myxoma virus-infected cells, was shown to be expressed in significant biological amounts as a typical poxvirus early gene product, efficiently secreted at early times of infection to levels that exceed 5 x 10⁷ molecules/cell, and function as a stable inhibitory protein in infected cell supernatants until late times of infection. M-T7 was specific in binding and inhibiting rabbit interferon-γ, and did not bind either human or murine interferon-γ. Scatchard analysis of rabbit interferon-γ binding curves yielded a single high affinity binding site on M-T7, with a K(d) of 1.2 x 10^-9 M, which is comparable to the affinity between soluble forms of cellular interferon-γ receptors and their cognate ligands. In comparison, rabbit interferon-γ was shown to bind its cellular receptor with a K(d) of 5.9 x 10^-10 M, again comparable to the affinity of membrane bound forms of other mammalian interferon-γ receptors for interferon-γ. Thus, the myxoma virus M-T7 protein is a functional soluble interferon-γ receptor homolog which binds and inhibits interferon-γ with high affinity in a species- specific manner.