Targeting activated lymphocytes with an entirely human immunotoxin analogue: Human pancreatic RNase1-human IL-12 fusion

A hybrid human protein was produced in E. coli by fusing the genes encoding human pancreatic RNase1 (hpRNase1) and human IL-2 (hIL-2). The recombinant hpRNase1-hIL-2 inhibited protein synthesis in HTLV-1-infected, malignant T cells, which hyperproduce high affinity IL-2 receptors, with an IC₅₀ of 2 x 10^-8 M, whereas no inhibition was detectable in control cells with lower affinity receptors. HpRNase1 alone had an IC₅₀ of almost 10^-3 M. A molar excess of hIL-2 blocked the protein synthesis inhibition dose-dependently. In a human mixed lymphocyte culture, hpRNase1-hIL-2 inhibited the proliferation of responder cells with potency comparable to that of cyclosporine, while non-effective doses of FK506 importantly improved its potency. Despite its short half-life in animals, hpRNase1-hIL-2 rapidly enters cells in a few minutes and arrests the protein translation in less than 10 h. Thus, hpRNase1-hIL-2 may be useful to selectively eliminate activated lymphocytes hyperproducing high affinity IL-2 receptors, as in allograft rejection, gaft-versus-host disease, autoimmune disorders, adult T cell leukaemia and other lymphoproliferative or retroviral malignancies including HIV infection, without inducing general immunosuppression. As an entirely human 'immunotoxin analogue' it may alleviate the dose limiting toxicity and immunogenicity of conventional immunotoxins. (C) 2000 Academic Press.