In vitro biologic activity and species specificity of mIFN-α₁. Analysis of [³H]-thymidine uptake in CRO-AP/3 cells treated with 100 IU/mL or 1000 IU/mL recombinant mIFN-α₁ (A), or hIFN-α₂b (B), and in MBL-2 cells treated with 100 IU/mL or 1000 IU/mL recombinant (C) or native mIFN-α₁ (D). Data are reported as ratio of values obtained in treated cells/untreated (A-C) or mock-treated (D) cells, and SDs are calculated as reported in the legend to Figure 1. Proliferation was not impaired in the PEL cell line after mIFN-α₁ treatment, whereas both human and murine IFN-α induced a dose-dependent species-specific reduction in [³H]-thymidine incorporation. The native murine cytokine exerted an antiproliferative activity on MBL-2 cells similar to that observed with the recombinant one, but its effects were already evidenced 24 hours after treatment. (E) RT-PCR analyses performed on RNA extracted from MBL-2 cells exposed to 100 and 1000 IU/mL native mIFN-α₁ for 72 hours showed the induction of murine interferon regulatory factor 7 (mIRF-7) expression in mIFN-α₁–treated cells but not in mock-treated MBL-2 cells, indicating that the native cytokine retains the ability to activate a classic IFN-inducible transcript. Expression of murine β-actin (mβ-act) is shown as control. Lanes: M, 1-kb DNA ladder marker; 1, water control; 2, RNA extracted from mock-treated MBL-2 cells; 3, 4, RNA from MBL-2 cells treated with 100 or 1000 IU/mL native mIFN-α₁; 5, positive control, RNA from MBL-2 cells exposed to 1000 IU/mL recombinant mIFN-α₁.