The Role of Interferon- α in a Successful Murine Tumor Therapy

Combination therapy using reovirus type 3 and the chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) is sufficient to cure approximately 80% of EL-4 lymphoma tumor-bearing BD2F1 male mice. Cured animals can be challenged with the EL-4 tumor, in the absence of the therapy, to yield 100% survival, whereas those challenged with heterologous tumor produce 0% survival. These results strongly suggest that a hostimmune response is responsible for the observed therapeutic effect. Reovirus, a double-stranded RNA virus, is an efficient inducer of type I interferon. In an effort to determine the role of virus in this therapy, we substituted interferon-a (IFN-a) for reovirus in the therapy. Doses of IFN-a from 1000-10,000 U were capable of replacing reovirus to produce cure rates similar to reovirus. Spleen cells isolated from therapy-treated animals demonstrated high levels of cytotoxicity against the natural killer cell-sensitive cell line YAC-1, but not against EL-4 tumor. In vitro stimulation of isolated spleen cells by IFN-a resulted in a high level of natural killer cell activity, but no cytotoxicity against the EL-4 tumor. A significant antiproliferative effect against the EL-4 tumor in cell culture was demonstrated by IFN-a. Finally, therapy-treated, tumor-bearing mice that were injected with anti-IFN-a + -β antibodies had similar survival levels as control mice, indicating that other cytokines might also play a role in promoting tumor killing. These investigations suggest that IFN-a may be a mediator of antitumor activity in the reovirus therapy system.