Systemic production of IL-12 by naked DNA mediated gene transfer: toxicity and attenuation of transgene expressionin vivo

Abstract

Background IL‐12 is a potent antitumor cytokine for cancer gene therapy. Previously, we demonstrated that single systemic administration of naked DNA (encoding IL‐12) could serve as a good model for in vivo evaluation of the antitumor effect of a candidate gene (unpublished data). In the present study, we propose that this gene delivery method could be a very useful model for in vivo evaluation of the toxicity of a given therapeutic gene (using IL‐12 as an example). By comparing the toxicities and the effects of initial IL‐12 administration on subsequent transgene expression, both IL‐12 gene delivery and recombinant murine IL‐12 protein (rmIL‐12) administration showed similar toxicity profiles. Methods Naked DNA encoding murine IL‐12 (mIL‐12) was delivered into mice by systemic administration. Toxicity profiles of mice treated with DNA or rmIL‐12 were compared. Results Systemic administration of naked DNA encoding mIL‐12 resulted in very similar toxicity as rmIL‐12 with respect to liver enzyme, hematological and immunological profiles. Repeated injection of mIL‐12 gene did not recover a high level of mIL‐12 production as the first injection. Moreover, initial mIL‐12 administration resulted in inhibition of subsequent reporter gene expression with both viral and non‐viral promoters (CMV, human α‐antitrypsin or chicken β‐actin promoter). This transgene inhibition effect was entirely mediated by IFN‐γ as the transgene expression was fully recovered in IFN‐γ knockout mice. Conclusions Systemic IL‐12 therapy, with either a protein or gene therapy approach, resulted in comparable liver and systemic toxicities. Refractoriness of mIL‐12 production by subsequent administration of mIL‐12 gene was observed. The transgene attenuation effect of IL‐12 pre‐dosing (either by IL‐12 or rmIL‐12), mediated by IFN‐γ, provided important insights for the design of IL‐12 combination gene therapy and the improvement of gene vectors for IL‐12 therapy. The present results show that simple injection of naked DNA could serve as a good model for in vivo evaluation of the toxicity of a candidate therapeutic gene. Copyright © 2001 John Wiley & Sons, Ltd.