Adenovirus is among the preferred vectors for gene therapy because of its superior in vivo gene-transfer efficiency. However, upon systemic administration, adenovirus is preferentially sequestered by the liver, resulting in reduced adenovirus-mediated transgene expression in targeted tissues. In the liver, Kupffer cells are responsible for adenovirus degradation and contribute to the inflammatory response. As scavenger receptors present on Kupffer cells are responsible for the elimination of blood-borne pathogens, we investigated the possible implication of these receptors in the clearance of the adenovirus vector. Polyinosinic acid [poly(I)], a scavenger receptor A ligand, was analysed for its capability to inhibit adenovirus uptake specifically in macrophages. In in vitro studies, the addition of poly(I) before virus infection resulted in a specific inhibition of adenovirus-induced gene expression in a J774 macrophage cell line and in primary Kupffer cells. In in vivo experiments, pre-administration of poly(I) caused a 10-fold transient increase in the number of adenovirus particles circulating in the blood. As a consequence, transgene expression levels measured in different tissues were enhanced (by 5- to 15-fold) compared with those in animals that did not receive poly(I). Finally, necrosis of Kupffer cells, which normally occurs as a consequence of systemic adenovirus administration, was prevented by the use of poly(I). No toxicity, as measured by liver-enzyme levels, was observed after poly(I) treatment. From our data, we conclude that poly(I) can prevent adenovirus sequestration by liver macrophages. These results imply that, by inhibiting adenovirus uptake by Kupffer cells, it is possible to reduce the dose of the viral vector to diminish the liver-toxicity effect and to improve the level of transgene expression in target tissues. In systemic gene-therapy applications, this will have great impact on the development of targeted adenoviral vectors.

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Currently, various therapeutic strategies are being explored as a potential means to immunize against metastatic malignant cells or even primary tumours. Using recombinant viral vectors systems or protein-based immunization approaches, we are developing immunotherapeutic strategies against cervical cancer or premalignant cervical disease, as induced by high-risk type human papillomaviruses (HPVs). We previously demonstrated that immunization of mice with recombinant replication-defective Semliki Forest virus (rSFV) encoding a fusion protein of HPV16 E6 and -E7 (SFV-eE6,7) induces strong cytotoxic T-lymphocyte (CTL) activity and eradication of established HPV-transformed tumours. In this study, we compared the antitumour efficacy of SFV-eE6,7 with that of a recombinant adenovirus (rAd) type 5 vector, expressing the same antigen construct (AdeE6,7). Prime-boosting with SFV-eE6,7 resulted in higher precursor CTL frequencies and CTL activity compared to prime-boosting with Ad-eE6,7 and also in murine tumour treatment experiments SFV-eE6,7 was more effective than Ad-eE6,7. To elicit a therapeutic effect with Ad-eE6,7, 100/1000-fold higher doses were needed compared to SFV-eE6,7. In vivo T-cell depletion experiments demonstrated that these differences could not be explained by the induction of a different type of effector cells, since CD8 + T cells were the main effector cells involved in the protection against tumour growth in both rSFV-and rAd-immunized mice. Also comparable amounts of in vivo transgene expression were found upon immunization with rSFV and rAd encoding the reportor gene luciferase. However, antivector responses induced by a single injection with rAd resulted in a more than 3-log decrease in luciferase expression after a second injection of rAd. With rSFV, transgene expression was inhibited by only one to two orders of magnitude in preinjected mice. As an antigen-specific booster immunization strongly increases the level of the CTL response and is essential for efficient induction of immunological memory, it is likely that (part of) the difference in efficacy between rSFV and rAd type 5 can be ascribed to a diminished efficacy of the booster immunization in the case of rAd due to anti-vector antibody responses.

Previously, we demonstrated potent tumor cell-selective pro-apoptotic activity of scFv425:sTRAIL, a recombinant fusion protein comprised of EGFR-directed antibody fragment (scFv425) genetically fused to human soluble TNF-related apoptosis-inducing ligand (sTRAIL). Here, we report on the promising therapeutic systemic tumoricidal activity of scFv425:sTRAIL when produced by the replication-deficient adenovirus Ad-scFv425:sTRAIL. In vitro treatment of EGFR-positive tumor cells with Ad-scFv425:sTRAIL resulted in the potent induction of apoptosis of not only infected tumor cells, but importantly also of up to 60% of noninfected EGFR-positive tumor cells. A single intraocular injection of Ad-scFv425:sTRAIL in tumor-free nu/nu mice resulted in predominant liver infection and concomitant high blood plasma levels of scFv425:sTRAIL. These mice showed no sign of Ad-scFv425:sTRAIL-related liver toxicity. Identical treatment of mice with established intraperitoneal renal cell carcinoma xenografts resulted in rapid and massive tumor load reduction and subsequent long-term survival. Taken together, adenoviral-mediated in vivo production of scFv425:sTRAIL may be exploitable for systemic treatment of EGFR-positive cancer.