Expression of human thrombopoietin transgene delivered by an adenov1rus based strategy is dependent on host immune competence

The clinical use of adenovirus mediated gene transfer is substantially limited by the short term gene expression that is inherent to this system. We present strong evidence, that by modulation of the host's immune system a prolonged, clinically relevant transgene expression can be obtained. An adenotypc 5 vector consisting of CMV promoter driven cDNA for human thrombopoietin (AdSCMV.TPO) has been shown to elevate circulating platelets (PLT) and to abrogate myeloablation induced thrombocytopenia in a murine model (Ohwada A. et al. submitted for publication). We applied AdSCMV.TPO in a dose of 109 pfu intraperitoneally per mouse to different strains of mice (Balb/c, Nude, SCID, NOD-SCID) to elucidate the influence of host's immune system on the expression of the transgene. Each group consisted of 6 animals. 3 animals were treated with ADSCMV.TPO and 3 mice were treated with AdSCMV.Nul]. The expression of the transgene was measured by following circulating PLT levels, which were obtained by manually counting of blood samples drawn retroorbitally. In Balb/c mice, characterized by a normal immune system, the circulating PLT peaked by day 7 after vector administration, reached a maximum of 4 to 6 times normal value and elevated PLT could be observed for ca I week. In SCID and NOD-SCID mice the PLT peak reached 6-9 times normal level and lasted for 6 weeks. Afterwords there was a gradual decrease of the PLT. In Nude mice, lacking T-cell function, there was a intermediate response with peak value of PLT 3 to 6 times normal level and a duration of elevated PLT for ca 3 weeks. A second elevation of circulating PLT was observed 35 days after vector administration in some of the Nude mice. In terms of granulocytes, we observed an increase of circulating PMNs up to 10 to 40 fold normal level lasting for 4 to 5 weeks in SCID and NOD-SCID mice only. During the study we observed reversible hepatosplenomegaly in all treated mice (Ad5CMV.TPO nd Ad5CMV.Null) which was of shortest duration in Balb/c mice compared to the other strains. These data show that longterm transgene expression using an adenovirus based strategy can be obtained by modulation of the host's immune system. Long term expression of human TPO stimulates not only PLT production effectively but also granulopoiesis as well. Together, these data suggest that AdSCMV.TPO may be beneficial for patients undergoing high dose chemo- and radiotherapy, abrogating myelotoxicity without causing major other toxicity.