Importantly, macaque 006 was free of neutralizing antibodies during the previous month before the fourth adenovirus injection, while 004 still maintained titers above 1/50,000. toward the viral capsids were sequentially monitored and found to be repressed by the drug combinations tested. Repeated liver transfer of theHSV1-tkreporter gene with the same recombinant adenoviral vector was achieved in macaques undergoing a clinically feasible immunosuppressive treatment that ablated humoral and cellular immune responses. This strategy allows measurable gene retransfer to the liver as late as 15 months following the first adenoviral exposure in a macaque, which has undergone a total of four treatments with the same adenoviral vector. == Introduction == The immune system has been evolutionarily selected to fight viruses and is a serious hurdle for gene therapies based on viral vectors.1,2Immunity against the vector K-252a precludes readministration as reported with recombinant adenovirus3and adeno-associated computer virus (AAV)based vectors.4Attempts to circumvent antivector immunity include pharmacological immunosuppression5or alternating different vectors with the same transgene.6In the case of adenoviruses, immunogenicity is very potent and has been exploited for vaccination.7,8 Liver tropism of adenoviruses is considered advantageous for gene therapy interventions in this organ. This is the case of helper-dependent (gutless) adenoviruses used to correct inherited disorders with a need for sustained expression that necessarily require repeated vector administrations.3Tumoricidal conditionally replicating adenoviruses may need immunosuppression to allow the agent to spread sufficiently within the malignancy. 9Adenoviruses turn on innate immune functions such as type I interferon and other proinflammatory cytokines.10,11,12Innate responses can primary for any humoral immune response generating neutralizing antibodies to adenoviral capsid antigens, which are augmented by T-cell help.1,2 Visualization and measurements of transgene expression are needed to refine and optimize gene therapy strategies. We have recently set up in K-252a nonhuman primates a semiquantitative transgene-specific positron emission tomography (PET) technique,13which allows detection and imaging of the reporter gene herpes simplex virus type 1 thymidine kinase (HSV1-tk).HSV1-tkas a reporter transgene can be traced by means of PET imaging in nonhuman primates13,14and in humans.15,16This imaging technique is based on measurements of retention of [18F]9-(4-[18F]-fluoro-3-hydroxymethylbutyl)-guanine ([18F]FHBG) in the liver parenchyma once phosphorylated by HSV1-tk. This noninvasive imaging technique is useful for assessing the possibility of repeated gene transfer. We reasoned that if first-generation recombinant adenovirus could be readministered, it would be easier to repeatedly use less immunogenic adenoviral vector generations. An additional advantage of our adenoviral system is the transient time course of transgene expression.15Once transgene expression is extinguished in about 2 weeks, the visualization of the efficiency of subsequent gene transfers is experimentally feasible. Transgene expression extinction is explained by immune removal of transduced cells and gene silencing affecting the cytomegalovirus (CMV) promoter.17 Many approved drugs exist5,18,19,20,21,22that thwart the K-252a immune response and might allow for viral vector readministration. A previous report23has shown the feasibility and security of pharmacological immunosuppression of three macaques in an attempt to block T-cell responses to AAV encoding factor IX coagulation factor. In this study, we have exhibited that pharmacological immunosuppression can achieve repeated liver gene transfer with the same first-generation adenoviral vector in spite of the high degree of immunogenicity attributed to this viral vector. B-cell depletion with anti-CD20 monoclonal antibody19,24and concomitant T-cell inhibition with clinically Rabbit Polyclonal to TOP2A available drugs permits repeated liver gene transfer in macaques. == Results == == Rituximab and FK506 treatment reduce the antiadenoviral humoral and cellular immunity that prevents gene-transfer repetition with the same vector == To test whether immunosuppression could allow repeated administration of an adenoviral vector, a group of macaques treated i.v. with the clinical-grade first-generation recombinant adenovirus AdCMVHSV1.tk25(Physique 1a) received two courses of Rituximab to deplete B cells26plus a conventional daily oral regimen with FK506 (ref.18) to repress T cells (Physique 1a). == Physique 1. == Immunosuppression of macaques with Rituximab and FK506 partially fails in an attempt to transfer twice a transgene to the liver with the same first-generation adenoviral vector.(a) Schematic time collection representation of two administrations of a first-generation adenovirus encoding HSV1-tk.