Substitution of the adenovirus serotype 5 knob with a serotype 3 knob enhances multiple steps in virus replication

Adenovirus (Ad) serotype 5 (Ad5) continues to be the predominant vector used for cancer gene therapy. However, many tumor types are reported to be relatively refractory to Ad5 infection because of low surface expression of the native Ad5 receptor, CAR. The observation that many tumor cells are CAR deficient has necessitated the development of CAR-independent infection strategies, including the introduction of heterologous ligand sequences into the virus fiber gene and immunological or chemical modifications of the capsid proteins. Alternatively, native Ad5 tropism can be modified by substituting the knob region from other Ad serotypes such as Ad type 3 (Ad3) into the Ad5 knob region. To date, the effect(s) of tropism modification on the replication and oncolytic capacity of these chimeric Ad vectors has not been fully evaluated. To address this issue, Ad5 vectors and isogenically matched chimeric vectors with Ad3 tropism (Ad5/3) were compared in this study. Various parameters of virus infection were compared, including binding, nuclear translocation, E1A transcription, transgene expression, de novo virus production, and oncolysis. Overall, the chimeric Ad5/3 virus was progressively more efficient at each step of the replication cycle compared with its Ad5 counterpart. The higher replication efficiency of the chimeric Ad5/3 vector translated into improved therapeutic efficacy in a murine in vivo tumor rejection model. These findings suggest that in addition to the initial target cell interaction, multiple mechanisms contribute to the enhanced replication of the chimeric Ad5/3 vector. Furthermore, the data demonstrate that alternative Ad serotype receptors can be used to improve infection and subsequent oncolytic replication, which is particularly relevant in gene therapy applications for tumors that are inefficiently infected with Ad5.