Figure 1. Oncolytic control. Macrophages are first transfected with a vector (light grey rectangle) in which the gene encoding the adenovirus small early region 1A (E1A) protein (yellow oval) is under the control of a hypoxic response element (HRE). They are then co-transduced with a conditionally competent, oncolytic adenovirus (Ad; dark grey rectangle)-the replication of which can be regulated by either E1A protein or a regulatory sequence called PPT. PPT is a composite of promoter enhancers from three genes expressed in prostate tumor cells: TCR-g alternate reading frame protein (TARP), prostate-specific antigen (KLK3; PSA) and prostate-specific membrane antigen (PSMA; FOLH1; GCPII).

In regions of low oxygen, macrophages upregulate a transcription factor called hypoxia-inducible factor 1 (HIF1; light blue oval) which binds the HRE to produce E1A protein [a]. Production of E1A (yellow oval) then initiates adenovirus replication in the macrophage, leading to virus production [b].

The virus is then released and can enter surrounding cells. When the virus enters normal cells, lack of either E1A or the prostate-specific factors that stimulate the PPT sequence prevents further viral replication [c]. When the virus enters a prostate cancer cell, the PPT sequence is activated (dark green oval), leading to a further round of replication of the oncolytic virus and then cell death [d].

In regions of normal oxygen, macrophages do not upregulate HIFs. As a result, E1A and adenovirus production do not occur.