Entry may remain a scattershot affair for adenovirus. “I think it's intermittent [epithelial] breaks that all of us must have that gives the initial entry,” says Welsh. These hypothetical micro-injuries would allow the adenovirus capsid protein called Fiber to bind CAR on the basolateral surface.
After replication, large numbers of viral particles, defective viral particles, and excess Fiber protein are all released into the basolateral solution. The authors showed that the resultant binding of CAR disrupted cell–cell adhesion, thus allowing viral escape to the apical surface. Similar disruption of endothelial CAR may allow the virus to spread into the bloodstream.
The efficient spread of adenovirus is thus dependent on a replication step, which is not an option for most gene therapy vectors. Welsh suggests using chemicals such as calcium phosphate to induce uptake, or viruses such as adeno-associated virus type 5 that use apical receptors. Alternatively, calcium chelators such as EDTA can open the tight junctions allowing adenovirus to reach CAR. “When you open up the tight junction,” says Welsh, “adenovirus works great.” ▪