Influence of adenoviral fiber mutations on viral encapsidation, infectivity and in vivo tropism

Targeting of adenovirus (Ad)-encoded therapeutic genes to specific cell types has become a major goal in gene therapy. Redirecting the specificity of infection requires the abrogation of the natural interaction between the viral fiber and its cellular receptors (CAR) and the simultaneous introduction of a new binding specificity into the viral capsid. To abrogate the natural affinity of the fiber, we have mutated residues presumed to be directly or indirectly involved in CAR-binding in the knob domain of the fiber protein. These residues are located in the AB loop (Ser408) and in the DG loop (Tyr491, Ala494, Ala503). The mutations Ser408Glu, Tyr491Asp, Ala494Asp and Ala503Asp did not prevent the incorporation of trimeric fibers in the viral capsid but led to loss of CAR binding in vitro. Infectivity of the mutant viruses could be restored in vitro by introducing a ligand at the C-terminal end of the knob, confirming that the reduced infectivity of the fiber-modified virus was due to an impaired interaction of the viral particle with the CAR receptor. However, after systemic delivery, the in vivo biodistribution of impaired CAR-binding viruses without addition of a specific ligand was not altered when compared with wild-type Ad.