Neonatal cotton rats do not exhibit destructive immune responses to adenoviral vectors.

Recombinant adenoviruses are being considered for treatment of cystic fibrosis lung disease. The utility of this vector system in adult recipients is limited by the formation of destructive humoral and cellular immune responses to the vector and vector-infected cells. We hypothesized that the host immune responses could be avoided by administering the vector early in development when the immune system is naive. This was evaluated in the context of newborn and adult cotton rats administered E1-deleted recombinant adenoviral vectors into the trachea. The results with adult cotton rats were consistent with previous studies in that gene expression was extremely high in the conducting airway when evaluated 3 days after administration of the vector. However, expression diminished to undetectable levels within 2 weeks and was associated with substantial inflammation and formation of neutralizing antibodies. The level of expression achieved in newborn cotton rats was equally high with reporter gene expression detected in both the conducting airway and alveolar cells. The immune response in the newborn cotton rats was substantially blunted with diminished CD4+ T cell activation, based on in vitro studies, as well as essentially no formation of neutralizing antibody. This translated to a substantially improved outcome with transgene expression persisting for over 6 months. The window of optimal immune tolerance was within 3 days of birth. Vector was readministered into animals 8-10 weeks after they had received the neonatal challenge of the initial vector. This second delivery of vector was efficient, consistent with the low neutralizing antibody in these animals. However, the animals mounted a full T cell response which correlated with extinction of the transgene from the vector administered at birth, as well as that of the second vector. Neutralizing antibodies did eventually develop following the second vector administration. These studies indicate that immune responses can be avoided if gene therapy is administered early in development. However, this so-called "tolerance' is broken when the animal receives a second vector after the immune system matures.