Characterization of the Immune Response After Local Delivery of Recombinant Adenovirus in Murine Pancreas and Successful Strategies for Readministration

Abstract

The pancreas is an ideal organ for adenoviral gene therapy because of the high level of gene transfer that can be achieved and because of the many diseases that can potentially be treated using this technology. In this report, we characterize the immune response to direct pancreatic injection of adenovirus and we overcome some of the limitations it imposes by using immunosuppression. Direct injection of recombinant adenovirus into the pancreas leads to the production of neutralizing antibodies and to sensitized splenocytes which engage in increased cytotoxic, lymphoproliferative, and cytokine release activity when reexposed to adenovirus. Transgene expression is transient and the vector cannot be readministered. Deletion of CD4⁺ T helper cells improves expression over time (40% of pancreatic cells express transgene at day 28 vs. 5% in controls), and allows the vector to be readministered in the pancreas, albeit, inefficiently, when compared to naive animals. Similarly, blockade of CD40 ligand, which preserves the CD4⁺ T helper cell population, also improves expression over time (30% of pancreatic cells express transgene at day 28), and allows the vector to be readministered. With both approaches, neutralizing antibodies are decreased and the remaining splenocytes do not engage in activated immune responses. Thus, local delivery of the adenoviral vector induces a systemic response that prevents pancreatic readministration, even with direct injection. Blockade of CD40 ligand and T helper cell depletion are transient regimens that induce systemic immunosuppression. Until the development of newer strategies that selectively suppress adenoviral immune responses, these are viable alternatives for enhancement of pancreatic adenoviral delivery. Activation of the immune system limits the ability to express genes delivered by the adenoviral vector. In the pancreas, significant local infiltration of immune cells occurs with delivery of recombinant adenovirus. In this report, we demonstrate that a global immune response is also activated following local pancreatic delivery of the adenoviral vector, which prevents even direct readministration of vector. This response can be blunted and expression can be improved by either deleting the T helper cell population or by transiently blocking the initial T cell–B cell interaction. Furthermore, the vector can be readministered into the pancreas following either of these immunosuppressive regimens. However, expression is less optimal than that seen in naive animals. With the introduction of new vectors containing therapeutic genes, immune responses in targeted organs need to be evaluated and immunosuppressive regimens developed, such that appropriate immunosuppression can be administered to enhance the beneficial effects while minimizing systemic toxicity.