Byrnes, Andrew P.

ABSTRACT Adenovirus serotype 5 (Ad5) naturally infects the liver after intravenous injection, making it a candidate for hepatocyte-directed gene transfer. While Ad5 can be efficient, most of the dose is destroyed by liver Kupffer cells before it can reach hepatocytes. In contrast, Ad5 bearing the hexon from Ad6 (Ad5/6) evades Kupffer cells. While Ad5/6 dramatically increases hepatocyte transduction in BALB/c mice, it has surprisingly little effect on C57BL/6 mice. To determine the source of this strain-specific difference, the roles of Kupffer cells, liver sinusoidal endothelial cells (LSECs), hepatocytes, scavenger receptors, clotting factors, and immunoglobulins were analyzed. The numbers of Kupffer cells and LSECs, the level of clotting factor X, and hepatocyte infectibility did not differ between different strains of mice. In contrast, high levels of immunoglobulins correlated negatively with Ad5 liver transduction in different mouse strains. Removal of immunoglobulins by use of Rag-deficient mice restored Ad5 transduction to maximal levels. Removal of Kupffer cells by predosing or by testing in colony-stimulating factor knockout mice restored Ad5 transduction in the presence of immunoglobulins. Partial reconstitution of IgM in Rag mice resulted in significant reductions in liver transduction by Ad5 but not by Ad5/6. These data suggest a role for IgM-mediated clearance of Ad5 via Kupffer cells and may explain the mechanism by which Ad5/6 evades these cells. These mechanisms may play a vital role in Ad pharmacology in animals and in humans.

ABSTRACTUnderstanding innate immunity is key to improving the safety of adenovirus (Ad) vectors for systemic gene therapy. Ad has been shown to activate complement in vitro, but activation of complement after Ad injection in vivo has not been directly measured. Using complement protein C3a as a marker of complement activation, we show that types 2 and 5 human Ads cause rapid complement activation after intravenous injection in mice. Unexpectedly, the mechanisms in vivo were different than those in vitro. Antibodies were critical for the activation of complement by Ad in vitro, but antibodies were not required in vivo. The classical pathway was required in vitro, whereas complement activation in vivo involved both classical and nonclassical pathways as well as the reticuloendothelial system. Remarkably, the entry-deficient Ad mutantts1 was completely unable to activate complement in vivo even though it was fully able to activate complement in vitro. This result demonstrates that the complement system senses intravenously injected Ad primarily by detecting the effects of Ad on cells rather than through direct interaction of complement with virions. Encouragingly, shielding Ad with polyethylene glycol was effective at reducing complement activation both in vitro and in vivo. In summary, intravenously injected Ad rapidly activates complement through multiple pathways, but these pathways are different than those identified by in vitro studies. In vitro studies are poorly predictive of in vivo mechanisms because Ad virions activate complement through indirect mechanisms in vivo.