Transduction of Dendritic Cells by DNA Viral Vectors Directs the Immune Response to Transgene Products in Muscle Fibers

Somatic gene transfer is a powerful way to elicit cellular and humoral immune responses to a foreign protein. While this has been exploited for the development of vaccines for cancer (5, 29, 32–35, 43) and infectious disease (25, 37, 40), it is a substantial problem in the treatment of chronic diseases, such as autosomal recessive disorders, where prolonged transgene expression may be desired (4, 44). This problem has been most extensively documented following in vivo gene transfer with recombinant adenoviruses. Adenoviruses expressing the lacZ gene elicit vibrant cellular and humoral immune responses to cytosolic β-galactosidase following delivery to liver, lung, muscle, and joint that often contribute to destruction of the genetically corrected target cells and lead to inflammation and loss of transgene (14, 41, 46, 48, 52). Similar problems were encountered following low-density lipoprotein receptor gene transfer in a murine model of familial hypercholesterolemia (27). Target cell destruction is mediated, in part, by antigen-specific class I-restricted cytotoxic T lymphocytes (CTL) to both transgene product and newly expressed viral protein (39, 46, 47, 51, 52, 54). Activation of CD4+ T cells, presumably of the TH1 subset, is required for the full realization of the destructive CTL effect (22, 28, 48, 50). Initial studies with recombinant adeno-associated virus (AAV) delivered to skeletal muscle have yielded unexpected results in terms of the stability of gene transfer and ensuing immune responses. This human parvovirus can be rendered defective by completely eliminating all viral open reading frames, leaving the viral capsid proteins and the product of the transgene as the only sources of antigen (26). In most cases, gene transfer with AAV has been good; however, transgene expression is often poor (9, 10, 12, 31, 36, 42). Two exceptions are skeletal muscle (11, 23, 45) and the central nervous system (21), where postmitotic, differentiated cells such as muscle fibers and neurons are efficiently targeted with AAV, leading to high-level and stable transgene expression. It was particularly surprising that AAV failed to elicit immune responses to highly expressed neoantigenic transgene products when injected into muscle (11, 23, 45) whereas other vector systems expressing the identical transgene, such as adenovirus (52) and naked DNA, do. We have evaluated the mechanisms by which AAV evades immunologic responses following injection into muscle in the context of rate-limiting steps of immune activation by adenovirus.