Tolerance induction by viral in vivo gene transfer

Abstract

Treatment of genetic disease by protein or gene replacement therapy is hampered by immune responses to the therapeutic protein. An excellent example is formation of inhibitory antibodies to coagulation factors in treatment of the X-linked bleeding disorder hemophilia. Experiments in murine and canine models of hemophilia B (deficiency in factor IX) have demonstrated sustained therapeutic levels of factor IX transgene expression following hepatic adeno-associated viral gene transfer in animals with deletion and nonsense mutations in the factor IX gene. This article reviews experimental evidence for induction of immune tolerance to the factor IX transgene product by hepatic adeno-associated viral gene transfer, which has been shown to limit T helper cell responses and to substantially reduce the risk of antibody responses. Tolerance induction is associated with activation of regulatory CD4(+) T cells capable of suppressing antibody formation to factor IX protein. Hepatic administration of adeno-associated viral vector expressing ovalbumin in mice transgenic for a T cell receptor specific for this antigen provided direct evidence for induction of CD4(+) T cell tolerance, including T cell anergy and clonal deletion. Taken together, these data indicate the potential for viral in vivo gene transfer not only to provide sustained systemic expression, but moreover to induce immunological hypo-responsiveness to the therapeutic gene product.

Figure 1.

Model for induction of immune tolerance to a therapeutic gene product by hepatic adeno-associated viral-mediated gene transfer. Vector is infused into the hepatic circulation resulting in gene transfer and transgene expression in hepatocytes (green stain). Subsequent presentation of transgene product-derived peptides via MHC class II molecules (likely through professional antigen presenting cells) to naïve CD4⁺ T cells causes induction of T cell anergy and deletion of transgene product-specific T cells or causes activation of regulatory T cells. The latter suppress activation of any remaining responsive CD4⁺ T cells. As a consequence of these mechanisms, there is a lack of T helper cell responses, in turn causing failure to activate B cells and CD8⁺ T cells. Ultimately, antibody or cytotoxic T cell responses are not observed or are limited.