Liver gene transfer keeps immune system in check

Comment on Dobrzynski et al, page 969

In this issue, Dobrzynski and colleagues unraveled some of the mechanisms of antigen-specific immune tolerance that occur following liver-directed gene delivery, which has important implications for gene therapy of genetic diseases, including hemophilia.

Gene therapy for hereditary diseases typically requires long-term expression of therapeutic proteins. Immune responses to the therapeutic gene product are a potential complication of gene therapy, since the recipient's immune system may not be tolerant to the functional protein. In particular, induction of neutralizing antibodies against circulating proteins encoded by the therapeutic transgene would compromise the effectiveness of the gene therapy approach. To maintain long-term transgene expression, it is therefore critically important to induce and maintain immunologic hyporesponsiveness to the therapeutic antigens.FIG1 

A number of recent preclinical studies suggest that hepatic gene delivery and restricted transgene expression in hepatocytes^1-4  seem to lower the risk of inducing neutralizing antibodies to the corresponding transgene product. This may involve induction of antigen-specific immune tolerance, which does not seem to occur following gene transfer in other tissues. This immune tolerance was associated with the generation of CD4⁺ T cells that suppressed antibody formation following adoptive transfer into naive recipients and subsequent challenge with the cognate antigen.³  However, detailed and more direct studies on the precise mechanisms of tolerance induction following hepatic gene delivery were hampered by the low frequency of antigen-specific T cells and the inability to physically identify them.

In this issue, Dobrzynski and colleagues managed to overcome some of these limitations and uncovered some of the mechanisms of antigen-specific immune unresponsiveness following hepatic gene delivery. The authors chose to perform gene transfer experiments using chicken ovalbumin (ova) as transgene in BALB/c mice that were transgenic for a T-cell receptor (TCR), specific for ova. This ova-specific TCR is expressed in the majority of T cells in the thymus of transgenic animals and allows for the physical identification of antigen-specific T cells. Using this model, Dobrzynski and colleagues demonstrated that hepatic gene delivery using an adeno-associated viral vector (AAV) resulted in long-term transgene expression and immune tolerance due to peripheral CD4⁺ T-cell deletion in secondary lymphoid organs and central CD4⁺ T-cell deletion in the thymus. Interestingly, the remaining CD4⁺ T cells were anergic and enriched for CD25⁺ regulatory T cells.

This study may provide an explanation of why liver-directed gene transfer keeps the immune system in check, which has important implications for gene therapy of various genetic diseases, including hemophilia. It is tempting to speculate that similar mechanisms would account for immune tolerance following hepatic gene delivery in larger animal models,⁵  and ultimately in patients. However, it cannot be excluded that some of the conclusions drawn from this study may be antigen dependent and not necessarily applicable to other transgene products. Nevertheless, the study by Dobrzynski and colleagues provides a useful conceptual framework that sheds some light into the possible mechanisms of immune tolerance following AAV-mediated hepatic gene delivery. One would predict that the same mechanisms may apply to other vectors that are commonly used for hepatic gene delivery,^2,4  which warrants further investigation.