Therapeutic gene transfer holds the promise of providing lasting therapies and even cures for diseases that were previously untreatable or for which only temporary or suboptimal treatments were available. For some time, clinical gene therapy was characterized by some impressive but rare examples of successes and also several setbacks. However, effective and long-lasting treatments are now being reported from gene therapy trials at an increasing pace. Positive outcomes have been documented for a wide range of genetic diseases (including hematological, immunological, ocular, and neurodegenerative and metabolic disorders) and several types of cancer. Examples include restoration of vision in blind patients, eradication of blood cancers for which all other treatments had failed, correction of hemoglobinopathies and coagulation factor deficiencies, and restoration of the immune system in children born with primary immune deficiency. To date, about 2,000 clinical trials for various diseases have occurred or are in progress, and many more are in the pipeline. Multiple clinical studies reported successful treatments of pediatric patients. Design of gene therapy vectors and their clinical development are advancing rapidly. This article reviews some of the major successes in clinical gene therapy of recent years.

Adeno-associated virus (AAV) is a replication-deficient parvovirus that is extensively used as a gene therapy vector. CD8 T-cell responses against the AAV capsid protein can, however, affect therapeutic efficacy. Little is known about the in vivo mechanism that leads to the crosspriming of CD8 T cells against the input viral capsid antigen. In this study, we report that the Toll-like receptor 9 (TLR9)-MyD88 pattern-recognition receptor pathway is uniquely capable of initiating this response. By contrast, the absence of TLR2, STING, or the addition of TLR4 agonist has no effect. Surprisingly, both conventional dendritic cells (cDCs) and plasmacytoid DCs (pDCs) are required for the crosspriming of capsid-specific CD8 T cells, whereas other antigen-presenting cells are not involved. TLR9 signaling is specifically essential in pDCs but not in cDCs, indicating that sensing of the viral genome by pDCs activates cDCs in trans to cross-present capsid antigen during CD8 T-cell activation. Cross-presentation and crosspriming depend not only on TLR9, but also on interferon type I signaling, and both mechanisms can be inhibited by administering specific molecules to prevent induction of capsid-specific CD8 T cells. Thus, these outcomes directly point to therapeutic interventions and demonstrate that innate immune blockade can eliminate unwanted immune responses in gene therapy.

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