Distinguishing immune responses from vascular endothelial injury is key to managing AAV gene therapy toxicity
Safety cannot be explained by delivery efficiency alone
A review suggests that AAV gene therapy toxicity is difficult to explain solely by pre-existing antibodies or excessive immune responses, and that the possibility of direct injury to vascular endothelial cells also needs to be considered. In a paper published in the International Journal of Molecular Sciences, Parajuli, Gallagher, and Flotte summarize toxicity syndromes reported in clinical settings and strategies for addressing them. This is a literature review synthesizing existing clinical and preclinical studies and case reports, not a new patient trial or a comparative study of therapies.
Recombinant adeno-associated virus (rAAV) vectors are vehicles for delivering therapeutic genes into cells. Although they showed a relatively favorable safety profile in preclinical studies, hepatotoxicity, myocarditis, thrombotic microangiopathy (TMA), acute respiratory distress syndrome (ARDS), cytokine storm, and hemophagocytic lymphohistiocytosis (HLH), among other toxicities, have been reported in clinical trials and after market approval. The review particularly highlights high-dose systemic administration as an important risk context. It also explains that risk assessment should consider the extent of disease progression, pre-existing organ damage, vector design, and route of administration alongside dose.
Antibody and T-cell responses do not necessarily mean severe toxicity
Pre-existing anti-AAV neutralizing antibodies can interfere with vector entry into cells and reduce gene delivery. Antibody binding to the vector can also activate complement, a protein-based immune defense system, and contribute to inflammation and tissue damage. However, not all antibodies that bind AAV are neutralizing antibodies, and antibody status alone cannot establish that severe toxicity will occur.
After administration, immune responses may develop against the viral protein shell, or capsid, or against the protein produced by the therapeutic gene. In a hemophilia B study described in the review, elevated liver enzymes and reduced factor IX expression were observed alongside capsid-reactive T cells. This suggests an immune attack on liver cells that had received the gene. By contrast, another study using intramuscular vector administration found that low but measurable gene expression persisted even when an immune response occurred. This is why detection of an immune response must be distinguished from clinically harmful toxicity.
Complement-related toxicity and the possibility of direct injury
TMA is characterized by hemolytic anemia, thrombocytopenia, and the formation of microthrombi. The review describes it as generally occurring 1–2 weeks after administration. It may be accompanied by kidney injury, and both antibody-mediated and antibody-independent complement activation have been proposed as contributing mechanisms.
However, not all severe toxicity arises through the same pathway. In a case of a patient with advanced Duchenne muscular dystrophy discussed in the review, no anti-AAV9 antibodies were detected before high-dose systemic AAV9 administration, and tests for T-cell responses to AAV9 and the therapeutic gene product were also negative. Despite prophylactic immunosuppression, the patient died on day 8 after administration from ARDS and multiorgan failure. Subsequent tissue analysis found high therapeutic gene expression in pulmonary vascular endothelial cells, raising the possibility of direct injury. Whether innate immunity was involved, among other questions, remains uncertain. This single case cannot be generalized, but it illustrates the limitation that antibody testing and prophylactic immunosuppression cannot predict or prevent all toxicities.
Mitigation strategies exist, but there is no standard regimen
The authors state that no universally established immunomodulatory regimen is yet available. Corticosteroids have been used for immune-mediated elevations in liver enzymes, while complement inhibitors and cytokine inhibitors, among other agents, have also been used or studied according to the suspected mechanism and symptoms. There are reports of improved hematological and renal markers after complement inhibitor treatment in patients with TMA, but there are also cases in which several drugs failed to stop severe toxicity from progressing.
The development direction proposed in the review is to identify risks specific to each patient, disease, and vector, and to respond to changes after administration. Vector designs that reduce off-target expression are also part of this approach. However, because the review synthesizes evidence from different diseases and administration conditions, it should not be interpreted as establishing overall incidence rates or the superiority of a particular preventive regimen. Further details are available in the full review and the bibliographic and abstract record.
Gene therapy using recombinant adeno-associated virus (rAAV) vectors has emerged as a transformative therapeutic modality for genetic disorders, demonstrating high transduction efficiency and a generally favorable safety profile during pre-clinical development. However, serious adverse events, including thrombotic microangiopathy, acute respiratory distress syndrome, hepatotoxicity, myocarditis, cytokine storm, and hemophagocytic lymphohistiocytosis, have been observed across multiple gene therapy clinical trials. Significant efforts have been made to understand the toxicities that cause these adverse events and clinical care for patients receiving gene therapies has evolved to mitigate their effects. These toxicities arise from a complex interplay between the innate and adaptive immune responses directed against the viral capsid and transgene products and are often compounded by pre-existing anti-AAV immunity. Immunomodulatory strategies have been developed to combat these responses to improve the long-term success of gene therapies, and this review provides clinicians managing gene therapy patients with an overview of mechanisms underlying AAV-associated immunotoxicities and a discussion of syndromes and mitigation strategies that have been reported in the clinical care of patients.
For researchers developing AAV therapies, the practical implication of this review is that safety assessment cannot readily be reduced to antibody status or whether an immunosuppressant was given. Anti-capsid responses, responses to the therapeutic protein, complement activation, and possible injury from off-target gene expression call for different evaluation and response priorities. The review therefore points to the need for a development strategy that connects vector design, route of administration, and patients’ pre-existing organ status with post-administration monitoring. In particular, a case of severe toxicity despite prophylactic immunosuppression shows that suppressing antibody formation alone cannot ensure overall safety. However, improvement in an individual case should not be interpreted as a universal treatment effect, and this review alone cannot determine clinical prescribing.