Immune Response Mechanisms against AAV Vectors in Animal Models

Abstract

Early preclinical studies in rodents and other species did not reveal that vector or transgene immunity would present a significant hurdle for sustained gene expression. While there was early evidence of mild immune responses to adeno-associated virus (AAV) in preclinical studies, it was generally believed that these responses were too weak and transient to negatively impact sustained transduction. However, translation of the cumulative success in treating hemophilia B in rodents and dogs with an AAV2-F9 vector to human studies was not as successful. Despite significant progress in recent clinical trials for hemophilia, new immunotoxicities to AAV and transgene are emerging in humans that require better animal models to assess and overcome these responses. The animal models designed to address these immune complications have provided critical information to assess how vector dose, vector capsid processing, vector genome, difference in serotypes, and variations in vector delivery route can impact immunity and to develop approaches for overcoming pre-existing immunity. Additionally, a comprehensive dissection of innate, adaptive, and regulatory responses to AAV vectors in preclinical studies has provided a framework that can be utilized for development of immunomodulatory therapies to overcome or bypass immune responses and for developing strategic approaches toward engineering stealth AAV vectors that can circumvent immunity. Adeno-Associated VirusAdeno-associated virus (AAV) is a single-stranded DNA dependovirus and member of the parvovirus family. The wild-type genome of AAV is 4.7 kb coding for replication (rep) and structural (cap) proteins. AAV infection is not associated with any disease in humans and other mammals, which are natural hosts for AAV, and the wildtype virus is weakly immunogenic. However, AAV replication is dependent on immunogenic helper viruses that promote inflammation, resulting in humoral and cell-mediated immune responses directed against the AAV capsid proteins. Thus, from natural infection, humans may have pre-exiting immunity with antibodies and immunological memory against the AAV capsid. AAV as a Gene Therapy VectorAAV vectors are generated by replacing the rep and cap genes with a transgene expression cassette, while retaining the flanking cis viral inverted terminal repeats (ITRs). 1 Capsids from different AAV serotypes, natural or engineered, can be used to cross-package AAV genomic DNA with AAV2 ITRs to direct vector tropism to a target tissue or organ. 1 The AAV vector genome can be packaged as single-stranded (ssAAV) DNA, similar to wild-type AAV or selfcomplementary (scAAV) with double-stranded DNA. 1 The viral capsid is made up from three proteins VP1, VP2, and VP3, in which VP2 and VP3 are shortened versions of VP1. Thus, the capsid proteins and transgene product constitute the only immunological antigens. However, since the viral capsids are derived from wildtype AAVs, AAV vectors can be recognized by pre-existing adaptive immune responses.

T cells limit accumulation of aggregate pathology following intrastriatal injection of α-synuclein fibrils

“…This is in contrast to what has been reported recently, in where T cells are necessary for the upregulation of MHC II in microglia, which then leads to loss of dopamine neurons in the substantia nigra in response to viral overexpression of α-syn in rats [34]. The absence of MHC II upregulation in response to α-syn PFFs (in contrast to the model with viral overexpression) could be due to recombinant adeno-associated virus being inflammatory itself [67][68][69][70].…”

Genetic Rescue of X-Linked Retinoschisis Mouse (Rs1−/y) Retina Induces Quiescence of the Retinal Microglial Inflammatory State Following AAV8-RS1 Gene Transfer and Identifies Gene Networks Underlying Retinal Recovery

“…AAV vectors (AAV capsid, CpG containing AAV genome) were shown to trigger immune responses in mice, 42,43 and it is rather surprising that the immune response transcripts are dampened by the administration of AAV8-Null control vector ( Figure 4A). Following .…”

Alginate hydrogel polymers enable efficient delivery of a vascular-targeted AAV vector into aortic tissue

“…Cellular and humoral immunity induced by AAV vectors play an increasing role in clinical translation. 40 , 41 As analysis of cellular immunity against AAV capsids in mice is not predictive for patients, 40 we focused on measuring nAbs in serum samples of mice. Interestingly, both AAV9SLR hydrogel implantation and high-titer systemic injections of AAV9SLR resulted in induction of high nAb titers, indicating that humoral immunity is not reduced by hydrogel-supported delivery.…”