It's all about the clothing: capsid domination in the adeno‐associated viral vector world

Abstract

The Chinese have an old saying, Ôdon't judge a person by his clothingÕ, equivalent to Ôdon't judge a book by its coverÕ. For years, scientists in the gene transfer field have focused on the contents/nucleic acid sequences as the major determinants for vector performance. A series of recent publications indicate that the truth might be otherwise: ÔclothingÕ does matter when it comes to adeno-associated viral (AAV) vectors.Vector biologists have long categorized vectors according to their nucleic acid content, and have adopted them unchanged. Retroviral and lentiviral vectors are RNA-based vectors; adenoviral and herpes viral vectors are double-stranded DNA vectors; while parvoviral (AAV) vectors contain singlestranded DNA. The rate of vector transduction and the success of long-term expression of vectors have been mainly attributed to the ways the viral genomes integrate into the host cells.In the case of AAV vectors, AAV serotype 2 (AAV2) has been one of the few serotypes that has been exploited as a gene therapy vector since 1999, even though several AAV serotypes have been known and characterized. The discovery that AAV1-based vectors performed greater than tenfold better in skeletal muscle than the canonical AAV2 [1] sparked an interest in studying alternate AAV serotypes. To date, there are 11 wellcharacterized AAV serotypes [2] and many more under investigation (partial representation shown in Fig. 1). Theoretically, the ability to generate pseudotyped recombinant AAV by switching capsid or DNA [inverted terminal repeats (ITRs)] sequences from different AAV serotypes allows the creation of an unlimited number of novel vectors. However, attempts by Kay and colleagues to improve liver-directed AAV gene transfer efficiency by modifying the vector DNA (i.e. ITRs) among serotypes AAV1 to AAV6 were unsuccessful, [3] whereas switching vector capsid significantly influences vector performance [2,4,5].Cumulative evidence from preclinical and clinical studies using AAV vectors has demonstrated that the vector capsid is not only critical in determining the tropism and transduction efficiency, but also is a major antigen whose interaction with host immune responses will determine both the ability of the vector to access the target tissue and the resulting duration of expression.The work by VandenDriessche and colleagues in this issue of the Journal [6], together with recent reports [7][8][9], characterizes a new player in the AAV arena: AAV9 isolated from human tissues, another vivid example of what a capsid change can achieve in gene transfer experiments. These authors demonstrate that, similar to AAV8, AAV9 is effective in transducing hepatocytes of hemophilia B mice, which resulted in supraphysiological levels of biologically active factor (F) IX. These data are in agreement with early data reported by Haig Kazazian's group in hemophilia A mice and dogs, which documented long-term expression of therapeutic levels of canine FVIII following portal vein injection of AAV9 at levels comparable with those of a dog injecte...

LuIII Parvovirus Selectively and Efficiently Targets, Replicates in, and Kills Human Glioma Cells

“…This is consistent with previous studies showing the critical importance of parvoviral capsids at determining host cell range, evidently operating at a postentry, pregene-expression step early in the viral life cycle (6,12,36). The role of the autonomous parvoviral capsid in governing species and tissue tropism is thus analogous to the key role that the capsid of parvovirus adeno-associated virus (AAV) plays in defining its tissue tropism (2). Although the LuIII capsid enhanced fitness of MVMp in human glioma MO59J, the entire LuIII genome, including untranslated regions and nonstructural protein genes, was required for a robust oncolysis of U373 and A172 gliomas.…”

Immune responses to AAV vectors: overcoming barriers to successful gene therapy

“…[57][58][59][60] Thus, although antibodies to AAV2 are clearly the most prevalent in humans (up to 70%), the natural host for this serotype, antibodies recognizing virtually all AAV serotypes can be found in a large proportion of individuals. Among the most commonly used AAV vectors, antibodies to AAV5, carrying one of the leastconserved capsid sequences, 61 and to AAV8, 56 are among the least prevalent.…”

Gene Therapy for Coagulation Disorders

“…56,70,71 This complication is the result of CD8 + cytotoxic T cell responses to AAV capsid peptides being presented on the surface of transduced hepatocytes ( Figure 4A). The timing of the subsequent transaminitis that develops has varied from as early as 3 weeks to as late as 10 weeks post vector delivery, probably reflecting either a secondary recall response or a later primary immune response to the AAV capsid.…”

“…In each instance, strong tissue-specific promoter/enhancer 56 Thus, AAV8 possesses strong hepatotropism and can be delivered efficiently to the liver via peripheral vein infusion without the risk of significant transduction of other tissue types.…”

“…[56][57][58] The vector particles are taken into endocytic vesicles, where, in the acidic milieu of the endosome, the capsid is removed and the nucleic acid released for transport to the nucleus. 59 The capsid proteins are then degraded by the proteasome, and the resultant peptides presented on the transduced cell's surface, in association with major histocompatibility complex class I molecules ( Figure 4B).…”