We describe a method, based on single-molecule imaging, that allows the real-time visualization of the infection pathway of single viruses in living cells, each labeled with only one fluorescent dye molecule. The tracking of single viruses removes ensemble averaging. Diffusion trajectories with high spatial and time resolution show various modes of motion of adeno-associated viruses (AAV) during their infection pathway into living HeLa cells: (i) consecutive virus touching at the cell surface and fast endocytosis; (ii) free and anomalous diffusion of the endosome and the virus in the cytoplasm and the nucleus; and (iii) directed motion by motor proteins in the cytoplasm and in nuclear tubular structures. The real-time visualization of the infection pathway of single AAVs shows a much faster infection than was generally observed so far.

The human parvovirus adeno-associated virus type 2 (AAV2) has many features that make it attractive as a vector for gene therapy. However, the broad host range of AAV2 might represent a limitation for some applications in vivo, because recombinant AAV vector (rAAV)-mediated gene transfer would not be specific for the tissue of interest. This host range is determined by the binding of the AAV2 capsid to specific cellular receptors and/or co-receptors. The tropism of AAV2 might be changed by genetically introducing a ligand peptide into the viral capsid, thereby redirecting the binding of AAV2 to other cellular receptors. We generated six AAV2 capsid mutants by inserting a 14-amino-acid targeting peptide, L14, into six different putative loops of the AAV2 capsid protein identified by comparison with the known three-dimensional structure of canine parvovirus. All mutants were efficiently packaged. Three mutants expressed L14 on the capsid surface, and one efficiently infected wild-type AAV2-resistant cell lines that expressed the integrin receptor recognized by L14. The results demonstrate that the AAV2 capsid tolerates the insertion of a nonviral ligand sequence. This might open new perspectives for the design of targeted AAV2 vectors for human somatic gene therapy.

The unique region of the VP1 protein of parvoviruses was proposed to contain a parvoviral phospholipase A2 (pvPLA2) motif. Here, PLA2 activity is shown in the unique region of adeno-associated virus type 2 (AAV-2) VP1 when expressed as an isolated domain in bacteria. Mutations in this region of the capsid protein strongly reduced the infectivity of mutant virions in comparison to wild-type AAV-2. This correlated with effects on the activity of PLA2. The mutations had no influence on capsid assembly, packaging of viral genomes into particles or binding to and entry into HeLa cells. However, a delayed onset and reduced amount of early gene expression, as measured by Rep immunofluorescence, was observed. These results suggest that pvPLA2 activity is required for a step following perinuclear accumulation of virions but prior to early gene expression.

Improving the efficiency and specificity of gene vectors is critical for the success of gene therapy. In an effort to generate viral mutants with controlled tropism we produced a library of adeno-associated virus (AAV) clones with randomly modified capsids and used it for the selection of receptor-targeting mutants. After several rounds of selection on different cell lines that were resistant to infection by wild-type (wt) AAV, infectious mutants were harvested at high titers. These mutants transduced target cells with an up to 100-fold increased efficiency, in a receptor-specific manner and without interacting with the primary receptor for wt AAV. The results demonstrate for the first time that a combinatorial approach based on a eukaryotic virus library allows one to generate efficient, receptor-specific targeting vectors with desired tropism.

Recombinant adeno-associated virus type 2 (rAAV2) is a promising vector for human somatic gene therapy. However, its broad host range is a disadvantage for some applications, because it reduces the specificity of the gene transfer. To overcome this limitation, we sought to create a versatile rAAV vector targeting system which would allow us to redirect rAAV binding to specific cell surface receptors by simple coupling of different ligands to its capsid. For this purpose, an immunoglobulin G (IgG) binding domain of protein A, Z34C, was inserted into the AAV2 capsid at amino acid position 587. The resulting AAV2-Z34C mutants could be packaged and purified to high titers and bound to IgG molecules. rAAV2-Z34C vectors coupled to antibodies against CD29 (␤ 1 -integrin), CD117 (c-kit receptor), and CXCR4 specifically transduced distinct human hematopoietic cell lines. In marked contrast, no transduction was seen in the absence of antibodies or in the presence of specific blocking reagents. These results demonstrate for the first time that an immunoglobulin binding domain can be inserted into the AAV2 capsid and coupled to various antibodies, which mediate the retargeting of rAAV vectors to specific cell surface receptors.The human parvovirus adeno-associated virus type 2 (AAV2) has many features that make it attractive as a vector for human somatic gene therapy (9, 11). However, its broad host range might represent a limitation for some applications, because recombinant AAV (rAAV)-mediated gene transfer would not be specific for the tissue or cell type of interest. The host range is determined by the interaction of the AAV2 capsid with specific cellular receptors and coreceptors (18,26,27).Recently, a hypothetical model of the AAV capsid was generated, and several regions which were exposed on the viral capsid accepted the insertion of an integrin-specific 14-aminoacid (aa) RGD ligand (L14) and bound to target cells expressing the corresponding receptor (6). Moreover, AAV2 vectors with a ligand insertion at site 587 infected wild-type AAVresistant B16F10 melanoma cells with infectious targeting titers of 5 ϫ 10 4 LacZ expression-forming units (EFU) per ml (multiplicity of infection, 1), indicating that the susceptibility of these cells to AAV2 infection was increased by at least 4 orders of magnitude (6).However, with this approach it remained difficult and laborious to generate targeting vectors, because the design and optimization of new AAV capsid mutants were required for each specific receptor and cell type. Thus, it seemed desirable to generate a universal AAV targeting capsid on which different ligands could bind and redirect the virus to specific cell surface receptors (Fig. 1A). Such a vector would allow rapid screening of appropriate receptors mediating virus binding, uptake, and correct intracellular processing, which are all prerequisites for successful retargeting of AAV-based vectors.For this purpose, an immunoglobulin G (IgG) binding domain was introduced into the capsid to enable AAV to bind different ...

Adeno-associated virus (AAV) is a promising vector for human somatic gene therapy. However, its broad host range is a disadvantage for in vivo gene therapy, because it does not allow the selective tissue-or organ-restricted transduction required to enhance the safety and efficiency of the gene transfer. Therefore, increasing efforts are being made to target AAV-2-based vectors to specific receptors. The studies summarized in this review show that it is possible to target AAV-2 to a specific cell. So far, the most promising approach is the genetic modification of the viral capsid. However, the currently available AAV-2 targeting vectors need to be improved with regard to the elimination of the wild-type AAV-2 tropism and the improvement of infectious titers. The creation of highly efficient AAV-2 targeting vectors will also require a better understanding of the transmembrane and intracellular processing of this virus.

Approach, contact, retreat; approach, contact, penetrate. With a number of fleeting subsecond contacts, the single virus moves across the cellular membrane (paths 1, 2 in the graphic) before it is suddenly engulfed (path 3). Although only a few viruses are successful, once inside they enter the nucleus in a matter of minutes (path 4), much faster than previously thought. Single virus tracing, the application of real‐time single‐molecule techniques to this biomedical problem, enables us to view the live scenes of viral infection with high spatial and temporal resolution and under ideal physiological conditions in a living cell. In this way, the “movie script” of a viral infection is obtained.

On page 1052, the last two column headings in Table 1 lacked the correct superscripting. The correct column headings are "particles/ml a " and A20+capsids/ml b ." On page 1053, the column headings for Table 2 were shifted. The correct table is printed below.