The adeno-associated virus type 2 (AAV) genome can be successfully rescued from recombinant plasmids following transfection in adenovirus-infected human cells. However, following rescue, the AAV genome undergoes preferential replication and encapsidation, whereas little replication and packaging of the vector DNA sequences occur. In view of the crucial role in the rescue, replication, and packaging of the proviral genome played by the AAV inverted terminal repeats (ITRs), which consist of a palindromic hairpin (HP) structure and a 20-nucleotide stretch, designated the D-sequence, that is not involved in the HP formation, we evaluated the involvement of the individual ITRs as well as their components in the selective viral DNA replication and encapsidation. A number of recombinant AAV plasmids that contained deletions-substitutions in different regions of the individual ITRs were constructed and examined for their potential to allow rescue, replication, and/or packaging in adenovirus-infected human cells in vivo. The results reported here document that (i) two HP structures and one D-sequence are sufficient for efficient rescue and preferential replication of the AAV DNA, (ii) two HP structures alone allow a low-level rescue and replication of the AAV DNA, but rescue and replication of the vector DNA sequences also occur in the absence of the D-sequences, (iii) one HP structure and two D-sequences, but not one HP structure and one D-sequence, also allow rescue and replication of the AAV as well as the vector DNA sequences, (iv) one HP structure alone or two D-sequences, but not one D-sequence alone, allow replication of the full-length plasmid DNA, but no rescue of the AAV genome occurs, (v) no rescue-replication occurs in the absence of the HP structures and the D-sequences, (vi) in the absence of the D-sequences, the HP structures are insufficient for successful encapsidation of the AAV genomes, and (vii) the AAV genomes containing only one ITR structure can be packaged into biologically active virions. Thus, the D-sequence plays a crucial role in the efficient rescue and selective replication and encapsidation of the AAV genome. Furthermore, the D-sequence specifically interacts with a hitherto unknown host-cell protein that we have designated the D-sequence-binding protein (D-BP). These studies illustrate that the D-sequence-D-BP interaction constitutes an important step in the AAV life cycle.

Parvovirus B19 infection leads to transient aplastic crises in individuals with chronic hemolytic anemias or immunodeficiency states. An additional unexplained sequela of B19 infection is thrombocytopenia. Because B19 is known to have a remarkable tropism for human erythropoietic elements, and is not known to replicate in nonerythroid cells, the etiology of this thrombocytopenia is uncertain. We sought to define the pathobiology of B19-associated thrombocytopenia by examining the role of B19 on in vitro megakaryocytopoiesis. B19 infection of normal human bone marrow cells significantly suppressed megakaryocyte (MK) colony formation compared with mock-infected cells. No such inhibition was observed with a nonpathogenic human parvovirus, the adeno-associated virus 2 (AAV). The B19-MK cell interaction was also studied at the molecular level. Whereas low-density bone marrow cells containing erythroid precursor cells supported B19 DNA replication, no viral DNA replication was observed in B19-infected MK-enriched fractions as determined by the presence of viral DNA replicative intermediates on Southern blots. However, analysis of total cytoplasmic RNA isolated from B19-infected MK fractions showed a low-level expression of the B19 genome as detected by quantitative RNA dot blots as well as by Northern analysis. Furthermore, a frame-shift mutation in a recombinant AAV-B19 hybrid genome segment that encodes the viral nonstructural (NS1) protein significantly reduced the observed inhibition of MK colony formation. These studies indicate tissue- tropism of B19 beyond the erythroid progenitor cell, and lend support to the hypothesis that B19 genome expression may be toxic to cell populations that are nonpermissive for viral DNA replication.

The adeno-associated virus type 2 (AAV) genome contains inverted terminal repeats (ITRs) of 145 nucleotides. The terminal 125 nucleotides of each ITR form palindromic hairpin (HP) structures that serve as primers for AAV DNA replication. These HP structures also play an important role in integration as well as rescue of the proviral genome from latently infected cells or from recombinant AAV plasmids. Each ITR also contains a stretch of 20 nucleotides, designated the D sequence, that is not involved in HP structure formation. We have recently shown that the D sequence plays a crucial role in high-efficiency rescue, selective replication, and encapsidation of the AAV genome and that a host cell protein, designated the D sequence-binding protein (D-BP), specifically interacts with this sequence (X.-S. Wang, S. Ponnazhagan, and A. Srivastava, J. Virol. 70:1668-1677, 1996). We have now performed mutational analyses of the D sequences to evaluate their precise role in viral DNA rescue, replication, and packaging. We report here that 10 nucleotides proximal to the HP structure in each of the D sequences are necessary and sufficient to mediate high-efficiency rescue, replication, and encapsidation of the viral genome in vivo. In in vitro studies, the same 10 nucleotides were found to be required for specific interaction with D-BP, but viral Rep protein-mediated cleavage at the functional terminal resolution site is independent of these sequences. These data suggest that AAV replication and terminal resolution functions can be uncoupled and that the lack of efficient replication of AAV DNA may not be a consequence of impaired resolution of the viral ITRs. These studies further illustrate that the D sequence-D-BP interaction plays an important role in the AAV life cycle and indicate that it may be possible to develop the next generation of AAV vectors capable of encapsidating larger pieces of DNA.

The pathogenic human parvovirus B19 has been shown to undergo productive replication in the erythroid lineage in primary normal human hematopoietic progenitor cells. However, none of the established erythroleukemia cell lines has allowed B19 virus replication in vitro. The remarkable erythroid tissue tropism of B19 virus was evaluated with a human megakaryocytic leukemia cell line, MB-02, which is dependent on the growth factor granulocyte-macrophage colony-stimulating factor but can be induced to undergo erythroid differentiation following treatment with erythropoietin (Epo). Whereas these cells did not support B19 virus DNA replication in the presence of granulocyte-macrophage colony-stimulating factor alone, active viral DNA replication was observed if the cells were exposed to Epo for 5 to 10 days prior to B19 virus infection, as detected by the presence of the characteristic B19 virus DNA replicative intermediates on Southern blots. No replication occurred if the cells were treated with Epo for 3 days or less. In addition, complete expression of the B19 virus genome also occurred in Epo-treated MB-02 cells, as detected by Northern blot analysis. B19 progeny virions were released into culture supernatants that were biologically active in secondary infection of nonnal human bone marrow cells. The availability of the only homogeneous permanent cell line in which induction of erythroid differentiation leads to a permissive state for B19 virus replication in vitro promises to yield new and useful information on the molecular basis of the erythroid tissue tropism as well as parvovirus B19-induced pathogenesis.

The adeno-associated virus 2 (AAV) contains a single-stranded DNA genome of which the terminal 145 nucleotides are palindromic and form T-shaped hairpin structures. These inverted terminal repeats (ITRs) play an important role in AAV DNA replication and resolution, since each of the ITRs contains a terminal resolution site (trs) that is the target site for the AAV rep gene products (Rep). However, the Rep proteins also interact with the AAV DNA sequences that lie outside the ITRs, and the ITRs also play a crucial role in excision of the proviral genome from latently infected cells or from recombinant AAV plasmids. To distinguish between Rep-mediated excision of the viral genome during rescue from recombinant AAV plasmids and the Repmediated resolution of the ITRs during AAV DNA replication, we constructed recombinant AAV genomes that lacked either the left or the right ITR sequence and one of the Rep-binding sites (RBSs). No rescue and replication of the AAV genome occurred from these plasmids following transfection into adenovirus type 2-infected human KB cells, as expected. However, excision and abundant replication of the vector sequences was clearly detected from the plasmid that lacked the AAV left ITR, suggesting the existence of an additional putative excision site in the left end of the AAV genome. This site was precisely mapped to one of the AAV promoters at map unit 5 (AAV p5) that also contains an RBS. Furthermore, deletion of this RBS abolished the rescue and replication of the vector sequences. These studies suggest that the Rep-mediated cleavage at the RBS during viral DNA replication may, in part, account for the generation of the AAV defective interfering particles.