The evolution of defective and autonomous parvoviruses.

Because of the small size and genetic simplicity of small DNA viruses, parvoviruses would appear to be excellent models for studying viral evolution and adaptation. In an earlier publication we hypothesized the evolution of sequences of cellular "junk" DNA into protective interfering transposons. These transposons would interfere with invading pathogenic viruses by competing with the pathogen DNA for replicative enzymes. We speculated that a small, defective parvovirus, the adeno-associated virus (AAV), which usually requires the presence of a pathogenic helper virus to replicate, may have evolved from such a piece of cellular "junk" DNA. Our theory predicted that AAVs, as a consequence of their defective nature, developed under pressures favoring maintenance of their transposon like qualities. In contrast, disease-causing, autonomous, non-defective parvoviruses such as the B19 agent of humans and the canine parvovirus, even though their origins may have been in cellular DNA, would appear to have developed under totally different evolutionary pressures. In this paper we will present evidence for a common ancestry for the defective and autonomous parvoviruses and discuss the divergent paths this evolution may have taken in establishing the two genera.

Evolution of a defective virus from a cellular defense mechanism.

Adeno-associated virus is a defective DNA virus, requiring the presence of a helper virus in order to replicate. In this paper we consider its origin in light of several observations, most notably the following: its own replication inhibits that of the helper virus; its DNA structure resembles that of transposable (moveable) elements; and extrachromosomal circles of DNA, about the size of adeno-associated virus DNA, have been found recently in eukaryotic cells. We have arrived at a hypothesis consisting of two main ideas: (1) that cells may use transposable DNA as a mechanism of defense against viral attack, and (2) that adeno-associated virus may have evolved directly from this cellular defense mechanism.

Defective parvoviruses acquired via the transplacental route protect mice against lethal adenovirus infection.

Adeno-associated virus type 1 (AAV-1) interfered with the replication of its murine adenovirus (MAV) helper in primary mouse kidney cells and in 1-day-old ICR mice. Mice carrying AAV-1 acquired via the transplacental route were protected against lethal infection with MAV. The replication of AAV-1 in these mice could be triggered by multiple challenges with MAV, and antibodies to AAV-1 were subsequently detected.

Characterization of heavy particles of adeno-associated virus type 1.

The temperature-sensitive mutant ts4 of adenovirus type 2 (Ad-2) is capable of complementing adeno-associated virus type 1 (AAV-1) in HEp2, KB and HEK cells at 34 degrees C and 39 degrees C when used as a helper virus. Heavy non-infectious AAV-1 particles can be generated by using the mutant ts4 in HEp2 cells. When AAV-1 is grown in serial passages in HEp2 cells, both the wild-type Ad-2 and the mutant ts4 give rise to heavy, less infectious AAV-1 particles. The heavy AAV-1 particles generated by Ad-2 in advanced serial passages retain the property of having CF and IF antigens, but the AAV-1 generated by the mutant in advanced serial passages lose this property. There is no appreciable difference in the particle counts made by electron microscopy of AAV-1 preparations generated either by Ad-2 or the mutant ts4. Analysis by polyacrylamide gel electrophoresis of purified heavy AAV generated by ts4 indicates that in late passage an additional polypeptide of higher mol. wt. than the three structural polypeptides is detected.

Properties of adeno-associated virus (type 1) replicated in rodent cells by murine adenovirus.

We report for the first time the replication of infectious adeno-associated virus type 1 (AAV-1) in rodent cells [primary mouse kidney (PMK) and mouse L929 cells] using murine adenovirus (MAV) as a helper virus and also the production of AAV-I virus antigen by herpes simplex virus type I (HSV-I) with its temperature-sensitive mutant ts 200 in mouse neuroblastoma (NB) cells. The infectious AAV virions produced by MAV on L cells had a buoyant density of 1.41 2ml in caesium chloride gradients.

Transplacental infection with adeno-associated virus type 1 in mice.

Adeno-associated type 1 parvovirus (AAV) was detected in the kidneys and lungs of fetuses and newborns, when pregnant mice were injected subcutaneously with AAV type 1 and murine adenovirus as a helper virus. These findings clearly indicate that transplacental infection with AAV in rodents has been achieved.

Inhibition of the replication of parvovirus X14 by 5-iodo-2'-deoxyuridine pre-treatment of cell cultures.

Pre-treatment of rat embryo cell cultures with 5-iodo-2'-deoxyuridine (IdUrd) inhibits the replication of parvovirus X14. Reduced yields of haemagglutinating and infectious particles were observed. Adsorption of virus to cells was not affected, but both virus protein and DNA synthesis were inhibited. Fewer cells were capable of supporting protein or antigen synthesis as determined by immunofluorescence. Virus-specific DNA was detected in IdUrd pre-treated cells, but the amount synthesized was considerably less than that from control cultures. Cellular DNA synthesis was also inhibited in IdUrd pre-treated cells. Therefore, the replication of parvoviruses appears dependent upon host cell factors involved in cellular DNA synthesis.

Isoelectric focusing of parvoviruses.

Adenovirus-associated virus type 4 and X14 migrate during electrophoresis to pH 2.6 in sucrose-stabilized, pH 2.5-6.0 gradients. Naturally occurring empty capsids appear to have the same isoelectric point as complete virus particles.

Complementation of adeno-associated satellite virus (AAV) by temperature-sensitive mutants of adenovirus type 31.

Temperature-sensitive (ts) matants of human adenovirus type 31 were able to complement adeno-associated satellite virus (AAV) antigen production in both HEK and KB cells at both permissive and non-permissive temperatures. However, mutant ts 94, an adenovirus 31 mutant which produces apparently normal amounts of structural protein and DNA but is defective in maturation, was significantly inhibited in its ability to potentiate AAV infectivity at the non-permissive temperature. Normal AAV DNA and adenovirus DNA were isolated from co-infections with AAV and mutant ts 94 at the non-permissive temperature. We suggest that an adenovirus-coded maturation function common to both adenovirus and AAV maturation is defective in the ts 94 system.

Hamster embryo cells transformed by herpes simplex virus: reactions with adeno-associated satellite virus (AAV) and its adenovirus helpers.

We have studied the reactions of hamster embryo cells transformed by ultraviolet-inactivated herpes simplex type 2 (333-8-9 T cells) to infections with adeno-associated satellite virus (AAV) and its adenovirus helpers. Resident HSV structural antigens were not detectable in early or late passage of 333-8-9 T cells. AAV structural antigens were not detected in these cells unless the cells were coinfected with a helper adenovirus. In early passage 333-8-9 T cells were permissive to infections with simian adenovirus SV15 whereas normal hamster cell line LSH was nonpermissive. In some late passages of 333-8-9 T cells infections with SV15 adenovirus led to the production of viruslike particles whose morphology was identical with reoviruses.

Antibodies to adeno-associated satellite virus and herpes simplex in sera from cancer patients and normal adults.

The ecologic aspects of the distribution of adeno-associated satellite virus (ASV) in the human population are of great interest because of its unconditional defectiveness and dependence on adenovirus for full and herpesvirus for partial complementation. Adenoviruses and herpesviruses are extremely common and persistent infections in man. We have developed immunofluorescent procedures for detecting the presence of satellite virus antibodies in human sera. The percentage of sera with antibodies to the ASV 2-3 complex was significantly higher in the normal group than in the cancer patients whereas there were no significant differences in herpes antibodies between the groups. The low incidence of satellite antibodies was particularly striking in patients with genital malignancies. The role of ASV's in human disease is not known. Their role in possible abrogation of oncogenesis mediated through adenoviruses or herpesviruses is worthy of further investigation.

The replication of rodent parvovirus X14.

The replication of rodent parvovirus X14 DNA has been studied in rat embryo tissue culture cells. Virus DNA was isolated from I M-NaCl-SDS-pronase supernatant fluids from 24 h after infection. The majority of this DNA was 1-7 mum in length and double-stranded, indicating that it was an intermediate in the replication cycle of this single-stranded DNA virus. Single-stranded DNA of equivalent length was isolated directly from X14 virions. The buoyant density of this DNA was 1-728 g/ml whereas the double-stranded form banded at 1-714 g/ml in caesium chloride gradients. Difficulties in detecting significant amounts of single-stranded viral DNA directly from infected cells would appear to indicate that progeny single-stranded DNA is rapidly encapsidated after synthesis.