Myxomatosis in European rabbits is a severely debilitating disease characterized by profound systemic cellular immunosuppression and a high rate of mortality. The causative agent, myxoma virus, is a member of the poxvirus family and prototype of the Leporipoxvirus genus. As a major step toward defining the genetic strategies by which the virus circumvents host antiviral responses, the genomic DNA sequence of myxoma virus, strain Lausanne, was determined. A total of 171 open reading frames were assigned to cover the 161.8-kb genome, including two copies each of the 12 genes that map within the 11.5-kb terminal inverted repeats. Database searches revealed a central core of approximately 120 kb that encodes more than 100 genes that exhibit close relationships to the conserved genes of members of other poxvirus genera. Open reading frames with predicted signal sequences, localization motifs, or homology to known proteins with immunomodulatory or host-range functions were examined more extensively for predicted features such as hydrophobic regions, nucleic acid binding domains, ankyrin repeats, serpin signatures, lectin domains. and structural cysteine spacings. As a result, several novel, potentially immunomodulatory proteins have been identified, including a family with multiple ankyrin-repeat domains, an OX-2 like member of the neural cell adhesion molecule family, a third myxoma serpin, a putative chemokine receptor fragment, two natural killer receptor-like species, and a variety of species with domains closely related to diverse host immune regulatory proteins. Coupled with the genomic sequencing of the related leporipoxvirus Shope fibroma virus, this work affirms the existence of a conserved complement of poxvirus-specific core genes and expands the growing repertoire of virus genes that confer the unique capacity of each poxvirus family member to counter the immune responses of the infected host.

Murine gammaherpesvirus 68 (␥HV68; also known as MHV-68) can establish a latent infection in both inbred and outbred strains of mice and, as such, provides a tractable small-animal model to address mechanisms and cell types involved in the establishment and maintenance of chronic gammaherpesvirus infection. Latency can be established at multiple anatomic sites, including the spleen and peritoneum; however, the contribution of distinct cell types to the maintenance of latency within these reservoirs remains poorly characterized. B cells are the major hematopoietic cell type harboring latent ␥HV68. We have analyzed various splenic B-cell subsets at early, intermediate, and late times postinfection and determined the frequency of cells either (i) capable of spontaneously reactivating latent ␥HV68 or (ii) harboring latent viral genome. These analyses demonstrated that latency is established in a variety of cell populations but that long-term latency (6 months postinfection) in the spleen after intranasal inoculation predominantly occurs in B cells. Furthermore, at late times postinfection latent ␥HV68 is largely confined to the surface immunoglobulin D-negative subset of B cells.Members of the gammaherpesvirus subfamily represent important pathogens that infect a wide variety of mammalian species, including humans. Representative biological characteristics include the ability to establish a latent infection within lymphocytes and association with lymphomas and lymphoproliferative diseases. ␥HV68 is closely related to the human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (also known as human herpesvirus 8) (11,12,44). Infection of mice with ␥HV68 provides a tractable model system for characterizing fundamental aspects of gammaherpesvirus infection, pathobiology, host control, and tumor induction (for reviews, see references 9, 10, 25, 26, 30, 31, 34, and 43). As such, it is a potentially valuable tool for the development of strategies to disrupt or prevent the establishment of latency by gammaherpesviruses.␥HV68 was originally isolated from a bank vole (Clathriomys glariolum), is a natural pathogen of free-living murid rodents (6), and is able to infect both inbred and outbred mice (6,23,28,36). Inbred mice inoculated with ␥HV68 establish productive virus replication in the lung (36) and establish a latent infection in the spleen (36,37,45). Establishment of splenic latency is characterized by a transient phase of CD4 ϩ -T-celldependent splenomegaly involving proliferation of both B and T lymphocytes (13,33,36,41) and by lymph node enlargement, which peaks 2 to 3 weeks postinoculation. The virus is also able to establish a latent infection in the thymus (28) and bone marrow (47, 48) and may also establish a persistent infection in the lungs (35).Analogous to EBV infection, B lymphocytes appear to be the major reservoir harboring latent ␥HV68 (14, 37), although macrophages (14, 48), splenic dendritic cells (14), and lung epithelial cells (35) have been shown to harbor laten...

The gammaherpesviruses include the human pathogens Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV, or human herpesvirus 8). These viruses establish life-long infection of the host and are associated with a number of malignancies. To better understand gammaherpesvirus pathogenesis, we and others have studied infection of mice with murine gammaherpesvirus 68 (␥HV68, also referred to as MHV-68), a member of the ␥ 2 -herpesvirus family based on genome sequence (17).The pathogenesis of ␥HV68 has been reviewed recently (32,41). Briefly, ␥HV68 infection of inbred mice results in an acute, productive infection of multiple organs and a CD4 ϩ

We have determined the complete DNA sequence of the Leporipoxvirus Shope fibroma virus (SFV). The SFV genome spans 159.8 kb and encodes 165 putative genes of which 13 are duplicated in the 12.4-kb terminal inverted repeats. Although most SFV genes have homologs encoded by other Chordopoxvirinae, the SFV genome lacks a key gene required for the production of extracellular enveloped virus. SFV also encodes only the smaller ribonucleotide reductase subunit and has a limited nucleotide biosynthetic capacity. SFV preserves the Chordopoxvirinae gene order from S012L near the left end of the chromosome through to S142R (homologs of vaccinia F2L and B1R, respectively). The unique right end of SFV appears to be genetically unstable because when the sequence is compared with that of myxoma virus, five myxoma homologs have been deleted (C. Cameron, S. Hota-Mitchell, L. Chen, J. Barrett, J.-X. Cao, C. Macaulay, D. Willer, D. Evans, and G. McFadden, 1999, Virology 264, 298-318). Most other differences between these two Leporipoxviruses are located in the telomeres. Leporipoxviruses encode several genes not found in other poxviruses including four small hydrophobic proteins of unknown function (S023R, S119L, S125R, and S132L), an alpha 2, 3-sialyltransferase (S143R), a protein belonging to the Ig-like protein superfamily (S141R), and a protein resembling the DNA-binding domain of proteins belonging to the HIN-200 protein family S013L). SFV also encodes a type II DNA photolyase (S127L). Melanoplus sanguinipes entomopoxvirus encodes a similar protein, but SFV is the first mammalian virus potentially capable of photoreactivating ultraviolet DNA damage.

During poxvirus infection, both viral genomes and transfected DNAs are converted into high-molecular-weight concatemers by the replicative machinery. However, aside from the fact that concatemer formation coincides with viral replication, the mechanism and protein(s) catalyzing the reaction are unknown. Here we show that vaccinia virus DNA polymerase can catalyze single-stranded annealing reactions in vitro, converting linear duplex substrates into linear or circular concatemers, in a manner directed by sequences located at the DNA ends. The reaction required > or =12 bp of shared sequence and was stimulated by vaccinia single-stranded DNA-binding protein (gpI3L). Varying the structures at the cleaved ends of the molecules had no effect on efficiency. These duplex-joining reactions are dependent on nucleolytic processing of the molecules by the 3'-to-5' proofreading exonuclease, as judged by the fact that only a 5'-(32)P-end label is retained in the joint molecules and the reaction is inhibited by dNTPs. The resulting concatemers are joined only through noncovalent bonds, but can be processed into stable molecules in E. coli, if the homologies permit formation of circular molecules. This reaction provides a starting point for investigating the mechanism of viral concatemer formation and can be used to clone PCR-amplified DNA.