Table of Contents

OVERVIEW ON FOAMY RETROVIRUSES

Foamy viruses (FV), also designated spuma- or spumaretroviruses are the least studied group within the family of the Retroviridae. During the last years, interest in FVs increased for certain reasons. First, FVs are considered promising vectors for the targeted delivery and expression of therapeutic genes or antigens, e.g. for the transduction of human hematopoietic stem cells or as vaccine vectors. Secondly, the FVs of wild-ranging and captive non-human primates, the simian FVs (SFV), have repeatedly been shown to cross host-range barriers resulting in zoonotic transmission to humans. Finally, various features of the FV replication pathway, morphogenesis, and particle structure are different from those of the other retroviruses. These FV-specific differences even resulted in a novel systematic placement within the family of the Retroviridae that is now subdivided into two major subfamilies: the Spumavirinae and the Orthoretrovirinae which comprise all other known retroviruses.

Like human immuno-deficiency and human T-cell leukemia virus (HIV and HTLV), FVs are complex retroviruses in terms of their genetic make-up, their strategies of gene expression, and their interactions with the infected hosts and the host cells. However, besides these apparent similarities, FVs have several unique features that set them clearly apart from the classical oncoretroviruses and the lentiviruses: In addition to the gag, pol and env genes and the regulatory elements located in both LTRs which are common to all retroviruses, the FV genome contains up to three additional open reading frames (bel1 to bel3) that are expressed predominantly by the internal promoter (IP) localized in the env gene. The presence of a functionally active and essential IP is a distinguishing feature of FVs. Both FV promoters, the LTR-promoter and the IP require the expression of the Bel1 post-transcriptional transactivator for high-level gene expression and infectivity. Whereas the activities of Bel1 are well established, the function of the abundantly expressed, cytoplasmic Bet protein that consists of bel1 and bel2 sequences is not fully understood and thus studied in the lab.

Unlike other retroviruses, the Pol proteins are expressed independently of Gag from a spliced, sub-genomic mRNA. Furthermore, the FV Pol precursor is incompletely cleaved leading to the unique situation in FVs of a protease (PR) - RT fusion protein detectable in infected cells and virions. Another unique feature is the onset of reverse transcription of the genomic RNA during the viral life cycle that occurs within the virus-producing cells before release rather than after entry of the new host cell. Thus, a substantial portion of released, FV particles already contains proviral DNA genomes.

In contrast to the other retroviruses, FV Env proteins are absolutely required for particle budding most probably due to a highly specific interaction between a distinct structure formed by the N-terminal Gag sequences, the FV MA-layer and the novel Env leader protein that is a component of released FV particles.

FVs have a wide tissue tropism and can be propagated in vitro in a variety of cell types from different species. The large range of cells permissive for FV infections probably reflects the ubiquity of the viral receptor. In cell cultures, FVs are often highly cytophatic/lytic. In naturally infected hosts, FVs cause a life-long persistent infection without obvious disease symptoms. Due to this apparently benign persistence in the naturally infected host, FVs are considered non-pathogenic. Although apparently a-pathogenic under natural conditions, FVs possess a significant disease potential as determined in transgenic mice. However, transgenic animals can be only considered as an experimental system that may not reflect the authentic situation.



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PRESENT RESEARCH PROJECTS

1. Construction and characterization of FV-based vectors for targeted gene delivery and vaccination
Different replication-competent and replication-deficient vectors based on the genome of the feline FV have been established and analyzed in cell cultures and in cats, our animal model to determine the efficiency and applicability of these vectors. Recent data show that i. replication-competent vaccine vectors actually induce a partially protective immunity in cats whereas ii. third-generation replication-deficient vectors display a high degree of biological safety and allow long-term expression of a heterologous gene. We are actually developing novel vectors for therapeutic application in defined human cancer(s).



Click to enlarge the figure


2. FV morphology and morphogenesis with special emphasis on particle budding and the molecular interaction between Env and Gag proteins
During the past years, our ultra-structural, biochemical and virological investigations on FVs have gained new insights into i. the composition of virus particles (FVs contain a unique glycoprotein product), ii. the spatial organization of structural proteins (identification of the novel matrix-layer by cryo-electron-microscopy), and iii. the interaction of defined domains in Gag and Env essential for FV particle budding. These data are presently used to achieve pseudotyping of foamy virus vector particles and to understand the mechanisms and cellular requirements for particle budding.




Click to enlarge the figure


3. Determination of the zoonotic potential of FVs from cats, cattle and horses
Transmission of animal viruses to humans (zoonoses) has been regularly observed for different simian and ape FVs. Such zoonotic events pose serious threads to human health due to the emergence of novel human viruses. At present, almost nothing is known whether FVs from cats, cattle, and horses are transmitted to humans. We presently establish high-throughput screening systems to detect such events. In addition, the biology, spread and replication of feline and bovine FVs is studied in their natural hosts.



Click to enlarge the figure


4. Characterization of cellular anti-viral restriction factors and identification of FV proteins counteracting these defense mechanisms
Recent studies have demonstrated that the replication of several retroviruses is, at least in part, restricted by several independent cellular anti-viral factors. We obtained strong evidence that similar mechanisms are also restricting FV replication and that, in response, FVs have developed specific means to overcome this restriction. We are underway to molecularly define the underlying proteins and mechanisms.

PRESENT RESEARCH PROJECTS

1. Construction and characterization of FV-based vectors for targeted gene delivery and vaccination
Different replication-competent and replication-deficient vectors based on the genome of the feline FV have been established and analyzed in cell cultures and in cats, our animal model to determine the efficiency and applicability of these vectors. Recent data show that i. replication-competent vaccine vectors actually induce a partially protective immunity in cats whereas ii. third-generation replication-deficient vectors display a high degree of biological safety and allow long-term expression of a heterologous gene. We are actually developing novel vectors for therapeutic application in defined human cancer(s).

2. FV morphology and morphogenesis with special emphasis on particle budding and the molecular interaction between Env and Gag proteins
During the past years, our ultra-structural, biochemical and virological investigations on FVs have gained new insights into i. the composition of virus particles (FVs contain a unique glycoprotein product), ii. the spatial organization of structural proteins (identification of the novel matrix-layer by cryo-electron-microscopy), and iii. the interaction of defined domains in Gag and Env essential for FV particle budding. These data are presently used to achieve pseudotyping of foamy virus vector particles and to understand the mechanisms and cellular requirements for particle budding.

3. Determination of the zoonotic potential of FVs from cats, cattle and horses
Transmission of animal viruses to humans (zoonoses) has been regularly observed for different simian and ape FVs. Such zoonotic events pose serious threads to human health due to the emergence of novel human viruses. At present, almost nothing is known whether FVs from cats, cattle, and horses are transmitted to humans. We presently establish high-throughput screening systems to detect such events. In addition, the biology, spread and replication of feline and bovine FVs is studied in their natural hosts.

4. Characterization of cellular anti-viral restriction factors and identification of FV proteins counteracting these defense mechanisms
Recent studies have demonstrated that the replication of several retroviruses is, at least in part, restricted by several independent cellular anti-viral factors. We obtained strong evidence that similar mechanisms are also restricting FV replication and that, in response, FVs have developed specific means to overcome this restriction. We are underway to molecularly define the underlying proteins and mechanisms.