Human foamy virus DNA forms and expression in persistently infected Dami megakaryocytic cells

Potent neutralizing antibodies in humans infected with zoonotic simian foamy viruses target conserved epitopes located in the dimorphic domain of the surface envelope protein

Human diseases of zoonotic origin are a major public health problem. Simian foamy viruses (SFVs) are complex retroviruses which are currently spilling over to humans. Replication-competent SFVs persist over the lifetime of their human hosts, without spreading to secondary hosts, suggesting the presence of efficient immune control. Accordingly, we aimed to perform an in-depth characterization of neutralizing antibodies raised by humans infected with a zoonotic SFV. We quantified the neutralizing capacity of plasma samples from 58 SFV-infected hunters against primary zoonotic gorilla and chimpanzee SFV strains, and laboratory-adapted chimpanzee SFV. The genotype of the strain infecting each hunter was identified by direct sequencing of the env gene amplified from the buffy coat with genotype-specific primers. Foamy virus vector particles (FVV) enveloped by wild-type and chimeric gorilla SFV were used to map the envelope region targeted by antibodies. Here, we showed high titers of neutralizing antibodies in the plasma of most SFV-infected individuals. Neutralizing antibodies target the dimorphic portion of the envelope protein surface domain. Epitopes recognized by neutralizing antibodies have been conserved during the cospeciation of SFV with their nonhuman primate host. Greater neutralization breadth in plasma samples of SFV-infected humans was statistically associated with smaller SFV-related hematological changes. The neutralization patterns provide evidence for persistent expression of viral proteins and a high prevalence of coinfection. In conclusion, neutralizing antibodies raised against zoonotic SFV target immunodominant and conserved epitopes located in the receptor binding domain. These properties support their potential role in restricting the spread of SFV in the human population.

Replication of primate foamy viruses in natural and experimental hosts

Foamy viruses (FVs) are common apathogenic retroviruses readily spread by horizontal transmission in nonhuman primate and some other mammalian host populations. Primate FV infections have been known for half a century, i.e., 15 years before the definition of retroviruses and another 15 years before the detection of primate immune deficiency viruses. The emerging interest in human retroviruses included primate FV, and although the role of human hosts for FV was greatly overestimated temporarily, enthusiastic researchers compiled invaluable data on molecular biology and classic as well as molecular epidemiology of these viruses. It has been shown that lytic FV infection in a wide range of cell cultures is in great contrast to the silent state of the infection in animals. Once transmitted by saliva via biting, FVs reside in all tissues as DNA copies, but their replication is untraceable except in oral submucosal cells, which are thought to supply the virus for transmission. FVs have not definitely been associated with any disease, regardless of viral phylogenetic differences. Various primate and nonprimate species have been used for studies on the natural carrier state and primary infection. Experimental infections have mostly proven to be inefficient in primates as well as lower laboratory animals. However, investigation of the immune response in FV-infected animals has only partly explained the control of FV replication in the animal host. Thus, the biological role of FV remains an enigma to be resolved in the future.

Cell-type-specific regulation of the two foamy virus promoters

The foamy virus (FV) genome contains two promoters, the canonical long terminal repeat (LTR) promoter, containing three consensus AP-1 binding sites, and an internal promoter (IP) within the env gene. We investigated the regulation of the two promoters in lytic and persistent infections and found that in the presence of a constitutive source of the viral transactivator protein Tas, transactivation of the LTR promoter and that of the IP differ. In lytic infections, both the LTR promoter and the IP are efficiently transactivated by Tas, while in persistent infections, the IP is efficiently transactivated by Tas, but the LTR promoter is not. Analysis of proteins expressed from the LTR promoter and the IP during infection indicated that IP transcription is more robust than that of the LTR promoter in persistently infected cells, while the opposite is true for lytically infected cells. Coculture experiments also showed that LTR promoter transcription is greatest in cells which support lytic replication. Replacement of much of the LTR promoter with the IP leads to increased viral replication in persistent but not lytic infections. We also found that the induction of persistently infected cells with phorbol 12-myristate 13-acetate (PMA) greatly enhanced viral replication and transcription from the SFVcpz(hu) (new name for human FV) LTR promoter. However, mutation of three consensus AP-1 binding sites in the FV LTR promoter did not affect viral replication in lytically or persistently infected cells, nor did the same mutations affect LTR promoter transactivation by Tas in PMA-treated cells. Our data indicate that differential regulation of transcription is important in the outcome of FV infection but is unlikely to depend on AP-1.

Historical perspective of foamy virus epidemiology and infection

Foamy viruses (FV) are complex retroviruses which are widespread in many species. Despite being discovered over 40 years ago, FV are among the least well characterized retroviruses. The replication of these viruses is different in many interesting respects from that of all other retroviruses. Infection of natural hosts by FV leads to a lifelong persistent infection, without any evidence of pathology. A large number of studies have looked at the prevalence of primate foamy viruses in the human population. Many of these studies have suggested that FV infections are prevalent in some human populations and are associated with specific diseases. More recent data, using more rigorous criteria for the presence of viruses, have not confirmed these studies. Thus, while FV are ubiquitous in all nonhuman primates, they are only acquired as rare zoonotic infections in humans. In this communication, we briefly discuss the current status of FV research and review the history of FV epidemiology, as well as the lack of pathogenicity in natural, experimental, and zoonotic infections.

Persistent zoonotic infection of a human with simian foamy virus in the absence of an intact orf-2 accessory gene

Although foamy viruses (FVs) are endemic among nonhuman primates, FV infection among humans is rare. Recently, simian foamy virus (SFV) infection was reported in 4 of 231 individuals occupationally exposed to primates (1.8%). Secondary transmission to spouses has not been seen, suggesting that while FV is readily zoonotic, humans may represent dead-end hosts. Among different simian species, SFV demonstrates significant sequence diversity within the U3 region of the long terminal repeat (LTR) and 3' accessory open reading frames (ORFs). To examine if persistent human SFV infection and apparent lack of secondary transmission are associated with genetic adaptations in FV regulatory regions, we conducted sequence analysis of the LTR, internal promoter, ORF-1, and ORF-2 on a tissue culture isolate and peripheral blood mononuclear cell samples from a human infected with SFV of African green monkey origin (SFV-3). Compared to the prototype SFV-3 sequence, the LTR, internal promoter, and FV transactivator (ORF-1) showed sequence conservation, suggesting that FV zoonosis is not dependent on host-specific adaptation to these transcriptionally important regions. However, ORF-2 contains a number of deleterious mutations predicted to result in premature termination of protein synthesis. ORF-2 codes in part for the 60-kDa Bet fusion protein, proposed to be involved in the establishment of persistent cellular SFV infections. These results suggest that persistent human infection by SFV and reduced transmissibility may be influenced by the absence of a functional ORF-2.

Study of human foamy virus proviral integration in chronically infected murine cells

This report describes integration sites of human foamy virus (HFV) in chronically infected BALB/c murine cells that we isolated by inverse PCR and characterized. We show that integration of HFV proviral genome mainly occurs in highly repetitive and/or transcriptionally active regions and leads to the formation of a 4-bp cellular direct repeat sequence at each provirus extremity. As non-random deletions were previously described in the HFV be/1 transactivator gene as well as in the long terminal repeats (LTRs), these regions were verified in integrated HFV. The analysis reveals that, in the studied chronic state, the defective interfering virus (delta HFV) is the main integrated proviral form and is always associated with a small LTR. Our results show that HFV can use a classic retroviral integration process to enter the host cell genome and stress the importance of delta HFV and the short LTRs in the establishment of the chronic state of infection.

Inhibition of the in vitro infectivity and cytopathic effect of human foamy virus by dideoxynucleosides

Human foamy virus (HFV) is a human retrovirus that has not been clearly associated with human disease. In this study, we tested the capacity of nucleoside derivatives to inhibit the infectivity and cytopathic effect of HFV in T-lymphoblastoid cells in vitro. H9 cells showed a dramatic cytopathic effect 3 weeks after exposure to HFV. At this time, viral infection was demonstrated by detection of viral antigens by immunofluorescence staining, release of reverse transcriptase activity (RT) in the supernatant, detection of typical viral particles by electron microscopy, and presence of proviral DNA by Southern blot analysis. H9 cells were pretreated with dideoxycytidine (ddC), dideoxyinosine (ddI), or azidothymidine (AZT) at various concentrations before HFV infection. ddC could not completely suppress viral replication at low concentrations, and inhibited cell proliferation at higher concentrations. ddI partially inhibited the formation of giant cells at 10 microM, with 95% inhibition of RT in the supernatant. AZT induced a complete inhibition of cytopathic effect at concentrations > or = 1 microM, with more than 95% inhibition of RT in the supernatant. Moreover, the synthesis of proviral DNA was completely suppressed by 10 microM AZT. These results show that AZT and ddI can inhibit HFV replication in vitro.

Isolation and characterization of infectious full-length DNA clones of chimpanzee foamy viruses SFV6 and SFV7: evidence for a Taf-dependent internal promoter

We have cloned complete viral genomes directly from Hirt supernatant DNAs of simian foamy virus types 6 and 7 (SFV6 and SFV7) -infected cells. These clones were shown to be infectious by transfection into cells and subsequent infection of susceptible cells either by cocultivation or by passage of cell-free supernatants. The presence of virus particles, suggested by a typical cytopathic effect, was confirmed by electron microscopy. These viruses were characterized at different levels of the replication cycle. The proviral genomes revealed a taf deletion comparable to that previously described in the human foamy virus (HFV) bel1 gene. Analysis of viral RNAs revealed similar patterns of transcripts for SFV6- and SFV7-infected cells, with predominant expression of accessory genes. Characteristic major viral polypeptides were identified by radioimmunoprecipitation for both isolates. Sequences homologous to the gene encoding Taf and to a potential internal promoter were identified in the infectious clones and subcloned into expression vectors. Their functional properties were tested by transfection assays, which provided evidence for the presence of a Taf-dependent internal promoter in both SFV6 and SFV7 isolates.

Molecular biology of the human foamy virus

Foamy viruses also known as spumaretroviruses are complex retroviruses infecting cell lines with no apparent specific cellular tropism and induce the formation of multinucleated cells with numerous vacuoles. Far less well characterized than oncoviruses and lentiviruses, this class of viruses is thought to be innocuous in vivo. However, several important discoveries on foamy viruses brought new insights in the field of retrovirology.

In vitro studies on interferon-inducing capacity and sensitivity to IFN of human foamy virus

We demonstrate in this article that human foamy virus (HFV) fails to induce interferon (IFN) production in two different human tissue culture cell lines: U373-MG and AV3. We also show the effect of human alpha-, beta- and gamma IFN on the multiplication cycle of HFV. Treatment of cells with 100 IU/ml of any IFN led to strong inhibition of an HFV-induced cytopathic effect. This effect was associated with a significant diminution of reverse transcriptase activity in supernatant fluids of IFN-treated infected cultures, and a substantial decrease in viral particle production, as detected by electron microscopy. All these effects were accompanied by strong inhibition of both viral proteins and RNA synthesis, as well as almost total disappearance of free and integrated proviral DNA. In light of our data, human IFN action on HFV seems to be mediated by a mechanism which differs from that observed in the case of other retroviruses (type C and D for instance); however, it evokes that described for HIV.

Involvement of a spliced and defective human foamy virus in the establishment of chronic infection

Human foamy retrovirus (HFV) is found as two proviruses (HFV and delta HFV) which differ by a splice-induced deletion within the bel1 transactivator gene. The defective delta HFV (which lacks a functional Bel1 but harbors an intronless bet gene) is predominantly found in nonlytic infections in vitro as well as in vivo. Here, we show that infection of cell lines stably transduced by delta HFV DNA with the highly lytic HFV leads to chronic infections characterized by an absence of lysis, a balanced ratio of HFV to delta HFV, and a persistent Bet expression accompanied by a shutoff of structural genes. While this system only partially reflects the natural situation, in which target cells are infected by HFV and delta HFV simultaneously, it strongly suggests that delta HFV is a defective interfering retrovirus. Accordingly, previous or concomitant exposure to delta HFV viruses greatly enhances the formation of lysis-resistant clones in culture after HFV infection. The inability of delta HFV proviruses encoding a mutated bet gene to induce chronic infection suggests a role for Bet in this process. Through a specific, splice-induced, genomic deletion, resulting in a switch from Bel1 to Bet expression, the lytic properties of HFV are progressively lost. Such programmed inactivation of a key gene represents a new regulatory mechanism of gene expression in retroviruses.