No evidence of XMRV infection in immunocompromised patients and HIV-positive individuals from Germany.

BACKGROUND: Xenotropic murine leukaemia virus-related virus (XMRV) has been detected in patients with prostate cancer and chronic fatigue syndrome (CFS). The detection of XMRV in healthy individuals has raised concern about a possible virus transmission by blood products. However, recent studies challenge the association between XMRV and human disease. This study investigated whether or not XMRV is present in patients with altered immune function and individuals at increased risk of blood-borne viral infections in Germany. METHODS: We investigated 503 peripheral blood mononuclear cell (PBMC) samples from 240 patients with iatrogenic immune suppression (71 haematopoietic stem cell recipients, 132 solid organ transplant recipients, 37 others) and 311 PBMC samples from 302 patients with HIV-1 infection for the presence of proviral XMRV by real-time polymerase chain reaction (PCR). RESULTS: All 814 PBMC samples from 542 patients tested negative for XMRV DNA and positive for an internal herpesvirus saimiri (HVS) control. Human genomic DNA was detected in all samples, and 90% of the samples contained >10,000 cell equivalents per XMRV PCR reaction. CONCLUSIONS: Our failure to detect proviral XMRV provides evidence against the presence of XMRV in patients at increased risk of viral infections in Germany.

Efficient HIV-1 replication can occur in the absence of the viral matrix protein.

Matrix (MA), a major structural protein of retroviruses, is thought to play a critical role in several steps of the HIV-1 replication cycle, including the plasma membrane targeting of Gag, the incorporation of envelope (Env) glycoproteins into nascent particles, and the nuclear import of the viral genome in non-dividing cells. We now show that the entire MA protein is dispensable for the incorporation of HIV-1 Env glycoproteins with a shortened cytoplasmic domain. Furthermore, efficient HIV-1 replication in the absence of up to 90% of MA was observed in a cell line in which the cytoplasmic domain of Env is not required. Additional compensatory changes in Gag permitted efficient virus replication even if all of MA was replaced by a heterologous membrane targeting signal. Viruses which lacked the globular domain of MA but retained its N-terminal myristyl anchor exhibited an increased ability to form both extracellular and intracellular virus particles, consistent with a myristyl switch model of Gag membrane targeting. Pseudotyped HIV-1 particles that lacked the structurally conserved globular head of MA efficiently infected macrophages, indicating that MA is dispensable for nuclear import in terminally differentiated cells.

CD4 expressing human 293 cells as a tool for studies in HIV-1 replication: the efficiency of translational frameshifting is not altered by HIV-1 infection.

An adherent human cell line (293) was made susceptible for HIV-1 infection by transfer of a CD4 expression plasmid. These cells could be infected with HIV-1 and produced infectious virus up to a titer of 10(6) TCID50/ml releasing p24 protein up to 1 micrograms/ml. Since they can be efficiently transfected with reporter genes, these cells are a suitable model system to monitor biochemical events during productive infection of HIV-1 and can be used for antiviral drugs. Translational frameshifting determines the balance of the structural Gag versus the catalytic Pol proteins which is probably crucial for correct virus assembly. We have genetically engineered CD4 expressing 293 cells with a sensitive in vivo reporter system to monitor the extent of frameshifting in HIV-1-infected versus uninfected cells. During the time course of productive HIV-1 infection the low efficiency of ribosomal frameshifting is not altered.

A heptanucleotide sequence mediates ribosomal frameshifting in mammalian cells.

Ribosomal frameshifting is an essential requirement for replication of many viruses and retrovirus-like elements. It is regarded as a potential target for antiretroviral therapy. It has been shown that the frameshifting event takes place in the -1 direction within a sequence, the slippery sequence, which is usually followed by structured RNA. To distinguish between the basic sequence requirements and the modulating elements in intact cells, we have established a sensitive assay system for quantitative determination of ribosomal frameshifting in mammalian cell culture. In this assay system, the gag and pol genes of human immunodeficiency virus type 1 are replaced by the genes for the functional enzymes beta-galactosidase and luciferase, respectively. The sensitivity of the test system allows us to demonstrate for the first time that the slippery sequence, a heptanucleotide, is sufficient to mediate a basal level of ribosomal frameshifting independent of its position within a gene. The stem-loop sequence serves only as a positive modulator. These data indicate that frameshifting could also occur during translation of cellular genes in which a slippery sequence is present within the reading frame. The resulting putative transframe proteins might have a functional importance for cellular processes.

Expression and frameshifting but extremely inefficient proteolytic processing of the HIV-1 gag and pol gene products in stably transfected rodent cell lines.

Expression, ribosomal frameshifting, and proteolytic processing of HIV-1 GAG and POL proteins were investigated in heterologous mammalian cells in order to elucidate the influence of the cellular background on these events. DNA fragments encoded by the gag and pol region were expressed in two rodent cell lines, LTK- and BHK. Both stably transfected cell lines continuously produce recombinant proteins which react with HIV-specific antisera. The GAG precursor and a 39-kDa proteolytic fragment thereof were the major recombinant proteins detected. Expression of the gag-pol region leads to the production of the GAG-POL precursor. Ribosomal frameshifting at the HIV-1 shifty sequence to a typical extent could be positively demonstrated by an enzyme assay. Despite the presence of the viral protease within the GAG-POL precursors, proteolytic processing of the HIV-derived polyproteins was extremely inefficient. The efficiency could not be enhanced by overexpression of the HIV-1 protease encoding region.

Test system for determination of HIV-1 frameshifting efficiency in animal cells.

We have developed a system in animal cells which allows the quantification of frameshifting determined by specific mRNA sequences. The method is based on the expression of an N-terminally extended firefly luciferase gene which requires frameshifting in order to be translated as a functional enzyme. The systems sensitivity is such that it allows the detection of even low efficiency of frameshifting. Our results show that the HIV-1 frameshift sequence including the 3' located stem-loop structure leads to ribosomal frameshifting at a lower level than that described for in vitro systems when tested in several fibroblastoid cell lines.