Abstract
The gammaretrovirus termed xenotropic murine leukemia virus-related virus (XMRV) was described to be isolated from prostate cancer tissue biopsies and from blood of patients suffering from chronic fatigue syndrome. However, many studies failed to detect XMRV and to verify these disease associations. Data suggesting the contamination of specimens in particular by PCR-based methods and recent reports demonstrating XMRV generation via recombination of two murine leukemia virus precursors raised serious doubts about XMRV being a genuine human pathogen. To elucidate cell tropism of XMRV, we generated replication competent XMRV reporter viruses encoding a green fluorescent protein or a secretable luciferase as tools to analyze virus infection of human cell lines or primary human cells. Transfection of proviral DNAs into LNCaP prostate cancer cells resulted in readily detectably reporter gene expression and production of progeny virus. Inoculation of known XMRV susceptible target cells revealed that these virions were infectious and expressed the reporter gene, allowing for a fast and highly sensitive quantification of XMRV infection. Both reporter viruses were capable of establishing a spreading infection in LNCaP and Raji B cells and could be easily passaged. However, after inoculation of primary human blood cells such as CD4 T cells, macrophages or dendritic cells, infection rates were very low, and a spreading infection was never established. In line with these results we found that supernatants derived from these XMRV infected primary cell types did not contain infectious virus. Thus, although XMRV efficiently replicated in some human cell lines, all tested primary cells were largely refractory to XMRV infection and did not support viral spread. Our results provide further evidence that XMRV is not a human pathogen.
Highlights
In 2006 the first human gammaretrovirus within the retroviridae family was reported to be identified in human prostate cancer tissue
For sensitive and convenient monitoring of xenotropic murine leukemia virus-related virus (XMRV) infection and replication, we sought to generate replication-competent XMRV reporter viruses encoding an enhanced form of the green fluorescence protein [39], or the secreted Gaussia princeps luciferase (GLUC) [40] via an internal ribosomal entry site downstream of the XMRV env frame (Figure 1)
A PCR amplified IRES-green fluorescence protein (GFP) cassette was inserted into the unique PmlI site 3' of the env ORF resulting in the generation of the plasmid XMRV-GFPΔ5U3 (Figure 1)
Summary
In 2006 the first human gammaretrovirus within the retroviridae family was reported to be identified in human prostate cancer tissue. XMRV prevalence applying well-controlled PCR protocols to prostate cancer [5,6], CFS [7] or diagnostic samples from other disorders like e.g. autism [8] failed to detect XMRV (reviewed here [9,10,11,12]). In support of these findings, it was recently demonstrated that the virus originated when two mouse leukemia viruses underwent recombination during experimental passage of a human prostate tumor xenograft in mice [13]. Subsequent studies cast massive doubts on the association of XMRV with CFS or prostate cancer [14,15,16,17,18] but XMRV remains a replication competent virus with novel biological features
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