In 2009, attention became more intensely focused on XMRV when Judy Mikovits’ group at the Whittemore Peterson Institute in Reno, Nevada, reported in Science
that 67% of chronic fatigue syndrome (CFS) patients were XMRV positive [3
] and that the virus was also present in the blood of 3.7% of healthy controls. The study used multiple lines of evidence (PCR, virus isolation and detection of specific antibodies) to demonstrate XMRV sequences and proteins in peripheral blood mononuclear cells (PBMC) and the presence of cell-free transmittable virus in patients’ plasma.
This study was immediately challenged by other reports, initially from Europe [16
] and later from the US [19
] that could not confirm the findings. The potential association between a new pathogen and CFS and the implied spread of a previously unrecognized gammaretrovirus in humans were nevertheless of great concern. Despite all warnings that the link between CFS and XMRV could not be confirmed, many patients suffering from this debilitating disease and desperately hoping for a cure started taking anti-retroviral drugs. However, the increasing number of reports failing to find a retroviral etiology for CFS and other disorders [23
] cast further doubt on the reliability of the original findings. The authors and enthusiasts of the original CFS study [3
] attempted to dismiss such negative results on the grounds of inadequate diagnostic procedures or inappropriate selection of patients [27
], demanding that the methods used in the original paper be meticulously replicated. However, a recent study [28
] again found no evidence for XMRV despite using virtually identical PCR, serology and virus isolation methodologies to investigate samples from some of the same CFS patients found to be positive in the initial study [3
The prevalence of XMRV in CFS patients was also addressed by Knox and colleagues [1
] who investigated a total of 61 CFS patients of which 43 had previously been found to be XMRV-positive by a commercial (VIPDx, Reno, NV, USA) or research laboratory (Whittemore Peterson Institute, Reno, NV, USA) using the methodology of Lombardi et al
]. Duplicate blood samples drawn on the same day by the same phlebotomist were available for 19 of these patients and, remarkably, in 10/19 cases samples tested positive for XMRV DNA by VIPDx while 0/19 tested positive by Knox et al
]. To broaden the study, samples from a subset of 29 CFS patients were subjected to a barrage of different tests: PCR and RT-PCR on nucleic acids from stimulated or unstimulated PBMC, RT-PCR for the detection of viral RNA in the plasma and even incubation of susceptible mink lung cells with patients’ plasma or PBMC culture supernatants to facilitate enhanced detection of infectious virus particles. Despite 26 of these 29 CFS patients having been previously tested positive by VIPDx or the Whittemore Peterson Institute using one or more of these methods, no trace of XMRV could be detected. Furthermore, when Knox et al.
investigated 60 plasma samples for the presence of XMRV-specific antibodies using an established chemiluminescence immunoassay [1
], just one was weakly reactive for XMRV gp70 and the specificity of this reaction could not be confirmed by Western blot.