The growing importance of non-HTLV human retroviruses was highlighted by the Scientific Committee organizing two sessions entitled "Endogenous retrovirus, foamy viruses and XMRV". Debate and discussion of whether Xenotropic Murine leukemia virus (MLV) Related Virus (XMRV) represents genuine human viruses or laboratory artefacts continued during these two sessions.
Two presentations did not find XMRV or related viruses in persons with chronic fatigue syndrome (CFS) or multiple sclerosis (MS) while a third report found only polytropic MLV in both CFS patients and healthy controls [120–123]. However, two articles published in Science the week of the conference helped elucidate the origin and significance of XMRV. Propotka
et al. provided strong evidence that XMRV was not present in the human prostate cancer cell line 22Rv1, but rather is a novel mouse gammaretrovirus generated via recombination during the passage of human prostate tissue xenografts in nude mice, containing near perfect chimeras of the 5' and 3' halves of the consensus XMRV genome . Knox et al. using serology, culture, and PCR methods, failed to confirm XMRV infection in persons with chronic fatigue syndrome (CFS) , including those found positive in the original study by Lombardi et al. . Knox
et al. showed that sera containing complement from CFS patients and matched controls both inactivated XMRV and MLV in vitro, restricting infection of human cells with these viruses, suggesting that sustained infection in humans is highly unlikely . They reported evidence of contamination of mouse monoclonal antibodies used in hot start PCR enzymes and cell sorting/staining. Contamination and non-specific serologic reactivity, as sources of false positive results in XMRV studies, were further emphasized in presentations by Erlwein et al. and Qiu et al. . Qiagen columns used for DNA extraction from paraffin-embedded tissues contained both MLV and XMRV gag sequences, adding to the plethora of reagents, such as PCR enzymes, monoclonal antibodies and cell lines found to be contaminated with these viruses and sequences. Qiu
et al. presented data showing cross-reactivity of sera from HTLV-1-infected persons with a conserved peptide region in the XMRV p15E envelope surface protein. They also reported an absence of reactivity in a large number of US blood donors and HIV-1-infected Africans using well-validated assays, suggesting that previous sero-reactive results obtained by others may be due to cross-reactivity or non-specific binding using incompletely validated tests .
Combined with the majority of the negative reports, these findings have questioned the findings from laboratories previously reporting positive results which now needs to exclude the possibility that the positive findings may have arisen from contamination. Such was the case in the presentation by Hanson et al. who described the extensive testing done and still to be performed to rule out false positive MLV results in the persons with CFS and healthy controls in their study . Although the field is now heavily favouring an end to the investigation of XMRV and MLV as human viruses, two ongoing studies at multiple institutions, one led by the National Heart, Lung, and Blood Institute (NHLBI) of the NIH as presented by Graham Simmons  and one led by Ian Lipkin at Columbia University, should finally clarify whether XMRV and MLV are present in blood donors and persons with CFS, respectively.
Simian foamy viruses
While the end is seemingly near for XMRV, much more research is needed to better understand the public health importance of human infection with other retroviruses, including simian foamy virus (SFV), a common infection in nonhuman primates (NHPs) . From the meeting, it is clear that SFV is widely distributed across central Africa (Cameroon, Democratic Republic of Congo, Gabon), mostly in persons with exposure to NHPs, but also in persons without reported NHP exposure [123, 131, 132]. For example, the wife of an SFV-infected hunter in Cameroon was reported to be seropositive, raising the possibility of person-to-person transmission . However, additional work is needed to determine if SFV is transmitted secondarily and if so, at what frequency.
In addition to SFV, other simian retroviruses are known to cross species and infect humans . For example, Calvignac-Spencer
et al. provided evidence for zoonotic transmission of Simian T-Lymphotropic Virus type 1 (STLV-1) from Colobus monkeys and chimpanzees in Côte d'Ivoire . In contrast, the lack of a closely related STLV has been an enigma for understanding the evolutionary history and origin of HTLV-2. STLV-2 in captive and wild bonobos is genetically similar, but very distinct from HTLV-2 . The finding of only HTLV-2 subtype b strains in Baka Pygmies, some of the oldest inhabitants of Cameroon, phylogenetically related to HTLV-2b in Amerindians, suggests an African origin for both STLV-2 and HTLV-2. HTLV-2b may have co-migrated with the ancient movement of Africans through Asia, across the Bering Strait, and into the Americas . Alternatively, HTLV-2b in the Baka may be due to a more recent introduction into Africa. An expanded search for STLV-2-like viruses in NHPs and testing of other ancient African and Asian populations will help clarify these possibilities.
ERVs integrated into the human genome millions of years ago and have been associated with a variety of illnesses, including cancer and multiple sclerosis (MS), though these associations remain controversial . At the conference, several presentations [138–141] addressed the role of ERVs in MS and various cancers. However, case-control studies and animal model studies using infectious molecular clones are required to fully understand what role, if any, ERVs may play in human disease.
Screening people for retroviruses in conjunction with other blood borne and sexually transmitted infections in endemic areas would help to determine if these viruses are entering the general population and if co-infections facilitate such transmission. Longitudinal, long-term follow-up studies will determine the pathogenicity of these viruses in humans. Establishing the prevalence of so called "harmless" retroviruses, such as SFV, in specific patient cohorts with common chronic conditions, malignant and inflammatory, in endemic regions, would add to our understanding of how these viruses might be contributing to human diseases. Genetic characterization of complete genomes and studying the intra-host and inter-host evolution of these viruses, coupled with animal model studies, will help understand the possible pathogenicity of these novel human viruses. Together, population-based and expanded molecular epidemiologic studies will determine how widespread these viruses are and determine their natural history in their non-human and human primate hosts.