Our study is consistent with previous PCR-based tissue surveys. Using a serologic test, we are able to examine comparable tissues (i.e., sera) from cases and controls as well as to assay for past and current MCV exposure. Our results are all consistent with MCV having a causal role in most but not all MCC tumors. Antibodies specific to MCV are found at high levels in sera from most MCC patients and do not cross-react with BK virus or other polyomaviruses. The robust antibody response against MCV VLP among these patients frequently distinguishes them from MCC patients whose tumors are not infected with MCV. The reasons why MCV-positive MCC patients have elevated MCC IgG antibody levels compared with persons who have been asymptomatically-exposed to MCV are not known but may represent either a continued viral antigen production during tumor development or a possible higher virus burden being a risk factor for MCC tumorigenesis. It is unlikely that MCC patients newly acquire MCV infection since MCV IgM is not significantly elevated among these patients. Presence of anti-VLP antibodies among MCC patients suggests that humoral immunity is not protective once a tumor is formed, but it remains possible that neutralizing immunity induced by MCV VLP vaccination might prevent primary infection.
Average MCV antibody levels are highest among MCV-positive MCC patients, but a substantial proportion of asymptomatic adults in each control group that we examined also have elevated MCV antibodies. Several studies have identified MCV genome in tissues from persons without MCC tumor consistent with MCV carriage without symptoms.4,6,7,20,21
Detection of MCV in respiratory aspirates suggests that the virus might be readily transmitted to and among children by a respiratory route.22,23
This is similar to studies for other human polyomaviruses. Stolt et al
report near universal prevalence for BKV antibodies among Swedish children under age 10 years and 72% prevalence for JCV antibodies among women age greater than 25 years old. If MCV behaves like other human polyomaviruses, persistent infection after initial exposure may be prolonged or even life-long. Given the rarity of MCC compared with the prevalence of MCV exposure, the large majority of persons who become infected with MCV do not develop MCC and other factors (e.g.
, sun exposure, immune deficiency) probably determine the risk for cancer development.
By testing sera at 1:500 dilution, we used a stringent protocol that reduces the possibility of cross-reactivity to other circulating human polyomaviruses. Even under these conditions, our test achieved 100% sensitivity in correctly identifying MCV-positive MCC patients. Antibodies reactive to the MCV VLP are not cross-reactive to BKV, a member of the SV40 family, or to the more closely-related murine polyomavirus, indicating that this test is highly specific for MCV. None of the children in our study under age 1 were positive for MCV antibodies and so vertical MCV transmission, if it occurs, is not likely to be common. MCV infection rates, however, are high even among very young children consistent with a casual transmission mechanism. Two other recently-discovered human polyomaviruses belonging to the SV40 subgroup, KI and WU viruses, have been found in respiratory secretions from symptomatic children and adults25,26
suggesting respiratory transmission as one possible route for polyomavirus infection. We do not know the transmission mechanisms for MCV but use of the MCV VLP EIA in longitudinal studies will help to reveal the transmission dynamics for this virus. Additional studies are also needed to determine if this first generation assay is sufficiently sensitive to detect all exposures to MCV among asymptomatically infected adults.
Our data support MCV being the seventh known human cancer virus and that it is directly involved in the pathogenesis of most MCC, a tumor for which prevention options are limited. Given the efficacy of human papillomavirus vaccines based on VLP antigens,27
our assay may be useful for measuring immune responses to MCV VLP-based vaccine candidates. This assay will also be useful for surveys of other neoplastic or nonneoplastic disorders to determine if MCV contributes to diseases beyond MCC. Added in proof: Kean et al
. have recently reported a 25–42% MCV prevalence rate among adults using a VP1-GST recombinant antigen assay (Kean JM, Rao S, Wang M, Garcea RL. Seroepidemiology of human polyomaviruses. PLoS Pathog 2009;5(3):epub).