Genus- and species-specific antigens have been identified on and within the JCV and BKV capsids. Members of the polyomavirus genus share a cross-reacting antigenic determinant that is not exposed on the virion surface. Antibodies directed against this determinant are produced by immunizing rabbits with disrupted virions or purified VP1. At least one species-specific antigenic determinant is found on the surface of the JCV, BKV and SV40 capsids which allows them to be distinguished from one another by serological means. Antibodies to this antigen are generated during an infection by the viruses of their natural hosts.
Antigenic similarities between the large and small T proteins of these three viruses can be demonstrated by a variety of immunological methods. Polyclonal antiserum raised against the early antigens of one virus generally crossreacts with those of the other two viruses, whereas monoclonal antibodies are less likely to recognize all three sets of proteins. Cytotoxic T lymphocytes (CTL) also have the ability to identify shared and distinct epitopes of T antigen presented on the surface of JCV- and SV40-transformed cells-, most CTL clones directed against determinants of the SV40 protein lyse cells expressing the JCV protein.
One major serotype of JCV (represented by Madl) appears to circulate in the population, although a distinct antigenic variant JCV(Madll) has been recognized. The Madl and Madll VP1 proteins only differ at amino acid position 66 (aspartic acid vs. histidine) which lies within a region (amino acids 40-80) predicted to be an antigenic epitope. Recently, two genotypic variants have been identified with deletions in their VP1 gene. It is not known whether these alterations signal the existence of additional JCV serotypes. BKV has been grouped into four serotypes. Sequence differences observed between amino acids 61 and 83 of their VP1 proteins are predicted to have altered a dominant epitope on the capsid surfaces, resulting in antigenic variability.
An important consequence of having multiple JCV and BKV serotypes circulating in the population might be the underestimation of the prevalence of these viruses in their human host if a single viral strain is used as the source of antigen in serological testing.
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