Human herpesvirus-6 strain groups: a nomenclature
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Human herpesvirus-6 strain groups: a nomenclature Human herpesvirus-6 (HHV-6) was first isolated from individuals with lymphoproliferative disorders (strain GS) [1]. Subsequent isolations of HHV-6 included those from AIDS patients in Uganda (strain U 1102) [2] and Zaire (strain Z 29) [3], and the virus has now been isolated in many laboratories. Accumulating evidence has indicated that HHV-6 isolates form at least two groups, which can be distinguished by differences in molecular and biological properties. The differences between the two groups of HHV-6 have led to a number of informal designations, including " U l l02-1ike and Z29-1ike" [4], "Type I and Type II" [5], "Group A and Group B" [6], and to a proposal [4] that the classification of HHV-6 strains be reconsidered within the formal nomenclature system of the International Committee for Taxonomy of Viruses (ICTV). The ICTV procedure for naming herpesviruses involves numerical designations for species defined on the basis of "readily assayable and distinctive" differences spanning the entire genome, as well as differences in biology and epidemiology [7]. The need for a consistent, formally agreed nomenclature for each group of HHV-6 isolates was discussed in special sessions at the 16th International Herpesvirus Workshop (Asilomar, July 1991) and at the First International Herpesvirus Symposium in Japan (Osaka, June 1992). Recently, at the 17th International Herpesvirus Workshop (Edinburgh, August 1992) a satellite symposium on HHV-6 was held to specifically address aspects of strain variation and was attended by 65 scientists. We report here a consensus reached by a group of more than 50 scientists present at this symposium and at a subsequent session on nomenclature. Workers not in attendance have been invited to join this consensus. It was agreed that all HHV-6 isolates examined to date can be assigned to one of two groups on the basis of criteria summarised as follows: (/) All HHV-6 isolates are tropic for CD 4 ÷ T-lymphocytes, but each group, characterised either by strains GS and U 1102 or by strain Z 29, may preferentially infect different T cell lines [6, 8-10]. (i/) Although human sera are reactive with both groups of isolates, each group has a distinct pattern of reactivity with monoclonal antibodies. Four of seven monoclonal antibodies specific for different proteins or glycoproteins of the GS strain reacted with all 31 isolates so far tested, including U 1102 and Z 29, but three recognised epitopes present only in the group which contains GS and U l 1 0 2 [4, 6, 8-11].
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(iii)
The two groups can be clearly distinguished by analysis of restriction endonuclease sites. Restriction fragment variations were first reported in studies on strains GS and Z29 [12] and strains U l102 and Z29 [13]. Subsequent work has shown that isolates can be discriminated into two groups on the basis of restriction site polymorphisms [4-6, 11, 14], which extend across the entire genome [4]. (iv)
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