Human herpesvirus 6 (HHV-6) like other herpesviruses, expresses sequentially immediate early (IE), early, and late genes during lytic infection. Evidence of ability to establish latent infection has not been available, but by analogy with other herpesviruses it could be expected that IE genes that regulate and transactivate late genes would not be expressed. We report that peripheral blood mononuclear cells of healthy individuals infected with HHV-6 express the U94 gene, transcribed under IE conditions. Transcription of other IE genes (U16͞17, U39, U42, U81, U89͞90, U91) was not detected. To verify that U94 may play a role in the maintenance of the latent state, we derived lymphoid cell lines that stably expressed U94. HHV-6 was able to infect these cells, but viral replication was restricted. No cytopathic effect developed. Furthermore, viral transcripts were present in the first days postinfection and declined thereafter. A similar decline in the level of intracellular viral DNA also was observed. These findings are consistent with the hypothesis that the U94 gene product of HHV-6 regulates viral gene expression and enables the establishment and͞or maintenance of latent infection in lymphoid cells.

The presence of human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7) was investigated by the polymerase chain reaction in saliva specimens from healthy persons, donors affected by common cold or recurrent aphthous ulceration (RAU), and human immunodeficiency virus (HIV) positive patients, and in salivary gland biopsies. The sensitivity of the technique made it possible to detect as few as 5-10 target molecules in 15 microliters of saliva. HHV-6 was present in 63% of salivary gland biopsies and in 3% of salivas from healthy persons. No significant difference in the presence of HHV-6 was detected in specimens from donors with common cold, RAU, or HIV-infected patients. HHV-7 was present in 75% of salivary glands and in 55% of salivas from healthy persons. HHV-7 was detected with similar frequency in salivas from donors with common cold or RAU. Salivas from HIV-infected patients harbored HHV-7 with higher frequency (81%) and increased viral load. These results show that salivary glands are a site of persistent infection for both HHV-6 and HHV-7. However, the two viruses seem to differ in their biological properties: 1) HHV-6 is rarely present in saliva in detectable amounts, while HHV-7 is frequently detected; and 2) immunosuppression by acquired immunodeficiency syndrome (AIDS) increases the frequency of detection and the viral load of HHV-7, but does not have a significant effect on HHV-6 shedding in saliva.

To define the molecular features characteristic of the early stages of infection of lymphocytes with human herpesvirus 6 (HHV-6) variant A or B, we studied the temporal regulation of expression of selected sets of viral genes. Thus, U42, U94, U89-U90, U73, and U39 are α genes since their transcripts (i) were made in the presence of inhibitors of protein synthesis and (ii) were detected 3 h after infection of untreated cells. U41, U53, U31, and U19 are β genes since their expression is inhibited by cycloheximide but not by phosphonoacetate, an inhibitor of DNA synthesis. U100 is a γ gene since its spliced transcript encoding the structural glycoprotein gp82/105 was first detected 16 h after infection of untreated cells but could not be detected in cells treated with phosphonoacetate. HHV-6 variants differ in the transcription patterns of their genes. U16-U17 originates a splice transcript and is regulated as α in HHV-6B and as β in HHV-6A. U91 generates two transcripts, amplified as 476- and 374-bp PCR fragments. The 476-bp fragment is α in HHV-6A-infected cells but β in HHV-6B-infected cells. Conversely, the 374-bp fragment is β in HHV-6A-infected cells and α in HHV-6B-infected cells. Furthermore, the spliced product of U18-U19-U20 (526 bp) is β in HHV-6A-infected cells, but only a partially spliced form (1.9 kb) was detected at late stages of infection in HHV-6B. HHV-6 transcription was also studied in nonproductive lymphoid cells, and the same transcription pattern detected during lytic infection was observed. Also, HHV-6 variants maintain the differences in U91, U16-17, and U18-U19-U20. We conclude that, as expected from the sequencing data, gene expression is generally similar in HHV-6 variants. However, transcription of selected genes in HHV-6A and HHV-6B differs with respect to temporal regulation and splicing pattern. Furthermore, the identification of viral functions expressed during the different stages of lytic replication suggests that reverse transcription-PCR for HHV-6 genes is a useful diagnostic approach to differentiate between latent and productive HHV-6 infection.

Transcription of human herpesvirus-7 (HHV-7) in cultures of productively infected T-cells was studied. Transcription of HHV-7 was regulated by the typical herpesvirus cascade in which α, β and γ genes are sequentially transcribed. Transcripts of U10, U14, U18, U31, U39, U41, U42, U53, U73 and U89/90 were detected 3 h after infection and were not inhibited by the absence of protein synthesis and therefore were α functions. U19 and U18/20 were β genes; their transcription was inhibited by cycloheximide but not by phosphonoacetate, an inhibitor of DNA synthesis. U60/66 and U98/100 were γ genes since their spliced transcripts were not detected in cells treated with phosphonoacetate. HHV-7 transcription was regulated by complex mechanisms, which involve the temporal coordinated activation of specific viral promoters and post-transcriptional processing. Splice mechanisms were also temporally regulated. Transcription of U89/90 pre-mRNA and splice took place simultaneously in the immediate-early phase. On the other hand, U16/17 pre-mRNA was synthesized with typical α kinetics, but the spliced product was regulated as a β function. Likewise, the primary transcripts of U60/66 and U98/100 were α and β, respectively, but both spliced products were synthesized in the late phase of virus replication. Finally, HHV-7 supported a bona fide latent infection in the adult population, since viral transcripts were not detected in peripheral blood mononuclear cells of healthy donors infected with HHV-7.