Sera from normal subjects were examined for reactivity to human herpesvirus 6 (HHV-6) by the anticomplement immunofluorescence test. Of a total of 179 serum specimens from donors aged from under 10 to 59 years, 141 specimens showed positive reactivity against HHV-6. The positive rate was 70 to 83% for all age groups, and there were no substantial differences in the positive rates. Sera from younger children aged from 0 to 21 months were then examined in detail. The antibody-positive rate of children aged from 0 to 5 months decreased from 52 to 5%, but it gradually increased by 12 months. Almost all children had the antibody against this virus after 13 months of age.

SUMMARYSpleen cells primed by Prospect Hill (PH) or Puumala (Pu) virus could cross-react with Hantaan virus (HV) 76-118 strain-infected target cells after in vitro stimulation with HV-infected cells, although anti-PH or anti-Pu immune serum showed no crossreactive neutralizing (NT) activity to HV without complement. These results and our previous findings with cross-reactive cytotoxic T lymphocytes (CTLs) suggest that some epitopes recognized by CTLs might be common among the hantavirus genus, while the epitopes related to NT activity were mainly specific to each virus of this genus. Next, to evaluate the cross-reactive immunities demonstrated by in vitro study, we investigated the effect of transferring T lymphocytes and sera from BALB/c mice immunized with PH or Pu virus into nude mice before HV inoculation. Transferring T lymphocytes primed by PH or Pu virus reduced HV titres in lungs and spleens of nude mice, corresponding with the results of the in vitro CTL assays. Transferring anti-Pu immune serum also decreased HV titres in nude mice, which seemed to reflect complement-dependent NT activity. Moreover ICR mice previously immunized with PH or Pu virus showed resistance to challenge with a lethal dose of the HV KHF strain, indicating that cross-reactive immunity induced by PH or Pu virus could protect ICR mice against pathogenic HV infection.

Clinical isolates of human herpesvirus 7 (HHV-7) from the saliva of healthy individual were investigated for genetic variations in the regions of two immediate-early (IE) genes, the glycoprotein B (gB) and glycoprotein H (gH) genes, and in R2-repeat. The genomic DNA of 24 isolates from citizens of Thailand, Japan, and the United States was amplified to detect size variations in the IE-1 and IE-2 loci, but none was observed, suggesting that there was no deletion or insertion in these genes, in contrast with an IE gene of human herpesvirus 6 (HHV-6). The sequences of the gB gene from isolates acquired from 5 Japanese and 8 Thai subject were then compared with those of American strains JI and RK with respect to codons that are known to differentiate gB alleles. All the isolates were found to have gB allele C except for the JI strain, which has allele F. Variability was also observed in five specific gH codons, resulting in 6 different groups. The HHV-7 isolates might be classified into two major genetic variants by combining their gB and gH allelic groupings. In the present study, only JI belonged to variant 1, while the rest of the isolates appeared to belong to variant 2. In the R2-repeat region, size heterogeneities were observed among the 24 isolates, due to different repeat numbers (17, 15, 14, 13, or 12 repeats). Therefore, we used the R2-repeat to identify the origins of isolates in a study of HHV-7 transmission, and found HHV-7 to be transmitted within a family from both mothers and fathers to their children.

The susceptibilities of seven T-cell lines to human herpesvirus 6 (HHV-6) infection were examined. MT-4 cells were the most susceptible of these lines to infection with this virus. Therefore, chemically adhered MT-4 cell monolayers were used for infections HHV-6 assay by indirect immunofluorescent-antibody (IFA) staining. When cell monolayers were fixed 30 to 45 h postinfection, the foci stained with IFA were easy to count and a linear relationship was observed between the number of foci and the virus concentration. MT-4 cell monolayers were also used for a focus reduction neutralizing-antibody test. In this test, sera from patients in the convalescent stage of exanthem subitum all showed significant neutralizing activity (1:80 to 1:320), whereas sera from patients in the acute stage of disease showed no detectable neutralizing activity. The titers of neutralizing antibody correlated well with the levels of anti-HHV-6 antibodies detected by IFA.