0022-538X/93/106259-06$02.00/0
Copyright © 1993,American SocietyforMicrobiology
Specificity Analysis
of
Human
CD4+
T-Cell Clones
Directed
against
Human
Herpesvirus 6 (HHV-6), HHV-7,
and
Human
Cytomegalovirus
MASAKI YASUKAWA,* YOSHIHIRO YAKUSHIJIN, MAHO FURUKAWA,AND SHIGERU FUJITA
FirstDepartmentofInternalMedicine, Ehime University School of Medicine, Shigenobu, Ehime
791-02,
JapanReceived 14April1993/Accepted 21 June 1993
In order to clarify antigenic variations among various isolates of human herpesvirus 6 (HHV-6) and
cross-reactivity among HHV-6, HHV-7, and human cytomegalovirus (HCMV)in the T-cell immuneresponse,
the antigenic specificity ofthe proliferative response mediated by 232 CD4+ human T-cell clones directed against HHV-6, HHV-7, orHCMV was examined. The results obtained were as follows. (i) Although the majorityof T-cell clones directed against HHV-6 proliferated in response to stimulation with all strains of
HHV-6 used (U1102, Z29, SF, and HST), 7% (8 of 122) of the T-cell clones showed distinct patterns of proliferative response against strain U1102 (group A) and other strains of HHV-6 (group B). (ii) Of 99 T-cell clones, 71 showedadistinctproliferative responsetoHHV-6 and HHV-7, whereas 28 proliferatedinresponse
to stimulation with both HHV-6 and HHV-7. (iii) A small number of T-cell clones (9 of 232) showed
cross-reactivity
againstHHIV-6andHCMV, and 2 of the 232 clones were reactive with HCMV as well as withHHV-6 and HHV-7. (iv) The specificity of gamma interferon production by T-cell clones following the
stimulation with virusantigen was identical to that of their proliferative response. These data thus indicate the presence of antigenic variations among isolates of HHV-6 and also epitopes common toHHV-6 and HHV-7 and
toHHV-6,HHV-7, andHCMV whichare recognized byCD4+T cells.
Human herpesvirus 6 (HHV-6) and HHV-7 are newly identified lymphotropic herpesviruses (12, 17). It has been foundthat HHV-6 isacausative agent of exanthem subitum (25) and that various clinical symptoms may result from reactivation of HHV-6(4, 9, 11, 19). On the other hand, the potential association of HHV-7 with diseases hasnot been clarified.The isolates of HHV-6 appeartoformtwoclusters, differingwith respecttotheirtropismfor certain T-cell lines, therestriction enzyme cleavagepatternsoftheir DNAs, and their reactivities with somemonoclonal antibodies (1, 2, 5, 13, 18, 20). It has been reported that HHV-7 differs from HHV-6onthe basis ofcomparative analysisoftheir DNAs andtheir reactivities with monoclonal antibodies(3, 21, 22). Compellingevidence that T-cell-mediated immunity plays
acrucialrole in resistancetoherpesviruses exists, and thus, investigation of the T-cellimmune responseagainst HHV-6 and HHV-7 is essential. In the present study, we investi-gatedthe immunological relationshipsbetween various iso-lates of HHV-6 andHHV-7,focusingontheir T-cellepitopes recognized by human CD4+ T cells. The results obtained withalarge number of CD4+ human T-cell clones indicated that HHV-6 group-common and group-specific T-cell epitopesarepresentand that HHV-6 and HHV-7 sharesome
T-cell epitopes. The immunological relationships among various HHV-6isolates, HHV-7, and human
cytomegalovi-rus (HCMV) are discussed on the basis of the present findings, together with previous reports concerning DNA
analysisandreactivitywith monoclonal antibodies.
The U1102(8),Z29(16),SF(15), and HST (25) strains of HHV-6and the RK strain of HHV-7(12)weregrown in cord blood mononuclear cells, whichwere stimulated with phy-tohemagglutinin (GIBCO,GrandIsland, N.Y.)andcultured
* Correspondingauthor.
in RPMI 1640 medium (GIBCO) supplemented with 10% heat-inactivated fetal calf serum (GIBCO) and 0.5 U of human recombinant interleukin-2 (Takeda Chemical Indus-tries, Osaka, Japan)permlasdescribedpreviously (25).The KOSstrain ofherpes simplexvirus type 1(HSV-1) and the AD169strain of HCMVweregrowninHEp-2 cells and Flow 7000humanembryonicfibroblasts, respectively.
Virusantigenswerepreparedasdescribedpreviously(23). Briefly, virus-infected cells were sonicated with an ultra-sonic disruptor. After centrifugation, the supernatant was
exposedtoUVlighttoinactivate the viruses. Mock-infected cellswere also treated asdescribed above and preparedas
the control antigen. The optimal dilutions of HHV-6, HHV-7, HSV-1, and HCMV antigens for stimulation of T
cells determined in previous experiments were 1:20, 1:20, 1:200, and 1:200, respectively.
T-cell clones reactive with HHV-6, HHV-7, or HCMV
weregeneratedasdescribedpreviously (24). Briefly, periph-eral blood mononuclear cells(PBMC)from6-, HHV-7-, and HCMV-seropositive healthyadultswere suspended in RPMI 1640 mediumsupplemented with 10% heat-inacti-vated pooled human AB serum (referred to here as the culture medium). HHV-6, HHV-7, or HCMV antigen was
then added to the cells, which were seeded in a 96-well round-bottom microtiterplate at37°C ina 5% CO2
incuba-tor. After 6 or7 days, the blastswere seeded at a
concen-tration ofonecell per well in round-bottom microtiterwells
containing0.2 ml ofculture medium supplementedwith 0.5 U of human recombinant interleukin-2 per ml and
105
mitomycin (MMC)-treated autologousPBMC. The growing cellswerethen transferredto16-mm-diameter wells.
Theproliferativeresponse of T-cell clonestothe stimula-tion with virus antigens was assessed as described
previ-ously (23). Briefly, 104 clone cells and 105 MMC-treated autologousPBMC(asa sourceofantigen-presenting
cells)
in6259
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TABLE 1. ProliferativeresponseofT-cell clones directed against HHV-6
Incorporation of[3H]TdR(cpm) bycellsstimulatedwitha: Clone
U1102 Z29 SF HST HCMV HSV-1 Novirusb
Group1
FU.SF-12 16,500(1,272) 20,892(3,810) 25,089 (2,973) 20,051 (3,868) 892 (8) 239 (66) 281 (70) MY.U1102-2 15,786(2,293) 16,225 (652) 32,178(4,244) 29,184 (4,607) 604 (111) NDC 696 (94) Group2
FU.Z29-11 898(164) 9,158(312) 10,181(300) 5,949(849) 588 (10) 292(38) 477 (80) MY.HST-5 835 (31) 16,072(657) 14,556(2,582) 9,967(85) 844 (124) 937(98) 898 (270) Group3
MY.U1102-4 14,636(1,495) 714(256) 748 (114) 788 (40) 973 (129) ND 726 (233)
Group4
FU.Z29-1 6,346 (558) 6,648(1,246) 7,124 (460) 3,553 (641) 3,828 (367) 692 (154) 751(38) FU.Z29-15 6,749 (785) 6,834(1,399) 7,505(871) 2,137 (192) 5,118 (422) 414(132) 471 (232)
MY.CMV-4 898(160) 1,033 (98) ND ND 12,309 (4,381) ND 885(90)
MY.HSV-4 880(112) ND ND ND ND 13,446(859) 437(129)
aIncorporationof
[3HJTdR
into104clonecellsin thepresenceof 105MMC-treated autologous PBMC withvirusantigenorcontrolantigenwasdeterminedduringthefinal16 hof a 72-h incubation. Values are means fortriplicatewells (standard deviationsare inparentheses).
bMock-infectedcells. cND,not done.
0.2 ml of culture medium were seeded in round-bottom microtiter wellstowhich 0.02 ml of virusantigenorcontrol antigenwasaddedattheoptimaldilution. The cultureplates werethenincubated at 37°C in a 5%CO2 incubator for 72 h. For the final 16 h of incubation, 1 ,uCi of
[3H]thymidine
([3H]TdR)
(New England Nuclear, Boston, Mass.) wasadded to eachwell, and the incorporation of [3H]TdRwas
determinedbyliquid scintillationcounting. The stimulation index (SI) was defined as the mean count per minute of cultures incubated with virus antigen divided by the mean countperminute of cultures incubatedwith controlantigen. Forthe assay of gamma interferon (IFN--y)production, 5
x 105clone cells and 1 x
106
cellsthat did not form rosettes withsheep erythrocytes(E-cells)weresuspendedin 2 ml of RPMI1640 mediumsupplementedwith10%fetalcalf serum and cultured in 16-mm wells with virus antigen. The cells which had beencompletelydepleted ofTcellsbytreatmentwith OKT3 monoclonal antibody and complement were
cultured for 72h, and then supernatantswere collected for the IFN--y assay, which was performed by enzyme-linked immunosorbent assay(ELISA) (Genzyme,Boston, Mass.).
As we described previously (26), the vast majority of T-cell clonesestablished fromperipheral blood lymphocytes (PBL)which had been stimulated with cell-freevirusantigen
werepositivefor CD4 butnotfor CD8. In the present series of experiments, a total of 232 CD4+ T-cell clones were
established fromPBLoffivehealthy adults by seeding blasts
at onecell per well inatotalof 6,912 wells. Among them, 58 cloneswere generated fromTcells which had been stimu-lated with HHV-6 strain U1102, 68 clones were from PBL stimulated withHHV-6strainZ29,16clones werefromPBL
stimulatedwith HHV-6 strain SF, 34 clones were from PBL stimulated with HHV-6 strain HST, 45 clones were from PBLstimulated with HHV-7 strain RK, and11clones were fromPBLstimulated with HCMV strainAD169.
Wefirst examined theantigenic variations among different
HHV-6 strains and the antigenic relationships between HHV-6 and HCMV by using 122 T-cell clones directed against HHV-6.
Representative
data on the proliferativeresponsesofT-cell clonesto stimulation with various virus antigensareshown in Table 1. FU.SF-12,whichwas gener-ated from PBL stimulgener-ated with the SF strain of HHV-6, proliferated in response to stimulation with all strains of HHV-6 examined but not to stimulation with HCMV or
HSV-1. This type of clone (group 1) seems to be HHV-6 specific and HHV-6 strain common. FU.Z29-11 and MY.HST-5 (group 2) proliferatedin responsetostimulation with all HHV-6 strains except for U1102. In contrastwith group 2clones, MY.U1102-4 (group 3) responded to stimu-lation with strain U1102 butnotwith other strains of HHV-6. Group 2 and 3 clones thus seemto recognize theantigenic difference between strain U1102 and other strains of HHV-6 used in the presentstudy.FU.Z29-1 and FU.Z29-15 (group 4)proliferated in response to stimulation with HCMVaswell
aswith all HHV-6 strains butnot tostimulation with HSV-1. Thereactivitypatternsof these clonesindicate the presence of T-cell epitopes that are cross-reactive between HHV-6 andHCMV.
The frequency of T-cell clones showing certain specifici-ties (whichwere established from threeindividuals, donors 1,2, and 3) isindicated in Table 2. All of 24 T-cell clones generated from PBL of donors 1 and 2 which had been stimulated with U1102 responded to stimulation with all HHV-6strains. Oneof12 clones from PBLof donor3 which had been stimulated with U1102wasU1102 specific (group 3), but the other cloneswereHHV-6 straincommon
(group
1). Similarly, group 1 cloneswere frequentlyisolated from
PBLof threedonorsfollowingstimulationwithZ29, SF,or
HST. However, seven group 2clones, whichresponded to
all HHV-6 strains except forU1102, were found among 86 clones generated from PBL stimulated with Z29, SF, or
HST. Group4clones,whichrespondedto stimulation with HCMV as well as with HHV-6, were found in 6 of 122 clones.
We next addressed the question of whether there are
T-cellepitopescommon toHHV-6and HHV-7. As shown in Table 3, HHV-6-specific andHHV-7-specific T-cell clones
were generated from PBLwhich had been stimulated with
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[image:2.612.64.563.89.285.2]TABLE 2. FrequencyofT-cellclonesdirected against HHV-6 with variousantigenspecificities
Specificityofproliferativeresponseb Frequency
Donor
Stimulatioenaeuec
U1102 Z29 SF HST HCMV (%)C
1 U1102 + + + + - 100 (11/11)
Z29 + + + + - 100 (12/12)
SF + + + + - 100 (9/9)
HST + + + + - 71(10/14)
- + + + - 29 (4/14)
2 U1102 + + + + - 92(12/13)
+ + + + + 8(1/13)
Z29 + + + + - 72(10/14)
+ + + + + 21(3/14)
- + + + - 7(1/14)
SF + + + + - 100(7/7)
HST + + + + - 100(10/10)
3 U1102 + + + + - 92(11/12)
+ - - - - 8(1/12)
Z29 + + + + - 90(9/10)
+ + + + + 10(1/10)
HST + + + + - 70(7/10)
- + + + - 20(2/10)
+ + + + + 10(1/10)
aVirusantigen used for thestimulation of PBMC during the T-cell cloning.
b+,SI was more than5;-, SI was less than 2.
cNumbersinparenthesesarenumbersof T-cell clones withthe indicated antigen specificity/totalnumbers of clonesestablished.
HHV-6 antigen or with HHV-7 antigen, respectively.
Fur-thermore, cloneMY.Z29-4, generated from PBL stimulated with HHV-6, showed an apparent proliferativeresponse to
stimulation both with HHV-6 and with HHV-7. Similarly, clones MY.RK-5, NA.RK-15, and NA.RK-16, generated from PBL stimulated with HHV-7, proliferatedin response to both HHV-6 and HHV-7 antigen stimulation. These
clones did not respond to stimulation with HCMV. Clone MY.Z29-7 showed a proliferative response to stimulation
with HCMVaswellaswith HHV-6 andwith HHV-7. These
data thussuggestthat HHV-6and HHV-7 sharesomeT-cell
epitopes recognized by human CD4+ Tcells.
The frequency of T-cell clones reactive with HHV-6
and/or HHV-7 is shown in Table 4. We examined the
reactivity of 54 T-cell clones which were generated from
HHV-6-stimulatedPBL oftwoindividuals. Among them,46 clonesresponded onlytoHHV-6 and 8 clonesproliferatedin
response tostimulationboth with HHV-6 and with HHV-7. Similarly, 25 HHV-7-specific T-cell clones and 20
HHV-6-and HHV-7-common T-cell cloneswerefoundamongatotal
of45 clonesgenerated from PBLstimulatedwith HHV-7.
ThefrequencyofHHV-6-andHHV-7-common T-cell clones
among all the clones generated from PBL stimulated with
HHV-6orHHV-7was28% (28of99).
We further examined the antigenic relationships among
HHV-6, HHV-7, and HCMV by studying the antigenic specificityof T-cell clones generatedfrom PBL stimulated
with HCMV. We established 11 CD4+ T-cell clones directed against HCMV. As shown in Table 5, the vastmajorityof clones (9 of 11), such as YY.CMV-1, YY.CMV-9, and
YY.CMV-13, proliferated in response to stimulation only withHCMV andnotwithHHV-6orHHV-7. However, one
clone, designated YY.CMV-12, responded to stimulation
with HHV-6 and with HHV-7 as well aswith HCMV. In
addition,cloneYY.CMV-7proliferatedinresponse to stim-ulation both with HCMV and with HHV-6 but not with HHV-7. These data suggest that HCMV is distinct from HHV-6 and HHV-7 but that thereare somecommonT-cell
epitopes recognized byCD4+ Tcells.
Wereported previouslythatHHV-6-specific CD4+ T-cell clonesproduced IFN-,y following stimulation with HHV-6
TABLE 3. ProliferativeresponseofT-cell clonesdirectedagainstHHV-6 or HHV-7
Incorporation of[3H]TdR(cpm) bycellsstimulated with': Clone
U1102 Z29 SF HST HHV-7 HCMV Novirusb
Group1
MY.U1102-3 34,822 (1,586) 41,171 (1,378) 59,275 (2,997) 51,303 (6,240) 855(267) 1,240(45) 1,565 (357) MY.U1102-9 15,786 (2,293) 16,225 (652) 32,178 (4,244) 29,184 (4,607) 534(127) 604 (111) 696 (94) Group2
MY.Z29-4 21,769(580) 24,040 (2,730) 25,441 (3,596) 27,642(599) 32,268(673) 1,290 (66) 1,194 (150)
MY.RK-5 19,208 (4,276) 23,584 (998) NDC ND 39,434(1,052) 2,557 (60) 1,647(160)
NA.RK-15 12,757(138) 12,030(1,858) ND ND 15,200(2,023) 482 (41) 403(69)
NA.RK-16 21,177 (1,700) 18,433 (2,098) ND ND 24,082(1,515) 613 (189) 411(98) Group3
MY.RK-2 466(99) 711 (118) ND ND 22,608(923) 748(58) 760(279)
NA.RK-6 215(13) 252(80) ND ND 39,222(3,930) 240(48) 146(9)
Group4
MY.Z29-7 6,343 (311) 6,200 (544) 7,474 (951) 4,869 (383) 5,870(216) 5,814(745) 974(144) aIncorporationof
[3H]TdR
into104clone cells in the presence of105
MMC-treatedautologousPBMCwith virusantigenorcontrolantigenwasdetermined duringthefinal 16 h of a 72-h incubation. Values are means fortriplicatewells(standard deviationsareinparentheses).b Mock-infectedcells. cND,notdone.
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[image:3.612.66.303.103.411.2] [image:3.612.61.564.514.690.2]TABLE 4. FrequencyofT-cell clones directed against HHV-6or
HHV-7 withvarious antigenspecificities
Specificityofproliferative responseb Frequency
U1102 Z29 HHV-7 HCMV (%)C
3 U1102 + + _ _ 75 (9/12)
+ - - - 8(1/12)
+ + + - 17 (2/12)
Z29 + + - - 73(11/15)
+ + + - 20(3/15)
+ + + + 7(1/15)
HHV-7 - - + - 41 (9/22)
+ + + _ 59(13/22)
4 U1102 + + - - 60(6/10)
+ - - - 20 (2/10)
+ + + - 20(2/10)
Z29 + + - - 100(17/17)
HHV-7 - - + - 70(16/23)
+ + + - 30(7/23)
a Virusantigen used for the stimulation of PBMC during T-cell cloning.
b+,SIwas morethan5; -,SIwasless than 2.
cNumbers in parentheses arenumbers ofT-cellclones with the indicated
antigenspecificity/total numbers of clones established.
antigen (24). On the basis of thisfinding, we examined the relationshipbetween theantigenicspecificityof the prolifer-ative response and that ofIFN--yproduction byT-cell clones (Table 6).It wasfound that thespecificity of the proliferative response of T-cell clones against stimulation with various virus antigens was identical to the specificity of
IFN-y
production, i.e., HHV-6-specificT-cellclones,which prolif-erated in response to stimulationwith HHV-6 but notwith HHV-7 or HCMV, produced IFN--y following stimulation with HHV-6 but not with HHV-7 or HCMV. Similarly, HHV-6- and HHV-7-common T-cellclones, which prolifer-ated in response to stimulation with either HHV-6 or
HHV-7, produced IFN--y following stimulation with either HHV-6 or HHV-7. Such identity of the specificity in the proliferative response and IFN--y production was observed in all T-cellclones examined.
Inthe presentstudy, we demonstrated that some human
T-cell clones responded distinctively to stimulation with different strains of HHV-6. It was also demonstrated that some T-cell clones responded to stimulation with HHV-6, HHV-7, and HCMV. Although the antigenstructures recog-nized by these T-cell clones areobscure, the present data strongly suggest the presence of specific T-cell epitopes amongthe strains ofHHV-6 and T-cellepitopescommonto HHV-6, HHV-7, and HCMV. The antigenic specificity of T-cell cloneswasconfirmed by analysis of
IFN-y
productionin additiontotheproliferativeresponse.It is postulated that various isolates of HHV-6canbe divided into two distinct groups, designated group Aand group B, on the basis of
their reactivities with monoclonal antibodies and DNA poly-morphism(1, 2, 5, 13, 18, 20). Group A contains strains GS and U1102. On the other hand, strains Z29 and SF and various other strainsisolated fromchildren with exanthem subitum, such as strain HST, are in group B (5). In the present study, fourU1102-specific T-cell clones and seven T-cell clones reactive withstrainsZ29, SF,and HST butnot with strainU1102weregenerated, suggestingthatU1102 is
distinct from other isolates of HHV-6. In addition to the previous serological study and DNA analysis, the present study focusingon T-cell-mediated immunity also indicated that the subclassification of HHV-6 into twoclustersseems reasonable.
The strains of HHV-6 isolated from the patients with
exanthem subitum are classed in group B, and the vast majority of healthy individualsareseropositiveforgroupB HHV-6 (7). On theotherhand, thegroupAstrains, GS and U1102, were both isolated from patients with immunodefi-ciency. Thus, it is of considerable interest to determine whether healthy general populationsare infectedonly with groupBHHV-6orwhethergroupA is alsoprevalentinthe
general population. It isimpossible to clarify this issue by
conventional serological methods because of the cross-reactivityofserumagainstgroupAandgroupBHHV-6.On
the other hand, since the T-cell proliferative response to HHV-6antigensresults from the in vitrosecondaryresponse ofHHV-6-specificmemoryT cells(23), this T-cellresponse seemstoreflectahistoryofinfection with each virus. In the
presentstudy,sevengroupB-specificT-cell clones andfour U1102-specificT-cell clonesweregeneratedfrom four
indi-viduals.Group B-specific T-cellclonesweregeneratedfrom
three individuals, donors 1, 2, and 3. On the other hand,
U1102(group A)-specificT-cell clonesweregenerated from
two individuals, donors 3 and 4. These data suggest that
TABLE 5. ProliferativeresponseofT-cell clones directed againstHCMV
Incorporation of[3H]TdR(cpm) by cells stimulatedwitha:
Clone
HCMV U1102 Z29 HHV-7 Novirus'
Group 1
YY.CMV-1 18,686(1,252) 1,025 (22) 843(126) 438 (23) 1,091(48)
YY.CMV-9 20,112(3,045) 780(74) 1,023 (89) 579(60) 908(154)
YY.CMV-13 15,597(574) 717(52) 755(60) 598(15) 813(90)
Group2
YY.CMV-12 15,144(4,527) 34,501 (824) 20,041(9,045) 34,665 (3,556) 1,013(346)
Group3
YY.CMV-7 32,093 (2,171) 4,625(751) 10,511 (1,380) 747(116) 703 (1)
aIncorporationof
[3HJTdR
into104clone cells in the presenceof105MMC-treated autologousPBMCwithvirusantigenorcontrolantigen wasdetermined duringthe final 16 hofa72-hincubation. Valuesaremeansfortriplicate wells (standard deviationsare inparentheses).bMock-infectedcells.
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[image:4.612.64.300.99.314.2] [image:4.612.59.559.573.695.2]TABLE 6. Production ofIFN-yby T-cell clones
Specificityofproliferativeresponse"bycelsstimulated IFN--y production
(pg/mi)
bycells stimulatedwithb: with:Clone HHV-6 7_____________6HHV
Clone____Ul102__HHV-6__HST__(RK)7HCMV Ul02 HV-_HT HHV-7
HCMV
NoU1102 Z29 SF HST (RK) HCV U1102 729 SF HST (RK) HCMVs
MY.U1102-3 + + + + - - 1,300 1,100 870 3,200 <100 170 <100
MY.U1102-4 + - - - 2,300 220 250 320 180 310 270
MY.Z29-4 + + + + + - 3,200 1,800 3,000 4,400 1,600 230 150
MY.RK-6 - - - - + - <100 <100 180 180 4,200 320 <100
a+,SIwas more than5; -,SIwaslessthan 2.
bClone cells(5 x 105) andautologousnon-Tcells (1 x 106)wereculturedwith virus antigenorcontrolantigen for72h,andIFN-y production inculture supernatantswasassayedby ELISA. The valuesarethe meansforduplicateassays.
cMock-infectedcells.
donors 1 and 2 had a history of infection with group B HHV-6 and that donor 4 had been infected with group A. Donor 3 seemstohave been infected with both group A and groupBHHV-6.Although the number of clones established is limited, the present data and the evidence that most isolates of HHV-6 from thepatients with exanthem subitum belong to group B (7) strongly suggest that the general population is infected with group B HHV-6 but that groupA
is also prevalent in healthy individuals and that some are coinfected with both group A and group B.
Another important finding in the present study is that HHV-6 andHHV-7 share T-cell epitopes. HHV-7 was first isolated from PBL of a healthy individual (12), and its potential association with diseases has not been clarified. Comparative analysis of DNAs from HHV-6 and HHV-7 and of reactivities with monoclonal antibodies revealed that HHV-7 is a virus distinct from HHV-6 (3, 21, 22). In the presentstudy,atotalof99T-cell cloneswereexamined for their reactivity with HHV-6 and HHV-7. Among them, 71 clones (72%) showed an HHV-6- orHHV-7-specific prolif-erative response, but the other 28 clones(28%)reacted both with HHV-6 and with HHV-7. Thefrequency of HHV-6- and HHV-7-commonT-cell clonesgenerated in the present study
seems to be somewhat higher than that expected from previousserological studiesand DNAanalysis.Although the clinical features of primary infection with HHV-7 are un-known, it has been found that HHV-7 is a prevalent virus and that the age at primaryinfection with HHV-7 is
some-what later than that with HHV-6(22). On the basis of these findings, it can be postulated that T cells directed against HHV-6- and HHV-7-common epitopes are generated in primaryinfection with HHV-6 and thatmostprimary infec-tions with HHV-7 produce no symptoms because of the protective roles of these HHV-6- and HHV-7-common T
cells.
The present study also revealed the presence of T-cell epitopes common to HHV-6, HHV-7, andHCMV. This is compatiblewiththerecentfindingof DNA sequence homol-ogy amongHHV-6, HHV-7,and HCMV(3, 6, 10, 12, 14). It is ofconsiderable interest toclarify the functional roles of
suchcross-reactive Tcells inprotection againstand
recov-eryfromherpesvirusinfection. In ordertoclarifythisissue, further studiesfocusingontheeffectsof such T cellsonthe replication of various herpesviruses and the peptide
se-quences oftheepitopesrecognized by T-cell clonesare now
under way.
We thank K. Yamanishi, Osaka University; Y. Minamishima, MiyazakiMedicalCollege;J. B.Black,Centers for DiseaseControl;
J. Nicholas, Medical Research Council;J. A. Levy, Universityof California, San Francisco; and N. Frenkel, National Institutes of Health for generouslyproviding viruses. Wealso thank Y. Koba-yashi, Ehime University, for valuablesupportin thepresentwork. This work was supportedbyagrant-in-aidfor scientific research from theMinistryofEducation, Science and Culture of Japan and bya grantfromtheUeharaMemorialFoundation.
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