In the present study, 3 carrier children, who were longitudinally followed for more than 10 years, were tested for PVL and clonality of HTLV-1-infected cells. This study gave us a unique opportunity to evaluate the HTLV-1 infection early in the life. Two asymptomatic carrier children (cases A and B) showed stable PVL and 1 carrier child (case C), who experienced lymphadenopathy, seborrheic dermatitis and hyperreflexia, showed a prominent increase of PVL. In a previous publication, we described the pattern of changes in PVL of 28 children, including the 3 in the current analysis [11
]. PVL in these vertically infected children increased up to 2 years after infection and plateaued thereafter, with an increase to a much higher PVL observed in the HTLV-1 carrier children with a diagnosis of eczema [11
]. Gabet et al. [24
] also reported the case of a 10-year-old girl, who was followed over a 2-year period, with infective dermatitis and infection with parasites. She had high PVLs and extensive and persistent oligoclonal expansion of infected lymphocytes, which was not influenced by clinical evolution or lamivudine treatment [24
In the present study, when the clonality of HTLV-1- infected cells was examined by IL-PCR, many bands of different sizes appeared on the gel for case A, who had relatively low PVL, representing polyclonal proliferation of HTLV-1-infected cells. Case B with relatively high PVL showed several major clones that were consistently detectable in the PBMC samples obtained at ages of 3, 5.6 and 13.1 years. The latter pattern was similar to that observed in the Japanese adult long-term carriers in the Miyazaki Cohort Study, who presumably were infected during the perinatal period [25
]. These data are consistent with the hypothesis that clonal expansion may occur concurrently with increase in PVL.
Case C had a history of lymphadenopathy, seborrheic dermatitis and hyperreflexia. These symptoms and signs are not uncommon in the HTLV-1 carrier children in Jamaica [14
]. Case C showed a very high PVL and 2 distinct clones (S and L) of HTLV-1-infected cells at age 13 years. The appearance of these clones coincided with a sharp increase in PVL between the ages of 5 and 13 years. DNA sequences of clones S and L were detectable by real-time PCR in case C's sample as early as 5 years of age and expanded continuously to 426 and 554 copies in 100,000 PBMCs, respectively, at 13 years of age. However, these 2 clones only accounted for less than 10% of the total PVL of case C (13,358 copies in 100,000 PBMCs). Many additional major bands were observed on the gel of the sample at the same age. Therefore, the marked increase of PVL in case C was considered to be due to the sum of these clones, although clone S and clone L were very evident.
It has been reported that HTLV-1 provirus is randomly integrated into the human genome. Nevertheless, proviral DNA sequences found in the ATL cells have been reported to be preferentially integrated into transcriptional units [26
]. It was thought that parts of corresponding genes are dysregulated, leading to proliferation of HTLV-1-infected cells. Expansion of clone S may be associated with the interaction between HTLV-1 provirus and host gene PTDSS1
. However, proviral DNA of clone L was not integrated into the transcriptional unit. These findings suggest that integration of proviral DNA into transcriptional units is not essential for the occurrence of clonal expansion.
The high PVL and detection of 2 distinct clones of HTLV-1-infected cells in case C were also evident at age 16 years, 3 years later. The analysis of the TCRγ gene rearrangement, which does not depend on HTLV-1 proviral integration, also detected an oligoclonal expansion of 5 T cell clones, which were repeatedly detected by PCR in the background of many T cells without clonality. The occurrence of ATL in patients with a history of infective dermatitis was reported [15
]. The shortened period of latency was observed in ATL patients with chronic infection, such as strongyloidiasis [28
]. Fortunately, case C did not show any signs of HTLV-1-associated disease, especially ATL, at age 16 years. Our data suggest that detection of clonal expansion of HTLV-1-infected cells does not always indicate the onset of clinically evident ATL. However, continuous clinical monitoring is warranted in this case, for any early signs of HTLV-1-associated disease.
Active HTLV-1 infection may have resulted in lymphadenopathy and inflammatory diseases in case C. Alternatively, these inflammatory diseases in childhood may have stimulated the immune system of case C, resulting in the expansion of certain T cells. If these activated T cells were infected with HTLV-1, DNA sequences of HTLV-1 provirus, especially the Tax-coding region, could have been transcribed at the same time. Therefore, it is possible that inflammatory antigenic stimulation, such as co-infection of other pathogens or allergic diseases in childhood, may induce the early clonal expansion of HTLV-1-infected T cells, leading to eventual development of ATL. The findings from the present study are suggestive, but warrant further study that includes larger number of cases.
Children infected with HTLV-1 during early childhood have an increased risk of developing ATL [9
]. In the present study, high PVL and the clonal expansion of HTLV-1-infected cells were seen in 2 of 3 children who were infected from their mothers. The clonal expansion accompanied by high PVL was most evident in a child who had skin manifestations. Youths with high PVL should be carefully monitored for any signs and symptoms, because the concurrent presence of high PVL and skin manifestations in carrier children could represent underlying oligoclonal expansion of clones, and thus future risk for clinically overt HTLV-1-associated diseases [29