Our results indicate that LIA performs well in confirming HTLV-1 seropositivity by showing a low incidence of indeterminate results and good agreement with PCR using samples in Japan, although the number of samples tested was small. In addition, semi-quantitative antibody titer to gp21 correlated well with HTLV-1 PVLs. Further study including larger samples is necessary to determine the positioning of LIA for HTLV-1 detection in Japan.

A human T-lymphotropic virus Type 1 (HTLV-1) positive cell line, MT-2, derived from human cord leukocytes co-culturing with adult T cell leukemia/lymphoma (ATL) cells is commonly used in HTLV-1 research; however, the details of provirus integrated in MT-2 genome have not yet been characterized. In this study, five types of HTLV-1 proviral sequences were detected in 11 different sites of the genome in a reference MT-2 cell line. The five types of HTLV-1 proviral sequences were one complete proviral genome, two types of proviruses with deletion of large internal viral sequences (5.3 and 3.9 kB), one provirus with a large deletion (6.2 kB) from 5'LTR to position 6257, and one provirus of LTR only. The provirus with identical deletion of large internal viral sequence (5.3 kB) was found to be integrated into six different sites (chromosomes). A complete provirus and three of four types of defective provirus were consistently detected in two other MT-2 cell lines cultured in different laboratories. Not only Tax/Rex RNA and HBZ RNA, but also the transcriptional product for a specific defective provirus, were detectable in all three MT-2 cell lines. Because it has been reported that defective provirus is frequently detected in ATL cells, these results may be important in understanding the mechanism of HTLV-1 proviral polymorphism, which may be related to leukemogenesis. In addition, the large variation in integrated HTLV-1 proviruses makes it important for researchers to exercise caution in their assessment and interpretation of results using MT-2 cell lines.

In a previous study, we reported that an identical defective provirus had integrated into multiple sites of the genome of a representative human T-lymphotropic virus type 1 (HTLV-1) cell line, MT-2. A possible explanation for this may be the repeated infection of this defective provirus to a cell. Therefore, we attempted to determine whether a defective provirus could transmit during the co-culture of HTLV-1 uninfected human T-cell line, Jurkat, with MT-2 cells treated with mitomycin C. As a result, we established not only a cell line with the integration of one complete provirus, but also a cell line with the integration of one defective provirus. The rearrangement of the T-cell receptor -γ gene of these cell lines showed them to be derived from Jurkat cells. Both HTLV-1 Tax/Rex and HBZ RNA were detected in the cell line, which harbors a complete provirus. On the other hand, HBZ RNA and transcriptional product specific for the defective provirus were detected in the cell line, which harbors a defective HTLV-1 provirus only. These results suggested that a defective HTLV-1 provirus with large depletion of internal sequence could transmit to other cells. Moreover, the defective provirus can be transcriptionally active. This suggested the possibility that the defective HTLV-1 provirus found in the lymphocytes of HTLV-1 carriers and patients with adult T-cell leukemia may transmit to other T-cells in vivo. The results also suggested that defective provirus in HTLV-1 carriers could be functional and may play a role in leukemogenesis.

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