Adult T-cell leukemia (ATL) cells carry the HTLV-1 provirus in the same genomic site in each case, indicating that provirus insertion, demonstrated by Southern blot hybridization (SBH), is an excellent biomarker for the cellular clonality. However, it is unavailable for samples including small clones with 5% or fewer monoclonal cell populations. In this study, we analyzed cell-surface markers of 313 ATL cases (smoldering type 74cases, chronic type 60 cases, lymphoma type 27cases, and acute type 152 cases) and 70 cases of HTLV-1 carrier at our hospital between March, 1997 and March, 2012. Typical ATL cells indicated significantly higher levels of CD4, CCR4 and CD25 and down-regulation of CD3, CD7 and CD26. These markers were useful for clinical diagnosis and classification for ATL sub-types. CD38 was also useful to the differential diagnosis of acute and chronic types. To better understand indeterminate HTLV-1 carriers and smoldering ATL, cell-surface markers, SBH, proviral load (PVL), and ATL-related biomarkers were examined focusing on carriers and smoldering cases. We found that the antigen modulation rates of CD26 and CD7 and the increasing rate of CD25 and CCR4 cells were closely correlated to clonal size. In particular, the ratio of CD26/CD25 had a predictive detection of clonal band. Conclusively, CD26 plays a central role in the evolution from early to overt smoldering ATL.
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