A Unique T-Cell Receptor Amino Acid Sequence Selected by Human T-Cell Lymphotropic Virus Type 1 Tax-Specific Cytotoxic T Cells in HLA-A24:02-Positive Asymptomatic Carriers and Adult T-Cell Leukemia/Lymphoma Patients.

We previously reported that the T-cell receptor (TCR) repertoire of human T-cell lymphotropic virus type 1 (HTLV-1) Tax-specific CD8 cytotoxic T cells (Tax-CTLs) was highly restricted and a particular amino acid sequence motif, the PDR motif, was conserved among HLA-A24:02-positive (HLA-A24:02) adult T-cell leukemia/lymphoma (ATL) patients who had undergone allogeneic hematopoietic cell transplantation (allo-HSCT). Furthermore, we found that donor-derived PDR CTLs selectively expanded in ATL long-term HSCT survivors with strong CTL activity against HTLV-1. On the other hand, the TCR repertoires in Tax-CTLs of asymptomatic HTLV-1 carriers (ACs) remain unclear. In this study, we directly identified the DNA sequence of complementarity-determining region 3 (CDR3) of the TCR-β chain of Tax-CTLs at the single-cell level and compared not only the TCR repertoires but also the frequencies and phenotypes of Tax-CTLs between ACs and ATL patients. We did not observe any essential difference in the frequencies of Tax-CTLs between ACs and ATL patients. In the single-cell TCR repertoire analysis of Tax-CTLs, 1,458 Tax-CTLs and 140 clones were identified in this cohort. Tax-CTLs showed highly restricted TCR repertoires with a strongly biased usage of BV7, and PDR, the unique motif in TCR-β CDR3, was exclusively observed in all ACs and ATL patients. However, there was no correlation between PDR CTL frequencies and HTLV-1 proviral load (PVL). In conclusion, we have identified, for the first time, a unique amino acid sequence, PDR, as a public TCR-CDR3 motif against Tax in HLA-A*24:02 HTLV-1-infected individuals. Further investigations are warranted to elucidate the role of the PDR CTL response in the progression from carrier state to ATL. ATL is an aggressive T-cell malignancy caused by HTLV-1 infection. The HTLV-1 regulatory protein Tax aggressively promotes the proliferation of HTLV-1-infected lymphocytes and is also a major target antigen for CD8 CTLs. In our previous evaluation of Tax-CTLs, we found that a unique amino acid sequence motif, PDR, in CDR3 of the TCR-β chain of Tax-CTLs was conserved among ATL patients after allo-HSCT. Furthermore, the PDR Tax-CTL clones selectively expanded and showed strong cytotoxic activities against HTLV-1. On the other hand, it remains unclear how Tax-CTL repertoire exists in ACs. In this study, we comprehensively compared Tax-specific TCR repertoires at the single-cell level between ACs and ATL patients. Tax-CTLs showed highly restricted TCR repertoires with a strongly biased usage of BV7, and PDR, the unique motif in TCR-β CDR3, was conserved in all ACs and ATL patients, regardless of clinical subtype in HTLV-1 infection.