Background
Mature T-cell lymphomas (TCLs) are rare, clinically heterogeneous hematologic cancers of high medical need. TCLs have inferior prognosis compared with their B-cell counterparts, which is attributed to poor understanding of their pathogenesis. Based on phenotypic similarities between normal and neoplastic lymphocytes it has been assumed that TCLs develop in the periphery, directly from various subtypes of normal T-cells.
Methods and findings
To address the debated question of the cell of origin in TCLs we analyzed to identify the highly variable complementarity determining regions (CDR3) regions of T-cell receptor (TCR) to trace the clonal history of the T-cells. We have collected previously published whole genome -exome, and -transcriptome sequencing data from 574 TCL patients comprising five nodal lymphomas [anaplastic large cell lymphoma (n=67), peripheral T-cell lymphoma (PTCL, n=55), adult T-cell lymphoma/leukemia (n=135), natural killer T-cell lymphoma (NKCL, n=25), not specified/other (n=30)] and three extranodal, cutaneous T-cell lymphomas [mycosis fungoides (n=122), Sezary syndrome (n=130), and subcutaneous panniculitis like T-cell lymphoma (n=10)]. TCR clonotypes contained in the tumor cell fraction, representing the clonotypes of malignant cells, were identified by de novo assembly of CDR3 regions of TCRγ, β and α. We have found that the vast majority of TCLs are clonotypically oligoclonal, although the pattern oligoclonality varied. Anaplastic large cell lymphoma was most diverse comprising multiple clonotypes of TCRγ, β and α whereas adult T-cell lymphoma/leukemia and peripheral T-cell lymphomas often showed monoclonality for TCRγ and β but had diverse TCRα clonotypes. These patterns of rearrangements were not compatible with the current mature T-cell precursor model and indicated that TCLs are initiated at the level of the lymphoid precursor. In keeping with this hypothesis, TCR rearrangements in TCLs resembled the pattern seen in the human thymus showing biased usage of V and J segments of high combinatorial probability resulting in recurrent, public CDR3 sequences shared between unrelated patients and across different clinical TCL entities. Frequencies of malignant clonotypes followed Zipf-Mandelbrot scaling law suggesting that TCLs comprise an interconnected system of expanding tumor clones.
The major limitation of this study is that it is based on the analysis of the TCR clonotypes and does not directly inform about developmental trajectories of cellular clones.
Conclusions
Lymphoid precursors are the likely cells of origin for mature T-cell lymphomas. Anaplastic large cell lymphoma seems to be derived from the most immature precursors with germline TCR whereas peripheral T-cell lymphoma and adult T-cell lymphoma/leukemia map to the later stages after TCR lower case Greek beta rearrangement stage. Clonotypically diverse initiating cells may seed target tissues being responsible for disease relapses after therapy.
Peripheral T-cell lymphoma (excluding anaplastic large-cell lymphoma): results from the Non-Hodgkin’s Lymphoma Classification Project