Identification of the Lymphoma-Initiating Cell Population in a T Cell Lymphoma Mouse Model Resembling Human Anaplastic Large Cell Lymphoma

In 60% of anaplastic large cell lymphoma (ALCL) patients a translocation t(2;5) (p23;q35) is found, which results in NPM-ALK fusion gene expression and constitutive activation of the ALK tyrosine kinase. Immunophenotypic characterization of human ALCLs revealed highly CD30-positive cells of T- or Null-cell-origin. However, the origin of the lymphoma initiating cell population as well as NPM-ALK signal transduction in course of the disease remains unclear.

In this regard, we established a retroviral murine bone marrow transplantation model resembling human ALCL. Therefore we use an inducible Cre/loxP system where NPM-ALK expression is restricted to early T cells. We infected bone marrow of Lck-Cre transgenic mice with our MSCV-Stop-NPM-ALK-IRES-EGFP vector and transplanted it into lethally irradiated recipient mice. With a latency of 4-5 months, these mice developed Thy1.2-positive lymphomas and died from neoplastic T cell infiltration of bone marrow and lymphatic organs. Immunophenotypic analysis confirmed T cell origin of the lymphomas with the characteristic high CD30 expression. Staining of the T cell subpopulations demonstrated high NPM-ALK expression in immature CD4^-/CD8^- double negative T cells and undifferentiated CD4⁺/CD8⁺ double positive T cells. Interestingly, FACS-staining for the proliferation marker Ki-67 as well as the activation marker CD30 revealed highest expression in the CD4^-/CD8^- double negative T cells. Therefore we hypothesized that the lymphoma-initiating cell must be within this early T cell population.

To substantiate our hypothesis we performed secondary transplantations with sorted T cell subpopulations and indeed, only the CD4^-/CD8^- double negative population was able to initiate T cell lymphoma in the recipient mice. Immunophenotypic characterization of the lymphoma population of these secondary transplanted mice revealed undifferentiated T cells of all CD4/CD8 subtypes, which argues for the existence of a lymphoma initiating cell population, which can still partly differentiate. Interestingly the CD4^-/CD8^- double negative lymphoma population aberrantly expressed the T cell receptor alpha/beta chain, which may allow these early T cells to establish a systemic lymphoma. Further analysis of the lymphoma population showed lymphatic precursors (CLP) as well as multipotent progenitors (MMP) and haematopoetic stem cells (LSK), which suggests early bone marrow or thymic progenitor cells as the pool of the lymphoma-initiating cell population.

We therefore were able to prove the existence of lymphoma initiating stem cells in a highly relevant NPM-ALK positive CD30 expressing mouse model of ALCL. Further analysis will give insides into eradication of the identified lymphoma stem cell population by clinical relevant NPM-ALK inhibitors and CD30 immunotoxins.