Conditional immortalization of lymphoid progenitors via tetracycline-regulated LMO2 expression.

Conditional immortalization of hematopoietic progenitors via lentiviral expression of selected transcription factors in hematopoietic stem and progenitor cells provides a promising tool to study stem cell and leukemia biology. Here, to generate conditionally immortalized lymphoid progenitor (ciLP) cell lines, murine hematopoietic progenitor cells were transduced with an inducible lentiviral "all-in-one" vector expressing LMO2 under doxycycline (DOX) stimulation and the reverse tetracycline-regulated trans-activator (rtTA3). For selection of LMO2-expressing ciLPs (LMO2-ciLPs) and longitudinal manipulation in T-cell differentiation lymphoid conditions, we developed a robust approach based on co-culture with OP9 DL1 stromal cells and improved cytokine conditions allowing a controlled balance between cell proliferation and differentiation in vitro. LMO2-ciLP cell lines with the highest proliferation, vector copy number and similar insertion pattern were selected for LMO2 "on/off" in vitro study. LMO2 expression under DOX induction resulted in a double negative (DN) 2 differentiation arrest and a propagation of CD44+CD25- myeloid cell population characterized by lymphoid and myeloid phenotypes. Both DN2 and CD44+CD25- myeloid cell subpopulations expressed c-KIT, suggesting that LMO2-ciLPs were similar to uncommitted progenitors under DOX supplementation. DOX removal resulted in cessation of ectopic LMO2 expression and LMO2-ciLPs continued T-cell lymphoid differentiation accompanied by c-KIT down-regulation and interleukin 7 receptor expression. Switching off LMO2 expression was accompanied by increased Notch-signaling and significant reduction of the CD44+CD25- myeloid cell population under T-cell differentiation lymphoid conditions. Although vector insertions in cooperation with LMO2 expression could influence the fate of LMO2-ciLPs and additional experiments are required to evaluate it, our approach provides a promising tool to investigate mechanisms underlying stem cell, leukemia and lymphocyte biology, leading to novel approaches for disease modelling and therapy evaluation.