Spleen route accelerates engraftment of human hematopoietic stem cells.

Intravenous injections of human hematopoietic stem cells (hHSCs) is routinely used in clinic and for modeling hematopoiesis in mice. However, unspecific dilution in vascular system and non-hematopoietic organs challenges engraftment efficiency. Although spleen is capable of extra medullar hematopoiesis, its ability to support human HSC transplantation has never been evaluated. We demonstrate that intra-splenic injection results in high and sustained engraftment of hHSCs into immune-deficient mice, with higher chimerisms than with intravenous or intra-femoral injections. Our results support that spleen microenvironment provides a niche for HSCs amplification and offers a new route for efficient HSC transplantation.

Leukemia-initiating cell activity requires calcineurin in T-cell acute lymphoblastic leukemia.

Despite their initial efficient response to induction chemotherapy, relapse remains frequent in patients with T-cell acute lymphoblastic leukemia (T-ALL), an aggressive malignancy of immature T-cell progenitors. We previously reported sustained calcineurin (Cn) activation in human lymphoid malignancies, and showed that Cn inhibitors have antileukemic effects in mouse models of T-ALL. It was unclear, however, from these studies whether these effects resulted from Cn inhibition in leukemic cells themselves or were an indirect consequence of impaired Cn function in the supportive tumor microenvironment. We thus generated a Notch (intracellular Notch 1, ICN1)-induced T-ALL mouse model, in which conditional Cn genetic deletion is restricted to leukemic cells. Ex vivo, Cn deletion altered the adhesive interactions between leukemic cells and their supportive stroma, leukemic cell survival, proliferation, migration and clonogenic potential. In vivo, Cn activation was found to be critical for leukemia initiating/propagating cell activity as demonstrated by the failure of Cn-deficient leukemic cells to transplant the disease to syngeneic recipient mice. Importantly, combination of vincristine treatment with Cre-mediated Cn ablation cooperated to induce long-term remission of ICN1-induced T-ALL. These findings indicate that Cn is a promising target in T-ALL relapse prevention, and call for clinical trials incorporating Cn inhibitors during consolidation therapy.

Expression of CD34 and CD7 on human T-cell acute lymphoblastic leukemia discriminates functionally heterogeneous cell populations.

Leukemia-initiating/repopulating cells (LICs), also named leukemic stem cells, are responsible for propagating human acute leukemia. Although they have been characterized in various leukemias, their role in T-cell acute lymphoblastic leukemia (T-ALL) is unclear. To identify and characterize LICs in T-ALL (T-LIC), we fractionated peripheral blood cell populations from patient samples by flow cytometry into three cell fractions by using two markers: CD34 (a marker of immature cells and LICs) and CD7 (a marker of early T-cell differentiation). We tested these populations in both in vitro culture assays and in vivo for growth and leukemia development in immune-deficient mice. We found LIC activity in CD7(+) cells only as CD34(+)CD7(-) cells contained normal human progenitors and hematopoietic stem cells that differentiated into T, B lymphoid and myeloid cells. In contrast, CD34(+)CD7(+) cells were enriched in LICs, when compared with CD34(-)CD7(+) cells. These CD34(+)CD7(+) cells also proliferated more upon NOTCH activation than CD34(-)CD7(+) cells and were sensitive to dexamethasone and NOTCH inhibitors. These data show that CD34 and CD7 expression in human T-ALL samples help in discriminating heterogeneous cell populations endowed with different LIC activity, proliferation capacity and responses to drugs.