Sphingosine-1-phosphate facilitates trafficking of hematopoietic stem cells and their mobilization by CXCR4 antagonists in mice.

CXCL12 and VCAM1 retain hematopoietic stem cells (HSCs) in the BM, but the factors mediating HSC egress from the BM to the blood are not known. The sphingosine-1-phosphate receptor 1 (S1P(1)) is expressed on HSCs, and S1P facilitates the egress of committed hematopoietic progenitors from the BM into the blood. In the present study, we show that both the S1P gradient between the BM and the blood and the expression of S1P(1) are essential for optimal HSC mobilization by CXCR4 antagonists, including AMD3100, and for the trafficking of HSCs during steady-state hematopoiesis. We also demonstrate that the S1P(1) agonist SEW2871 increases AMD3100-induced HSC and progenitor cell mobilization. These results suggest that the combination of a CXCR4 antagonist and a S1P(1) agonist may prove to be sufficient for mobilizing HSCs in normal donors for transplantation purposes, potentially providing a single mobilization procedure and eliminating the need to expose normal donors to G-CSF with its associated side effects.

Potentiating effects of RAD001 (Everolimus) on vincristine therapy in childhood acute lymphoblastic leukemia.

Despite advances in the treatment of acute lymphoblastic leukemia (ALL), the majority of children who relapse still die of ALL. Therefore, the development of more potent but less toxic drugs for the treatment of ALL is imperative. We investigated the effects of the mammalian target of rapamycin inhibitor, RAD001 (Everolimus), in a nonobese diabetic/severe combined immunodeficiency model of human childhood B-cell progenitor ALL. RAD001 treatment of established disease increased the median survival of mice from 21.3 days to 42.3 days (P < .02). RAD001 together with vincristine significantly increased survival compared with either treatment alone (P < .02). RAD001 induced a cell-cycle arrest in the G(0/1) phase with associated dephosphorylation of the retinoblastoma protein, and reduced levels of cyclin-dependent kinases 4 and 6. Ultrastructure analysis demonstrated the presence of autophagy and limited apoptosis in cells of RAD001-treated animals. In contrast, cleaved poly(ADP-ribose) polymerase suggested apoptosis in cells from animals treated with vincristine or the combination of RAD001 and vincristine, but not in those receiving RAD001 alone. In conclusion, we have demonstrated activity of RAD001 in an in vivo leukemia model supporting further clinical development of target of rapamycin inhibitors for the treatment of patients with ALL.

CXCR4 mediates the homing of B cell progenitor acute lymphoblastic leukaemia cells to the bone marrow via activation of p38MAPK.

The mechanisms regulating the migration of leukaemic cells between the blood and bone marrow compartments remain obscure, but are of fundamental importance for the dissemination of the disease. This study investigated the in vivo homing of human B cell progenitor acute lymphoblastic leukaemia (ALL) cells to the femoral bone marrow of non-obese diabetic severe combined immunodeficient (NOD/SCID) mice. It was demonstrated that patient ALL cells use the chemokine axis, chemokine (CXC motif) receptor 4 (CXCR4)/ chemokine (CXC motif) ligand 12 (CXCL12), to home to the femoral marrow. CXCL12-mediated signalling through p38 mitogen-activated protein kinase (MAPK) was required for optimal homing. In contrast, the homing of normal peripheral blood CD34(+) cells and the cytokine-dependent CD34(+) cell line Mo7e was independent of p38MAPK, consistent with the dependence of these cells, as well as normal CD34(+) CD19(+) B cell progenitors, on PI-3K/AKT signalling. Altogether, our data provide clarification of the direct role of CXCL12 in the bone marrow homing of ALL cells and demonstrate unique signalling molecule usage that may have therapeutic implications for this disease.

BAFF selectively enhances the survival of plasmablasts generated from human memory B cells.

The generation of Ig-secreting cells (ISCs) from memory B cells requires interactions between antigen-specific (Ag-specific) B cells, T cells, and dendritic cells. This process must be strictly regulated to ensure sufficient humoral immunity while avoiding production of pathogenic autoantibodies. BAFF, a member of the TNF family, is a key regulator of B cell homeostasis. BAFF exerts its effect by binding to three receptors - transmembrane activator of and CAML interactor (TACI), B cell maturation antigen (BCMA), and BAFF receptor (BAFF-R). To elucidate the contribution of BAFF to the differentiation of B cells into ISCs, we tracked the fate of human memory B cells stimulated with BAFF or CD40L. BAFF and CD40L significantly increased the overall number of surviving B cells. This was achieved via distinct mechanisms. CD40L induced proliferation of nondifferentiated blasts, while BAFF prevented apoptosis of ISCs without enhancing proliferation. The altered responsiveness of activated memory B cells to CD40L and BAFF correlated with changes in surface phenotype such that expression of CD40 and BAFF-R were reduced on ISCs while BCMA was induced. These results suggest BAFF may enhance humoral immunity in vivo by promoting survival of ISCs via a BCMA-dependent mechanism. These findings have wide-ranging implications for the treatment of human immunodeficiencies as well as autoimmune diseases.

mTOR inhibition by everolimus in childhood acute lymphoblastic leukemia induces caspase-independent cell death.

Increasingly, anti-cancer medications are being reported to induce cell death mechanisms other than apoptosis. Activating alternate death mechanisms introduces the potential to kill cells that have defects in their apoptotic machinery, as is commonly observed in cancer cells, including in hematological malignancies. We, and others, have previously reported that the mTOR inhibitor everolimus has pre-clinical efficacy and induces caspase-independent cell death in acute lymphoblastic leukemia cells. Furthermore, everolimus is currently in clinical trial for acute lymphoblastic leukemia. Here we characterize the death mechanism activated by everolimus in acute lymphoblastic leukemia cells. We find that cell death is caspase-independent and lacks the morphology associated with apoptosis. Although mitochondrial depolarization is an early event, permeabilization of the outer mitochondrial membrane only occurs after cell death has occurred. While morphological and biochemical evidence shows that autophagy is clearly present it is not responsible for the observed cell death. There are a number of features consistent with paraptosis including morphology, caspase-independence, and the requirement for new protein synthesis. However in contrast to some reports of paraptosis, the activation of JNK signaling was not required for everolimus-induced cell death. Overall in acute lymphoblastic leukemia cells everolimus induces a cell death that resembles paraptosis.

Excess BAFF rescues self-reactive B cells from peripheral deletion and allows them to enter forbidden follicular and marginal zone niches.

The role of BAFF in B cell self tolerance was examined by tracking the fate of anti-HEL self-reactive B cells in BAFF transgenic mice using four different models of self-reactive B cell deletion. BAFF overexpression did not affect the development of self-reactive B cells normally deleted in the bone marrow or during the early stages of peripheral development. By contrast, self-reactive B cells normally deleted around the late T2 stage of peripheral development were rescued from deletion, matured, and colonized the splenic follicle. Furthermore, self-reactive B cells normally selectively deleted from the marginal zone repopulated this compartment when excess BAFF was present. Self-reactive B cells rescued by excess BAFF were not anergic. BAFF overexpression therefore rescued only self-reactive B cells normally deleted with relatively low stringency and facilitated their migration into otherwise forbidden microenvironments. This partial subversion of B cell self tolerance is likely to underlie the autoimmunity associated with BAFF overexpression.