IntroductionHumoral immunity is a highly orchestrated process involving antigen-specific T-B cell interactions leading naive B cells to (1) rapidly become activated, proliferate and differentiate into short-lived plasma cells secreting low affinity antibodies, and (2) generate high-affinity antigen-specific antibody secreting B cells after somatic hypermutations and recombination of immunoglobulin genes in the germinal center. 1 This cellular process allows for the formation of memory B cells and long-lived antibodyforming cells (AFCs). 2 Generation and persistence of these cells are critical for the life-long production of high-affinity antibodies against the immunizing antigen which is an important component of immunologic memory. Apoptosis is indispensable for selection of high-affinity effector cells and for maintenance of self-tolerance. B cells expressing low affinity antibodies are deleted by apoptosis, whereas clones expressing BCR with enhanced affinity for the immunogen are positively selected. 1,3,4 Apoptosis is also crucial for immune system homeostasis by inducing the death of the clonally expanded lymphocytes once the antigen has been eliminated. 5 Proteins of the Bcl-2 family play a critical role in controlling the humoral immune response. Immunized transgenic mice overexpressing antiapoptotic Bcl-2 or Bcl-xL in their lymphocytes exhibit a profound increase in the numbers of antigen-specific B cells and antibody secreting plasma cells compared with wild type mice. 6-9 The Bcl-2 proteins are key regulators of cell survival and are classified into 3 sub-groups. 10 The pro-survival members (Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1) are essential for cell survival. Bax, Bak are proapoptotic and required for activation of the downstream phases of apoptosis, including permeabilization of the outer mitochondrial membrane (MOMP) with consequent activation of the caspase cascade that elicits cellular demolition.The so-called BH3-only proteins (Bad, Bid, Bim/Bod, Bik/Blk/ Nbk, Hrk/DP5, Bmf, Noxa, and Puma/Bbc3) share with each other and the wider Bcl-2 family only the BH3 region and are essential for initiation of apoptosis signaling. 11 BH3-only proteins play an important role in the homeostasis of the immune system. 5 For instance, Bim-deficient mice accumulate abnormally increased numbers of B cells and develop hypergammaglobulinemia, which, on a mixed C57BL/6 ϫ 129SV background, progresses to fatal immune complex mediated systemic lupus erythematosus (SLE)-like autoimmune kidney disease. 12 Moreover, immunized Bim Ϫ/Ϫ mice exhibited an abnormal excess of antigen-specific memory B cells and antibody-forming cells. 13 However, the less marked phenotype of the Bim Ϫ/Ϫ mice compared with the Bcl-2 transgenic mice indicates that other BH3-only proteins may also contribute to the apoptosis of activated B cells during humoral immune responses. Analyses of Bid Ϫ/Ϫ , Bad Ϫ/Ϫ , Bik Ϫ/Ϫ single knock-out as well as Bim Ϫ/Ϫ Bid Ϫ/Ϫ , Bim Ϫ/Ϫ Bad Ϫ/Ϫ and Bim Ϫ/Ϫ Bik Ϫ/Ϫ mice have demonstrated that Bid, Bad and Bik are not...

Aryl Hydrocarbon Receptor (AHR) is an ubiquitous basic helix-loop-helix transcription factor, which is ligand-activated and involved in numerous biological processes including cell division, cell quiescence and inflammation. It has been shown that AHR is involved in normal hematopoietic progenitor proliferation in human cells. In addition, loss of AHR in knockout mice is accompanied by a myeloproliferative syndrome-like disease, suggesting a role of AHR in hematopoietic stem cell (HSC) maintenance. To study the potential role of AHR pathway in CML progenitors and stem cells, we have first evaluated the expression of AHR in UT-7 cell line expressing BCR-ABL. AHR expression was highly reduced in UT-7 cell expressing BCR-ABL as compared to controls. AHR transcript levels, quantified in primary peripheral blood CML cells at diagnosis (n = 31 patients) were found to be significantly reduced compared to healthy controls (n = 15). The use of StemRegenin (SR1), an AHR antagonist, induced a marked expansion of total leukemic cells and leukemic CD34+ cells by about 4- and 10-fold respectively. SR1-treated CML CD34+ cells generated more colony-forming cells and long-term culture initiating cell (LTC-IC)–derived progenitors as compared to non-SR1-treated counterparts. Conversely, treatment of CML CD34+ cells with FICZ, a natural agonist of AHR, induced a 3-fold decrease in the number of CD34+ cells in culture after 7 days. Moreover, a 4-day FICZ treatment was sufficient to significantly reduce the clonogenic potential of CML CD34+ cells and this effect was synergized by Imatinib and Dasatinib treatments. Similarly, a 3-day FICZ treatment contributed to hinder significantly the number of LTC-IC-derived progenitors without synergistic effect with Imatinib. The analysis of molecular circuitry of AHR signaling in CML showed a transcriptional signature in CML derived CD34+ CD38- primitive cells with either low or high levels of AHR, with an upregulation of myeloid genes involved in differentiation in the “AHR low” fraction and an upregulation of genes involved in stem cell maintenance in the “AHR high” fraction. In conclusion, these findings demonstrate for the first time that down-regulation of AHR expression, a major cell cycle regulator, is involved in the myeloproliferative phenotype associated with CML. AHR agonists inhibit clonogenic and LTC-IC-derived progenitor growth and they could be used in leukemic stem cell targeting in CML.