CXCL12-Expressing Bone Marrow Niches Differentially Regulate CML Compared to Normal Stem Cells

CXCL12 plays a major role in hematopoietic stem cells (HSC) localization to regulatory niches in the bone marrow microenvironment (BMM). Previous studies indicate that CXCL12 deletion from Prx+ mesenchymal stem cells (MSC) reduces murine HSC numbers, and from Tek+ endothelial cells (EC) modestly reduces their repopulating activity. In contrast, CXCL12 deletion from Ocn+ osteoblasts or Osx+ osteoprogenitors does not affect HSC function (Greenbaum et al. Nature. 2013 Mar 14;495(7440):227-30; Ding et al. Nature. 2013 Mar 14;495(7440):231-5.). These studies identified MSC and endothelial cells as important HSC niche components. However, niche cells for CML leukemic stem cells (LSC) remain poorly characterized. To evaluate the contribution of CXCL12-expressing populations to LSC regulation, we transplanted normal and CML BM cells (CD45.1/2+; 2*10⁶/mouse) into CXCL12^f/f (loxP sites flanking exon 2) mice crossed with Tek-Cre and Prx-Cre mice ( reported in ASH 2016, Blood, Abstract: 26) . We showed that CXCL12 deletion from Prx+ MSC resulted in reduction in normal HSC (LSK Flt3- CD150+ CD48- cells) numbers and function compared to controls, but did not observe significant reduction of HSC with CXCL12 deletion from Tek+ EC. Interestingly, transplantation of CML cells into mice with CXCL12 deleted from Prx+ MSC led to reduced survival compared to control mice associated with increased WBC counts, BM cellularity, LSC numbers, and secondary repopulation capacity. In contrast, CXCL12 deletion from Tek+ cells led to enhanced survival, associated with reduced BM cellularity and reduced LSC numbers. In further studies, we transplanted mixtures of equal numbers of normal and CML cells into Prx-Cre and Tek-Cre mice, and found a marked growth advantage of LSC compared with HSC in Prx-Cre mice, and a growth advantage of HSC over LSC in Tek-Cre knockout mice. We also show that both HSC and LSC demonstrate increased cell cycling via EdU labeling. Consistent with this, we found that Nilotinib treatment leads to significantly greater reduction in BM cellularity, LSC numbers and secondary repopulation potential in Prx-Cre mice, compared to Tek-Cre or WT mice. To evaluate the effects of CML on CXCL12-expressing BMM cells, we crossed CXCL12^GFP (GFP reporter knocked into the CXCL12 locus) and SCL-tTA-BCR-ABL mice to generate CXCL12^GFP-BCR-ABL mice. CML development lead to significantly reduced numbers of GFP+ BM stromal cells (CD45-Ter119-31-) but increased GFP+ endothelial cells (CD45-Ter119-CD31+), and within the stromal population, the number of GFP+ MSC (PDGFRα+Sca-1+) were decreased compared to WT mice. To further understand niche mediated interactions that regulate LSC fate, we studied gene expression profiles of LSC from mice following Cre-mediated targeting of CXCL12, compared to LSC from Cre-negative mice. Our preliminary analysis indicates significant enrichment of genes associated with cell cycle progression and Myc activation, and negative enrichment of genes related to TGF-b, STAT3, and Polycomb Repressive Complex 2 (PRC2) activity in LSC from Prx1+Cre CXCL12^f/f mice. Reduced TGF-b, STAT3 and PRC2/EZH2 activity represent potential mechanisms for regulation of CML LSC maintenance and/or TKI resistance, and may potentially contribute to Prx1+ MSC-mediated LSC regulation. We conclude that CXCL12+ BM MSC inhibit CML development, maintain LSC quiescence, and enhance TKI resistance, while CXCL12+ EC enhance LSC maintenance and disease development. Leukemia associated reduction in CXCL12+MSC and increase in CXCL12+EC may enhance CML development. Collectively, these results reveal important and distinct niche functions for CXCL12 expressing BM MSC and EC for CML LSC compared with normal HSC.