Role of Enhanced Microenvironmental Interleukin-1 (IL-1) Expression and Increased IL-1 Responsiveness in Persistence of Leukemia Stem Cells in TKI Treated CML Patients

BCR-ABL tyrosine kinase inhibitors (TKI), although highly effective in the treatment of chronic myelogenous leukemia (CML) patients, fail to eliminate leukemia stem cells (LSC), which remain a potential source of relapse. In previous studies we have shown that altered expression of inflammatory cytokines in the CML bone marrow (BM) microenvironment provides a selective growth advantage to CML compared with normal long term hematopoietic stem cells (LTHSC) (Cancer Cell 2012, 21:577). Our studies suggest an important role for the pivotal pro-inflammatory cytokine Interleukin-1α/β (IL-1α/β) in selectively promoting growth of CML LTHSC. Using a transgenic BCR-ABL mouse model of CML and human CML and normal CD34+CD38- cells, we showed that inhibition of IL-1 signaling using recombinant IL-1 receptor antagonist (IL-1RA) in combination with nilotinib (NIL) resulted in significantly greater inhibition of CML LSC, compared with NIL alone (Blood 2013, abstract 512). To further investigate the mechanisms underlying increased IL-1 sensitivity of CML stem cells, we evaluated expression of the IL-1 receptor components, IL-1 receptor-associated protein (IL-1RAP) and IL-1R1, on CML and normal stem cells using flow cytometry. Expression of both IL-1RAP and IL-1R1 were increased on primary CML CD34+CD38-CD90+ cells compared to their normal counterparts (n=5, p<0.05). Exposure to IL-1α (10ng/ml) resulted in increased expression of p-NF-kB (p65), p-p38 MAPK and p-JNK in CML compared to normal CD34+CD38-CD90+ cells as evaluated by flow cytometry, indicating enhanced sensitivity to IL-1 induced signaling (n=5, p<0.05). The expression of p-NF-kB(p65), p-p38 MAPK and p-JNK in CML CD34+CD38-CD90+ cells cultured in CML BM conditioned medium (CM) was reduced after treatment with NIL or IL-1RA, and further reduced by the combination of NIL and IL-1RA (n=4, p<0.05). Immunohistochemistry (IHC) analysis showed that nuclear NF-κB p65 protein was reduced in NIL and IL-1RA treated CML CD34+CD38-CD90+ cells compared with controls. Treatment with NIL and IL-1RA also significantly reduced expression of the NF-κB target genes NFκB1A, BCL2L1, BIRC3 and CD83 (n=6, p<0.001), and of the inflammatory cytokines IL6, CXCL1, CXCL2, CCL2, CCL3, CCL4 and TNF-α (n=6, p<0.05), as assessed by Q-RT-PCR. We evaluated IL-1 expression in BM samples from CML patients with undetectable minimal residual disease (UMRD) using Q-RT-PCR. Interestingly IL-1α, but not IL-1β, expression was increased in BM samples from CML patients with UMRD compared to normal BM samples (n=12, p<0.05). To evaluate the source of increased IL-1α expression we analyzed selected monocyte (CD45+CD14+), non-monocytic myeloid cell (CD45+CD14-CD33+), T cell (CD45+CD14-CD33-CD3+), B cell (CD45+CD14-CD33-CD19+), endothelial cell (CD45-GPA-CD31+) and mesenchymal cell (CD45-GPA-CD31-) populations from BM samples obtained from CML patients with UMRD and from normal healthy controls. These studies revealed significantly elevated IL-1α expression in BM CD14+ monocytic and CD31+ endothelial cells from CML patients with UMRD compared to normal controls (n=12, p<0.05). Our studies indicate that CML LSC demonstrate increased IL-1 receptor expression and IL-1 induced NF-kB, p38 MAPK and JNK signaling. We also observe enhanced IL-1α expression in BM endothelial and monocytic cells from CML patients achieving UMRD, indicating persistence of an inflammatory microenvironment that may contribute to persistence of residual LSC. Our studies provide a strong rationale for the application of anti-IL-1 directed strategies to inhibit inflammatory signaling and enhance LSC elimination in TKI treated CML patients.