The CXCL10/CXCR3 axis cross-talk between emerging T cell acute lymphoblastic leukemia and thymic epithelial cells
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Within the thymus, the development of T-cells is controlled by specialized thymic epithelial cells (TECs). Based on their function and anatomic location TECs are separated into cortical and medullary subsets (cTECs and mTECs). cTECs express the indispensable NOTCH-ligand DLL4 controlling T cell lineage commitment while mTECs play a central role in T-cell negative selection. Acquired NOTCH1 gain-of-function mutations play a key role in acute T cell lymphoblastic leukemia (T-ALL) development. During T-ALL leukemogenesis aberrant expression of transcription, factors such as SCL and LMO1 block T cell differentiation and increased self-renewal. Since acquired NOTCH1 mutations are ligand-dependent to exert augmented signaling, we proposed DLL4-expressing TECs playing a critical role in T-ALL leukemogenesis. In the present study, we used the SCL/LMO1 T-ALL mouse model, murine TEC cell lines and human T-ALL cell lines to investigate TEC dynamics and function in the T-ALL context. First, we demonstrated in co-cultures that TEC cell lines possess in vitro T-ALL supporting potential, which was comparable to the mesenchymal cell line OP9. Next, we showed in the SCL/LMO1 T-ALL mouse model that preleukemic thymocytes displayed a striking upregulation of NOTCH1 target genes. Interestingly, fluorescence microscopy revealed a relative expansion of cortical and a relative reduction of the medullary thymic areas in SCL/LMO1 thymi. Correspondingly, absolute numbers of cTECs expanded while mTEC numbers declined. Gene expression profiling of sorted SCL/LMO1 cTECs revealed upregulation of the chemokines CXCL10 and CXCL12. Remarkably, CXCL12 produced by endothelial cells is a known factor controlling in vivo T-ALL maintenance. We moved on to study whether CXCL10 expression by TECs was T-ALL-dependent. Strikingly, we showed that CXCL10 upregulation in TEC cell lines could only be induced by direct co-culture with SCL/LMO1 cells while wild-type control cells did not alter TEC CXCL10 expression. Moreover, increased CXCL10 chemokine concentrations were detected in SCL/LMO1 thymic interstitial fluid. Next, the expression of the CXCL10 receptor CXCR3 was revealed on human T-ALL cell lines and on SCL/LMO1 thymocytes. Finally, we demonstrated a CXCL10 dependent pro-survival effect within cultured SCL/LMO1 thymocytes, which was associated with the activation of NOTCH1 signaling. In summary, the collected data support a novel T-ALL-promoting regulatory circuit in which emerging T-ALL lymphoblasts induce CXCL10 in expanding TECs which positively feeds back to T-ALL cells via the CXCL10 receptor CXCR3.
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