The cell-biological program termed the epithelial-mesenchymal transition (EMT) confers on cancer cells mesenchymal traits and an ability to enter the cancer stem cell (CSC) state. In addition, EMT upregulates the expression of cell surface receptors that enable CSCs to interact with their surrounding microenvironment in order to sustain their stem cell state. We discovered that EphA4 and CD90, two cell surface proteins upregulated in the carcinoma cells by the EMT, mediate juxtacrine signalling between the CSCs and tumor-associated monocytes and macrophages (TAMs). When engaged with its counter-receptor on TAMs, EphA4 on the CSCs activates Src and PLCγ1, the latter resulting in the translocation of NF-κB into the nucleus, which cooperates with Twist to drive the rapid induction of a variety of cytokines in the CSCs. Among the secreted cytokines IL-8 and IL-6 maintain the CSC-state by reinforcing the expression of the EMT-inducing transcription factors, whereas other cytokines GM-CSF, M-CSF and IL-10 recruit monocytes and induce their differentiation into M2-like macrophages. Together these cytokines perpetuate the CSC-promoting interactions between CSCs and TAMs. Indeed, admixed TAMs promote the tumor-initiating ability of CSCs whereas ablation of endogenous macrophages substantially diminished tumor initiation. Importantly, the juxtaposition of TAMs with the mesenchymal-like CD90high CSCs can be observed at the invading edge in primary human breast tumor sections and xenograft tumors. On the contrary, we rarely find TAMS infiltrating beyond the edge of tumors into the regions where the CD90neg non-stem carcinoma cells reside. These findings underscore the importance of TAMs as critical components of the CSC niche and highlight their potential as therapeutics targets.

Citation Format: Haihui Lu, Wai Leong Tam, Vera Donnenberg, Xin Ye, Rohit Bhargava, Robert Weinberg. Juxtacrine signaling between the tumor-associated macrophages and the breast cancer stem cells contribute to the cancer stem cell niche by inducing a cytokine signaling network. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr B150.