Figure 3.

IL-6 is a major mediator of the cross talk between tumors and MDPs. (A) Mapping upregulated cytokines in melanoma met-high TCM with their corresponding receptors, differentially expressed by progenitor populations, as identified by single-cell RNA sequencing (Wilcoxon rank-sum test followed by Benjamini-Hochberg correction). (B) IL-6 levels were quantified by ELISA in met-low and met-high TCM of melanoma and breast cancer models (n=5–8= mice/group). (C) IL-6Ra expression in the MDP population in the bone marrow of met-low and met-high melanoma-bearing mice, based on scRNA-seq data set (average fold-change=1.21, p=0.02, Wilcoxon rank-sum test). (D) Single-cell IL-6Ra messenger RNA expression labeling on the UMAP plot. (E) MDPs from naïve mice were grown in Methocult medium supplemented with met-low or met-high melanoma TCM in the presence or absence of anti-IL-6 neutralizing antibodies. MDP-derived colonies were counted (n=3 biological repeats/group). (F–G) MDPs obtained from the bone marrow of naïve mice were stimulated with escalating doses of IL-6. The levels of p-STAT3 (F) and IL-6Ra (G) were determined by flow cytometry. Statistical significance was assessed by one-way analysis of varaince, followed by Tukey post hoc test when comparing more than two groups or unpaired two-tailed t-test when comparing two groups. Asterisks represent significance from control, unless indicated otherwise in the figure. Significant p values are shown as *p<0.05; **p<0.01. BM, bone marrow; GMP, granulocyte-monocyte progenitor; IL-6, interleukin 6; IL-6Ra, interleukin 6 receptor; MDP, monocyte-dendritic progenitor; met-high, high metastatic potential; met-low, low metastatic potential; MLP, multilymphoid progenitor; SC.CMP.DR, stem cell - common myeloid progenitor; TCM, tumor-conditioned medium; UMAP, uniform manifold approximation and projection.