Fig. 2

MSCs increase the release of CCL2 upon contact with NK cells and CD56^bright NK cells express CCR2. a, b Conditioned media (CM) from NK cells, MSCs, MSC/NK cell co-cultures, and cell-free medium were prepared as described in Methods and the content of CCL2 was quantified. a Bar graph showing mean ± SD of 4–5 replicates from one representative experiment. b Cumulative data of 15 experiments. No CCL2 was detectable in CM from NK cells only. Statistical analysis was performed using one-way ANOVA followed by Bonferroni multiple comparison test. c, d PBMC were stained with fluorochrome-labeled antibodies against CD3, CD56, and CCR2. c Representative dot plot of CCR2 expression on gated CD3^–CD56^bright and CD3^–CD56^dim NK cells (gated as shown in Fig. 1d). Numbers indicate the percentage of cells positive for CCR2. The threshold of positive staining for CCR2 was set according to the isotype control (iso). d Scatter plot showing the percentage of CCR2⁺ cells among CD56^bright and CD56^dim NK cells from individual healthy donors (n = 27). Horizontal lines indicate the median with interquartile range. Statistical analysis was performed using the Wilcoxon signed-rank test. ***p < 0.001. MSC mesenchymal stromal/stem cell, NK natural killer, CCL2 C-C chemokine ligand 2, CCR2 C-C chemokine receptor 2