Fig. 4: CXCL4 induce myeloid-derived suppressor cells, which inhibit CD8⁺ T cells and promote metastasis.

CXCL4 expressed in metastatic tumours makes non-metastatic 67NR tumours to become metastatic in wild-type BALB/c mice (a–c). a Heatmap of mouse cytokine expression array shows CXCL4 (highlighted in yellow) being expressed selectively in metastatic tumours. 67NR cell line (i) and 4T1 cell line (ii); 67NR tumours (iii) and 4T1 tumours (iv) harvested from wild-type BALB/c mice; 67NR tumours (v) and 4T1 tumours (vi) tumours harvested from Nude mice lacking T cells. b Flow cytometry analysis for MDSCs in bone marrow cell samples treated with vehicle, CXCL4, platelets or GM-CSF. c Quantification of MDSCs in bone marrow cell samples treated with vehicle, CXCL4, platelets or GM-CSF (n = 4). In wild-type BALB/c mice, CXCL4 induces non-metastatic 67NR cells to become metastatic from the mammary fat pad to the lung (d–k). d A cartoon showing the mammary fat pad injection (top). Representative images of whole-body bioluminescence of BALB/c wild-type mice harbouring 67NR tumour in the mammary fat pad and treated with vehicle or CXCL4 (middle) and of tumours excised from these mice (bottom) (n = 10). e The mean tumour volume of the tumours excised from these mice (n = 10). f Representative H&E staining of tumours excised from these mice (n = 5). Scale bar: 200 µm. g Representative bioluminescence images of lungs excised from BALB/c wild-type mice harbouring 67NR tumour in the mammary fat pad and treated with vehicle or CXCL4 (top). h The mean total flux from these lungs (bottom) (n = 10). i The number of visible lung nodules in vehicle- or CXCL4-treated tumour-bearing BALB/c wild-type mice (n = 7). j H&E staining of representative lungs from BALB/c mice harbouring 67NR tumour in the mammary fat pad and treated with vehicle or CXCL4. Scale bar: 200 µm. k The number of CTCs isolated from BALB/c wild-type mice harbouring 67NR tumour in the mammary fat pad and treated with vehicle or CXCL4 (n = 5). Quantitative data are mean ± SEM. Statistical analyses were performed by unpaired two-tailed Student’s t test for panels c, e, i and k or by F test for panel h; ns indicates not significant, ^∗P ≤ 0.05, ^∗∗P ≤ 0.01, ^∗∗∗∗P ≤ 0.0001.