PF4 and cancer: We also analysed TCGA transcriptomics data for correlations between CXCL4-producing platelets, MDSCs, and CD8+ T cells using the Gene Set Variation Analysis method39 with immune marker gene sets previously described.40,41 We observed a striking negative correlation between CD8+ T cells and CXCL4-producing platelets in breast tumours (R = −0.32, P < 2.2 × 10−16, Fig. 5f, Table 2) as well as a strong positive correlation between platelets and MDSCs (R = 0.67, P < 2.2 × 10−16) across cancer types (Table 2).