Triple-negative breast cancer (TNBC) is an aggressive tumor entity, in which immune checkpoint (IC) molecules are primarily synthesized in the tumor environment. Here, we report that procoagulant platelets bear large amounts of such immunomodulatory factors and that the presence of these cellular blood components in TNBC relates to pro-tumorigenic immune cell activity and impaired survival. Mechanistically, tumor-released nucleic acids attract platelets into the aberrant tumor …