PF4 and head and neck squamous cell carcinoma: Moreover, ELISA measurements of cytokine CXCL4 (ng/mL) in cell culture supernatants revealed significantly increased levels of CXCL4 secretion by THP-1 monocytes in response to incubation with plasma-derived sEVs from HNSCC patients (pre- and post-RCT) compared to sEVs from healthy donors (Figure 5).