Ceramide and sphingosine-1-phosphate act as photodynamic therapy-elicited damage-associated molecular patterns: Cell surface exposure

•Ceramide and sphingosine-1-phosphate (S1P) cell levels change after photodynamic therapy (PDT).•Both ceramide and S1P appear on the surface of tumor cells treated by PDT.•Cell surface-exposed ceramide and S1P on PDT-treated tumor cells allow them to act as DAMPs.

Molecules that appear on the surface of tumor cells after their therapy treatment may have important roles either as damage-associated molecular patterns (DAMPs) or signals for phagocytes influencing the disposal of these cells. Treatment of SCCVII and CAL27 cells, models of mouse and human squamous cell carcinoma respectively, by photodynamic therapy (PDT) resulted in the presentation of ceramide and sphingosine-1-phosphate (S1P) on the cell surface. This was documented by anti-ceramide and anti-S1P antibody staining followed by flow cytometry. The exposure of these key sphingolipid molecules on PDT-treated tumor cells was PDT dose-dependent and it varied in intensity with different photosensitizers used for PDT. The above results, together with the finding that both ceramide and S1P can activate NFκB signaling in macrophages co-incubated with PDT-treated tumor cells, establish that these two sphingolipids can act as DAMPs stimulating inflammatory/immune reactions critical for tumor therapy response.