HMGB1 Mediates Post-PDT Immune Checkpoint Upregulation on T Cells Through NF-κB Signaling Pathway

The limited abscopal effect of photodynamic therapy (PDT)-induced immune responses underscores the need to investigate mechanisms underlying PDT's failure to inhibit distant tumors. A breast cancer dual-tumor mouse model was established. Post-PDT, flow cytometry, ELISA, immunofluorescence, and Western blot were employed to analyze HMGB1 release, T-cell profiles, immune checkpoint molecules (PD-1/LAG-3), NF-κB signaling (p-p65, p-p65/p65 ratio), and survival outcomes. PDT failed to suppress distant tumor growth or prolong survival in dual-tumor models. However, it increased tumor-infiltrating T cells while upregulating PD-1/LAG-3 expression on T cells. PDT-treated tumor cells released elevated HMGB1, which promoted PD-1/LAG-3 upregulation on T cells and impaired their cytotoxic function. HMGB1-treated T cells exhibited activated NF-κB signaling (increased p-p65 and p-p65/p65 ratio), correlating with PD-1/LAG-3 overexpression. HMGB1 may drive T-cell exhaustion via NF-κB-mediated PD-1/LAG-3 upregulation, attenuating the abscopal effect. These findings reveal a novel immunosuppressive axis limiting PDT efficacy in metastatic settings.