Figure 2.

Mechanism of photodynamic immunotherapy for gliomas. (A) ⁰PS is activated to a singlet state (¹PS*) after absorbing photons (hv). ¹PS* can lose energy through internal conversion to heat and fluorescence. ¹PS* can form a triplet state (³PS*) through the intersystem crossing process (ISC). ³PS* can be restored to 0PS by emitting phosphorescence and can also react with neighbouring molecules in two types of reactions (type I and type II). In type I reactions, ³PS* transfers an electron or a proton to form organic free radicals (O^2-, OOH·, H₂O₂, OH·, etc.) that interact with cellular oxygen to produce cytotoxic ROS. In a type II reaction, the energy of ³PS* can be directly transferred to molecular oxygen (³O₂) to form singlet oxygen (¹O₂). This results in various biological effects (264, 282, 283). (B) The most common types of cell death induced by PDT include apoptosis (284), autophagy (285), necrosis (286), necroptosis (287), and ferroptosis (270). (C) ROS produced by PDT can also cause vascular occlusion, leading to vascular damage, thereby affecting the blood supply of tumour cells (288). (D) PSs can induce immunogenic cell death (ICD), resulting in the exposure and release of DAMPs, such as ATP, HMGB1, CALR, HSP70/90, etc (262). The released DAMPs promote DC recruitment and maturation and present tumour antigens to T cells, leading to the activation of CD8⁺ T cells, which subsequently migrate in vivo to kill tumour cells (264).