In addition, studies have shown that 5-aminolevulinic acid photodynamic therapy (ALA-PDT) can induce M1 polarization of THP-1 macrophages by regulating the PTGS2/PGE2/TLR4/TREM1 axis, thereby exacerbating the inflammatory response, and this is used as a therapeutic approach for chronic inflammatory skin conditions such as acne. Here, TLR4 is linked to acne.