Abstract
Photodynamic therapy (PDT) is a clinically approved treatment for various types of cancer. Besides killing the tumor cells directly, PDT has also been reported to trigger anti-tumor immunity. In our previous study, BAM-SiPc-based PDT was shown to induce immunogenic cell death on CT26 murine colon tumor cells in vitro. Using the BALB/c mouse animal model and a vascular-PDT (VPDT) approach, it could also eradicate tumor in ∼ 70% of tumor-bearing mice and elicit an anti-tumor immune response. In the present study, the serum obtained from the VPDT-cured mice was studied and found to possess various immunomodulatory properties. In in vitro studies, it stimulated cytokine secretions of IL-6 and C-X-C motif chemokine ligands 1-3 in CT26 cells through the NF-κB and MAPK pathways. The complement protein C5a boosted in the serum was shown to be involved in the process. The serum also induced calreticulin exposure on CT26 cells and activated dendritic cells. It contained CT26-targeting antibodies which, through the Fc region, induced macrophage engulfment of the tumor cells. In in vivo studies, inoculation of the serum-treated CT26 cells to mice demonstrated a retarded tumor growth with leukocytes, particularly T cells, attracted to the tumor site. In addition, the VPDT-cured mice showed different degrees of resistance against challenge of other types of murine tumor cells, for example, the breast tumor 4T1 and EMT6 cells.