Low-dose photodynamic therapy promotes angiogenic potential and increases immunogenicity of human mesenchymal stromal cells.

Photodynamic therapy (PDT) is a non-invasive FDA and EMA-approved anticancer treatment modality. Initially developed for elimination of malignant cells, PDT affects all cells in the tumor bed including stromal cells. Stroma represents not only an important component of tumor microenvironment, but has a significant impact on tumor susceptibility to PDT and other anticancer therapies. However, the effects of PDT on stromal cells are poorly investigated. During PDT the tumor stroma can receive low-dose irradiation as a result of chosen regimen or limited depth of light penetration. Here, we characterized response of human mesenchymal stromal cells (MSCs) to low-dose PDT. In an in vitro model we demonstrated that low-dose PDT resulted in activation of Erk1/2 and inhibition of GSK-3 signaling in MSCs. PDT-mediated induction of intracellular reactive oxygen species (ROS) resulted in reorganization of MSC cytoskeleton and decreased cell motility. More importantly, low-dose PDT dramatically upregulated secretion of various proangiogenic factors (VEGF-A, IL-8, PAI-1, MMP-9, etc.) by MSCs and improved MSC ability to promote angiogenesis suggesting an increase in the pro-tumorigenic potential of MSCs. In contrast, co-cultivation of PDT-treated MSCs with lymphocytes resulted in significant decrease of MSC viability and potential increase in MSC immunogenicity, which may lead to increased anti-tumor immunity. Low-dose PDT in MSCs significantly inhibited secretion of CCL2 (MCP-1) potentially limiting infiltration of pro-tumorigenic macrophages. Altogether, our findings demonstrate that low-dose PDT significantly modifies functional properties of MSCs improving their pro-tumorigenic potential while simultaneously increasing potential immune stimulation suggesting possible mechanisms of stromal cell contribution to PDT efficacy.