Introduction: Malignant pleural mesothelioma (MPM) is a deadly disease with a dismal prognosis and limited treatment options. Among the potential therapeutic approaches, chemotherapy, radiation therapy, photodynamic therapy, immunotherapy and surgery can be attempted. Multimodal approaches are often preferred as they provide the best results. In the past, it was shown that a single treatment of MPM by photodynamic therapy could lead to interesting long-term survival in patients although the exact mechanism was never demonstrated. Photodynamic therapy (PDT) is a dual component treatment where a photosensitizer is activated by light and leads to pleiotropic biological effects. It was recently shown that a single low dose of PDT (L-PDT) could enhance tumour infiltration by lymphocytes and antigen presenting cells which leads to better tumour response in the presence of immune checkpoint inhibition therapy. Because immune cell infiltration of tumours requires vascular modulation and activation, we hypothesized that L-PDT affected endothelial expression of diapedesis proteins, which correlated with tumour immune infiltration of various effector cells including cytotoxic and helper T lymphocytes and antigen presenting cells such as macrophages.
Methods: We first treated endothelial cell cultures with L-PDT and assessed at 24 hours the expression of the diapedesis proteins E-Selectin, ICAM-1 and VCAM-1 by western blot. We then treated our murine MPM tumour model by L-PDT and harvested the tumours 24 hours after therapy. We co-stained the tumour sections with the CD144 endothelial cell marker and E-Selectin, ICAM-1 and VCAM-1. We then determined the tumour infiltration of leucocytes by staining the tumour sections for cytotoxic (CD8+CD3+) and helper (CD4+CD3+) lymphocytes and F4/80+CD45+ antigen presenting macrophages. The sections were imaged by fluorescence microscopy and images were quantified using ImageJ. Statistics were performed using Excel (student T test).
Results: L-PDT significantly enhanced E-selectin expression of endothelial cells in vitro and in vivo but did not affect ICAM-1 and VCAM-1 levels. Interestingly, while L-PDT was delivered at the surface of tumours, E-selectin induction was homogenous throughout the tumour. Also, L-PDT caused a significant and homogenous increase in CD8+CD3+ and CD4+CD3+ lymphocytes but did not affect CD45+F4/80+ macrophages of MPM tumours.
Conclusion: A single L-PDT surface treatment of MPM caused a homogenous increase in endothelial E-selectin expression which correlated with a homogenous infiltration of CD8+CD3+ and CD4+CD3+ lymphocytes but did not affect CD45+F4/80+ macrophages. This finding combined with previous data suggests that L-PDT favours an elaborated cytotoxic immune response in MPM. Also, the observation that a localized L-PDT treatment leads to a generalized tumour response could suggest the involvement of a paracrine mechanism which requires further studies.