The impact of macrophage-cancer cell interaction on the efficacy of photodynamic therapy

Macrophages are one of the principal host cell populations in solid tumors. They are capable, due to their plasticity, of acquiring phenotypes that either combat (M1 type) or promote (M2 type) neoplastic growth. These cells, known as tumor-associated macrophages (TAMs), play complex but pivotal roles in the outcome of photodynamic therapy (PDT) of malignant lesions. Among the various parenchymal and stromal cell populations found in tumors, TAMs have been shown to have the greatest capacity for the uptake of systemically administered photosensitizers. Both the tumor-localizing property of photosensitizers and their tumor-localized fluorescence could be partly attributed to the activity of TAMs. Since resident TAMs with accumulated high photosensitizer content will sustain high degrees of PDT damage, this population (predominantly M2 in most tumors) is selectively destroyed, and during the ensuing inflammatory reaction is replaced with newly invading macrophages of M1 phenotype. These macrophages are sentinels responding to DAMP signals from PDT-treated tumor cells and in turn are mobilized to generate a variety of inflammatory/immune mediators and opsonins. They have a critical role in contributing to the therapeutic effect of PDT by mediating disposal of killed cancer cells and by processing/presenting tumor antigens to T lymphocytes. However, TAMs accumulating in the later post-PDT phase can acquire the M2 (healing) phenotype, and could have a role in tumor recurrence by releasing factors that promote angiogenesis and the survival/proliferation of remaining cancer cells. Various therapeutic strategies modulating TAM activity in the PDT response have potential for clinical use for improving PDT-mediated tumor control.