Photodynamic Therapy and Immune Checkpoint Blockade†

Abstract

Immune checkpoints including PD-1 and CTLA-4 help to regulate the intensity and timeframe of the immune response. Since they become upregulated in cancer and prevent sufficient antitumor immunity, monoclonal antibodies against these checkpoints have shown clinical promise for a range of cancers. Multimodal treatment plans combining immune checkpoint inhibitors with other therapies, including photodynamic therapy (PDT), may help to expand treatment efficacy and minimize side effects. PDT's cytotoxic effects are spatially limited by the light activation process, constraining PDT direct effects to the treatment field. The production of damage-associated molecular patterns and tumor-associated antigens from PDT can encourage accumulation and maturation of antigen-presenting cells and reprogram the tumor microenvironment to be more susceptible to therapies targeting immune checkpoints.

Figure 1.

Spatial arrangement of immune checkpoints. APCs like dendritic cells capture tumor-associated antigen signals, present them on MHC complexes and carry them to T cells in lymph nodes. CTLA-4 on T cells competes with CD28 for binding with ligands CD80/CD86 on APCs. αCTLA-4 can prevent CTLA-4 binding to CD80/CD86, encouraging T-cell activation. Activated T cells migrate to the tumor, engaging with MHC-antigen complexes on cancer cells. PD-1 can interact with PD-L1/PD-L2 expressed on tumors or myeloid cells, inhibiting T-cell targeting. Monocytic and granulocytic myeloid-derived suppressor cells (M-MDSCs/G-MDSCs) within the tumor microenvironment also express immune checkpoints that inhibit T-cell activity. αPD-1 can prevent PD-1/PD-L1 interactions, encouraging continued cytotoxic T-cell response.

Figure 2.

PDT immunomodulation. PDT of a tumor causes direct cell death through various mechanisms, leading to release of antigen and damage-associated molecular patterns (DAMPs), vasculature damage/platelet aggregation and release of inflammatory cytokines such as IL8 and IL6. This promotes the recruitment of immune cells like neutrophils which then modulate the subsequent immune system response (maturation of dendritic cells and T-cell activation) that can promote antitumor immunity.

Figure 3.

PDT increases the proportion Ly6G+ cells with low PD-L1 expression. AE17ova,meso (ovalbumin and mesothelin transfected) tumors were grown to 80–100 mm³ in C57BL/6 mice prior to Photofrin-PDT. PDT was delivered with Photofrin (i.v. at 5 mg kg⁻¹) with a 24-h drug-light interval and a 632 nm light dose of 135 J cm⁻² (75 mW cm⁻²). The next day, tumors were processed via enzymatic and mechanical digestion into single-cell suspensions for antibody staining and flow cytometry. Tumors were combined into groups of up to 2 for sufficient cell numbers, for a total of n = 6 for PDT and 5 for tumor control (TC). (A) Cells of interest were gated as single cells based on forward scattering, viable based on negative Aqua Live-Dead staining, CD45+, CD3−, CD11b+ and Ly6G+. (B) PDT significantly increases the percentage of CD11b+/Ly6G+ immune cells (as a percentage of all CD45+ immune cells) at 24 h after treatment compared to tumor control (TC) (P < 0.0001). (C) PD-L1 expression histograms show representative data for 1 of 2 independent experiments, including a PD-L1 fluorescence minus one (FMO) control. (D) TC tumors have an average PD-L1 geometric mean (gMFI) of 1265 (s.d. 294), and PDT-treated tumors have a PD-L1 gMFI of 713 (s.d. 309). Thus, Ly6G+ cells in PDT-treated tumors express significantly less PD-L1, albeit the presence of these cells is greatly increased (P = 0.015).