Targeting immunogenic cancer cell death by photodynamic therapy: past, present and future

Abstract

The past decade has witnessed major breakthroughs in cancer immunotherapy. This development has been largely motivated by cancer cell evasion of immunological control and consequent tumor resistance to conventional therapies. Immunogenic cell death (ICD) is considered one of the most promising ways to achieve total tumor cell elimination. It activates the T-cell adaptive immune response and results in the formation of long-term immunological memory. ICD can be triggered by many anticancer treatment modalities, including photodynamic therapy (PDT). In this review, we first discuss the role of PDT based on several classes of photosensitizers, including porphyrins and non-porphyrins, and critically evaluate their potential role in ICD induction. We emphasize the emerging trend of ICD induction by PDT in combination with nanotechnology, which represents third-generation photosensitizers and involves targeted induction of ICD by PDT. However, PDT also has some limitations, including the reduced efficiency of ICD induction in the hypoxic tumor microenvironment. Therefore, we critically evaluate strategies for overcoming this limitation, which is essential for increasing PDT efficiency. In the final part, we suggest several areas for future research for personalized cancer immunotherapy, including strategies based on oxygen-boosted PDT and nanoparticles. In conclusion, the insights from the last several years increasingly support the idea that PDT is a powerful strategy for inducing ICD in experimental cancer therapy. However, most studies have focused on mouse models, but it is necessary to validate this strategy in clinical settings, which will be a challenging research area in the future.

Keywords: adaptive immunity; alarmins; cytotoxicity; immunogenicity; immunologic; immunotherapy; vaccine.

Figure 1

Mechanisms of photodynamic reaction during photodynamic therapy (PDT). (1) Following the absorption of photons (hv), one of the electrons of the photosensitizer (PS) is boosted into a high-energy orbital (S₁ or S₂) and activated to the short-lived (nanoseconds) excited singlet state (¹PS^·). ¹PS^· can lose its energy by internal conversion into heat (2) or by emitting light (fluorescence) (3). Alternatively, ¹PS^· transforms into a relatively long-lived (microseconds) excited triplet state (³PS^·) via an intersystem crossing process (4). ³PS^· moves directly from a triplet to a singlet state (¹PS) by emission of light (phosphorescence) (5) or undergoes two kinds of reactions with surrounding molecules. In the type I photochemical reaction (6), ³PS^· reacts directly with a substrate (eg, polyunsaturated fatty acids in cell membrane lipids) and transfers an electron or a proton, forming organic radicals. These radicals may further react with cellular oxygen to produce reactive oxygen species (ROS), such as superoxide anion (O₂^–·), hydroperoxide radical (HOO^·), peroxides (H₂O₂, ROOH) and hydroxyl radical (HO^·), as well initiate free radical chain reactions. In the type II photochemical reaction (7), the triplet ³PS^· can undergo triplet−triplet energy transfer to molecular oxygen (triplet in the ground state) to form excited-state singlet oxygen (¹O₂). Type I and type II photochemical reactions can be simultaneous, and the ratio between them depends mainly on the type of PS used, the concentrations of substrate and the availability of oxygen. As a result of the photodynamic reaction, various molecular mechanisms are activated, leading to different cell death modalities, recruitment and activation of immune cells and vascular damage.

Figure 2

Photodynamic therapy (PDT)-induced immunogenic dell death at a glance. Photosensitizers (PSs) used in PDT have various chemical structures and can be divided into non-porphyrin and porphyrin (or tetrapyrrole) compounds. It has been experimentally proven that after accumulation in tumor cells and excitation by light of appropriate wavelength (hv), some PSs in each group of PSs can induce immunogenic cell death (ICD) (1). ICD refers to an immunological feature of cell death and is observed in immunogenic apoptosis and immunogenic necroptosis, as well as in mixed cell death types (2). The role of PDT in the induction of ferroptosis in cancer cells needs to be further clarified . Importantly, only a fraction of cancer cells can be reached by light during PDT because light can penetrate only to a limited depth. ICD stimulates innate and adaptive immune responses, resulting in long-term immunological memory. Of note, the immunogenicity of ICD is mediated by the antigenicity (3) and adjuvanticity (4) of dying/dead cancer cells. The antigenicity of tumor cells is determined by the presence of tumor-associated antigens and tumor neoantigens (3). However, tumor-associated antigens usually fail to drive efficient immunity in the absence of additional adjuvants required to recruit and activate antigen-presenting cells. It is currently not known how PDT in combination with the above-mentioned PSs can modulate the antigenicity of dying cancer cells. The adjuvanticity of ICD resides in the release of damage-associated molecular patterns (DAMPs) such ATP, HMGB1 and HSP and CRT exposure on the outer cell surface (4). The emitted DAMPs promote the recruitment and maturation of antigen-presenting cells (eg, DCs) (5, 6), which leads to optimal antigen presentation to CD8⁺ T cells (7) and induction of antitumor immunity (8), resulting in significant suppression of tumor growth and/or regression of cancer and decreased risk of metastasis. The activated anticancer immunity aims to eradicate cells deep within the primary tumor and, therefore, significantly enhance PDT efficiency. The ‘gold standard’ for determining the true immunogenicity of cell death requires the conduction of experimental studies in vivo (mouse prophylactic tumor vaccination model) (9). For this, immunocompetent mice are first vaccinated with PDT-treated cancer cells in one flank and 1 week later rechallenged with living cells of the same type in the other flank (10). Protection against tumor growth at the challenge site is interpreted as a sign of successful priming of the adaptive immune system (11). *Examples of PSs with presumed but not fully proven immunogenic properties (lack of DAMPs expression and/or lack of immunogenicity either in vitro or in vivo). CD, cluster of differentiation; CRT, calreticulin; DC, dendritic cell; HMGB1, high-mobility group protein box 1; HSP, heat shock protein; hv, photons, IFN, interferon; IL, interleukin;.