The Course of Immune Stimulation by Photodynamic Therapy: Bridging Fundamentals of Photochemically Induced Immunogenic Cell Death to the Enrichment of T-Cell Repertoire

Abstract

Photodynamic therapy (PDT) is a potentially immunogenic and FDA-approved antitumor treatment modality that utilizes the spatiotemporal combination of a photosensitizer, light and oftentimes oxygen, to generate therapeutic cytotoxic molecules. Certain photosensitizers under specific conditions, including ones in clinical practice, have been shown to elicit an immune response following photoillumination. When localized within tumor tissue, photogenerated cytotoxic molecules can lead to immunogenic cell death (ICD) of tumor cells, which release damage-associated molecular patterns and tumor-specific antigens. Subsequently, the T-lymphocyte (T cell)-mediated adaptive immune system can become activated. Activated T cells then disseminate into systemic circulation and can eliminate primary and metastatic tumors. In this review, we will detail the multistage cascade of events following PDT of solid tumors that ultimately lead to the activation of an antitumor immune response. More specifically, we connect the fundamentals of photochemically induced ICD with a proposition on potential mechanisms for PDT enhancement of the adaptive antitumor response. We postulate a hypothesis that during the course of the immune stimulation process, PDT also enriches the T-cell repertoire with tumor-reactive activated T cells, diversifying their tumor-specific targets and eliciting a more expansive and rigorous antitumor response. The implications of such a process are likely to impact the outcomes of rational combinations with immune checkpoint blockade, warranting investigations into T-cell diversity as a previously understudied and potentially transformative paradigm in antitumor photodynamic immunotherapy.

Figure 1.

Overview of this review article discussing the fundamentals of PDT and how they relate to the stimulation of the innate and adaptive immune responses that are becoming increasingly critical in achieving control over distant metastases and disease recurrence. Finally, speculation on the effect of PDT on T Cell Repertoire will be discussed.

Figure 2.

Photosensitizers and photodynamic therapy. (a) A simplified workflow of in vivo PDT that starts with systemic PS administration and distribution followed by tumor accumulation, photoactivation and local and systemic management of the disease. (b) Representations of classical and cutting-edge nano-sized carrier systems often leveraged to improve PDT efficacy. (c) A simplified Jablonski diagram portraying the energetics of PS molecules following non-thermal photoexcitation. The excitation of a sensitizer in its singlet ground state (PS¹) with red light (most common for in vivo PDT), results in the molecule rising to a higher energy level singlet excited state (PS¹*). The molecule then undergoes non-radiative intersystem crossing to the long-lived triplet excited state (PS³*) whereby type I and type II photochemical reactions proceed. These reactions ultimately lead to therapeutic antitumor biological consequences.

Figure 3.

Mechanism of PDT-induced apoptosis. Photosensitizers that localize in mitochondria or lysosome can directly damage the cell organelles upon light irradiation. Lysosomal phototoxicity promotes the release of proteases from the lysosome to the cytosol which cleave BID to generate truncated BID (tBID). Next, tBID activates BAX and BAK (BH123 protein) to form mitochondrial outer membrane channels (pores). As a result, cytochrome c and second mitochondrion-derived activator of caspases (SMAC) are released leading to the activation of initiator caspase-8/9 and downstream executioner caspases (caspase 3/7). The release of SMAC leads to inhibition of IAPs (inhibitor of apoptosis proteins) preventing them from binding to and inhibiting activated caspases. Finally, the activated executioner caspases initiate the degradation of cellular components, including proteins and nucleic acids and commence cellular death.

Figure 4.

Innate immune stimulation by PDT. Light-irradiation on photosensitizer-loaded tumor cells leads to necrotic or apoptotic cell death. The dying cells express or secret DAMPs on the membrane or in the TME, respectively. These DAMPs include HMGB1, calreticulin (CRT), Hsp70, extracellular ATP, etc. At the site of light irradiation, various cytokines and chemokines are released by the photodamaged tumor cells and tissue-resident immune cells. These cytokines such as IL-1β, IL-6, TNF-α, etc recruit more innate immune cells such as neutrophils, macrophages, natural killer cells and cause local inflammation. The DCs connects the innate immune system with the adaptive one. DCs engulf the dying cells and undergo maturation. They migrate to the dLN to activate the adaptive immune system as shown in figure 5.

Figure 5.

Steps for activation of the adaptive immune system by PDT. (1) Photosensitizers accumulate in the malignant tissue. (2) Upon light irradiation (PDT), tumor cells undergo apoptosis and necrosis. DAMPs are expressed on the surface of dying cells. TSAs are released in the TME. (3) DCs internalize the TSAs and become activated and mature. (4) DCs migrate to the dLN and present the processed TSAs to the naïve T cells. (5) Activated T cells undergo clonal expansion and are disseminated into the systemic circulation. (6) Activated T cells reach the primary or metastatic tumor sites. (7) T cell receptor recognizes TSA presented by the tumor cells and form the immunological synapse. (8) Toxic granules containing granzyme B and perforin are secreted at the synapse to specifically kill the tumor cell.

Figure 6.

T cell repertoire. (a) T cell receptors (TCR) undergo VDJ recombination in a primary lymphoid organ (thymus) which results in T cell specificity. This somatic rearrangement of a variable (V), joining (J), and diversity (D) gene segments to a constant region (C) in the case of the β-chain of the TCR, and V-J-C recombination (not shown in the diagram) in the case of the α-chain of TCRs, constitute a functional TCRαβ heterodimer of a T cell. A region of the TCRαβ heterodimer, known as Complementarity Determining Region 3 (CDR3) accounts for the highest diversity due to the gene recombination. (b) VDJ recombination takes place before T cells encounter an antigen. Thus a pool of initial repertoire of naïve T cells exists in the body. TSAs released following PDT are carried by the dendritic cells (DCs) to the draining lymph nodes where the DCs activate naïve T cells. TSA-reactive T cells probably undergo clonal expansion and enrich a pool of activated T cells that eliminate the tumors.