The current elaboration on the mechanism of PDT antitumor immune response tends to focus on PDT-induced oxidative stress in tumor cells, causing ICD and release of TAAs and damage-associated molecular patterns (DAMPs) (56, 57). ICD is a specific cell death mode that translocates calreticulin (CRT) to the cell surface and releases high mobility group box 1 protein (HMGB1), adenosine triphosphate (ATP), and heat shock proteins (HSPs) to the extracellular surface (58, 59). Inflammation-related signaling pathways, the release of immune-related cytokines, neutrophil infiltration, and the complement cascade are all triggered by these DAMPs (60, 61). DCs are specialized antigen-presenting cells that link the innate immune response to the adaptive immune response, taking up TAAs, binding DAMPs through pattern recognition receptors (PRRs), and processing antigens as they migrate to lymph nodes and mature. The antigens are presented to T cells for proliferation and differentiation into CTLs, which exert antitumor immune effects (62–64).

After the determination of the important role of immunogenic DAMPs in the PDT-mediated antitumor immune response, many studies have been conducted to improve the ICD triggered by PDT. Deng et al. (65) designed reduction-sensitive Ds-sP nanocarriers loaded with an efficient endoplasmic reticulum (ER)-targeting PS TCPP-T^ER. The unique ER targeting ability of PS TCPP-T^ER results in an elevated level of oxidative stress in the ER of tumor cells, which in turn releases more DAMPs and enhances the immune effect. This strategy can effectively address the problems of short ROS half-life and limited intracellular diffusion depth. However, a hypoxic tumor microenvironment (TME) can limit the efficacy of PDT and reduce the efficiency of ICD induction (66). Therefore, increasing the oxygen content of tumor tissues is essential to improve the efficiency of PDT treatment. Liang and colleagues (67) developed gold nanocages (AuNCs) with hollow structures and coated them with a layer of manganese dioxide to synthesize core-shell nanoparticles (AuNC@MnO₂). In the acidic microenvironment of tumor tissue rich in H₂O₂, manganese dioxide reacts as follows: MnO₂ + H₂O₂ + 2H⁺→Mn²⁺ + 2H₂O + O₂↑ generates a large amount of oxygen to promote the accumulation of ROS in the tumor and enhances the efficacy of PDT by improving tumor hypoxia to achieve ICD. The released oxygen and Mn²⁺ can provide fluorescence (FL)/photoacoustic (PA)/magnetic resonance multimodal imaging function to evaluate the integration of tumor diagnosis and treatment. In short, the induction of ICD by enhanced PDT to promote antitumor immune response is a promising tumor treatment strategy ( Figure 3 ).

Tumor cell lysates and TAAs produced by PDT can also induce specific immune responses and are more effective than tumor cell lysates produced by ionizing radiation and ultraviolet rays (68). Similar to the inoculation mechanism of conventional vaccines that directly introduce microorganisms into the body to produce protective antibodies, cancer vaccines stimulate the activation of the body’s immune system through tumor cell death (69, 70). DCs have played a major role in the development of cancer vaccine therapy as critical mediators of antigen presentation, reversing a major component of tumor-mediated immune suppression (71, 72). Tumor residues after PDT can be used as a cancer vaccine to dramatically increase DC activation and release inflammatory cytokines to boost immune response in a mouse breast cancer model, according to a study using chlorin e6 (Ce6) as a PS (73). A promising tumor treatment strategy is to use PDT-treated tumor cells as a DC vaccine to develop a PDT-DC vaccination that can more efficiently destroy tumors and trigger a powerful antitumor immune response (74, 75).

lAntigens produced by PDT ablation of tumors may have insufficient immunogenicity as a DC vaccine and are limited to immunosuppressed “cold” tumors (76). Korbelik made a vaccine against SCCVII cells using PDT to destroy the cells and used this vaccine in the SCCVII tumor model to show that it inhibits tumors growth (77). However, the levels of splenic myeloid-derived suppressor cells (MDSCs) were significantly enhanced. Therefore, the immune adjuvant N-dihydrogalactochitosan (GC) was added to the PDT vaccine group to reduce the number of MDSCs (precursors of DCs, macrophages, and granulocytes) and alleviate immunosuppression (78). Ni et al. (79) used the amphipathic 4T1 breast cancer cell membrane to load PS Ce6 and the chemotherapeutic drug, i.e., doxorubicin hydrochloride (Dox), and coated the cell membrane surface with calcium carbonate to construct nanodrug delivery systems. Ce6-based PDT and Dox cause DNA damage, induce tumor ICD, and release TAA. The ROS generated during this process is expected to form in situ PDT-DC vaccination by mimicking inflammatory mechanisms to recruit DCs. In PDT-DC vaccinated mice, the growth of both primary 4T1 and untreated distant tumors was suppressed, indicating the establishment of an efficient immune response. Moreover, serum levels of inflammatory cytokines in mice increased continuously, peaking and then stabilizing the day after vaccination. It provides a novel antitumor combination therapy for improving the immunogenicity of the PDT-DC vaccine by introducing adjuvants or chemotherapeutic drugs. This therapy enhances body-specific immune responses, eliminates tumors, and builds long-term immunological memory.

Immune adjuvants are chemicals that boost the cellular or humoral immune response to an antigen (80, 81). Vaccines, which are one of the most successful medicinal discoveries against a variety of infectious diseases, occasionally require a molecule in conjugation to boost the immune response (82). It is therefore expedient to co-administer these with an adjuvant to ensure a high-quality/high-quantity, memory-enhanced antibody response. In chronological order of appearance, the first immune adjuvant to be used clinically was Alum, followed by the development of oil-in-water emulsions and toll-like receptor (TLR) agonists (83, 84). TLR agonists are currently being used as immune adjuvants to activate TLR signaling pathways and boost immunological responses and are found to be promising agents for cancer treatment (85, 86). This review focuses on summarizing the strategies of TLR agonists in combination with PDT.

TLRs are one of the PRRs that are expressed by a wide range of immune cells and have received more attention. To date, 13 different TLRs (TLR1–13) have been identified in mammals (87). The TLR7 agonist imiquimod (R837) is a synthetic imidazoquinoline-like molecule, approved by the US Food and Drug Administration (FDA) as a single drug and commonly used in the treatment of various skin diseases, including basal cell carcinomas (88). R837 interacts with TLR7 on the DC surface and endosomes that results in the stimulation of DC maturation and release of pro-inflammatory cytokines through elevated expression of co-stimulatory molecules (89). PDT using the PS 5-aminolevulinic acid (ALA) in combination with imiquimod cream has been proven to be useful in the treatment of squamous cell carcinoma of the skin (90). This combination therapy ameliorates the poor oncogenic effect caused by insufficient local penetration of the PS into the tumor. Because of the limitations of topical use of imiquimod cream, this agent can only be used for superficial skin cancer treatment. If tumors in internal organs of the body are to be destroyed, R837 must be delivered locally to the tumor through blood circulation. R837, being a small-molecule, is diffused after local injection, and few of them eventually reach the tumor site (91). Furthermore, R837 causes direct cell death by inducing autophagy and has concentration-dependent cytotoxicity (92–94). The use of nanomaterials to encapsulate immune adjuvants and decrease their harmful effects is a viable technique for addressing such issues. Xu et al. (95) used the hydrophobic region between UCNP and PEG to load the PS chlorin e6 (Ce6) and the TLR7 agonist R837. The results showed that TAA released from PDT-killed tumor cells and R837 induced DC maturation and released cytokines related to innate and adaptive immunity, such as TNF-α and IL-12, by upregulating the expression of co-stimulatory molecules such as CD80 and CD86. Resiquimod (R848), a second-generation derivative of R837, shares a similar structure and properties with R837. However, in contrast to R837, R848 can be used as an agonist of both TLR7 and TLR8 (96, 97). Many studies on R848 immunotherapy, both alone and in combination with chemotherapy and photothermal therapy, have been reported, suggesting that R848 can enhance immunity and improve anticancer therapeutic effectiveness (98–100). However, studies on the combined application of R848 with PDT have not been reported, which may be a promising direction for future research.

Other TLR agonists have been used in PDT immunotherapy to enhance the immune response. According to the reported study, the combination of PDT and TLR5 agonist flagellin (FlaB-Vax) effectively inhibited bilateral melanoma in mice, enhanced TME tumor antigen cross-presentation, and promoted tumor CD8⁺ T-cell infiltration and systemic IFN-γ secretion (101). CpG oligodeoxynucleotides (CpG ODN) are synthetic DNA fragments that function as TLR9 agonists by interacting with DC-expressed TLR9 and enhancing antigen-specific immune responses (102). Ni and co-workers (103) used the cationic PS 5,10,15,20-tetra(p-benzoato)porphyrin (TBP) to adsorb the anion CpG to achieve efficient PDT and effective delivery of CpG. In a mouse breast cancer model, the combination of PDT and CpG was found to provide excellent tumor suppression, with about 97% of tumors being eliminated. Cai et al. (104) designed a metal–organic framework (MOF) nanoparticle formed by the self-assembly of the PS H₂TCPP and zirconium ions. The porous internal structure of MOF was used to load the TLR9 agonist CpG ODN. When compared with the control and the treatment group alone, the combination of CpG ODN and PDT dramatically increased the expression of MHC-II, CD317, and co-stimulatory molecules including CD80/CD86. Remarkably, CpG ODN also reduced the immunosuppressive activity of MDSCs and improved the tumor immunosuppressive microenvironment (105). However, CpG ODN is still mostly used as an immunostimulant in PDT immunotherapy, and there is a significant research gap in improving immunosuppression.

Many immune adjuvants such as glycated chitosan (GC), lactobacillus BCG, mycobacterial cell wall extract (MCWE), complete Freund (CF) adjuvant, and incomplete Freund (IF) adjuvant can stimulate the immune response similar to that of TLR agonists (106). Previously, it was demonstrated that GC, a water-soluble compound synthesized from galactose and chitosan, was shown to stimulate TNF-α secretion by macrophages and induce tumor-specific immune responses (107). Cai’s group (108) found the highest levels of apoptotic and inflammatory responses and the highest infiltration of immune cells in tumors in the synergistic treatment group of PDT and GC. Additionally, mice treated with a combination of PDT and GC had a significantly better survival rate in the EMT6 mammary tumor and 4T1 metastatic mammary tumor models. In short, PDT adjuvant by immune adjuvant has promising research potential because of its ability to inhibit tumor metastasis and recurrence.

PDT can activate the immune system to some extent, but the intensity of the PDT-induced immune response may not be sufficient to destroy the tumors or prevent their metastasis and recurrence due to the immunosuppressive effect of the TME and immune escape of tumor cells (109, 110) ( Figure 4 ). Moreover, PDT causes local inflammation and immunosuppression due to contact hypersensitivity (CHS) (111). Therefore, it is necessary to explore suitable immune inhibitors to inhibit tumor immunosuppressive signals and enhance PDT-induced immune responses.

Immunosuppressive cells such as MDSCs and regulatory T cells (Tregs) in the TME suppress the antitumor immune responses and promote tumor progression and invasion (112, 113). Specifically, MDSCs suppress T-cell function through multiple mechanisms, including production of nitric oxide and immunosuppressive metabolites, secretion of immunosuppressive cytokines such as TGF-β and IL-10, and upregulation of cyclo-oxygenase 2 (Cox2) and prostaglandin E2 (PGE2) (114, 115), while Tregs suppress T-cell function through CTL antigen 4 to inhibit the expression of DC co-stimulatory molecules including CD80 and CD86 (116). Recently, it has been found that PDT vaccination significantly increased MDSC and Treg levels, while low doses of GC and cyclophosphamide were found to reduce the elevated levels of immunosuppressive cells (77, 117). The findings demonstrated the feasibility of using small-molecule inhibitors to weaken immunosuppressive cells and alter the immunosuppressive TME, thereby enhancing the PDT-DC vaccine-induced immune responses.

Apart from their role in tumor development and immune escape, the reprogramming of tumor cells’ metabolism influences immune cell metabolism (118). Tryptophan (Trp) metabolism in T cells is mediated by tumor cells through an elevated level of indoleamine 2,3-dioxygenase (IDO) expression, which converts Trp to kynurenine (Kyn) (119). Lack of Trp inhibits CTL activation, while abnormal accumulation of Kyn recruits Treg to suppress effector T-cell function (120, 121). Reducing the level of immunosuppression by PDT nanoparticles loaded with IDO inhibitors is a more appropriate strategy. Zhao et al. (122) constructed self-delivery photo-immune stimulators (iPSs) through non-covalent interactions between the PS Ce6 and IDO inhibitor, i.e., NLG919. They first demonstrated that iPSs can promote DC maturation by inducing apoptosis and ICD in CT26 cells via PDT and cellular release of CRT and HMGB1. Following PDT treatment with iPSs, increased CD4⁺/CD8⁺ T-cell infiltration was observed in the mouse CT26 tumor tissues, while a significant decrease in Kyn/Trp ratio was observed in the serum. Additionally, transcriptomic analysis of mouse tumor tissues demonstrated that iPS-based PDT could effectively stimulate the tumor immune microenvironment and enhance tumor immunotherapy efficacy. A variety of PSs and IDO inhibitors, such as self-assembled nanoparticles, are being developed extensively. Yang’s group (123) designed pH-responsive nanovesicles (pRNVs) as carriers to synthesize PRNVS/HPPH/IND smart nanoparticles by encapsulating the PS HPPH and the IDO inhibitor indoximod (IND) through hydrophobic interactions. They found that PDT treatment of the nanoparticles inhibits mouse melanoma growth, and the release of IND stimulates CD8⁺ T cells to destroy distant tumors by increasing P-S6K phosphorylation.

Tumor tissues secrete large amounts of vascular endothelial growth factor (VEGF) to promote the proliferation of immunosuppressive cells and inhibit DC maturation by NF-κB pathway activation (124, 125). Excess VEGF leads to abnormalities in tumor vascular structure and function, exacerbating the hypoxic state of the TME and affecting the efficacy of PDT therapy (126). Zhou et al. (127) developed a self-assembled nanoplatform containing the PS Ce6, the VEGF receptor (VEGFR) inhibitor axitinib (AXT), and the IDO inhibitor dextro-1-methyl tryptophan (1MT) to alleviate immunosuppression by promoting vascular normalization and improving the tumor hypoxic microenvironment, thereby enhancing PDT immunotherapy. According to the obtained results, the enhanced PDT immunotherapy has significant effects on both primary melanoma and lung metastases in mice.

The PS-based PDT induces tumor ICD and stimulates immune activation, while the inhibitor of immune suppression promotes PDT-induced immune response by weakening tumor immune escape (128). It is about the immunosuppressive TME of MDSCs and Tregs and immunosuppressive molecules like IDO-1 and VEGF. PDT should be combined with compounds that can inhibit MDSCs and/or Tregs and IDO or VEGF.

Various immunosuppressive mechanisms can impair the efficiency of antitumor immunotherapy during tumor progression. For example, immune checkpoint molecules are considered to be the primary anticancer immunotherapy targets, as they have a negative immunomodulatory effect (129). The development of target-specific antibodies to block the underlined immune checkpoints is a hot topic in immunotherapy. However, due to the low tumor immunogenicity, the response rate of some patients to ICB therapy is unsatisfactory (130). The efficiency of ICB therapy can be improved by enhancing tumor immunogenicity and sensitivity through PDT-mediated ICD induction (131, 132). Herein, three immune checkpoints, including CTLA-4, PD-1/PD-L1, and CD47, have been described, followed by summarizing their combined treatment strategies along with PDT.