Apoptosis is the most extensively studied form of RCD characterized by morphological alterations of cell rounding, cytoplasmic shrinkage, plasma membrane blebbing and chromatin condensation, that participates in various physiological and pathophysiological processes (2). Apoptosis is usually divided into two forms: the intrinsic pathway (mitochondria) and the extrinsic pathway (death receptors). The intrinsic pathway is initiated by various internal traumas that eventuate in mitochondrial outer membrane permeabilization (MOMP), cytochrome c being released from the intermembrane space, and caspase 9 being activated by the formation of the apoptosome complex (2). The extrinsic pathway is mediated by death receptors, mainly Fas cell surface death receptor and TNF receptor superfamily members. When binding with ligands, the intracellular structural domains of these death receptors interact with adaptor proteins through the interplay of death structural and death effector domains, resulting in the activation of caspase 8 in a cytosolic complex called the death-inducing signaling complex (DISC). Both caspase 8 and caspase 9 are initiator caspases that can cleave and activate executioner caspases such as caspase 3, 6 and 7, leading to apoptosis. Surface-exposed calreticulin (ecto-CRT) and secreted ATP are crucial DAMPs for immunogenic apoptosis. It was reported that dying tumor cells induced by photodynamic therapy (PDT) underwent immunogenic apoptosis characterized by phenotypic maturation and functional stimulation of dendritic cells (DCs) as well as induction of a protective anticancer immune response (5). Early after PDT, cancer cells exhibited ecto-CRT and secreted ATP before displaying a biochemical feature of apoptosis, via a novel PERK-dependent subroutine that necessitate a competent secretory pathway and PI3K-mediated plasma membrane/extracellular trafficking (5).

Necroptosis is considered as a regulated necrotic cell death modality driven by receptor-interacting kinase protein 1 (RIPK1) via forming complex IIB (necrotic body), requiring RIPK3 activity and being executed by mixed lineage kinase domain-like pseudokinase (MLKL) protein (6). Cancer cells undergoing necroptosis are characterized by cell membrane perforation and increased intracellular osmotic pressure, morphologically being cell rounding and swelling, and explosive rupture of the plasma membrane, resulting in releasing cell contents into extracellular space and exacerbating inflammatory response (7). Natural compounds, kinase inhibitors, irradiation and chemotherapy can induce necroptosis (8). Shikonin, an effective anticancer constituent, has been reported to inhibit both primary and metastatic osteosarcoma by inducing necroptosis (9). Necroptosis has been reported to promote or suppress cancer progression, contextualized in different types of cancers (8). Necroptotic pancreatic cancer cells can facilitate their migration and invasive abilities via CXCL5-CXCR2 axis (10). Necroptosis acts as an inflammatory mode of cell death with several signaling pathways activation, such as NF-κB and MAPK pathways, favoring cancer development (11).

Pyroptosis is an inflammatory caspase-dependent cell death modality that requires members of the gasdermin protein family, characterized by pore-formation, cell swelling and lysis, and release of proinflammatory mediators, including IL-1β, IL-18, ATP, and HMGB1 (12). Chemotherapy drugs can trigger tumor cell pyroptosis through caspase 3 cleavage of gasdermin E (GSDME) (13). Metformin treatment induces gasdermin D (GSDMD)-mediated pyroptosis of esophageal squamous cell carcinoma (ESCC) by impacting miR-497/proline-, glutamic acid- and leucine-rich protein-1 (PELP1) axis (14). The involvement of pyroptosis in cancer development appears to be contingent on the types of cancer, genetics, and the duration of pyroptosis induction (15). As the activation of pyroptosis is often accompanied by an inflammatory microenvironment, it is evident that pyroptosis contribute to cancer progression under some circumstances (15).

Ferroptosis is a newly defined form of RCD characterized by iron overload, lipid reactive oxygen species (ROS) accumulation, and lipid peroxidation (16). When cells undergo ferroptosis, mitochondria are smaller than normal, with a reduction or loss of mitochondrial cristae and rupture of the outer mitochondrial membrane, but the nuclear size is still normal, without chromatin condensation (17). Ferroptosis can be regulated via extrinsic and intrinsic pathways. The extrinsic pathway (transporter-dependent pathway) is triggered by repression of cell membrane transporters such as the cystine/glutamate antiporter system or by initiation of the iron transporters serotransferrin and lactotransferrin. The intrinsic pathway (enzyme-regulated pathway) is activated through blockade of intracellular antioxidant enzymes such as glutathione peroxidase 4 (GPX4) (18). However, the effector molecules of ferroptosis remain to be determined (19). The role of ferroptosis in tumorigenesis depends on the release of DAMPs and the activation of immune response within the TME. Many antitumor therapies have been reported to induce ferroptosis (17–19). Cisplatin induces ferroptosis through depleting intracellular glutathione (GSH) and inhibiting GPXs (20). The dysregulation of ferroptosis contributes to antitumor therapy resistance and failure.

Notably in 2022, Tsvetkov et al. discovered that excess intracellular copper could cause a proteotoxic stress response and ultimately cell death through direct binding to lipid acylated components of the tricarboxylic acid (TCA) cycle, leading to abnormal aggregation of lipid acylated proteins and loss of iron-sulfur (Fe-S) cluster proteins in the respiratory chain complex. The team found that this cell death is not blocked by apoptosis, necroptosis, pyroptosis and ferroptosis-related inhibitors, and therefore, is considered as a novel cell death mode and named as cuproptosis (21). This study shows that ferredoxin 1 (FDX1, a reductase that reduces Cu²⁺ to the more toxic Cu¹⁺) and protein lipoylation are pivotal modulators of cuproptosis (21). However, there are few studies related to cuproptosis, and its overall regulatory mechanism in cancer remains to be further investigated.

Phagocytosis of dying tumor cells by DCs results in cross-presentation of cell-associated antigen, and the priming of CD8⁺ T cells, which has been implicated in the maintenance of immune homeostasis. When mice were immunized with cells that experienced RIPK3-mediated necroptosis, CD8⁺ T cells cross-priming was significantly higher compared to immunization with caspase-8-mediated apoptotic cells (23). Both RIPK1 signaling and NF-κB activation within the dying tumor cells are required for efficient cross-priming and antitumor immunity (23). It was demonstrated that ATP released by dying cancer cells activated P2RX₇ receptors on DCs and induced the nucleotide-binding oligomerization domain (NOD)-like receptor family, pyrin domain containing-3 protein (NLRP3) inflammasome, linking the innate and adaptive immune responses against dying cancer cells via secretion of IL-1β (24). Cytosolic DNA released by dying tumor cells induces the production of type I IFN (IFN-I) by DCs through cGAS-STING pathway, contributing to antitumor immunity (25). However, it is also reported that DAMPs released from dying tumor cells upregulate T cell immunoglobulin and mucin domain protein-4 (TIM-4) expression levels on TAMs and DCs, which activating autophagy and impeding anti-tumor immunity via TIM-4-AMPKα1 interaction (26). Conventional type 1 dendritic cells (cDC1) are vital for antitumor immunity and the accumulation of cDC1 relies on chemoattractants CCL5 and XCL1 produced by NK cells (27). Dying tumor cells produced prostaglandin E2 (PGE₂) to evade from the NK cell-cDC1 axis by reducing NK cell viability and chemokine production, as well as by downregulating chemokine receptor expression in cDC1 (27). MDSCs are a heterogenous group of myeloid cells that are highly suppressive to antitumor lymphocyte function and trafficking to tumor (28). Two major subsets of MDSCs have been identified in humans: polymorphonuclear MDSCs (PMN-MDSCs) and monocytic MDSCs (M-MDSCs) (28, 29). Translationally controlled tumor protein (TCTP) released by dying tumor cells induces expression of CXCL1 and CXCL2, which then recruits PMN-MDSC population to TIME and exerts immunosuppressive function (29). The resultant accumulation of PMN-MDSCs also strongly attenuates the activation and aggregation of CD8⁺ T and NK cells in the TIME (29–31). Inhibition of TCTP averts PMN-MDSC accumulation and tumor growth (29). Additionally, the immunomodulatory activity of irradiation-induced cell death varies. High non-fractionated radiation doses above the threshold (ranging from 12 to 18 Gy in different cancer cells) induce the expression of the DNA exonuclease Trex1, which degrades cytoplasmic DNA and mitigates the production of IFN-I and the immunostimulatory effects on DCs (32, 33). In contrast, radiation given in fractionated doses below the dose threshold for Trex1 induction stimulates dying tumor cells to release IFN-β cytokine, which recruits Batf3-dependent DCs and activates anti-tumor CD8⁺ T cells (33).

Emerging evidence demonstrates that cancer development and progression are associated with a tumor subpopulation, termed cancer stem cells (CSCs). CSCs not only show treatment resistance, but also regulate the immune system in order to avoid being eliminated by immune cells (34). CSCs have been proved to facilitate macrophage differentiation toward an immunosuppressive M2 phenotype by secreting macrophage-colony stimulating factor (M-CSF), which depends on the constitutive activation of interferon-regulatory factor 5 (IRF5) (35). CSCs also promote the production of IL-6 and IL-1β to mediate Th17 cell differentiation, as well as IL-8 and TNF-α to stimulate neutrophil tumor infiltration and angiogenesis (35–37). In turn, TAMs promote the expansion of CSCs via IL-6 and STAT3 signaling, which stimulate further cytokine production, generating a positive feedback loop for CSC self-renewal (38, 39). Moreover, CSCs secrete several cytokines to exert inhibitory effects on immune cells in TME, including TGF-β, IL-10, IL-4 and IL-13 (40). Although chemotherapy effectively elicited apoptosis, associated PGE₂ released by these dying tumor cells could paradoxically facilitate neighboring CSC repopulation (41). Blockade of PGE₂ production using cyclooxygenase-2 (COX2) inhibitors abrogated this effect (41).

Several studies have highlighted the interconnections between cell death and inflammatory signal transduction (42). Inflammasomes are cytoplasmic multimeric protein complex that provide molecular platforms for activation of caspase-1, contributing to pyroptosis (43). The formation of inflammasome requires cytosolic sensing of pathogen-associated molecular patterns (PAMPs) or DAMPs by the nucleotide-binding domain and leucine-rich repeat receptors (NLRs) or absent in melanoma 2 (AIM2)-like receptors (ALRs) (43). Various innate immune receptors also assemble inflammasome complexes (42). It is widely believed that inflammasomes either favor or suppress carcinogenesis in a highly context-dependent manner. Among NLRs, NLRP3 senses the broadest array of stimuli and participates in adaptive immunity. It is testified that NLRP3 inflammasome drives a T-cell response towards dying tumor cells (24). However, the detrimental effect of NLRP3 is observed when mice are treated with DC vaccination against the poorly immunogenic melanoma cell line B16-F10, via enhancing the accumulation of MDSCs (44). NLRP3 is also an important suppressor of NK cell-mediated responses to carcinogen-induced tumors and metastases (45).

A functional vascular network delivers continued oxygen and nutrients to cells, however, the vasculature of tumor is structurally and functionally abnormal, such as poorly organized vasculature, tortuous and dilated vessels with uneven diameter, excessive branches and shunts (46). The impaired vessel perfusion and high vascular permeability increase tissue hypoxia, acidosis, and necrosis, which could reprogram the resident macrophages into a pro-tumorigenic and immunosuppressive phenotype (47, 48). Also, the hypoxic and acidic microenvironment could up-regulate programmed cell death ligand 1(PD-L1) on MDSCs, DCs and cancer cells, polarize TAMs to the immune inhibitory M2-like phenotype to suppress T-effector cell function. Chronic hypoxia and microenvironment acidity contribute to immune evasion by inducing T cells differentiation into immunosuppressive Tregs (49, 50). Actually, hypoxia is often found in solid tumors due to the rapid growth of cancer cells (51). Hypoxia could influence monocytic cells with respect to morphology, antigen expression, and cytokine secretion, and to induce various genes including vascular endothelial growth factor (VEGF), fibroblast growth factor-3 (FGF-3), BCL2-interacting killer (BIK), and MMPs (52–54). Hypoxia-inducible factors (HIFs) are major transcription factors for cancer cells to adapt hypoxic stress and the activation of HIF pathway results in angiogenesis (55). VEGF family members play important roles in HIF induced angiogenesis. There is mounting evidence to support the perspective that angiogenesis and inflammation are interdependent (56, 57). It is well established that cancer cells are able to secrete pro-angiogenic factors as well as inflammatory mediators (37). Our previous studies have shown that apoptotic tumor cells mediate proangiogenic response via caspase 3/NF-κB/COX2/PGE₂ and caspase 3/p-eIF4E/VEGF-A pathways after irradiation (58). Additionally, normalizing blood vessels with anti-angiogenic therapy normalizes tumor vasculature, creates a more uniform distribution of perfused tumor vessels, and promotes infiltration of T effector cells while inhibiting the accumulation of MDSC (56, 57). Normalized vasculature also alleviates the hypoxia and acidity in TIME, which in term polarizing TAMs to an immunostimulatory M1-like phenotype (56, 57).

Emerging evidence has pointed out that the immune response is correlated with dramatic modifications in tissue metabolism, such as nutrients exhaustion, oxygen depletion, and increased accumulation of reactive nitrogen and oxygen intermediates (59). Given their coexistence, cancer cells and immune cells are in the competition for the local resources available in the microenvironment. When cancer cells underwent dying process, the nutrients competition would still exist. The dying tumor cells either utilized the nutrients around themselves to survival or against demise, or sacrificed themselves to support remnant tumor cells proliferation or escape from immune clearance. It has been reported that apoptotic cells can release metabolites to activate specific programs in neighboring cells (60). Cancer cell death leads to a local ionic imbalance within the TIME as well. In necrotic areas, dying tumor cells release a high number of cations, specifically potassium ions, causing functional starvation of the tumor-infiltrating T cells in parallel to suppression of AKT/mTOR phosphorylation (61). Overabundance of potassium in the TIME also limited T cell effector function by promoting autophagy and metabolic reprogramming, while decreased histone acetylation at effector and exhaustion loci, which in turn generating T cell stemness with improved persistence, engraftment, self-renewal, multipotency, and tumor clearance (62). Lactic acid, a metabolic product of cancer cells, is reported to act as an epigenetic regulator and elicit M2 macrophage polarization through epigenetic reprogramming (63). It binds straight to the histone lysine lactylation (Kla) sites and the transcription of downstream genes is stimulated, which then invokes M2 polarization and reinforces inflammation-independent biological pathways (63). G protein-coupled receptor 81 (GPR81) is a lactic acid receptor found on both immune cells and cancer cells (64). Its activation in cancer cells promotes proliferation, therapy resistance, and enhanced expression of PD-L1 (65–67). Its activation on DCs is associated with decreased cAMP, IL-6, IL-12, and prevents tumor-specific antigen presentation (67). The depletion of glutamine, arginine, and tryptophan also show immunosuppressive effects. It is demonstrated that cancer cells could express indoleamine 2,3-dioxygenase (IDO), an enzyme that catalyzing tryptophan (TRP) and suppresses immune response within the TME (68). Gut microbiota impacts the amenability of some tumor types to therapies by influencing regulatory aspects of the immune response. More clearly, the microbiome has been reported to play a critical role in determining DC functionality (69, 70).

Extracellular vesicles (EVs) are a heterogeneous class of cell-derived membranous structures produced by all cells that contain proteins, lipids and genetic material (71). EVs are essential mediators of cell-cell communication and can be broadly divided into two distinct classes: exosomes and microvesicles (MVs) (71). Tumor-derived exosomes are involved in many stages of tumor progression, including angiogenesis (72), therapy resistance (73), invasion and metastasis (74), and immune evasion (75). Dying tumor cells treated by antitumor agent topotecan (TPT) could release exosomes that contain DNA to activate DCs via a STING-dependent pathway (76). Exosomes derived from tunicamycin (TM)-treated cancer cells contain abundant miR-23a-3p, which inhibit PTEN expression and subsequently elevated phosphorylated AKT and PD-L1 expression in macrophages, thus helping cancer cells to escape from antitumor immunity (77). MVs arise via direct outward budding and fission of the plasma membrane and participate in cell-to-cell communication, immune regulation, and tumor progression (71). Dying tumor cells could release MVs loaded with PD-L1 to suppress the function of T cells and induce M2 phenotype macrophages via TBK1/STAT6 and AKT/mTOR signals, contributing to the suppressive TIME (78).