Non-apoptotic regulatory cell death in tumor immunotherapy

preface

In recent years, immunotherapy represented by immune checkpoint inhibitors (ICIs) has made unprecedented breakthroughs

Since anti-apoptosis is a common feature of cancer, inducing non-apoptotic regulatory cell death (RCD) is a new cancer treatment strategy

Autophagy

Autophagy is a regulatory mechanism that removes unnecessary or dysfunctional cellular components and recovers metabolic substrates

The tumor microenvironment (TME) plays a key role

Evading the anti-tumor immune response is an important survival strategy for various tumors

On the other hand, MDSCs exert an immunosuppressive effect in TME, and studies have demonstrated that autophagy in MDSC is a key mechanism

In addition, autophagy, as a mechanism by which (cancer) cells respond to threatening stressors, is considered an important mechanism

Autophagy inhibitors are divided into early inhibitors against ULK1/ULK2 or VPS34, such as SBI-0206965, 3MA and wortmannin, and advanced inhibitors against lysosomals, such as CQ, hydroxychloroquine (HCQ), parfamycin A1 and monensin, CQ and HCQ inhibit autophagosome degradation

chemotherapy

High autophagy flux in cancer is associated with a reduced response to chemotherapy and is associated with low survival rates in cancer patients

An early Phase II study in 2014 used HCQ monotherapy to treat patients with metastatic pancreatic cancer who had previously been treated with other methods with a primary endpoint of two months of progression-free survival

Radiotherapy

Autophagy plays a key role

In glioblastoma, radiation therapy induces autophagy by increasing the expression of mammalian STE20-like protein kinase 4 (MST4), which stimulates autophagy

immunotherapy

Harnessing the immune system is an important way to

For example, inhibition of VPS34 kinase activity using inhibitors SB02024 or SAR405 results in elevated levels of CCL5, CXCL10, and IFN-γ in TME, thereby elevating

Therefore, targeted autophagy may enhance immunotherapy

Iron dies

Iron death is a recently discovered type of programmed cell death that plays an important role

Iron death is a regulatory cell death
caused by iron-dependent lipid peroxidation.
Three key features of iron death have been cracked: membrane lipid peroxidation, availability of iron within cells, and loss
of antioxidant defenses.
(For details, see Iron Death in Tumor Immunotherapy.
)

Recently, iron death has been found to contribute to the antitumor effect of CD8+ T cells and to affect the effectiveness
of anti-PD-1/PD-L1 immunotherapy.
Immunotherapy, combined with ways to promote iron death, such as radiation therapy and targeted therapy, can create synergistic effects through iron death to promote tumor control
.

Combined use of immunotherapy with cystine restriction

Recently, it has been reported that CD8+ T cells activated by anti-PD-L1 immunotherapy secrete IFN-γ promote iron death
in tumor cells after PD-L1 blockade.
Secretory IFN-γ significantly downregulates the expression of SLC3A2 and SLC7A11 in tumor cells, resulting in decreased cystine uptake, increased lipid peroxidation, and subsequent iron death
.
Cystine/cysteine enzymes work synergistically with anti-PD-L1 to produce potent anti-tumor immunity
by inducing iron death.

Immunotherapy combined with targeted therapy

A recent study suggests that resistance to anti-PD-L1 therapy can be overcome by combining with a TYR03 receptor tyrosine kinase (RTK) inhibitor that promotes iron death
.
Increased
expression of TYR03 was found in anti-PD-1 resistant tumors.
Mechanically, the TYR03 signaling pathway modulates the expression of key iron death genes such as SLC3A2, thereby inhibiting neoplastic iron death
.
In the TNBC homogeneous mouse model, inhibition of TYR03 promotes iron death and makes tumors sensitive
to PD-1 therapy.
The study reveals that relieving iron death by using TYR03 inhibitors is an effective strategy for
overcoming immunotherapy resistance.

Immunotherapy combined with radiotherapy

Recent evidence suggests that the synergistic effects of radiation therapy and immunotherapy are associated
with increased sensitivity to iron death.
Radiation has been shown to induce iron death, and genetic and biochemical characteristics
of iron death have been observed in radiation-treated cancer cells.
Its mechanism involves radiation-induced ROS production and upregulation of ACSL4, leading to enhanced lipid synthesis, increased lipid peroxidation, and subsequent membrane damage
.
Therefore, the antitumor effects of radiotherapy can be attributed not only to cell death induced by DNA damage, but also to the induction
of iron death.
Radiation therapy and immunotherapy worked together to downregulate SLC7A11, mediated by DNA damage-activated kinase ATM and IFN-γ, resulting in decreased cystine uptake, increased iron death, and increased
tumor control.
These studies reveal that iron death is a novel mechanism
of synergy between immunotherapy and radiotherapy.

Combination of immunotherapy with inhibitors of iron death in T cells

In addition to inducing neoplastic iron death, iron death may also occur in T cells themselves, which may weaken their immune response
.
T cells lacking GPX4 rapidly accumulate membrane lipid peroxides, accompanied by iron death
.
Similar to cancer cells, ACSL4 is essential for iron death in CD8+ T cells and their immune function
.

Recently, two studies have shown an increase in
the expression of CD36 in CD8+ tumor-infiltrating lymphocytes.
CD36 inherent in T cells promotes uptake of oxidizing lipids and induces lipid peroxidation, resulting in CD8+ T cell dysfunction
.
These findings reveal that iron death in CD8+ T cells is a new model of tumor immunosuppression and highlight the therapeutic potential
of blocking CD36 to enhance anti-tumor immunity.
Notably, the study also shows that GPX4 plays a role
in regulating the anti-tumor function of CD8+TIL.
Therefore, therapeutic induction of iron death in cancer cells by GPX4 inhibitors may have unnecessary targeted effects on T cells and produce adverse toxicity
.

Cell scorching

In the 1990s, cytometry was
first described in macrophages infected with S.
typhurium and Salmonella fleuri.
Although initially thought to be a process of apoptosis, further research suggests that this bacterial-induced cell death relies heavily on caspase-1
.
Scorched cells share some common features with apoptotic cells, such as chromatin concentration and DNA fragmentation, but can be distinguished
by their intact nuclei, pore formation, cell swelling, and osmolysis.

Typically, focal cell rupture is achieved by activation of cysteine-mediated well-formed GSDM proteins after binding to the damage-associated molecular pattern (DAMP) or pathogen-associated
molecular pattern (PAMP).
These same cysteine proteases may also directly or indirectly promote the maturation of pro-inflammatory cytokines that, together with DAMP, initiate or maintain an inflammatory response
upon release.

Although the number of known pathways of coke death may increase in the future, two major pathways and several alternative pathways have been elucidated
.
Among the main pathways, cellular coke death is induced by GSDMD and involves inflammatory caspase-1 (classical pathway) or caspase-4/5 (non-classical pathway).

Among the alternative pathways, the most widely concerned is GSDME-induced cytofoulospiration
by caspase-3.
(For details, see Tumor Immunity and Cytokewal Death)

The ability of cell death to trigger an adaptive immune response is called immunogenic cell death (ICD
).
Unlike apoptosis, which is essentially an immune-tolerant process, cytofoulosphericity has a molecular mechanism that induces a strong inflammatory response and is in some cases considered a form
of ICD.
While the link between cytokewal and anti-cancer immunity is unclear, a growing body of research suggests that cytokeming-mediated tumor clearance is achieved
by enhancing immune activation and function.

For example, methotrexate is injected into cholangiocarcinoma (CCA) cells using tumor cell-derived microparticles (TPPs) to induce GSDME-mediated cellular scorch death, thereby activating patient-derived macrophages and recruiting neutrophils to the tumor site for drug-directed tumor destruction
.
In addition, when this methotrexate TMP delivery system is injected into the bile duct lumen of patients with extrahepatic CCA, 25% of patients observed neutrophil activation and a reduction
in biliary obstruction.

In addition, GSDME-mediated pyrrophoresis was found to cause immune cell infiltration/activation in melanoma through a combination of BRAF and MEK inhibitors and lead to regression of melanoma
.
In another strategy, metformin is the most commonly used drug for the treatment of type 2 diabetes, which can inhibit cancer cell proliferation
by indirectly activating cytofoliation through caspase-3.

A range of small molecule inhibitors against KRAS, EGFR, or ALK mutant lung cancers have also been found, inducing cellular focus death
through caspase-3-mediated GSDME lysis after activation of the intrinsic apoptosis pathway in mitochondria.
In breast cancer cells, treatment with RIG-1 agonists triggers exogenous apoptotic pathways and cytofoliation, activates STAT1 and NF-κB and upregulates lymphocyte recruitment chemokines
.
Thus, after RIG-1 activation in mice, a decrease in breast cancer metastasis and tumor growth is accompanied by an increase
in tumor lymphocytes.

Another major obstacle faced by almost all anti-cancer immunotherapy strategies is the dysregulation caused by the immunosuppressive tumor microenvironment
.
To solve this problem, Lu et al.
engineered NK92 cells containing a chimeric co-stimulated transformation receptor (CCCR), which converts an inhibitory PD-1 signal into an activation signal, effectively enhancing anti-tumor activity
.
In vitro, CCCR-NK92 cells rapidly kill H1299 cells through GSDME-mediated cytokelesis, significantly inhibiting tumor growth
in vivo.

In addition, a growing number of exciting studies suggest that cytofouls induction works synergistically with PD-1 inhibitors to change tumors from "cold" to "hot," suggesting the enormous potential
of this combination.

Necrotizing apoptosis

Necrotizing apoptosis proposed by Deggerev et al.
in 2005 is another form of ICD that includes CERTAIN death receptors (DRs) such as FAS and tumor necrosis factor receptor 1 (TNFR1) or PRR such as toll-like receptor 3 (TLR3) that recognize unfavorable signals from the microenvironment inside and outside the cell, triggering necrotizing apoptosis
.

There is evidence that necrotizing apoptosis plays a tumor suppressor role in most cases
.
In a study of more than 60 cancer cell lines, two-thirds of the samples showed reduced RIPK3 levels, suggesting that cancer cells tend to survive
by escaping necrotizing apoptosis.
In addition, necrotizing apoptosis is closely related to cancer prognosis, and the expression of RIPK3 is an independent prognostic factor for the overall survival and disease-free survival of colorectal cancer patients
.
Recently, a study showed that the expression of RIPK1, RIPK3, and MLKL was associated with better overall survival in liver cancer patients
.

Effectors in necrotizing apoptosis such as RIPK1 and RIPK3 can directly regulate the function of immune cells without relying on cell death
.
The study found that RIPK3-mediated activation of phosphoglycerolectionase 5 (PGAM5) promotes a naturally killer T cell-mediated antitumor immune response
in a process independent of necrotic pathways by activating nuclear translocation of nuclear factors (NFAT) of T cells and dephosphorylation of the dyname-related protein 1 (Drp1).
In addition, in the same gene melanoma and lung adenocarcinoma models, injecting necrotizing tumor cells activated by RIP3 into existing tumors can enhance anti-tumor immunity
.

Necrotizing apoptosis inducers may be synergistic with ICIs to fight tumors
.
In melanoma, the SMAC analogue Birinapant sensitizes tumor cells to TNF-α-mediated T cell killing and directly regulates immune cell function, including B cells, myeloid cells, and cytotoxic lymphocytes, by modulating the NF-κB signaling pathway, thereby improving response
to ICIs.
Similarly, in mouse tumor models, SMAC analogues activated CD8+ T cells and NK cells through RIPP1-dependent cell death to improve the survival benefits
of immune checkpoint blockade.
In addition, SMAC analogues are also thought to improve the efficacy of CAR-T cells in the treatment of acute lymphoblastic leukemia, because death receptor signals are a key mediator of
CAR-T cell cytotoxicity.

brief summary

The study of non-apoptosis RCDs is a broad and rapidly evolving field
.
A new view is that for autophagy, coke death, iron death and necrotizing apoptosis in local tumors, it profoundly affects the response of infiltrated immune cells and immunotherapy in TME
.
There is a wide range of interactions
between the mechanism of non-apoptotic cell death and anti-tumor immunity.

Although the role of autophagy, cytoke death, iron death, and necrotizing apoptosis in tumor immunity remains unclear, and some findings suggest a more complex interaction between non-apoptotic RCDs and immunity in different tumor types and backgrounds, targeted non-apoptotic cell death has increasingly become a promising strategy
for improving the efficacy of cancer immunotherapy.
Currently, it is imperative
to develop more specific cell death-inducing drugs that act on tumor cells with minimal side effects on normal tissues.

At the same time, preclinical testing of these drugs in combination with ICIs, as well as chemotherapy, radiotherapy, and targeted therapies, may be critical
to balancing treatment goals and possible adverse effects.
In the future, clinical trials of combination therapies should be actively encouraged to evaluate their efficacy and safety, and to provide more references for subsequent in-depth studies to benefit
more cancer patients.