Innovation: The application of nanotechnology in tumor immunotherapy

Cancer remains the second leading cause of death globally, with cancer spreading to distant organs accounting for more than
90% of cancer-related deaths.
Although immuno-oncology has shown great potential in prolonging patient survival, there is still a lack of effective metastatic therapies due to the difficulty of selectively targeting these small, non-localized tumors
distributed across various organs.

Nanotechnology holds great promise
in improving immunotherapy outcomes in patients with metastatic cancer.
Different from traditional cancer immunotherapy, rationally designed nanomaterials can trigger specific tumicidal effects, thereby improving the contact of immune cells to major metastatic sites such as bone, lung and lymph nodes, optimizing antigen presentation, and inducing a long-lasting immune response
.
Specifically, it can directly reverse the immune status of the primary tumor, harness the potential of peripheral immune cells, prevent the formation of premetastatic niches, and inhibit tumor recurrence
through postoperative immunotherapy.

In addition, compared to low molecular weight immunomodulators, nanoscale immunomodulators have controlled pharmacokinetic behavior and the potential to enhance immune activation through synergistic effects due to their unique size effects and co-loading capacity resulting from the presence of multiple functional domains, which may overcome barriers
to effective immunotherapy for solid tumors.
Nanotechnology has a wide range of application potential in tumor immunotherapy, especially for refractory and recurrent cancers
.

Reverse the immune status of the primary tumor

Tumorigenesis is a process that results from
deficits in host immune surveillance caused by a range of cancer escape mechanisms.
In primary solid tumors, due to the immunosuppressive state of the tumor microenvironment, the immunogenicity of neoantigens in cancer cells is too weak to effectively stimulate the immune response
.
Nanomaterials offer a new way to
overcome this barrier to therapeutic efficacy.

Alters the immunosuppressed tumor microenvironment

The characteristics of the tumor microenvironment play an important role
in determining the success of cancer immunotherapy.
In the tumor immunosuppressive microenvironment, multiple immune and non-immune cell types cause long-term inflammation and local immunosuppression, so that malignant cells are not detected and eliminated
by the host immune system.
To break this situation, nano-immunomodulators are designed to directly target the immunosuppressive microenvironment, thereby reactivating the immune system in situ and inhibiting tumor growth
.

Macrophages are one of the most abundant immune cell populations in the tumor microenvironment, and TAM is mainly M2 phenotype, so making TAMs complex from M2 to M1 has become one of
the immunotherapy strategies.
Recently, Chen and his colleagues constructed a programmable cellular vesicle to combat postoperative tumor recurrence and metastasis
.
Heterozygous cell membrane nanovesicles (HNVs) interact with circulating tumor cells (CTCs) in the vascular lumens and aggregate at the excision site, blocking CD47-SIRPα interactions, repolarizing the TAM from M2 to M1, thereby killing cancer cells
.
These nanovesicles can also improve the killing ability of T cells to malignant cells through antigen presentation, and significantly improve the survival rate
of malignant melanoma model mice by reducing local recurrence and postoperative distal spread.

In addition to TAM, MDSCs are also involved in the generation of immunosuppressive microenvironments
.
Shuai and colleagues designed a nanoregulator containing MnO2 particles and the PI3Kγ inhibitor IPI549 to reduce hypoxia while downregulating the expression of
immunosuppressive PD-L1 molecules.
At the same time, nanomodulators can activate MDSCs to accelerate the polarization of TAM to the M1 phenotype and reactivate cytotoxic T cells to stop the proliferation
of tumor cells.

In addition to affecting TAMs and MDSCs, NK cell activation is another strategy
to alleviate the immunosuppressive microenvironment.
Nano-assisted immunotherapies using multiple cytokines, such as IL-2 and IL-12, have been explored to prevent progression and metastasis
by activating NK cells.
Irvine and colleagues developed a combination therapy system
with agonistic anti-CD137 and IL-2 on the surface of polyethylene glycolated liposomes.
With this liposome-based treatment, immune stimulants can accumulate rapidly in tumors, thereby inducing efficient activation of NK cells and T cells, which in turn inhibits tumor progression at the site of primary tumor in mice and lung metastases
.
Similar approaches to nanomaterials activated by cytokines to assist NK cells have been widely developed for the treatment of metastatic cancer
.
In addition to directly activating NK cells, nanoparticles can be designed to improve the accumulation of NK cells at the tumor site, thereby improving the therapeutic effect
.

In general, by alleviating the immunosuppressive effect of tumor-associated immune cells and remodeling the immune status of tumor microenvironment, it provides a feasible way
for local regional tumor immunotherapy.
More importantly, this strategy has great potential
to prevent distant tumor metastasis and recurrence through systemic anti-cancer immune responses.

Immune cells are activated by ICD

In recent years, studies have shown that traditional local treatments, including local hyperthermia, radiotherapy or chemotherapy, not only destroy primary tumor cells but also induce tumor immunogenic cell death (ICD).

However, the immune response stimulated by the ICD is usually not sufficient to cause systemic effects against metastasis or to prevent tumor recurrence
.
Therefore, nanomaterials are designed to enhance the immune response
to traditional cancer therapies.

Local area heat treatment is the most common method for
inducing ICD.
In one study, Wang and colleagues used systematic, erythrocyte membrane-wrapped 2D polypyrrole nanosheets as NIR-II photothermal sensors, resulting in a synergistic photothermal and immune response that is beneficial in preventing metastasis and prolonging survival in
mice.

In addition to photothermal therapy, magnetothermal therapy is another viable method
of inducing ICDs.
Liang and colleagues designed a novel ferromagnetic iron oxide nanoring with a vortex basin that is able to mediate mild magnetic hyperthermia leading to calreticulin expression in 4T1 breast tumor cells and promote immune cell phagocytosis of tumor cells
.

In addition to local hyperthermia, nano-assisted radiation therapy is another treatment strategy that causes tumor ICDs and may inhibit distant metastases
.
In fact, the distant effects of radiation therapy have been observed in some clinical cases, more formally known as radiation-induced bystander effects (RIBE), which are triggered by an immune response to dead tumor cells
.
Through the combination of nano-assisted immunotherapy, RIBE can be significantly amplified
.
For example, Liu and colleagues developed a new radioisotope therapy
using a combination of 131I-Cat, natural polysaccharide alginate, and synthetic cytosine guanosine monophosphate (CpG).
After intratumoral injection, polysaccharides rapidly form a hydrogel due to the presence of endogenous Ca2+, which fixes 131I-Cat at the tumor site, so that the primary tumor
can be completely eliminated by low-dose radiotherapy.
Importantly, with the help of immunostimulatory oligonucleotide CpG, the production of tumor-associated antigens (TAAs) after local radiotherapy for primary tumors effectively triggers a systemic anti-tumor immune response, which, in combination with checkpoint blocking therapy, successfully prevents metastasis and recurrence
.

These studies help illustrate the great potential
of tumor ICDs combined with nano-assisted immunotherapy in preventing tumor growth and metastasis.
Combination therapy offers great opportunities
for the clinical application of nanomaterials-assisted immunotherapy.

Applications against immune cells

Immune cells are located in peripheral immune organs, including the lymph nodes, spleen, skin, and vascular system
.
They play an important role in generating an
immune response stimulated by foreign antigens.
Nanotechnology can be applied to immune cells
in a variety of ways.

Cancer vaccine against dendritic cells

Dendritic cell-based cancer vaccines have great potential in tumor prevention and treatment and have been shown to be effective in inhibiting tumor metastasis and recurrence
.
However, DC-based immunotherapy is still limited by an inadequate immune response, which makes it difficult to completely eradicate established solid tumors
.

Thanks to recent advances in nanotechnology, structures such as liposomes, polymer nanoparticles, and inorganic nanoparticles are capable of loading different components, including small molecules, peptides, nucleic acids, and cell membranes
.
This enables antigens and adjuvants to be co-loaded in nanovaccines, ensuring that these active ingredients are delivered to the same APC
at the same time.
In addition, nanovaccines can also prevent components such as antigens and adjuvants from rapidly spreading into the blood circulation and promote their effective accumulation
in draining lymph nodes.
Therefore, nanoparticle-based vaccines may be a valuable tool
to enhance the immune response and prevent tumor metastasis.

Zhou et al.
constructed an adjuvant/antigen co-delivery nanoplatform
by coating PLGA nanoparticles with phospholipid membranes.
This nanovaccine can effectively accumulate in the lymph nodes and trigger an antigen-specific T cell response, thereby inhibiting the metastasis of B16-OVA melanoma cells, and the number of metastatic nodules is greatly reduced
.
In another example, an anti-metastatic nanovaccine
based on PLGA nanoparticles was developed by encapsulating the novel TLR7/8 bispecific agonist 522NP.
After intravenous injection, 522NPs enter the draining lymph nodes and activate DC, thereby significantly enhancing the subsequent CTL response
.
Mouse immunized with OVA+522NP had approximately 75%
fewer lung metastatic nodules than the control group.

Artificial simulation of immune cells

Artificial APCs (aAPCs) based on micro- and nanomaterials are designed to inhibit tumors
by mimicking native APCs by presenting antigen signals to T cells and activating them.
To achieve the antigen presentation effect, the aAPC surface should include two parts: an MHC peptide complex and a co-stimulatory molecule
that binds to co-stimulatory receptors and activates T cells.
Compared with natural dendritic cells, aAPCs have a relatively well-defined composition and controllable biological behavior
.
In addition, aAPCs can be used for large-scale production, making ready-made vaccines
available.

Lu et al.
developed an aAPC vaccine in which microlatex beads are assembled with H-2Kb-Ig/pTRP2 dimer complexes, anti-CD28 antibodies, 4-1BB ligands, and CD83 molecules
.
In mouse animal models of B16 melanoma, intravenous administration of H-2Kb-Ig/pTRP2 aAPCs stimulated melanoma-specific CTLs
.
The results showed that lung metastasis was significantly reduced after H-2Kb-Ig/pTRP2 aAPC treatment, and only about 36 metastatic nodules appeared in the lungs after treatment, compared with about 330 lung metastatic nodules
in the control group mice.

Moreover, APC mimics are not the only immune cells
explored using nanomaterials.
Neutrophils, or Tregs, also play a key role in the innate immune response to tumors, and nanomaterials have been used to mimic them to inhibit tumor growth
.
It can be expected that new biomimetic nanomaterials
that can mimic more types of tumor-associated immune cells will be developed in future research.

Adoptive T-cell therapy

Adoptive T-cell therapy is one of
the main treatments for cancer.
The use of nanotechnology can effectively engineer lymphocytes to express T cell receptors or chimeric antigen receptors, further expanding the successful application
of ACT in cancer treatment.

For example, Irvine and colleagues demonstrated that modifying the surface of T cells with cytokines or drug-loaded nanoparticles can significantly improve the efficacy
of ACT.
They also used nanogels to load protein drugs onto T cells
.
This strategy significantly increased the number of T cells present in the tumor, thereby improving the safety of
ACT therapy.

In addition to T cells, platelets are also used for ACT
.
Due to their inherent properties, they can accumulate spontaneously in the wound area
.
Inspired by this ability, Gu and colleagues bind anti-PD-L1 to the surface of platelets that can successfully target surgical wounds after tumor resection and release anti-PD-L1
through platelet-derived particles after platelet activation in situ.
This strategy has been shown to successfully remove residual tumor cells and prevent the cancer from returning
.

Interference with the formation of ecological niches before transfer

Primary tumors need to change the microenvironment of distant organs to create favorable conditions for CTCs, called premetastatic niche (PMN).

In this phenomenon, primary tumor cells first secrete soluble components, such as exosomes (EVs), at the site of potential metastasis and regulate the microenvironment
in that region by transferring small nucleic acid fragments to normal cells.
Primary tumor-derived inflammatory factors simultaneously recruit inhibitory immune cells, including MDSCs, TAM, or Treg, which further secrete chemokines and cytokines to support PMN formation
.
Therefore, interfering with the formation of PMN is an opportunity
to prevent tumor cell colonization.

Remodeling the molecular composition of PMN

PMN can support extravasation, anchoring, survival, proliferation, and immune evasion
of CTCs by altering the vascular state of tissues.
In addition, PMN also exhibits inflammation and stromal reprogramming, which is also a key process for
CTC colonization and survival.

To prevent these processes, researchers have found molecules
that can remodel PMN components to inhibit metastasis.
Lysyl oxidase, an enzyme overexpressed in tumors and PMN, aids tumor cell colonization
by remodeling the ECM.
In addition to small molecules, nanopharmaceutical preparations that target the molecular components of PMNs are also beginning to emerge
.
Jiang et al.
designed metformin and docosahexaenoic acid mixed microparticles as remodeling agents for PMN, and these nanomedicines reduce adhesion between CTCs and endothelial cells, reverse abnormal expression of inflammatory molecules, including fibronectin, matrix metalloproteinase-9 (MMP-9), and S100A9 in PMN, and thus show metastasis-inhibiting effects
.

Inhibit MDSC

Among the immunosuppressive cells of PMN, MDSCs play a key role in the formation of PMN and inhibit the activity
of CD8+ T cells.
Therefore, inhibiting the recruitment of MDSCs is an effective way to
prevent metastasis.

Nanotechnology has been applied to intervene in the early recruitment of MDSCs
.
Low molecular weight heparin and tocopheryl succinate are used to self-assemble into micellar nanoparticles (LT NPs).

The former inhibits the extravasation of P-selectin/PSGL-1-mediated granulocyte-derived suppressor cells (g-MDSCs) through competitive binding, and the latter inhibits the expression
of MMP-9 in g-MDSCs through competitive binding.
In addition, the absence of MDSCs in PMN is another way to
inhibit metastasis.
Ni et al.
reported that hafnium DBP (5,15-bis(parabenzoic acid) porphyrin), a nanoscale metal-organic compound, in combination with αPD-L1 demonstrated excellent antitumor activity and anti-metastatic effects
in an in situ breast cancer lung metastasis model.
Further studies showed that the anti-metastatic effect came from a decrease in monocyte MDSC (mMDSC) and granulocyte MDSCs in the lungs, as well as a reduction
in mMDSCs in the primary tumor.

Blocks carcinogenic EVs

Exosomes are important messengers of primary tumor cells and stromal cells that remodel the microenvironment
of distal organs.
Interfering with EVs can block the signals transmitted by the primary tumor and may help with treatment
.

One promising strategy for interfering with exosomes is the direct elimination of circulating exosomes
.
Xie et al.
innovatively proposed dragging exosomes into the small intestine
through the hepatobiliary metabolic pathway of nanoparticles.
Specifically, they designed epidermal growth factor receptor-targeting aptamers that couple positively charged mesoporous silica nanoparticles to recognize and bind negatively charged exosomes
in the blood.
In vivo experiments have shown that these nanomaterials effectively increase the distribution
of exosomes in the liver and small intestine.

It is used in postoperative immunotherapy

To address recurrence after surgery, various nano-immunomodulators have been developed for immunotherapy
after surgical resection.
Gu's group developed a nanoformulation that can be sprayed into the tumor resection cavity, which can form immunotherapeutic fibrin gel in situ and induce macrophages to engulf tumor cells
by gradually releasing anti-CD47 antibodies to block the interaction between CD47 and SIRPα.
This nanoagent can effectively inhibit local and distant tumor recurrence
after surgery.

Nanomaterial-assisted postoperative immunotherapy has a greater advantage in preventing long-term recurrence of tumors that grow in specific sites, where it is highly undesirable to remove additional tissue, such as brain tumors
.
Jiang and colleagues proposed an injectable homemade oligopeptide hydrogel system that promotes tumor-specific immune responses after surgical resection of glioblastoma and effectively prevents brain tumor recurrence
in mice.
The hydrogel acts as a drug library
co-delivered by CXCL10 and tumor homing immune nanomodulators (THINR).
After administration in the surgical cavity, the precursor solution forms a hydrogel and is released over time, which includes mitoxantrone and small interfering RNA targeting IDO (siIDO1).

Mitoxantrone and siIDO1 are released by breakdown in the acidic environment after internalization and have an immunomodulatory effect on tumor cells, thereby activating circulating T cells and alleviating immunosuppression
of Treg.
The activated T cells are then recruited into the brain by CXCL10 to attack the remaining tumor cells
.

Compared with systemic administration, topical administration of nanomaterials has the potential to reduce adverse effects by limiting organ exposure while increasing drug concentrations
in the affected area.
Due to the activation of tumor-specific immunity and the formation of long-term immune memory, local nanomaterials-assisted postoperative immunotherapy is a feasible strategy
to prevent tumor recurrence after surgical resection.

brief summary

Nano immunomodulators can not only effectively eliminate primary tumors, but also have a good inhibitory effect on distant metastasis, which can prevent recurrence
.
In addition, nanomaterials can be used as a multifunctional platform to complement the shortcomings
of various immunotherapies.
The emergence of more immunotherapy combinations, artificial immune cells and new nanomaterials will profoundly affect the ability to
treat refractory and metastatic cancers.

References:

1.
Nanotechnology-enhanced immunotherapy formetastatic cancer.
Innovation (N Y).
 2021 Nov 28; 2(4): 100174.