Immune Checkpoint and Tumor Therapy

Abstract

Cancer immunotherapy employing immune checkpoint inhibitors (ICI) has revolutionized the tumor therapy far beyond their impressing clinical effects. Immune checkpoint therapy (ICT), which is directly involved in different immunosuppressive mechanisms at tumor sites, has been thoroughly studied. Nevertheless, the “off-target” effects of ICIs following systemic administration is still challenging. In addition, the clinical response rate of ICT is still unsatisfactory in that only a few patients hold lasting benefits. In this chapter, the mechanism of most widely used ICIs, including those based on CTLA-4 and PD-1/PD-L1, has been introduced. The approaches to enhancing the efficacy of ICT have been highlighted, namely improving targeted delivery of ICI by employing nanotechnology, modulating the immunosuppressive tumor microenvironment (TME), and combining ICT with other therapies. We hope advanced strategies summarized in this chapter would further inspire the development of ICT to boost their effectiveness while minimize unwanted side effects.

1. Introduction

Cancer immunotherapy is a promising strategy to combat cancer by leveraging host immune system, involving lymphocyte-promoting cytokines, cancer vaccines, immune checkpoint therapy (ICT), and engineered T cells [1, 2]. Among the diverse immunotherapeutic approaches, ICT is the most thoroughly investigated approach with broad impact. It can enhance antitumor immunity by inhibiting negative regulatory pathways. To date, several monoclonal antibodies against the cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed cell death 1 (PD1)–PD1 ligand 1 (PD-L1) axis have been clinically approved for various cancers, including melanoma, lung, and renal cancers (Figure 1) [3]. Some other checkpoint inhibitors are also in preclinical or earlier phases of clinical development, such as LAG3, TIGIT, TIM3, B7H3, CD39, CD73 and adenosine A2A receptors [4, 5].

Figure 1.

Timeline of significant milestones in the development of cancer immune checkpoint inhibitors. CD28: cluster of differentiation 28; CTLA-4: cytotoxic T lymphocyte-associated protein 4; FDA: US Food and Drug Administration; irRC: immune-related response criteria; PD-1: programmed cell death protein 1; and Tregs: regulatory T cells.

Despite substantial progress of immune checkpoint inhibitors (ICIs) in the cancer treatment, there are still several key limitations. Firstly, systemically delivery of checkpoint inhibitors may cause serious side effects in several major organs. In addition, many patients with nonimmunogenic tumor microenvironments (TMEs) showed therapeutic resistance to checkpoint inhibitors and do not respond to the treatment. The mechanism of the non-responsiveness to checkpoint inhibitors are still in investigation and may involve poor tumor-infiltration of T cells, checkpoints dysregulation in tumor cells and T cells, and adaptive resistance to checkpoint inhibition [6, 7, 8]. These drawbacks need to be overcome to achieve more satisfactory therapeutic outcomes against various cancer. In this chapter, we will introduce immunological mechanism of immune checkpoint blockade and highlight emerging approaches to enhancing ICT efficiency. It is foreseeable that ICT will lead to next-generation promising techniques and continuously contribute to the future cancer treatment.

2. Mechanisms of immune checkpoint therapy (ICT)

T cells enable to distinguish tumor cells from normal cells and launch attack accordingly, which plays a critical role in maintaining appropriate immune responses. However, such immunologic effects may be prevented in the TME. The prevention of T cells activation in the presence of antigen is related with the T cells dysfunction and inhibiting the receptors expression, such as those of CTLA-4 and PD-1 [9] ICP aim to block such inhibition and reverse the immunosuppressive TME, thus achieving functions mainly by activating normal immune system to eradicate cancer cells. Despite a few overlaps in inhibitory roles, each checkpoint inhibitor also performs some unique functions.

2.1 CTLA-4

The normal T cell activation requires the binding of CD28 on T cells with co-stimulatory B7 molecules (CD80 and CD86) on DC surface, also known as signal 2 of T-cell receptors (TCR) activation [10]. However, CTLA-4 which is expressed on the surface of late-stage T cells can competitively bind with CD80/CD86 to prevent T-cell activation [5]. Regarding this, blockade with monoclonal antibody against CTLA-4 enables to proceed CD28/B7 pathway and restore T-cells activity. CTLA-4 is also constitutively overexpressed on regulatory T cells (Tregs), which can mediate dendritic cell (DC) function inhibition and suppress the T cell response against tumors [11]. Additionally, CTLA-4 was reported to be expressed in some other cells such as activated B cells, placental fibroblasts and monocytes, and may playing roles in immune regulation of other cells. For instance, it is associated with decreased circulating B cell amounts and antibody expression levels [12]. Of note, the exact cellular mechanisms underlying CTLA-4 blockade remains to be investigated and different anti-CTLA-4 antibody has distinct properties (Figure 2) [13].

Figure 2.

Scheme illustration showing the mechanism of (A) CTLA-4 blockade; and (B) PD-1 Blockade.

2.2 PD-1/PD-L1

In normal physiological conditions, ICIs can modulate T cell activity and protect healthy tissues from immune attack. T cell activity can be suspended by the binding of PD-1 on T-cells with its ligands programmed death ligand 1 (PD-L1) and programmed death ligand 2 (PD-L2) which are largely distributed on tumor cells and DCs. Regarding this, PD1 PD-L1/2 blockade by using monoclonal antibody can lead to restoration of T cell activity. PD-L2 shows higher affinity to PD-1 but with more limited expression profile and is mainly expressed on activated DCs, macrophages, and some B cells [14]. PD-L1 is more widely expressed on DCs, macrophages, T and B cells, as well as some cell types in non-hematologic tissues, such as epithelia, endothelial cells, astrocytes and neurons.

2.3 Other immune checkpoints

Some other ICI molecules under investigation include positive regulators such as tumor necrosis factor receptor superfamily membrane 9 (4-1BB) and tumor necrosis factor receptor superfamily member 4 (OX-40), and negative regulators such as T-cell immunoglobulin and mucin domain (TIM-3) and lymphocyte activation gene-3 (LAG-3) [5]. These immune checkpoints also have been recognized for their roles in regulating tumor immunity and elicits antitumor response.

3. Approaches to enhancing ICB therapy efficiency

3.1 Improve targeted delivery of ICIs by employing nanotechnology

The ICP therapeutic effects depends on successful interaction of ICIs with the protein of interest. Nevertheless, the “off-target” effects of ICT therapeutics following systemic administration brings some side effects and limits the maximum allowable doses. Thus, it is significant to achieve targeted delivery and controlled release of ICIs in the desired cell types. To this end, several nanoparticle (NP) systems, such as liposome, polymeric NPs and inorganic NPs, have been used to achieve targeted delivery of ICIs to maximize the therapeutic effects while minimizing the unwanted side effects [15]. The nanotechnology-mediated ICT showed several advantages over traditional method and can improve therapeutic efficacy of ICT, as described below.

3.1.1 Passive targeting

Employing nanotechnology can improve the tumor accumulation of therapeutic ICIs via enhanced permeability and retention (EPR) effect, which refers to the higher permeability of tumor vessels to NPs than normal vessels and the increased retention of NPs in tumors due to the poor lymphatic clearance. For example, Nikpoor et al. developed PEGylated liposomes for the delivery of αCTLA-4 monoclonal antibodies [16]. They found that tumor accumulation of PEGylated liposomes encapsulated with anti-CTLA-4 antibodies was significantly greater than that of free antibodies in the CT26 colorectal tumor-bearing mice 18 h post-injection. Accordingly, the tumor-bearing mice receiving treatment of PEGylated liposomes loaded with antibody showed obviously extended survival time compared with free antibody group, suggesting that improved tumor accumulation led to greater therapeutic efficacy. It should be noted that the tumor accumulation effects of NPs via EPR effect are closely related with tumor type, heterogeneity, and perfusion.

The size and charges of NPs playing critical roles in passive targeting pattern by affecting the half-life and biodistribution. Such structure–activity relationships guide the rational design of targeted delivery nanoplatform. The size of the NPs should not be too small or too large. The NPs smaller than 7 nm tend to be cleared by renal filtration and urinary excretion [17, 18], while those larger than 200 nm are more likely to be cleared by the reticuloendothelial system (RES) [19]. As for the surface charge, the positively charged NPs show higher cellular uptake efficiency, while those slightly negatively charged and neutral NPs exhibit longer persistence during circulation. Besides, strongly positively or negatively charged NPs tend to be cleared by RES [17].

The surface properties of NPs also have impact on in vivo fate and performance by affecting their interaction with endogenous macromolecules. Poly(ethylene)glycol (PEG) is the most widely applied coatings to adjust the NPs surface properties. In a study, PEGylated and non-PEGylated liposomes in similar diameter of 140 nm were both applied to deliver anti-CTLA-4 mAb into C26 colon tumor-bearing mice to study antitumor therapeutic effects. PEGylated CTLA-4-liposomes were shown to prolong blood half-lives and induce higher intratumoral accumulation than free antibodies and non-PEGylated groups [16].

3.1.2 Active targeting

In addition to passive targeting via EPR effects, achieving active targeting by introducing targeting moieties into NPs also can facilitate target site accumulation. Active targeting approaches can promote targeted delivery by directing NPs to action sites, either a specific location or a specific cellular type, and reduce off-target side effects. This strategy often leads to better therapeutic effects compared with those without targeting moieties through passive targeting. For example, LinTT1 is an active targeting peptide which can promote cellular uptake and tumor tissue penetration by intervening low-affinity binding with p32 cell surface receptors on tumor cells, and tumor-associated macrophages [20]. Li et al. incorporated an active targeting peptide LinTT1 into self-assembled micelles for the co-delivery of siRNA for PD-L1 and an IDO inhibitor [21]. The results showed that intravenous administration of LinTT-1-targeted NPs significantly enhanced tumor delivery of the therapeutic cargos than free therapeutics.

Moreover, introducing multiple targeting molecules into one nanoplatform can further enhance the active targeting ability. For instance, Chiang et al. fabricated anti-CD3 antibodies modified magnetic NPs for anti-PD-1 mAb delivery [22]. In addition to facilitating T cells delivery mediated by anti-CD3 antibodies bounding to the CD3 T-cell surface marker, ferromagnetic properties also facilitated tumor targeting under an external magnetic field. This dual-targeting strategy improved tumor accumulation of anti-PD-1 mAb drugs and antitumor therapeutic effects compared with the anti-CD3 single targeting group. Multivalent active targeting strategies not only can promote the NPs transportation to targeting sites, but also enable to attract specific immune cells to the site of interest. For example, Au et al. established a PEG-PLGA based trispecific NK cell engager platform for combining targeted chemoimmunotherapy and co-stimulatory 4-1BB molecule-based ICT [23]. The NPs were functionalized with tumor targeting anti-epidermal growth factor receptor (α-EGFR) antibody and two NK-activating components, anti-CD16 (α-CD16) and anti–4-1BB (α-4-1BB) antibodies, and encapsulated chemotherapeutics epirubicin (EPI). This trispecific α-EGFR/α-CD16/α-4-1BB NPs not only can achieve targeted delivery of EPI to EGFR-overexpressed tumor cells and NK cells, but also can recruit and activate circulating NK cells to the TME following systemic delivery. This multifunctional and multivalent active targeting strategy led to the greatest therapeutic efficacy and extended survival in EGFR-overexpressing murine tumor model compared with other treatment groups. These finding demonstrated that multiple targeting strategy can be applied to improve targeting specificity or drive two different targets together into spatial proximity to improve treatment outcomes.

3.1.3 Controlled release

In addition to passive and active targeting strategy, the NPs also can be engineered to achieve selective and controlled release of ICI cargos at the action sites so as to maximize the therapeutic effects. Several NPs have been reported to utilize the characteristics of TME as triggers to realize controlled release of ICT drugs, such as acidic pH and matrix metalloproteinases (MMPs) in TME [24, 25]. For instance, Lang et al. encapsulated chemotherapeutic drug paclitaxel (PTX), anti- cancer stem cells (CSC) agent thioridazine (THZ), and the PD-1/PD-L1 inhibitor HY19991 (HY) into an MMPs enzymes as well as pH dual-responsive double-layer structured NPs [25]. The MMPs in TME triggered outer layer degradation and achieved release of HY, THZ, and PTX-loaded. Subsequently, the micelles internalized into cells and disrupted under endosomes/lysosomes acidic, leading to the PTX release and cancer cell death. This controlled release strategy controlled spatial and temporal delivery to showed powerful synergy among different therapeutic effects.

3.1.4 Codelivery of different therapeutics

Utilizing nanotechnology enable co-delivery of different therapeutics simultaneously. Mi et al. explored the dual immunotherapy nanoparticles (DINP) for the co-delivery of αPD-1 monoclonal antibodies and agonistic antibodies for the co-stimulatory receptor αOX40, to prevent T-cell inhibition and elicit T-cell activation simultaneously [26]. They proved that using DINP induced higher levels of T-cell activation compared with free immunotherapeutic antibodies or single therapeutic NPs. This NP-based co-delivery strategy enabled to increase T-cell activation, improve therapeutic efficacy and enhance immunological memory. Cheng et al. developed amphiphilic peptides containing NPs for the codelivery of PD-1/PD-L1 peptide ICI, DPPA-1, and an IDO inhibitor, NLG919 [27]. At neutral conditions, the hydrophobic segments of amphiphilic peptides formed a tight shell to protect hydrophobic cargos. At the weak acidic pH at TME, the NP swelled and MMPs diffusing into the internal hydrophobic domain, leading to the disassembly of NP and release of DPPA-1 and NLG919. This co-delivery of DPPA-1 and NLG919 enhanced tumor inhibition effects and survival in tumor-bearing mice compared to the delivery of either therapeutic alone. These finding confirmed the superiorities of nanotechnology in terms of integrating different therapeutic into a single platform.

3.1.5 Other superiorities of nanotechnology

The application of nanotechnology allows for real-time delivery monitoring. For instance, Meir et al. developed an integrated diagnostic and therapeutic nanoplatform by conjugating α-PD-L1 antibodies to gold nanoparticles (αPDL1-GNPs) to stratify patient response to ICIs [28]. αPDL1-GNPs were intravenously injected into subcutaneous MC38 colon tumors bearing mice and accumulated in tumor, which generated CT signal contrast and could be used to predict response to ICT. A strong correlation was observed between αPD-L1-GNPs related CT signal and tumor growth, leading to the facile precise prediction of the ICT response via CT signal levels. Although more validation in other tumor models is required, this proof-of-concept study suggested that nanotechnology may promote non-invasive monitoring of ICT response.

Additionally, the combination of nanotechnology can promote development of novel delivery approaches. As an example, Wang et al. established a microneedle patch coated with pH-sensitive dextran nanoparticles for the sustained delivery of αPD1 [28]. The αPD1 was encapsulated into the NPs, which can dissociate at acidic pH and achieve controlled and sustained released αPD-1 antibodies over 3 days. This sustained release of αPD-1 antibodies improved tumor retention of antibodies and prolonged the survival time of subcutaneous B16F10 melanomas bearing mice. Nevertheless, this delivery approach seems be limited to superficial tumors, such as melanomas, and need more investigation to confirm strategies.

3.2 Modulate the immunosuppressive tumor microenvironment (TME)

ICT have showed great potential in increasing survival rate in various cancers. However, the low response rate of patients to ICT, which is related to immunosuppressive tumor microenvironments, remains a challenge to be addressed. Several studies have aimed to target and reverse the immunosuppressive TME, so as to increase the therapeutic effects and decrease side effects of ICT (Figure 3) [29].

Figure 3.

Modulate the immunosuppressive tumor microenvironment (TME) to boost the efficacy of immune checkpoint therapy (ICT). Treg: regulatory T cell; CAF: cancer-associated fibroblast; and MDSC: myeloid treated suppressor cell.

3.2.1 Modulate or eradicate the fibrotic stroma

Fibroblastic stroma can reduce the efficacy of ICT by promoting tumor development and employing immunosuppressive immune cells. Several strategies have been applied to modulate or eradicate the fibrotic stroma to improve the effectiveness of ICT. For instance, Xu et al. constructed a puerarin loaded nano emulsion (nanoPue) for the targeted delivery of puerarin to the sigma receptor over-expressing cancer-associated fibroblasts (CAFs) and cancer cells [30]. Reactive oxygen species (ROS) play critical roles in activation of CAFs and puerarin was applied to decrease ROS production in the activated myofibroblast. In the desmoplastic triple-negative breast cancer (TNBC) model, nanoPue greatly reduced CAFs in mice and deactivated the stromal microenvironment, leading to enhanced chemotherapy effect of encapsulated paclitaxel. Importantly, combination therapy of nanoPue and α-PD-L1-based ICT induced more apoptosis and exhibited a significantly robust antitumor effect in 4 T1 tumor model compared to α-PD-L1 or nanoPue monotherapy. This suggests that CAFs deactivation is a promising approach to regulate tumor stroma and improve the efficacy of ICT. In another study, Zhao et al. established cyclopamine (CPA) and paclitaxel (PTX) co-encapsulated polymeric micelles (M-CPA/PTX) to modulate desmoplastic stroma in pancreatic ductal adenocarcinoma (PDAC) and improve the efficacy of ICT [31]. The (M-CPA/PTX) plarform could regulated the tumor stroma to enhance intratumoral vasculature density, leading to increased tumor-infiltrating CTLs and decreased hypoxia. In a Kras model, the combination treatment of M-CPA/PTX with α-PD-1-based ICT upregulated CD8⁺ T cells and IFN-γ levels in tumor tissues and prolonged the survival of mice compared to monotherapy of M-CPA/PTX or and α-PD-1 alone. These findings demonstrated the potential of fibrotic stroma modulation in enhancing the therapeutic efficacy of ICT.

3.2.2 Targeted modulation of T cells

Kosmides et al. fabricated an “immunoswitch” iron-dextran nanoparticle coated with two different antibodies which can simultaneously block the PD-L1 inhibitory signal and activate T cells through 4-1BB co-stimulatory pathway [32]. The intratumoral treatment with immunoswitch NPs suppressed tumor growth and prolonged survival time compared with direct co-injection of free antibodies in various tumor models-bearing mice. In addition, regulatory T (Treg) cells can induce immunosuppressive TME which is a major obstacle for cancer immunotherapy. In an exemplified study, Ou et al. constructed imatinib (IMT)-loaded, tLyp1 peptide-decorated hybrid NPs (IMT-loaded tLyp1-hNP) which exhibited good stability and effective targeting ability to neuropilin-1 (Nrp1) overexpressing Tregs in TME [33]. IMT was applied to reduce the proportion of Treg cells by blocking STAT3 and STAT5 signaling. The IMT-loaded tLyp1-hNP showed higher cellular uptake efficiency for Treg cells and boost the effect of imatinib in inhibiting Tregs-mediated suppression. The combination treatment of α-CTLA-4-based ICT and IMT-loaded tLyp-hNPs increased tumor-infiltrating CD8+ T cells and extended survival of B16BL/6 tumor-bearing mice, suggesting that reducing Tregs mediated by targeted IMT-loaded tLyp-hNPs enabled a synergistic effect with ICT.

3.2.3 Targeted modulation of tumor-associated myeloid cells (TAMCs)

To enhance the therapeutic effects of ICT and modulate the immunosuppressive TME, various approaches have been adopted for TAMCs targeting. As is known, polarization of macrophages in TME into M2 tumor-associated macrophages (TAMs) can facilitate tumor progression and inhibit antitumor efficacy of ICT by releasing anti-inflammatory cytokines and angiogenic factors. Based on this, Choo et al. exploited M1 macrophages derived nanovesicles (M1NVs) to repolarize M2 TAMs to M1 macrophages which can release pro-inflammatory cytokines and elicit antitumor immunity [34]. The results showed combination treatment of M1NVs and aPD-L1 significantly decreased the tumor volume compared to the treatment of M1NVs or aPD-L1 alone, proving that M1NV can repolarize M2 TAMs to M1 macrophages and potentiate the antitumor efficacy of ICT. Shae et al. constructed stimulator of interferon gene NPs (STING-NPs) based on endosomolytic polymersome possessing pH-responsive membrane [35]. The STING-NPs can enhance cytosolic delivery of 2′3’ cyclic guanosine monophosphate–adenosine monophosphate (cGAMP), which is an endogenous ligand for cyclic dinucleotide (CDN) agonists of stimulator of interferon genes (STING). They demonstrated that STING-NPs treatment could remodel the TME and repolarize macrophages to block immunosuppressive characteristics in melanoma bearing mice. Importantly, the combination of STING-NPs enhanced response to α-PD-1 and α-CTLA-4, prolonged the survival and inhibit tumor growth compared with ICT or free cGAMP-ICT treatment. These results confirmed that STING-NPs can activate STING pathways in myeloid cell populations in TME and increases the therapeutic efficacy of ICT.

3.2.4 Other approaches to regulating immune-suppressive TME

There are plenty of other immune suppressive mechanisms related with the reduced effectiveness of ICT. In view of this, several therapeutic agents have been applied to regulate various immune suppressive mechanisms and increase the efficacy of ICT, such as pro-stromal signaling modulators [36], exosome release inhibitors [37], tumor-associated myeloid cells (TAMCs) eliminators [38], TAMC recruitment inhibitors [39], and TAMC reprogrammers [40]. For instance, inhibiting C-X-C motif chemokine ligand 12 (CXCL12), which is secreted by CAFs and promotes cancer cell migration and proliferation, is another approach to regulate the fibrotic stroma. Shen et al. downregulated the CXCL12 expression by employing small trap proteins targeting IL-10 and CXCL12, leading to elevated tumor-infiltrating DCs, NK cells, and tumor-infiltrated T cells [36]. TLR7 and TLR8 are highly expressed in leukocytes and myeloid cells. Lee et al. revealed the treatment of TLR agonist resiquimod which binds to TLR7 and TLR8 can promote the differentiation of myeloid derived suppressor cells (MDSCs) into macrophages and dendritic cells [40]. MDSCs were shown to lost immunosuppressive ability in T cells and result in increased proliferation of T cells.

3.3 Combine ICT with other therapies

ICT can combine with other immunotherapies, such as cancer vaccines, to augment antitumor immunity. Moreover, conventional therapeutic approaches, including chemotherapy, phototherapy, radiotherapy, not only can kill cancer cells but also show immunomodulation effects. ICT can combine with these different treatments to boost the antitumor immunotherapeutic effects.

3.3.1 ICT combines with cancer vaccine

Combined immunotherapies of ICT and cancer vaccine can potentiate antitumor immune response. Kuai et al. developed a nanodisc to co-deliver anti-PD-1 and anti-CTLA-4, in combination with the sHDL-Ag/CpG mediated cancer vaccine, for the MC-38 colon tumors and B16F10 melanomas treatment [41]. The combination of ICT with neoantigen vaccination markedly inhibit tumor growth and eradicate established tumors, suggesting the superiority of combined immunotherapeutic strategy. Zhu et al. further explored this strategy by constructing endogenously self-assembled albumin/AlbiVax nanocomplexes [42]. The AlbiVax nanovaccines are composed of antigens and adjuvants conjugated with maleimide-functionalized Evans Blue (MEB), namely MEB-Ag and MEB-CpG. MEB can bind with endogenous albumin which can work as a natural carrier and enable to direct the nanovaccine trafficking to the lymph nodes. The anti-PD-1 based ICT could prevent exhaustion of CTL responses, in combination with AlbiVax nanovaccine which can efficiently traffic to lymph nodes, leading to induction of robust antitumor immune response. The found that vaccination with AlbiVax led to 12.5% tumors regression, while combination treatment of AlbiVax + anti-PD-1 increased tumor regression to 60% in mice.

3.3.2 ICT combines with chemotherapy

It has been realized that cytotoxic chemotherapy exerts therapeutic effects not only through direct tumor cells killing, but also may be related with immunoregulatory properties of chemotherapeutic agents. The chemotherapy achieve antitumor effects by facilitating tumor cell killing or inhibiting tumor cell division via multiple mechanism, such as causing DNA damage, disrupting DNA replication, preventing mitosis, cellular metabolism and microtubule assembly [43]. Although the precise mechanism remains further investigation, it is believed that chemotherapy can modulate T cell activity by promoting immunogenic cell death (ICD), increasing effector T-cell response, enhancing tumor antigenicity, or blocking immune suppressive pathways [44, 45]. Chemotherapy can induce ICD by releasing damage-associated molecular patterns (DAMP), which can be recognized by pattern-recognition receptors such as Toll-like receptors (TLRs) expressed on antigen-presenting cells (APCs). These DAMPs and tumor-associated antigens collectively elicit APCs maturation and induce a robust antitumor immunity. For instance, anthracycline-based chemotherapy has been shown to induce immunogenic cell death (ICD) which favors the DCs maturation and block immunosuppressive pathways in the TME [44]. In a genetically engineered mouse lung adenocarcinoma model, combined therapy of oxaliplatin and cyclophosphamide drive the T cell infiltration-lacking tumors sensitive to ICT based on PD1 and CTLA4 antibodies (Figure 4) [46].

Figure 4.

Scheme illustration showing the cancer treatment with chemotherapy which can elicit immune stimulation including: Secretion of ATP; expression of type 1 interferon (IFN); exposure of calreticulin (CALR) on the outer membrane; and release of high mobility group box 1 (HMGB1).

3.3.3 ICT combines with radiotherapy

When ICT combines with radiotherapy, abscopal effect can occur to facilitate regression of distant tumors or metastases. Specifically, APCs uptake tumor-associated antigens (TAAs) released by the dying tumor cells upon irradiation, accumulate to the lymph nodes and activate CD8⁺ T-cells to eradicate the tumor cells in primary and distant tumors. It has been recognized that the abscopal effect can be augmented by combining radiotherapy with ICT. Ni et al. established a radiosensitizer (Hf12-DBA) for radiotherapy in combination with anti-PD-L1 based ICT, resulting antitumor response both in primary and distant tumors [47]. In dual subcutaneous colorectal CT26 tumors bearing mice, monotherapies of ICT and radiotherapy just lead to delayed primary and distant tumors growth, while combination of ICT and radiotherapy elicit complete regression of primary, treated tumor and shrinkage of distant, non-irradiated tumors. Accordingly, the abscopal effect was boosted and the antitumor immune responses was potentiated by the combination of ICT and radiotherapy. Min et al. developed a new strategy to improve abscopal effect by constructing antigen-capturing NPs mediated combination therapy of anti-PD-1-based ICT and radiotherapy [48]. In bilateral B16F10 melanomas bearing mice, ICT and radiotherapy combination therapy mediated by antigen-capturing NPs induced a 20% complete response rate and tumor re-challenge resistant 3 months later. By contrast, mice receiving the combination treatment without antigen-capturing succumbed to disease within 40 days, suggesting that antigen-capturing strategy play a critical role in improving the abscopal effect and enhance therapeutic effects.

3.3.4 ICT combines with phototherapy

Combination of phototherapy can induce abscopal effect, reduce tumor burden, and boost antitumor responses in various tumor models. Phototherapy relies on photosensitizers which can generate reactive oxygen species for photodynamic therapy (PDT) or heat for photothermal therapy (PTT) upon laser irradiation to eradicate tumor cells. Chen et al. combined PLGA-ICG-R837-based PTT with anti-CTLA-4-based ICT to induce robust anti-tumor immune responses for cancer immunotherapy [49]. In a 4 T1 breast tumor model with lung metastases, the combination treatment of PTT and ICT could protect treated mice against tumor rechallenging 40 days post ablation, while surgery + anti-CTLA-4 treatment or PLGA-ICG-R837-based-PTT alone can lead to metastases.

3.3.5 Challenges for combination therapies

Combination therapy is crucial for increasing sensitivity of ICT and enhancing antitumor efficacy. However, there are still some challenges need to be addressed. First, careful consideration needs to be given to for which therapies to combine. Preclinically determining whether there is an additive or synergistic therapeutic effect and determining the optimal combination can help identify and drive combinations that produce synergistic therapeutic effects into clinical trials. Second, strong rationale is needed for the spatiotemporal factors of combination therapy administration. The half-life, tumor accumulation and kinetics for each monotherapy should be examined. Nanotechnology exhibit superiorities in this aspect by integrating multiple therapies into a single platform to promote accumulation and co-localization at the target sites. Additionally, optimizing the dose and scheduling of combination therapy is also needed when considering spatiotemporal factors. Of note, nanotechnology shows the potential to solve the challenges related to combination therapy in a number of ways: through integrating multiple therapies into a single nanoplatform, or optimizing dosage and therapeutic schedule, or exploring the potential therapeutic mechanisms.

4. Conclusion

ICT-mediated immunotherapy has attracted extensive research interest and pioneered a new paradigm for cancer treatment over the past decades. The final goal of ICT is to induce a robust antitumor response by interfering immunosuppressive TMEs while alleviating side effects. Various strategies have been investigated for enhancing efficacy of ICT, including nanotechnology-mediated targeted delivery of ICIs, regulation of the immunosuppressive TME, and combination therapies. Despite substantial progress, the issues of immune-related adverse events (irAEs) and therapeutic resistance may lead to the failure of therapy and even patient mortality in some cases. Biomarkers can be employed to predict the efficacy of ICI treatment and irAEs by distinguishing responders and non-responders, which would promote patient selection and decision-making. Abundant opportunities remain in ICT for maximizing therapeutic effects, improving safety profiles, and reducing recurrence. We believe that expanding the understanding of immune checkpoint biology and nanotechnology will improve the efficacy of current ICT and continuously contribute to the next generation of novel immunotherapy for clinical translation.

Acknowledgments

This research was supported by National Natural Science Foundation of China (NSFC) projects (Nos 32171320 and 81371667), Xi’an Association for Science and Technology young Talent Promotion Plan (Nos. 095920211324 and 095920221341), Qinchuangyuan cited the high-level innovation and entrepreneurship talent program (QCYRCXM-2022-27), State Key Laboratory of Veterinary Biotechnology Foundation (SKLVBF202207), the Fundamental Research Funds for the Central Universities (No. QTZX22067), the Opening Project of the State Key Laboratory of Microbial Resources (SKLMR-20220703).

Conflict of interest

The authors declare no conflict of interest.