Introduction

Lung cancer is the second most common malignancy and the leading cause of cancer-related deaths worldwide.[1] Owing to the lack of early symptoms, over 80% of lung cancer cases present with advanced/metastatic diseases.[2] Metastases spread from an initial primary tumor to distant sites where secondary tumors are seeded, and represent the main cause of cancer-related mortality.[3] Given that early stage cancer remains asymptomatic, most patients with lung cancer present with locally advanced, or widely metastatic tumors upon diagnosis, resulting in reduced life expectancy.[4] Despite remarkable advances in standard therapy, treatment remains complex and comprises surgical resection, radiation, chemotherapy, and targeted therapies. The dormancy of metastatic cancer cells within the metastatic microenvironment is a primary contributor to treatment failure as most current available therapeutic strategies target actively proliferating cells.[5] Therefore, novel therapeutic approaches must be established to deliver drugs based on a specific metastasis cascade phase.

Immunotherapy has proven to be a promising option for targeting metastasis as the activity of the immune system is less influenced by cancer cell metabolism.[6] Additionally, metastatic cancer cells within the tumor microenvironment (TME), including circulating tumor cells (CTCs) and disseminated tumor cells (DTCs), are readily cleared by the host immune response.[7] However, the antitumor immune response is restricted by immunosuppressive TMEs characterized by low pH, a reductive environment, high-level reactive oxygen species and glutathione, a hypoxic status, overexpressed enzymes, high-level adenosine triphosphate,[8] and abnormal expression levels of stress proteins,[9] thus facilitating tumor metastasis.[10] Mechanically, tumor cells can achieve immune escape by targeting platelets and macrophages.[11] Therefore, immunotherapy aimed at either activating the immune response or inhibiting immunosuppressive activity is a promising strategy for controlling metastasis and has revolutionized the metastatic non-small cell lung cancer (NSCLC) treatment landscape.[4] The ability of immune checkpoint inhibitors (ICIs) to restore the T cell-mediated antitumor response has transformed current approaches for treating multiple solid tumors, including lung cancer.[4] In 2015, nivolumab, a fully human immunoglobulin G4 monoclonal antibody targeting the programmed cell death protein 1 (PD-1)/programmed cell death ligand 1 (PD-L1) interaction, became the first PD-1 inhibitor approved for NSCLC treatment.[12] Subsequently, over the past 5 years, an increasing number of ICIs have been approved for monotherapy or combinatorial therapy with other agents for metastatic lung cancer [Table 1].

Table 1 - FDA-approved immunotherapies targeting metastatic lung cancer in the clinical stage over the past 5 years (2017–2022). Generic name (brand name) Description Trial name (ClinicalTrials.gov Identifier) Disease setting FDA approval Study phase Approval year Reference Pembrolizumab (Keytruda) Humanized IgG4 monoclonal antibody KEYNOTE-042 (NCT02220894) Stage III NSCLC not candidate for surgical resection or definitive chemoradiation, or mNSCLC; no EGFR or ALK genomic aberrations; PD-L1 expression (TPS ≥ 1%) First-line treatment of NSCLC (TPS ≥ 1%) III 2019 [85] KEYNOTE-407 (NCT02775435) Metastatic squamous NSCLC Pembrolizumab plus chemotherapy as first-line treatment of metastatic squamous NSCLC III 2018 [86] KEYNOTE-189 (NCT02578680) Metastatic non-squamous NSCLC; no EGFR or ALK genomic aberrations Pembrolizumab plus pemetrexed and platinum as first-line treatment of metastatic non-squamous NSCLC III 2018 [87] Nivolumab (Opdivo), Ipilimumab (Yervoy) Humanized IgG4 anti-PD-1 monoclonal antibody, human IgG1 kappa anti-CTLA-4 monoclonal antibody CheckMate 9LA (NCT03215706) mNSCLC or recurrent NSCLC; no EGFR or ALK genomic aberrations Nivolumab plus ipilimumab and chemotherapy as first-line treatment of mNSCLC III 2020 [88] CheckMate 227 (NCT02477826) mNSCLC expressing PD-L1 (TPS ≥ 1%), as determined via an FDA-approved test; no EGFR or ALK genomic aberrations Nivolumab plus ipilimumab as first-line treatment of mNSCLC (PD-L1 TPS ≥ 1%) III 2020 [89] CheckMate 032 (NCT01928394) Metastatic SCLC progressing after platinum-based chemotherapy and at least another line of therapy Third-line treatment of metastatic SCLC I/II 2018 [90] Atezolizumab (Tecentriq) Fully humanized anti-PD-L1 monoclonal antibody IMpower110 (NCT02409342) mNSCLC highly expressing PD-L1 (≥50% of PD-L1+ tumor cells or PD-L1+ tumor-infiltrating IC covering ≥10% of the tumor area); no EGFR or ALK genomic aberrations First-line treatment of mNSCLC highly expressing PD-L1 III 2020 [91] IMpower150 (NCT02366143) Metastatic non-squamous NSCLC; no EGFR or ALK genomic aberrations Atezolizumab plus chemotherapy and bevacizumab as first-line treatment of metastatic non-squamous NSCLC III 2018 [92] Durvalumab (Imfinzi) Selective, high-affinity human IgG1 monoclonal antibody CASPIAN (NCT03043872) Extensive-stage SCLC Durvalumab plus etoposide and either carboplatin or cisplatin as first-line treatment of extensive-stage SCLC III 2020 [93] PACIFIC (NCT02125461) Unresectable stage III NSCLC not progressing following concurrent platinum-based chemotherapy and radiation therapy Durvalumab after chemoradiation as treatment of unresectable stage III NSCLC III 2018 [94] Cemiplimab (cemiplimab-rwlc) Highly potent, fully human, hinge-stabilized IgG4 anti-PD-1 monoclonal antibody Study 1624 (NCT03088540) Advanced NSCLC (locally advanced NSCLC not candidate for surgical resection or definitive chemoradiation, or mNSCLC) highly expressing PD-L1 (TPS > 50%); no EGFR, ALK, or ROS1 genomic aberrations First-line treatment of NSCLC highly expressing PD-L1 III 2021 [95]

ALK: Anaplastic lymphoma kinase; CTLA-4: Cytotoxic T lymphocyte antigen-4; EGFR: Epidermal growth factor receptor; FDA: Food and Drug Administration; IC: Immune cells; IgG4: Immunoglobulin G4; mNSCLC: Metastatic NSCLC; NSCLC: Non-small cell lung cancer; PD-1: Programmed cell death protein 1; PD-L1: Programmed cell death ligand 1; ROS1: c-ros oncogene1 receptor tyrosine kinase; SCLC: Small cell lung cancer; TPS: Tumor proportion score.

Nanomaterial-based therapeutic strategies, classified as targeting either cancer cells, TME, or the immune system, have been utilized for various cancer therapies to overcome toxicity and lack of specificity, augmenting drug efficiency and bioavailability based on unique chemical and physical features.[13,14] Owing to the inherent capabilities of nanomaterials, or their delivered drugs, to shape the immune response, significant focus has been placed on developing nanotechnology that leverages immune cells to fight against metastasis.[11] In this scenario, nanotherapeutics can be roughly grouped into two strategies: (1) preventive, therapeutic approaches before tumor metastasis that remodel the TME, including maintaining primary tumors, and preventing CTCs from migrating to their preferred colonization organs; (2) targeting established metastases by remodeling the TME.[2] Notably, nanomedicine-based strategies have faced some limitations, including infusion reactions,[15] biological barriers to drug delivery,[16] inefficient targeting of nanomaterials to the desired tissues and cells, inter- and intra-tumor heterogeneity, variability among patients, and toxicity.[17] Nonetheless, various nanomedicines have been approved for marketing or are currently undergoing evaluation in clinical trials for metastatic lung cancer [Table 2].

Table 2 - Nanomedicine-based therapy for metastatic lung cancer approved for marketing or under clinical trial. Name Description Nanoparticle-associated advantage Patient population Therapy approval Primary outcome Most common grade ≥3 AEs Study phase/FDA approval year Ref./Clinical Trials.gov Identifier Nab-paclitaxel Low-dose nanoparticle albumin-bound paclitaxel Enhanced immune response to tislelizumab (anti-PD-1 antibody) 29 Patients aged ≥65 years with advanced NSCLC ≥1st Line of chemotherapy or targeted therapy Median PFS = 9.5 months, OS = 16.5 months Grade 3 AEs (n = 3, 10.3%) II [96] Genexol-PM Cremophor EL-free polymeric micelle formulation of paclitaxel Improved delivery 43 Patients with advanced NSCLC First-line treatment Median PFS = 4.0 months (95% CI 2.0–6.0 months), median OS = 14.8 months (95% CI 9.1–20.5 months) Neutropenia (n = 7, 16%), pneumonia (n = 5, 12%) II [97] Abraxane/ABI-007 Nanoparticle albumin-bound paclitaxel Improved solubility and delivery 1052 Untreated patients with stages IIIB–IV NSCLC First-line treatment Overall response rate higher than that for solvent-based paclitaxel (33% vs. 25%, P = 0.005) Grade ≥3 AEs (4%) III/2012 [98] NBTXR3 Hafnium oxide nanoparticles Enhanced radiation Patients with metastatic lung cancer Radiotherapy and anti-PD-1 antibody – – I/II NCT03589339

AE: Adverse event; CI: Confidence interval; FDA: Food and Drug Administration; NSCLC: Non-small cell lung cancer; OS: Overall survival; PD-1: Programmed cell death protein 1; PFS: Progression-free survival; Ref.: Reference.

Tremendous efforts have been undertaken to overcome the limitations of cancer immunotherapy for detecting and targeting metastatic lung cancer by harnessing the potential of nanomaterials.[18] This has included delivering therapeutic drugs to major metastatic sites, such as the lungs, liver, bones, brain, adrenal gland, and lymph nodes, and targeting specific cell populations.[19] An ideal nanoscale drug delivery system should combine the following features: (1) excellent stability in circulation with a long biological half-life, (2) capability of evading clearance by the reticuloendothelial system, (3) high accumulation within the tumor, (4) ability to penetrate through blood vasculature and tumor stroma, (5) effective cell uptake and controlled payload release at target sites, and (6) excellent biocompatibility and biodegradability.[18]

Recently, several nanotechnology-based approaches have successfully shaped the TME in the invasion-metastatic cascade, including primary tumors, CTCs, and metastatic organs.[2] The primary objective of this review is to summarize the current state of nanomedicine-based immunotherapy in terms of its application in preventing and eliminating lung cancer metastasis via TME modulation. We also briefly discuss the challenges in this research area and future clinical translation.

Role of the Microenvironment in Lung Cancer Metastasis

Tumor metastasis occurs in a series of discrete steps: (1) local tumor invasion from the primary tumor into the surrounding stroma through basement membrane degradation; (2) tumor cell migration, and survival, into the blood or lymphatic vessels (i.e., CTCs); (3) initial extravasation and survival of tumor cells in a supportive tissue microenvironment in preparation for metastatic tumor cells (i.e., premetastatic niche); (4) formation and progression of distant metastases.[2] Metastasis is an inefficient process with <0.1% of tumor cells surviving for 24 h and <0.01% undergoing metastasis after entering the circulation.[20,21] Research on NSCLC metastasis has focused on both non-tumor cell components and extracellular matrix components comprising the TME.[22] Micro-environmental interactions of solid cancer assist each of these steps.[23] Therefore, reconstruction of the TME could help regulate tumor cells from the source in a holistic manner, rather than single targeting.

The TME of solid cancer contains tumor cells and host stromal cells and proceeds through three phases, namely, elimination, equilibrium, and escape.[23] During the elimination phase, the innate and adaptive immune responses cooperate to recognize and kill cancer cells. Tumors capable of surviving this phase enter the equilibrium phase, wherein the tumor presents a partially immune indolent state. Finally, cancer cell growth is unrestricted due to the immunosuppressive TME, leading to metastasis in the escape phase.[24] Following adaptation to the new microenvironment of the primary tumor, metastatic tumor cells can disseminate and colonize to form secondary tumors in the organs, such as the brain, bones, liver, adrenal gland, and lymph nodes [Figure 1].

Figure 1:

Steps and micro-environmental interactions of lung cancer metastasis. Schematic representation of the metastatic process from primary lung tumors to secondary tumors underlying the spread of tumor cells through circulation systems. CAFs: Cancer-associated fibroblasts; CTCs: Circulating tumor cells; CTLs: Cytotoxic T lymphocytes; MDSC: Myeloid-derived suppressor cell; NK: Natural killer; PMN: Polymorphonuclear neutrophil; Treg: Regulatory T cells.

The microenvironments of both the primary tumor and putative metastatic target organ facilitate local invasion, survival, and extravasation of CTCs, resulting in the promotion of tumor metastasis.[25] Each phase of metastasis involves various inflammatory factors and other non-tumor cells, including immune cells, cancer-associated fibroblasts (CAFs), endothelial cells, and bone marrow-derived cells, forming a complicated network[11] [Figures 1 and 2]. Thus, characterizing the mechanisms responsible for the cooperation of tumor cells with the TME will contribute to the prevention of tumor metastasis in primary tumors and metastatic organs.

Figure 2:

Nanotechnology-enabled immunotherapy by harnessing immune cells in primary and secondary lung cancer. Various nanomedicine-based immunotherapeutic strategies, such as liposomes, nanoemulsions, dendrimers, micelles, hydrogen, magnetic NPs, and MSN, target immunosuppressive components of primary and secondary lung cancer. ATP: Adenosine triphosphate; CAFs: cancer-associated fibroblasts; CRT: Calreticulin; CTLs: Cytotoxic T lymphocytes; DCs: Dendritic cells; HMGB1: High mobility group protein 1; MDSCs: Myeloid-derived suppressor cells; MSN: Mesoporous organosilica nanoplatforms; NPs: Nanoparticles; PD-1: Programmed cell death protein 1; PD-L1: Programmed cell death ligand 1; TME: Tumor microenvironment; Treg: Regulatory T cells.

Nanomedicine-based Strategy for Modulating the Immunosuppressive Microenvironment of Primary Tumors

Lung cancer originates from a microenvironment characterized as highly vascularized and oxygenated.[19] Primary lung cancer develops a pro-metastatic TME with specific changes, including angiogenesis, hypoxia, mild acidity, immune checkpoint expression, and immunosuppressive cell infiltration.[4] Various nanomedicine-based immunotherapeutic strategies, such as liposome, nanoemulsion, dendrimer, micelle, hydrogen, magnetic nanoparticles (NPs), and mesoporous organosilica nanoplatforms (MSN) have been established to target specific changes, for example, the immunosuppressive components of primary lung cancer [Figure 2, Table 3].

Table 3 - Nanomedicine-based immunotherapy for metastasis inhibition in primary lung cancer. Tumor subtype Cell type Name Nanomaterials Size (nm) Drugs AR Mechanisms NP-associated advantage Therapeutic effects Reference NSCLC A549 MnIIIPC@DTX@PLGA@Mn2+@HA (MDPMH) PLGA, HA-HDA solution, Mn2+ 210–230 Mn-modified phthalocyanine derivative (MnIIIPC), DTX, HA – Tumor immunity activation through cGAS-STING via Mn2+ release Convenient internalization by cancer cells Synergistic photothermal, chemotherapy, and immunotherapy effects [35] LUAD LLC IO-PG-GLU-Ce6 Tris(acetylacetonate) iron with tri(ethylene) glycol, carboxyl groups (–COOH), GLU 220 ± 30 Ce6 i.v. Lung cancer cell DNA damage; STING activation; IFN-β, HMGB1, and HSP90 upregulation; lung cancer cell immunogenicity increase Enhanced cell uptake Antitumor efficacy [36] Lung cancer LLC Au@PG Ortho-nitrophenyl-β-d-galactopyranoside, glucose, galactose, mannose 18.3–32.2 Au i.v. M1 macrophage polarization, tumor remodeling, and transformation of the tumor microenvironment from “cold” to “hot,” promoting cytotoxic T cell response and tumor inhibition Good stability and dispersion Tumor growth inhibition (combined with anti-PD-1 therapy) [39] Lung cancer TC-1 NP (DOX + pIC + R848 + MIP3α) PEG 180 DOX, immune adjuvants (endosomal TLR3 agonist pIC, TLR7/8 agonist R848, MIP3α) i.t. Enhanced DC activation, IL-12 production, and circulating CD3+, CD8+, and cancer antigen-specific CD8+ T cell numbers Biocompatibility Strong tumor regression and better overall survival than those for the free NP components [44] LUAD A549 Fe3O4@PDA NPs Polydopamine 50–60 Fe3O4 i.v. Ki-67 downregulation; increased A549 cell apoptosis; NK cell modulation Physiological stability, biocompatibility Tumor growth inhibition [49] NSCLC H460 miR-424@PPCN Podophyllotoxin 110 miR-424 i.v. PD-L1 production reduction and promotion of tumor cell targeting by T cells (miR-424); effective delivery of miR-424 and PPCNs into NSCLC cells Good biocompatibility Tumor growth inhibition, extended survival [27] NSCLC H460 Podophyllotoxin-loaded NPs Hydrophobic, hydrophilic 4-(4-methyl-1-piperazinylmethyl) benzoic acid, tripeptide cationic lipid 100–130 Podophyllotoxin i.v. PD-L1 production inhibition in lung cancer cells, tumor-specific immune response induction Better targeting activity, fewer side effects Increased antitumor efficacy [30] Lung cancer Urethane-induced orthotopic lung cancer aPD/IND@MON-aANN NH2-PEG-COOH ∼50 Anti-annexin A1 antibodies, anti-PD-L1 antibody (aPD-L1), indoximod i.v. CTL intratumoral infiltration facilitation, immunosuppressive tumor microenvironment reversion Rapid extravasation across the tumor endothelium, extensive accumulation in the tumor interstitium Superiority over the combination therapy with the free drugs in attenuating disease progression [33] LUAD LLC FX@HP Cystaminebisacrylamide 150 Boc-protected cyclam monomer, fluorinated polymerized CXCR4 antagonist (FX), paclitaxel-loaded HP, anti-PD-L1 small interfering RNA i.t.i. T cell infiltration induction, calreticulin upregulation in tumor cells, MDSC/Treg reduction; tumor fibrosis reduction, T cell infiltration facilitation, and immunosuppression relieving (CXCR4-inhibiting nanocomplex) Enhanced cell uptake Extended survival [31] LUAD LLC P-Cis – 14.40 Anti-PD1 antibody, BMS-202, P-Cis i.v. Sustained increase in tumor PD-L1 levels Prolonged plasma circulation, sustained cell uptake, increased tumor retention Improved therapeutic effect of PD-1/PD-L1 inhibitors [28] LUAD LLC DOX@LINV 1,2-dioleoyl-sn-glycero-3-phosphocholine, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-(methoxy[polyethylene glycol]-2000), cholesterol – DOX s.c. DC activation and induction of subsequent antigen-specific T cell response Improved drug loading stability Increased antitumor efficacy of ICIs [46] Lung cancer ASB-XIV NE (R848) – 78.5 ± 19.4 TLR7/8 agonist (R848) i.t. Polarization of TAMs and MDSCs into APCs, tumor-specific T cell activation, and mitigation of T cell exhaustion – Extended survival (combined with anti-PD-1 antibodies) [45] LUAD LLC HA-CDDP/PMet NPs – 166.5 Hyaluronic acid-cisplatin, polystyrene-polymetformin i.v. Increased CD4+ and CD8+ T cells, decreased Treg, increased IFN-γ and TNF-α Good self-assembling performance, excellent stability Enhanced overall synergistic efficacy [29]

APC: Antigen-presenting cell; AR: Administration route; Ce6: Chlorin e6; CTL: Cytotoxic T lymphocyte; DC: Dendritic cell; DOX: Doxorubicin; DTX: Docetaxel; HA: Hyaluronic acid; HP: Human serum albumin; s.c.: Subcutaneous; ICI: Immune checkpoint inhibitor; IFN: Interferon; i.t.: Intratumorally; i.t.i.: Intratracheal instillation; i.v.: Intravenously; LLC: Lewis lung cancer; LUAD: Lung adenocarcinoma; MDSC: Myeloid-derived suppressor cell; miR: MicroRNA; MPE: Malignant pleural effusion; NK: Natural killer; NP: Nanoparticle; PEG: Polyethylene glycol; PD-1: Programmed cell death protein 1; PD-L1: Programmed cell death ligand 1; SCLC: Small cell lung cancer; STING: Stimulator of interferon genes; TAM: Tumor-associated macrophage; TLR: Toll-like receptor; Treg: Regulatory T cell; –: Not available.

Boosting CD8+ T lymphocyte cytotoxicity

The main hallmark of the immunosuppressive TME is the inactivation of CD8+ cytotoxic T lymphocytes (CTLs).[26] Cationic NPs (PPCN) delivering both the herbal medicine podophyllotoxin and microRNA (miR)-424 to target PDL1 messenger RNA led to reduced PD-L1 production and enhanced CTL-mediated attack of cancer cells.[27] Consequently, the miR-424@PPCN complexes significantly suppressed tumor growth compared with either miR-424 or PPCNs alone in H460-bearing mice, without significant toxicity, and the survival of mice was extended.[27] Moreover, hyaluronic acid-cisplatin/polystyrene-polymetformin nano-prodrug treatment induced longer overall survival of Lewis lung cancer (LLC)-bearing mice compared to the parental drug, with tolerable systemic toxicity.[28] This effect partially resulted from an immune-active TME with increased proportions of CD4+ and CD8+ T cells, reduced number of regulatory T cells (Tregs), and elevated interferon (IFN)-γ and tumor necrosis factor-α (TNF-α) levels.[29] Hyaluronic acid-cisplatin and polystyrene-polymetformin synergistically induce LLC cell apoptosis.[29]

Blocking PD-1/PD-L1 interactions

Tumors can trigger immune-suppressive signaling, such as cyclooxygenase 2, prostaglandin E2, PD-L1, and indoleamine 2,3-dioxygenase, thereby suppressing the activity of CD8+ CTLs.[4] Thus, self-assembled lipid bilayer NPs encapsulating podophyllotoxin reportedly inhibit PD-L1 expression and promote intratumoral CTL infiltration in lung cancer.[30] These effects lead to significant tumor growth inhibition by blunting tumor immune escape, yielding extended survival with reduced systemic toxicity of podophyllotoxin.[30] Moreover, the FX@HP nanocomplex comprising fluorinated polymerized C-X-C chemokine receptor 4 antagonism (FX) and paclitaxel-carried human serum albumin (HP) used for pulmonary delivery of anti-PD-L1 small-interfering (si) RNA, also induces CTL infiltration, enhances calreticulin expression by tumor cells, and reduces myeloid-derived suppressor cell (MDSC)/Treg abundance in the TME, thus achieving greater tumor inhibition and a longer survival time than the control polyethyleneimine/siPD-L1 group in orthotopic lung tumors.[31] Tumor immune-modulatory therapies that induce immunogenic cell death or inhibit immunosuppressive signaling may be rendered more effective via NP-based targeted delivery than systemic administration of the drug when combined with the blockade of immunosuppressive signaling.[32] For instance, cisplatin, the first approved platinum drug, upregulates PD-L1 expression in a time-dependent manner.[28] Poly(L-glutamic acid)-graft-methoxy poly(ethylene glycol) complex NPs containing cisplatin improve the therapeutic response of PD-1/PD-L1 blockades partly via sustained elevated tumor PD-L1 expression in an LLC tumor mouse model.[28]

Targeting endothelial cells

Endothelial cells influence vascular endothelial growth factor (VEGF)-mediated angiogenesis in tumors via multiple cell types, including tumor cells, natural killer (NK) cells, and tumor-associated macrophages (TAMs).[4] Therefore, endothelial cells can function as promising targets of cancer immunotherapy. In fact, MSN carrying anti-PD-L1 antibody and indoximod were established to bind Annexin A1 expressed on caveolae of luminal tumor endothelial cells. Endocytosis of the NPs is initiated via caveolae, and the subsequent transcellular trafficking delivers nanocarriers that accumulate in the TME.[33] The MSN loaded with anti-PD-L1 antibody and indoximod elicits superior results than combination therapy of free drugs, leading to significant inhibition of orthotopic lung cancers with satisfying in vivo biocompatibility and tolerable toxicity.[33]

Regulating the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)/stimulator of interferon genes (STING) pathway

Activating the STING pathway in the TME generates a strong antitumor response.[34] MnIIIPC@DTX@PLGA@Mn2+@HA, a new nanotechnology platform comprising photothermal therapy, chemotherapy, and immunotherapy, activates tumor immunity through cGAS-STING via releasing Mn2+ in A549 NSCLC cells, ultimately exerting synergistic therapeutic effects against NSCLC.[35] The immunogenicity of chlorin e6 (Ce6)-mediated photodynamic therapy is limited by its poor solubility, rapid clearance, and insufficient accumulation in lung cancer.[36] Meanwhile, the nanocarrier iron oxide (IO)–polyaniline-based glyco (PG)–GLU containing Ce6 potently promotes Ce6 accumulation in lung cancer tissues, damages DNA, activates the cGAS/STING axis, and upregulates the level of IFN-β, high mobility group protein 1 (HMGB1), and heat shock protein 90 (HSP90), ultimately improving the immunogenicity of LLC and augmenting efficacy with little adverse effects in LLC-bearing mice.[36]

Promoting polarization of M2 to M1 macrophages

Emerging evidence has demonstrated that macrophages manifest a pro-tumor phenotype after entering the TME owing to their pro-tumor role and simultaneous immunosuppression.[37] Mechanistically, macrophages expressing the macrophage receptor with collagenous structure facilitate Treg proliferation and IL-10 production, reduce CD8+ T cell activities, and block NK cell activation,[38] thus providing opportunities for M1 TAM-polarized cancer immunotherapy. For instance, the gold (Au)-based Au@PG NPs exhibit an M1 macrophage phenotype, accompanied by immunogenic cytokine release (reduced levels of immunosuppressive IL-4, IL-10, and IL-13, and increased levels of immunogenic IL-12, IFN-γ, and TNF-α).[39] The gold (Au)-based Au@PG NPs lead to tumor remodeling by switching the TME from cold to hot, thus contributing to the CTL response and tumor suppression without significant cytotoxicity. These results support the potential of Au@PG NPs in lung cancer immunotherapy.[39] In addition, toll-like receptor (TLR) 3 agonists, such as polyriboinosinic-polyribocytidylic acid (poly [I:C]), are reportedly emerging as immunotherapy adjuvants for cancer.[40] Indeed, TLR3 agonist poly (I:C) NPs improve drug potency to induce M1 macrophages up to 100-fold via inducing autocrine type I IFNs and potentiating the synergistic interaction with TLR1/2 agonist Pam3CSK4 in an LLC murine model.[41]

Activating mature dendritic cells (DCs)

Immature DCs release transforming growth factor-β (TGF-β) and boost fork-head box P3 (FOXP3)+ Tregs, which in turn suppress CD8+ CTLs.[42] Multiple strategies have been established to target DCs to improve tumor immunotherapy.[43] Bio-compatible poly (lactic-co-glycolic acid)-polyethylene glycol NPs loaded with a doxorubicin (DOX) cytostatic agent and immune adjuvants (the endosomal TLR3 agonist poly [I:C] and TLR7/8 agonist R848, macrophage inflammatory protein-3 alpha, and C-C motif chemokine ligand 20 chemokine), strongly stimulate DC activation, secrete IL-12, and increase CD8+ CTLs.[44] This leads to stronger tumor inhibition and longer overall survival relative to that of free components in a TC-1-bearing mouse model. Meanwhile, a cancer vaccine adjuvanted with nanoemulsion carrying R848 elicits potent local and systemic antitumor immunity,[45] leading to the reprogramming of the immunosuppressive TME by activating tumor-specific T cells and reversing T cell exhaustion in subcutaneous and orthotopic lung cancer-bearing mice.[45] Moreover, its combination with anti-PD-1 antibodies synergistically prevents tumor recurrence and prolongs survival.[45] Additionally, DOX-loaded biomimetic hybrid nanovesicles (DOX@LINV) are an emerging drug delivery vehicle with augmented antitumor immune activity[46] that can be used for effective combination immunochemotherapy by applying artificial liposomes with tumor-derived nanovesicles.[46] The combination of DOX@LINV with ICIs boosts the antitumor effect, with 33.3% of mice being tumor-free following activation of DCs and subsequent triggering of antigen-specific T cell immunity. Additionally, the selective cell cytotoxicity of DOX@LINV on cancer cells instead of DC cells renders it a promising cancer-targeting