Current literature review on the tumor immune micro-environment, its heterogeneity and future perspectives in treatment of advanced non-small cell lung cancer

Introduction

Lung cancer is one of the most common malignancies causing cancer-related deaths, and non-small cell lung cancer (NSCLC) accounts for 85% of all newly diagnosed lung cancer cases (1,2). Immune checkpoint inhibitors (ICIs) were a major clinical advancement that provided an opportunity to improve the prognosis of patients with NSCLC. Patients who show high programmed death-ligand-1 (PD-L1) expression from tumor cells are more likely to benefit from ICIs (3-5); however, PD-L1 expression does not sufficiently predict ICI efficacy in NSCLC patients. In addition, PD-L1 expression does not fully explain the ICI mode of action, and complex underlying mechanisms likely exist. ICIs neutralize programmed cell death 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA4) pathways that directly and indirectly block the activation of T cells to restore anti-tumor activities of immune cells (6,7), but other non-immune cells are also significantly involved in anti-tumor responses.

In recent studies, the tumor immune microenvironment (TIME) was shown to have a central role in lung cancer development and to affect clinical outcome of patients diagnosed with lung cancer (8,9). The TIME is a collection of tumor and non-tumor cells, with involvement of various cytokines, extracellular matrix (ECM), and vessels. The TIME is dynamic, and involved cells participate in treatment resistance and show complex interactions during malignant progression by activation of inhibitory immune checkpoints. As development of new therapeutic targets to overcome ICI resistance is a priority, understanding the TIME is important. The heterogeneity of TIME can be spatial and temporal and significantly influence efficacy of anti-cancer treatment modalities, especially ICIs. Recently, a series of studies was conducted to target the immunosuppressive component of TIME to overcome resistance to tumor treatment. In this review, important features regarding the TIME and its heterogeneity and importance in practical NSCLC management are discussed. We present the following article in accordance with the Narrative Review reporting checklist (available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-22-633/rc).

Methods

PubMed and PMC were searched from January 1st, 2012 to August 16th, 2022 using the following key words: “NSCLC”, “Tumor microenvironment”, “Immune”, “Metastasis” and “Heterogeneity” (Table 1).

Overview of TIME

The TIME is comprised of both tumor cells and nonmalignant cells, including fibroblasts, pericytes, adipocytes, vascular endothelial cells, various immune cells, carcinoma-associated fibroblasts (CAFs), ECM, and blood and lymphatic vessels (10-12) (Figure 1). Secretion of cytokines and growth factors and tissue matrix remodeling by tumor cells are involved in suppression of immune cells in the TIME (10,13). For clinical application, each TIME component should be subgrouped into either pro-immunogenic or immunosuppressive to determine the potential target of clinical intervention.

Figure 1 Schematic illustration of the tumor immune microenvironment. TAM, tumor-associated macrophage; NK, natural killer; CAF, cancer-associated fibroblast.

Recognition of tumor-associated antigens (TAAs) is thought to trigger the tumor-immune cell response (14,15). Signals from proliferating tumor cells in the TIME may activate innate cells including natural killer (NK) cells (15). Debris from tumor cells and damage-associated molecular patterns further recruit antigen-presenting cells (APCs), mainly dendritic cells (DCs), into the tumor microenvironment (16). DCs capture tumor antigens; type I DCs initiate CD8 T cell responses (17), while type II DCs are responsible for initiation of CD4 responses (18). The activated cytotoxic T lymphocytes in the TIME are re-stimulated by tumor-resident APCs or major histocompatibility complex (MHC) class I molecules on tumor cells, with subsequent death of tumor cells (19).

However, tumors can progress despite the presence of tumor-antigen-specific CD8+ T-cell populations, largely due to immunosuppressive activities, both from tumor cells and the adjacent environment. Subtypes of cells in the TIME such as tumor cells, regulatory T (Treg) cells, suppressive myeloid cells, CAFs, vascular endothelial cells, and regulatory B cells can be immunosuppressive (20).

Treg cells are a subset of CD4-positive T cells and generally express the transcription factor Forkhead box protein P3 (FoxP3) (21). Treg cells suppress the activity of other immune cell subsets and prevent excessive immune responses to self-and non-self-antigens, contributing to immune homeostasis (22). Furthermore, Treg cells can promote carcinogenesis and cancer progression by inactivating anti-tumor immunity (23). Abundance of Treg cells is generally associated with poor clinical outcome in lung cancer. One meta-analysis that included 1,303 NSCLC patients from 11 studies showed association between increased tumor-infiltrating FoxP3+ Tregs cells and poor overall survival (OS) (24). Another study that included 196 NSCLC and 137 normal samples also showed strong association between tumor-infiltrating Treg-related genes and poor OS (25).

CAFs and the ECM also contribute to development and maintenance of an immunosuppressive microenvironment. CAFs are involved in production of ECM components of the tumor microenvironment (26) and secretion of paracrine ligands that promote tumor growth, vessel formation, and drug resistance (27). In various types of malignancies, CAFs play roles in recruiting Treg cells, myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs), contributing to formation of an immunosuppressive tumor microenvironment (28,29).

MDSCs derive from immature myeloid progenitors and, in general, are categorized into two subpopulations, polymorphonuclear MDSCs (PMN-MDSCs) and monocytic MDSCs (M-MDSCs) (30-32). MDSCs contribute to the control of anti-cancer immune responses and are involved in tumor progression by activating tumor angiogenesis, tumor cell proliferation, and formation of a premetastatic niche (33).

TAMs can have pro- or anti-tumor properties depending on phenotype (M1 vs. M2) (34). TAMs (M2 type) contribute to tumor growth, immunosuppression, and cancer cell invasion. In addition, they play a central role in therapeutic resistance by secreting transforming growth factor beta (TGF-β), chemokine (C-C motif) ligand 18 (CCL18), interleukin-10 (IL-10), matrix metalloproteases, vascular endothelial growth factor (VEGF), and platelet-derived growth factor (PDGF)-B (35). Tables 2,3 list the important immune and non-immune cells, cytokines, chemokines, and other related proteins based on the predominant immunogenic or immunosuppressive characteristics. Table 4 shows the major cells in the TIME and their related molecular markers, role in tumor microenvironment, and association with prognosis of lung cancer.

TIME heterogeneity

Overview of heterogeneity in the TIME

In management of advanced NSCLC, inhibiting cancer cell proliferation and if possible, killing them are the most important objectives. Therapeutic targeting of tumor cells becomes difficult, if cells within the same tumor exhibit various phenotypes, and all phenotypes simultaneously show different responses to anti-tumor treatment. Heterogeneity in TIME is important, because bigger the heterogeneity, more likely that tumor cells be irresponsive to anticancer treatment. Furthermore, when cancer progresses, more genetically and molecularly divergent lineages will come to exist, augmenting the TIME heterogeneity. For these reasons, it is important to understand the TIME heterogeneity and overcome it in order to increase treatment responses.

Several factors cause intra-tumoral heterogeneity including genetic and epigenetic alterations, extrinsic factors such as the tumor microenvironment, and interactions with other non-tumor cell types (e.g., CAFs and immune cells) (91). Heterogeneity in the TIME can be either spatial or temporal. Subsequent to heterogeneous changes in the TIME, treatment of lung cancer can be more challenging because drug resistance is more likely to occur.

Spatial heterogeneity in the TIME

The chemotactic factors secreted by the target organ and the intrinsic tendency of tumor cells to migrate to and proliferate at a specific site are important for metastatic potential (92). When tumor cells from primary lung cancer metastasize to distant sites, several steps are involved. Tumor cells travelling via the bloodstream escape immune surveillance and arrive at distant organs to form a metastatic niche. At the early phase of metastatic niche formation, infiltration by cancer cells involves degradation of the ECM. Cell-to-cell adhesions are weakened with increased levels of N-cadherin and integrin β1 while suppressing Serpin B2 (93,94). Tumor cells of metastatic lesions have a different tumor microenvironment from primary sites, and this heterogeneity can be a challenge in treatment of metastatic lung cancer.

Bone

Compared with other organs, bone is a relatively immunocompromised area and an environment in which cancer cells are more amenable to proliferation. In the pre-metastatic niche, there are large numbers of immature and inhibitory immune cells, a relatively smaller number of T cells, and a small proportion of NK cells in bone marrow (95,96). Conversely, Treg cells account for a large proportion of non-cytotoxic immune cells in bone, which co-exist with a large number of other inhibitory cells such as MDSCs (97).

Balance between osteoclasts and osteoblasts is an important feature in the tumor microenvironment. Cancer cells can induce imbalance between osteoblasts and osteoclasts and deter effective bone reconstruction (98). Lung cancer cells secrete IL-7, and T-cell-derived cytokines including TNF-α and receptor activator of nuclear factor-κB ligand (RANKL) are upregulated, further promoting osteoclast production (99). Osteolytic and osteogenic bone metastases can occur in lung cancer; osteolytic metastasis caused by osteoclasts is the predominant type (100). Osteoclasts can secrete various immunosuppression-inducing substances including indoleamine 2,3-dioxygenase-1 (IDO1) and IL-10. Bone resorption results in the release of TGF-β and IL-6 secretion, causing T cells to differentiate into T helper 17 and Treg cells, further contributing to formation of the TIME. The T helper 17 lymphocytes release IL-17 and IFN-γ, further promoting osteoclast differentiation (96).

Brain

The brain is one of the most frequently metastasized sites of NSCLC (101) and has several unique anatomic features. The blood-brain barrier (BBB) serves as protection against micrometastatic diseases; however, under certain circumstances, circulating tumor cells may cross the BBB via transendothelial migration (102,103). Subsequent to formation of the metastatic niche in the brain, cancer-associated angiogenesis, vascular remodeling, and changes in surface molecules occur (104). Despite the BBB, tumor-infiltrating T lymphocytes and other immune cells from systemic circulation can migrate to the brain metastatic lesions (105). Some subgroups of immune cells from the central nervous system (CNS) can also enter the endolymphatic system, into the cerebrospinal fluid, and travel to the lymphatic system. CD4-positive memory T cells and macrophages are present in the ventricle, pia mater, and other perivascular spaces and play important roles in immune monitoring in the brain (106).

Regarding management of brain metastatic disease in NSCLC, the disparity in tumor microenvironment between primary lung lesion and intracranial lesion should be considered. PD-L1 expression is an important biomarker predicting ICI efficacy in NSCLC. In a paired primary lung cancer and brain metastases study by Mansfield et al., the inconsistency rate of positive PD-L1 expression in paired cancer samples reached 14% despite some time interval between acquisition of the two samples (107). Compared with premetastatic lesions, patients with brain metastatic lung carcinoma show higher proportions of immunosuppressive peripheral monocyte PD-L1, MDSC, and Treg (108). Furthermore, brain metastatic lesions reportedly showed a larger fraction of tumor cells compared with primary lung tumors (109).

Liver

The hepatic metastatic site has a unique environment associated with local immune tolerance. The liver is a Kupffer cell-rich environment. Similar to other organs, metastatic cancer cells that reach the liver can trigger T-cell-mediated immune responses. Kupffer cells play various roles including cholesterol metabolism, pathogen removal, and initiation of local immunity (110). Mature Kupffer cells have a central role in immune surveillance by detecting, binding, and internalizing pathogens and other associated molecules such as lipopolysaccharide. Activated Kupffer cells release cytokines and chemokines to activate other immune cells (111). However, after invasion of cancer cells, Kupffer cells have both tumor-killing and pro-metastatic functions. In the early phase of metastasis, Kupffer cells kill and clear circulating metastatic cells. However, in the later phase, they can contribute to metastatic growth (112).

Despite various activities by immune cells in the tumor microenvironment, T cell-mediated anti-tumor activities can be deterred. Antigen presentation by local APC, usually a prerequisite for a cytotoxic T cell response, can also lead to immune tolerance. When expressing PD-1 ligands PD-L1 and PD-L2, the metastatic tumor cells can escape from CD4+ T helper cell- and CD8+ cytotoxic T lymphocyte (CTL)-mediated killing. Subsequent recruitment of MDSCs and Treg cells to the tumor microenvironment in the liver contributes to an immunosuppressive state. TGF-β and IL-2 further polarize naive T cells into inducible Treg cells, which further inhibit CD8+ T cell activity by releasing TGF-β, IL-10, granzymes, and perforin (81). The TGF-β-rich microenvironment induces neutrophils and monocytes to immunosuppressive (M2 and N2) phenotypes (113).

Similar to other metastatic organs, there are differences in the TIME between primary lung cancer and liver metastasis. In multiplexed IHC analysis of primary lung cancer and liver metastasis paired samples of 10 lung cancer patients, six immune markers (CD4, CD8, CTLA-4, granzyme B, FoxP3, and PD-L1) were evaluated. Primary lung cancer lesions showed higher number of tumor-infiltrating lymphocytes (TILs) and other T cells compared with liver metastasis. In addition, differences in CTLA-4 and PD-L1 expression were observed (114).

Changes in the TIME after anti-cancer treatment

When the tumor is exposed to systemic and local anti-cancer treatment and cancer cells survive, diverse subclones develop, temporal heterogeneity can occur (107). After patients undergo anti-cancer modalities such as chemotherapy, radiotherapy, and targeted therapies, PD-L1 expression show variable changes in tumor tissues, indicating the possibility of unpredictable immune-mediated cancer cell killing activities (115).

Reportedly, cytotoxic chemotherapy affects the TIME and, more specifically, the subpopulations of immune cells. Depletion of Treg cells, which play immunosuppressive roles in the TIME, can occur following cytotoxic chemotherapy regimens such as paclitaxel and cyclophosphamide (116,117). The analysis of peripheral blood samples of patients who underwent paclitaxel treatment showed that the inhibitory function of Treg was reduced, while the levels of interferon-γ (IFN-γ) and IL-2 were increased after paclitaxel treatment (116).

In a study including 138 epidermal growth factor receptor (EGFR) mutation-positive patients who underwent re-biopsy after progression while on EGFR-tyrosine kinase inhibitors (TKI) treatment, the treatment seemed to affect the TIME. The proportion of study patients with PD-L1 expression level of 50% or more significantly increased from 14% (before EGFR-TKI treatment) to 28% (after EGFR-TKI resistance). Densities of CD8+ and FoxP3+ TIL were significantly decreased after EGFR-TKI treatment (118).

In EGFR-mutated NSCLC, immunoinhibitory changes in the TIME reportedly occur in EGFR TKI-resistant NSCLC. Immunosuppressive cells were increased and immune-activated cells decreased in EGFR-TKI-resistant tumors compared with EGFR-TKI-sensitive tumors, and immune-inhibitory factors were more active in EGFR-TKI-resistant tumors. Furthermore, EGFR-TKI-resistant cancer cells exhibited epithelial–mesenchymal transition (119).

Radiotherapy induces different forms of changes in the TIME depending on radiation dose. Low-dose radiotherapy initiates the anti-tumor response, enabling NK and T cell infiltration into tumor cells by modulating the tumor stroma. However, there is minimal influence on cancer blood vessels or direct damage to the tumor. Conversely, high-dose radiotherapy is more likely to activate the strong anti-tumor immune response. Direct killing of tumor cells, subsequent antigen release, and T cell priming can occur. When used in combination, immunotherapy decreases T cell exhaustion and enhances lymphocyte activity against non-irradiated tumors. However, concurrent destruction results in large regions of hypoxia, which subsequently initiate processes leading to tumor regrowth (120,121).

Current strategies and future perspectives for TIME-targeting treatment

The TIME as a therapeutic target

In terms of the TIME, the main concept for increasing the chance of successful NSCLC treatment is to activate immune responses against tumor cells and inhibit immunosuppressive activities. In addition, relevant research is focused on normalizing an otherwise aberrant TIME in NSCLC patients. Potential therapeutic targets include immune cells, cytokine interactions, and non-immune cells such as fibroblasts or vessels.

Favorable TIME for immunotherapy

PD-L1 expression is the most widely known biomarker correlated with ICI response (122,123). However, based on real-life data, immunotherapy treatment can show outcomes inconsistent with PD-L1 expression, indicating that additional factors may affect immunotherapy mechanisms. Overall, high PD-L1 expression and TIL density in the tumor microenvironment are necessary conditions for favorable anti-PD-1 or anti-PD-L1 antibody efficacy (13,124).

In a retrospective cohort study of anti-PD-(L)1-treated NSCLC, the density of CD103+ CD8+ cells in tumor tissues showed significant association with improved progression-free survival (PFS) in patients receiving immunotherapy (75). In a retrospective study involving 39 NSCLC patients who received immunotherapy, tumoral CD8+ immune cell status was associated with the overall response (P<0.01). Conversely, 7 patients with high PD-L1 expression and low tumoral CD8+ did not show a significant response. Notably, all patients had EGFR mutations (125).

T effector (Teff) cells play a central role in cytotoxic cell death; however, Treg cells are associated with immune evasion of tumor cells. The Teff/Treg cell ratio showed potential prognostic and predictive values in many tumor types (126-128). Plasma cell signatures were also reported to have predictive value for improved OS in NSCLC patients receiving immunotherapy (129).

In general, active tumor killing T cell and low immunosuppressive cell signatures are favorable conditions for patients to receive immunotherapy. Table 5 shows a brief summary of current literature on favorable TIME for immunotherapy.

Radiation therapy as an immune enhancer

Radiotherapy is a treatment modality used for localized control of the tumor in management of NSCLC. Treatment mechanisms include direct killing of tumor cells via double-strand DNA damage induction and anti-tumor immune response modulation in both irradiated and non-irradiated tumors (130). Radiotherapy upregulates MHC class I expression, enabling the immune system to react to neoantigens released from tumor cells (131), and activates immunogenic cell death by inducing calreticulin expression on the tumor cell surface and releasing ATP and HMGB1 (132,133).

A difference in TIME between metastatic lesions and primary lung cancer has been suggested (109,134), and pre-emptive radiotherapy can be an effective tool in direct killing of tumor cells and activator of anti-tumor responses before subclone lineages of tumor cells become more complex.

A combinatorial approach using radiotherapy and ICIs in NSCLC patients with brain metastases resulted in a high intracranial local tumor control rate (135,136). In a retrospective study including 152 NSCLC patients with fewer than 4 metastatic lesions, the ICI plus radiotherapy group showed improved objective response rate (ORR) and PFS compared with the ICI only group. In addition, the out-of-field (abscopal effect) response rate reached 41.3% in the ICI plus radiotherapy group (137).

Abscopal effects are also important because radiotherapy tends to change the TIME to be more susceptible to ICIs. Irradiated tumor cells can play a role similar to that of a vaccine that activates the systemic adaptive immune response and promotes regression of a distant tumor (130). Often, ICIs are used in combination with radiotherapy, and non-irradiated sites may regress following radiation to a site of metastatic disease (138). The efficacy of ipilimumab and concurrent radiation treatment of a single metastatic site in 39 NSCLC patients who progressed on prior treatment was evaluated in a prospective study. The study showed the significant abscopal effect. Furthermore, increased serum interferon-β level after radiation and early dynamic changes in blood T cells were significant predictors of treatment response (139).

However, possibility of additional adverse events should be discussed at multidisciplinary team, because potential treatment-related toxicities of combinatorial management are much higher than the single-regimen therapy.

Overcoming immunosuppressive signals in the TIME

Inhibiting multiple immunosuppressive targets is another approach. A number of T cell inhibitory signals can be potential therapeutic targets. LAG3 is expressed by several immune cells such as CD4+ and CD8+ T cells and Tregs. In addition, LAG3 are an inhibitory regulator of T cell activity and related cytokine production (140,141). Combined blockade of LAG3 and PD-1 resulted in synergistic enhancement of CD8+ T cell cytotoxicity and reduced the number of Treg cells in the TIME (142).

Eftilagimod alpha, a soluble LAG-3 protein, plus pembrolizumab showed an overall response rate (ORR) of 33% in pembrolizumab-refractory melanoma patients and of 50% in PD-1-naïve melanoma patients (143). Furthermore, in PD-1/PD-L1 refractory metastatic NSCLC, therapy combining eftilagimod alpha and pembrolizumab was shown to have a favorable 6-month OS rate of 73% when used as second-line treatment in a phase II TACTI-002 trial (NCT03625323) (144).

Relatimab, a LAG-3-blocking antibody, was shown to improve PFS in melanomas when used in combination with nivolumab (145). NCT04623775 is a prospective randomized study in which the efficacy and safety of nivolumab plus relatlimab in combination with platinum doublet chemotherapy were compared with those of nivolumab plus platinum-doublet chemotherapy in patients with stage IV or recurrent NSCLC (146). This ongoing trial is expected to show new possibilities for overcoming the immunosuppressive component of TIME.

TGF-β is a cytokine involved in immunotherapy resistance (147). Bintrafusp alfa is comprised of the extracellular domain of the TGF-β receptor II fused to a human immunoglobulin G1 antibody blocking PD-L1. Bintrafusp alfa was assessed in the expansion cohort of NCT02517398, in which patients with advanced NSCLC were included. This phase I, open-label trial included 80 patients and showed a relatively favorable ORR efficacy of 17.5% and 25.0% for 500 mg and 1,200 mg doses, respectively (148). Galunisertib (LY2157299) is a selective TGF-β receptor type I kinase inhibitor. In an ongoing trial (NCT02423343), the efficacy and safety of orally administered galunisertib in combination with nivolumab were evaluated in refractory NSCLC and hepatocellular carcinoma (149).

Another strategy is to stimulate pro-immunogenic signals to enhance anti-tumor activities. CD40 is a member of the TNF receptor superfamily (150). APX005M, an agonistic antibody that binds to CD154, a ligand of CD40, activates the receptor and activates APCs, B cells, and monocytes. When administered in combination with other chemotherapy regimens, APX005M has shown promise in a metastatic pancreatic cancer phase Ib trial (151). NCT03123783, a phase I-II open-label study administering APX005M in combination with nivolumab to patients with NSCLC or metastatic melanoma is ongoing and expected to show results in the near future (152).

Management of angiogenesis

Maintaining constant migration of tumor-killing T cells into the TIME is important but not likely without a normal vascular supply. Tumor cells, especially when they are in hypoxic conditions, induce angiogenesis; however, leaky abnormal vessels are formed and contribute to further hypoxic and immunosuppressive conditions (153,154).

To induce more severe immunogenic conditions, it is important to normalize the vascular supply to the tumor microenvironment. Additional evidence indicates a clinically important relationship between angiogenesis and immune cell activities (155). In a 2-stage, phase II study, treatment outcomes were evaluated when adding bevacizumab to atezolizumab in metastatic NSCLC patients who showed disease progression after atezolizumab monotherapy. Enrolled patients received 1,200 mg atezolizumab every 3 weeks, and bevacizumab was combined with atezolizumab after radiographic progression was confirmed in the previous stage. The combination treatment showed a disease control rate of 87.5%, with a median PFS of 5.6 months (95% CI: 4.1–7.1) and OS of 14.0 months (156). In another phase II study, the lung-MAP nonmatch sub-study (S1800A), ramucirumab combined with pembrolizumab showed significant improvement in OS compared with investigators’ choice of treatment for patients with advanced NSCLC previously treated with ICI and chemotherapy (157). These two recent studies show the possibility of anti-angiogenic treatment combined with ICIs as a novel method for rendering the TIME more favorable to cancer management.

CAR-T therapy

The main concept for overcoming TIME heterogeneity is promoting immunogenicity-related cell death and subsequent T cell-dependent anti-tumor activity (158,159). The objective of chimeric antigen receptor (CAR)-T cell immunotherapy overlaps this concept. The emergence of CAR-T cell therapy provides a new approach for lung cancer management. Potential targets of CAR-T cell therapy in NSCLC include EGFR, human epidermal growth factor receptor 2 (HER2), mesothelin, mucin 1 (MUC1), prostate stem cell antigen (PSCA), tyrosine kinase-like orphan receptor 1 (ROR1), carcinoembryonic antigen, CD80/CD86, and PD-L1 (160), and many associated prospective trials are ongoing (161).

NCT03525782 is a trial that evaluates the safety of PD-1-knockout engineered anti-MUC1 CAR-T cells for treatment of stage IIIB–IV NSCLC. Among the 20 treated patients who received at least one cycle of anti-MUC1 CAR-T cell therapy, 11 showed stable disease and 9 had progressive disease. Common adverse events included systemic symptoms such as fever and chills (162). Several ongoing trials on efficacy of HER2-specific CAR-T cells (NCT03198052, NCT03500991, and NCT03696030) are expected to show results in the near future (163).

A combination of CAR-T cell therapy and immunotherapy has shown potential in overcoming tumor heterogeneity. In a phase I study, pembrolizumab combined with locally delivered, autologous, mesothelin-targeted CAR-T cell therapy, was effective and safe in malignant pleural diseases comprised of metastatic lung and breast cancers and malignant pleural mesothelioma. In 27 patients, the median OS was 23.9 months, with 2 patients showing complete metabolic response. In this phase I study, combination treatment was shown to induce polyclonal immunity to overcome heterogeneity in tumor antigens and a potential synergistic effect of combined regional CAR-T cells and PD-1 blockade. The study showed that combination of CAR T cells and pembrolizumab further expanded endogenous T-cell clones which can contribute to overcoming tumor antigen heterogeneity (164).

However, several challenges should be overcome for CAR-T therapy to become a mainstay treatment modality. First, lung cancer is a solid tumor. CAR-T cell therapy showed potential in the treatment of hematological malignancies. However, treatment of solid tumors using CAR-T showed relatively decreased efficacies due to poor trafficking, short-lasting effects, and limited T cell activities in the tumor microenvironment (165,166). Several T cell barriers including stromal tissue surrounding the tumor cells must be overcome. Second, lung cancer has a relatively small number of targetable antigens compared with other tumor types (167). Last, several potential target antigens are also expressed in normal tissues, imparting risk for CAR-T cells to attack normal cells (160).

Nanomedicine

Cancer nanomedicine has several advantages. Nanomedicine enables more accurate delivery to the target tissues. Due to its modular flexibility, anti-cancer medications can exist in various forms enabling more effective transportation and absorption. Nanomedicine has provided chances to promote anti-tumor immune responses and minimize unnecessary systemic side effects (168). Intratumor conditions can trigger anti-tumor actions of immunotherapy and may provide possible switch-on signals for localized tumor cell-killing effects. Tumor microenvironment-specific conditions regarding reactive oxygen species (ROS), pH, GSH, ATP, hypoxia, and cytokines are possible targets of cancer nanomedicine combined with immunotherapy (168). Acidic tumor environment is generally T cell inhibitory, and a pH-sensitive signaling pathway has recently been suggested as a candidate mechanism to balance localized activation of T cells in the tumor microenvironment while avoiding unwanted systemic immune responses (169).

Human studies on the efficacy of nanomedicine are limited and most were performed in vitro. Salvianolic acid B-loaded PEGylated liposomes were shown to induce inactivation of CAFs by inhibiting TGF-β1 secretion. Subsequently, collagen deposition in tumors was reduced and penetration of nanoparticles into tumors increased (170).

Conclusions

Immunosuppressive signals and heterogeneity in TIME are serious obstacles to treatment of NSCLC, and ICIs play essential roles. In management of lung cancer, understanding the heterogeneity and personalized analysis of lung cancer is significant for overcoming treatment resistance. Ongoing trials including combination therapy such as radiotherapy, cytotoxic chemotherapy, and anti-angiogenic treatment and regimens inhibiting other immunoinhibitory molecules are promising.

Acknowledgments

Funding: This research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Korean government (MSIT) (No. 2019M3A9H2032425).

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-22-633/rc

Peer Review File: Available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-22-633/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-22-633/coif). SJK reports that this research was supported by the Bio & Medical Technology Development Program of the National Research Foundation (NRF) funded by the Korean government (MSIT) (No. 2019M3A9H2032425). The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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