The activation of T cells requires two signals. The first signal is the T-cell receptor (TCR) signal formed by the combination of TCR and peptide-major histocompatibility complex (MHC) molecules, but this signal is not enough to activate resting T cells. Only in the case of the second costimulatory signal provided by CD28 and its receptor, T-cell activation-related RNA and proteins will be synthesized, the key cytokine interleukin (IL)-2 will be secreted, and cells will enter from G0 phase to G1 phase. Therefore, it is the costimulatory signal and TCR signal that complete the activation of T cells. Studies have shown that B2M, as an important part of human leukocyte antigen (HLA)-I molecules, participates in the folding and transport of MHC-I molecules and plays an important role in the processing and presentation of tumor antigens. B2M mutation can lead to impaired expression of MHC-I molecules on the surface of antigen-presenting cells (APCs) and then lead to impaired antigen presentation, resulting in immunotherapy resistance (2). In addition, negative costimulatory molecules such as cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death-1 (PD-1) and their ligands CD80, CD86, PD-L1, and PD-L2 can prevent the body from producing second signals, resulting in downregulation or termination of T-cell activation (3).

A large number of studies have shown that immune cells play a key role in tumor progression and inflammation. First, tumor-associated macrophages (TAMs) showed significant plasticity toward environmental cues (4, 5). In the early stage of the tumor, TAMS mainly showed M1 phenotype, while in the late stage, TAMS mostly belonged to M2 phenotype (6). M1 macrophages are pro-inflammatory cells, but they have an antitumor effect, which is related to the cytotoxicity and immunostimulatory function to cancer cells. M2 macrophages expressing anti-inflammatory cytokines such as IL-10, C-C motif chemokine ligand 22 (CCL22), and CCL18 can reduce inflammatory response but can promote tumors due to immunosuppression and angiogenesis induction (4, 7, 8). In tumors, microenvironment, such as hypoxia, nitric oxide (NO), can promote TAMs to M2 polarization. In addition, macrophage colony-stimulating factor (M-CSF) produced by tumor cells can also promote the polarization of TAMs to M2, resulting in tumor escape. The main secretion of M2 suppresses cytokines IL-10 and transforming growth factor-β (TGF-β), and the presence of antigen is weak, which inhibits T-cell activation and contributes to tumor immunity (7, 8).

Second, regulatory T (Treg) cells exert their immunosuppressive function through a variety of mechanisms. The high expression of IL-2 receptor on the surface of Treg cells can neutralize IL-2 to limit the proliferation and activation of T cells and produce inhibitory cytokines (TGF-β, IL-10, and IL-35) and cytotoxic substances (perforin and granulase) to inhibit and kill the excitation of effector T cells (9). CTLA-4 expressed by Treg cells binds to CD80/86 to impair APC maturation and inhibit T-cell proliferation, such as dendritic cells (DCs) (10). In addition, Treg cells had low expression of Nrf2, which is a key transcription factor of antioxidant reaction. Oxidative stress can induce apoptosis of Treg cells and release a large number of ATP, which is metabolized into adenosine by CD39 and CD73, which are highly expressed in Treg cells. Adenosine binds to A2A receptor (A2AR) to inhibit effector T cells (9, 11).

Last, myeloid-derived suppressor cells (MDSCs) can promote tumor growth through immunological inhibition and non-immunological inhibition, in which immunosuppressive mediator ARG1 and inducible NO synthase (INOS) can decompose L-arginine into 1-ornithine and urea, NO and nitrite, an important mediator of the IL-2 pathway, resulting in T-cell expression incompetence (12, 13). MDSC also expressed a high level of indoleamine 2mine3-dioxygenase (IDO), which can degrade 1-tryptophan to N-formylcanine, inhibit the proliferation and activation of T cells and NK cells, and promote CD4+ T cells to differentiate into Treg (14, 15). In addition, MDSC secretes immunosuppressive cytokines and growth factors (TGF-β and IL-10) to reduce the antitumor activity of effector T cells, recruit Treg cells, and increase reactive oxygen species (ROS) and NO in the microenvironment to inhibit the antitumor activity of natural killer (NK) cells and effector T cells (7, 16–18). The latest research shows that MDSCs can exert an immunosuppressive effect by upregulating PD-L1 (19). In addition, MDSCs can also promote tumor progression through non-immunological mechanisms (20). MDSCs can produce a large number of matrix metalloproteinases (MMPs), especially MMP9, to promote the infiltration of metastatic cells (21) and secrete high levels of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) to promote angiogenesis (22). To sum up, MDSCs play an important role in the occurrence and development of tumors.

Solid tumors tend to secrete a variety of pro-angiogenic factors, such as VEGF, hepatocyte growth factor, and platelet-derived growth factor. In 1971, Folkman (23) proposed that tumor growth was angiogenesis dependent, with further research. From 1983 to 1989, Senger et al. (24) proposed vascular permeability factor (VPF)/VEGF and Ferrara et al. (25) established the important position of VEGF until Terman isolated and purified VEGFR2 in the 1990s. These results fully indicate that VEGF (VEGFR2) plays an important role in the process of tumor growth, recurrence, and metastasis. VEGF/VEGFR is expressed in most tumors, including non-small cell lung cancer (NSCLC), and it has been found to increase the risk of tumor recurrence, metastasis, and death. Angiogenic factors are continuously secreted in the tumor microenvironment, resulting in abnormal angiogenesis. On the one hand, neovascularization usually lacks some adhesion molecules, and the downregulation of adhesion molecules leads to T-cell extravasation (26). At the same time, circulating VEGF hinders the maturation and function of DCs and helps tumors escape immune surveillance. On the other hand, neovascularization cannot offset the increase in oxygen consumption, so the hypoxia environment will directly damage the function of tumor-infiltrating lymphocyte (TIL). In addition, on the one hand, hypoxia can upregulate the inhibitory signals of antitumor immune response, such as PD-L1, IDO, IL-6, and IL-10 (27).

On the other hand, hypoxia induces upregulation of chemokine expression, which makes Treg cells reenter the tumor (28, 29). In addition, hypoxia can also promote the polarization of TAM to M2-like phenotype of TAMs (30). To sum up, angiogenesis can participate in tumor growth and immune escape through a variety of ways.

PTEN gene plays an important role in maintaining cell proliferation, differentiation, and apoptosis. PTEN can inhibit phosphoinositide 3-kinase (PI3K) pathway, which plays a regulatory role in some key cell processes such as tumor survival and proliferation. On the other hand, the lack of PTEN expression can activate the PI3K–AKT pathway, thus reducing the infiltration of lymphoid T cells and reduce the tumor killing effect of effector T cells (31). Similar to Janus kinase (JAK), it plays an important role in cytokine signal transduction. The JAK protein tyrosine kinase family consists of four members: tyrosine kinase (TYK)2, JAK1, JAK2, and JAK3. Patients with JAK1/2 gene mutations may be resistant to primary immunotherapy (32). Although JAK2 mutant tumor cells can produce interferon (IFN)-γ, the JAK2–signal transducer and activator of transcription _(STAT) signal pathway cannot be activated by IFN-γ and cannot upregulate the expression of PD-L1, which leads to the weak killing effect of IFN on JAK2 mutant tumor cells. However, the mutant cells of JAK1 were not sensitive to all kinds of effects of IFN. The above results suggest that the mutant tumor cells of the JAK1/JAK2 gene are not sensitive to the killing effect of IFN, and the expression level of PD-L1 is low, which makes the mutated tumor cells resistant to ICIs (33).

Immune combined chemotherapy can not only increase the cross-presentation of antigens by dendritic cells (34) but also weaken the immunosuppressive components of the tumor microenvironment (35), such as Treg cells, MDSCs, immunosuppressive cytokines, etc., and then increase toxic lymphocytes and the ratio of Treg cells (36). In 2019, the American Society of Clinical Oncology (ASCO) published a three-phase clinical trial KEYNOTE-189 to evaluate the efficacy of NSCLC first-line treatment for advanced non-squamous NSCLC: pablizumab combined with chemotherapy compared with chemotherapy alone. The results showed that the immune combined chemotherapy group could significantly double the levels of overall survival time (OS), progression-free survival (PFS), and PFS2 [mean OS (mOS): 22.0 vs. 10.7 months, hazard ratio (HR): 0.56, 95% confidence interval (CI): 0.45–0.70; mean PFS (mPFS): 9.0 vs. 4.9 months, HR: 0.48, 95% CI: 0.40–0.58; and mPFS2: 17.0 vs. 9.0 months, HR: 0.49, 95% CI: 0.40–0.59]. From the safety analysis, the incidence of treatment-related select adverse events (AEs) of any grade in the combination chemotherapy group and the simple chemotherapy group was 26.4% vs. 12.9%. The grade 3–5 treatment-related select AEs were 10.9% vs. 4.5%. It further confirmed the safety and efficacy of this first-line chemotherapy regimen combined with pablizumab in the treatment of non-squamous NSCLC (37). In addition, KEYNOTE407 3-year follow-up data were released at this year’s European Lung Cancer Congress (ELCC) to evaluate the efficacy of immunotherapy combined with chemotherapy and chemotherapy alone. Studies have shown that immunotherapy combined with chemotherapy provides more lasting benefits to patients than chemotherapy (mOS: 17.1 vs. 11.6 months, HR: 0.71, 95% CI: 0.58–0.88; mPFS: 8.0 vs. 5.1 months, HR: 0.57, 95% CI: 0.47–0.69; mPFS2: 13.8 vs. 9.1 months, HR: 0.59, 95% CI: 0.49–0.72). Three-year follow-up data showed that OS: 29.7% vs. 18.2%, PFS: 16.1% vs. 6.5%; in terms of safety, the two groups of 3 and above treatment related adverse reactions were 74.8% vs. 70.0% (38). In the latest Camel-sq study released by the ELCC in 2021, carrizumab combined with carboplatin and paclitaxel combined with carboplatin chemotherapy regimen in the treatment of advanced non-small cell lung squamous cell carcinoma, objective response rate (ORR) and PFS were significantly prolonged (ORR: 64.8% vs. 36.7%, PFS: 8.5 vs. 4.9 months) (39).

Preclinical evidence points to RT as a priming event for immunotherapy. By modulating the host’s immune system, RT can render tumor cells more susceptible to T cell-mediated attack. RT promotes the release of tumor neoantigens from dying tumor cells, enhances MHC-I expression, and upregulates chemokines, cell adhesion molecules, and other immunomodulatory cell surface molecules, thereby potentiating an antitumor immune response by triggering immunogenic cell death (40). Just as the PACIFIC trial evaluated the efficacy of durvalumab consolidation therapy in patients with NSCLC after simultaneous RT and chemotherapy, the results showed that the mPFS was 16.9 vs. 5.6 months (HR: 0.55, 95% CI: 0.45–0.68), and the mOS was 47.5 vs. 28.1 months (HR: 0.55, 95% CI: 0.45–0.68) (41). In addition, a retrospective study at the 2020 ASCO conference showed that local treatment can significantly improve survival benefits (42). After immune resistance, local therapy combined with immunotherapy can reverse drug resistance to some extent, providing new treatment ideas for patients with immune drug resistance.

Anti-angiogenic therapy can change the function of tumor vascular endothelial cells to regulate immunosuppression and reduce the inhibitory effect of VEGF on DC migration and immune function (43, 44). Studies have shown that CTLA-4 or PD-1 inhibitors can reduce tumor vascular density, improve vascular perfusion, relieve hypoxia of tumor tissue, normalize blood vessels, and reduce the immunosuppressive effect of Treg, TAMS, and MDSCs (44–46). Therefore, the combination of immunosuppressive and anti-angiogenic drugs may play a synergistic role. As the latest results of the IMpower150 trial were presented at the American Association for Cancer Research (AACR) 2020 meeting of the AACR, the mOS of ABCP (atezolizumab+carboplatin+paclitaxel+bevacizumab) vs. BCP (carboplatin+paclitaxel+bevacizumab) was 19.5 vs. 14.7 months (HR: 0.80, 95% CI: 0.67-0.95), the mPFS was 8.4 vs. 6.8 months, and the ORR was 63.5% vs. 48.0%, mOS was 29.4% vs. 18.1%, respectively. In the liver metastasis subgroup, the mOS of ABCP vs. BCP was 13.2 vs. 9.1 months, and the PFS was 8.2 vs. 5.4 months. From the data survey, it can be seen that compared with BCP, ABCP can significantly improve the PFS and OS of patients (47, 48). From the above trial results, for epidermal growth factor receptor (EGFR) and anaplastic lymphoma kinase (ALK) mutant population, the combination of four drugs can bring significant survival benefits to patients with advanced NSCLC (49).

The Phase I/II KRYSTAL-1 test (NCT03785249) included patients with KRASG12C mutation-positive, unresectable, or metastatic NSCLC. After adagrasib monotherapy, the results showed that in patients with NSCLC, the overall remission rate of adagrasib treatment reached 45% and the disease control rate (DCR) reached 96%, and for NSCLC patients with STK11 mutation, the overall remission rate reached 64%. In terms of safety, the most common AEs of adagrasib treatment included nausea (54%), diarrhea (51%), vomiting (35%), and fatigue (32%) (50, 51). The researchers indicated that they plan to verify the efficacy of adagrasib in combination with other drugs or treatments in future trials, such as pembrolizumab, Keytruda (52). The Phase II KRYSTAL-7 trial (NCT04613596) of adagrasib combined with pimumab is under preparation, and the results are worth looking forward to. And now there are more and more data supporting the combination of immunotherapy and other targeted treatments.