PD-L1 is a potential tumor biomarker, and its predictive value varies in different tumors (47). Pembrolizumab has been approved by the FDA as a first-line treatment for patients with NSCLC with PD-L1 expression ≥50%, and without EGFR or anaplastic lymphoma kinase (ALK) gene mutations (48). A meta-analysis revealed that the higher the expression level of PD-L1, the greater its benefit in patients using anti-PD-1/PD-L1 (49). However, patients with squamous NSCLC are treated with nivolumab regardless of PD-L1 expression level (50). As a biomarker, PD-L1 expression is not omnipotent. At present, PD-L1 detection is widely employed in the clinic, and can be used as a clinical auxiliary or supplementary diagnostic tool to predict the efficacy of immunotherapy (51,52); however, it is not without its challenges.

PD-L1 is a continuous variable, the levels of which can be altered by induction under constant conditions, for example, by cisplatin. Therefore, accurately detecting the level of PD-L1 is challenging (53). Furthermore, surgical resection and biopsy can impact PD-L1 expression, due to the heterogeneity of the tumor (54). The expression of PD-L1 in surgically-removed sections and lung biopsies was analyzed by SP142 immunohistochemistry (IHC) in 160 patients with NSCLC. The results showed that the expression of PD-L1 in lung biopsies was not consistent with that in the surgically-removed sections (overall inconsistency rate, 48%), and the expression of PD-L1 in the biopsies was generally lower than that in the sections (55). Finally, the difference between IHC assays also has important implications on PD-L1 detection, as results are inconsistent among different assays. Currently, four IHC assays have been approved by the FDA (28-8, SP263, SP142 and 22C3), and selecting the most suitable one for each patient is crucial. In addition, the scoring system for PD-L1 expression has not been standardized (56,57). In conclusion, clinical detection of PD-L1 demands standardization and improved accuracy.

TMB is a potential biomarker for immunotherapy, which is defined as the total number of detected somatic gene coding, base substitution, gene insertion or deletion errors in every million bases (Mb). The principle of TMB in predicting the efficacy of immunotherapy is that tumors with a high TMB may express a greater variety of antigens and have stronger immunogenicity, thus increasing their recognition by, and the killing effect of, cytotoxic CD8⁺ T cells. Therefore, tumors with a high TMB are suitable for immunotherapy (58). In terms of clinical trials, CheckMate-568 revealed that patients with a TMB of >10 mutations (mut)/Mb had higher ORR and longer progression-free survival (PFS) than those with a TMB of <10 mut/Mb, regardless of PD-L1 expression level. The results showed that TMB could be used as a prospective biomarker for patients treated with nivolumab combined with ipilimumab (59). However, the use of TMB as a biomarker has certain limitations.

In terms of detection methods, whole genome sequencing or whole-exome sequencing (WES) are the standard methods for calculating the TMB (58); their disadvantages include the need for high sample quality, long detection times and high cost, and as such, their wide application in clinical practice is limited (60). A promising and convenient alternative is next-generation sequencing (NGS) (61). In a study by Samstein et al (62), integrated mutation profiling of actionable cancer targets (MSK-IMPACT™) was used to sequence the genome. The results showed that for the majority of tumors treated with ICIs, patients with a higher TMB exhibited a higher survival rate (62). The relevant data also showed that the results of quantification of TMB detected by NGS and WES were correlated (63). The FDA has approved two NGS panels (FoundationOne^®CDx and MSK-IMPACT) to evaluate TMB. Although NGS has its advantages, in-depth research is required in order to improve accuracy in clinical practice before it can become the standard method for TMB determination (64). Secondly, there is no fixed standard TMB cut-off value. Generally speaking, a TMB of >20 mut/Mb is considered high, whereas a TMB of <10 mut/Mb is considered low. Therefore, it is necessary to determine the optimal cut-off value of each tumor type in more prospective clinical studies and clinical practice (65).

Additionally, 30% of cancer patients face further challenges, including the inability to obtain tumor tissue, insufficient tumor tissue samples and tumor tissue content not meeting the requirements for detection (66). Therefore, a blood-based assay was developed to measure blood tumor mutation burden (bTMB). Blood detection is safer, less costly, and blood samples are easier to obtain than tumor biopsies. Related studies have confirmed that the bTMB can predict the efficacy of atezolizumab in patients with advanced NSCLC (67). However, other studies have shown that the bTMB cannot evaluate the results of immunotherapy (PFS and ORR). Patients with a low bTMB may also benefit from immunotherapy (68). In addition, recent studies have reported that the maximum somatic allele frequency combined with bTMB has a higher predictive effect than bTMB alone in patients with advanced NSCLC treated with atezolizumab (69). bTMB is therefore not a mature biomarker, and further experimentation is required for clinical verification. Although both TMB and bTMB can predict the efficacy of immunotherapy, there are still several obstacles that need to be overcome, including the specification of the detection platform, and the lack of standardization in the evaluation of TMB and bTMB.

At present, there is no single biomarker that can accurately predict the efficacy and prognosis of immunotherapy. In a clinical setting, PD-L1 and TMB are the most commonly used biomarkers, but there are certain limitations regarding their clinical use. It is therefore crucial to continue investigating existing, and identify novel, biomarkers. For example, tumor-infiltrating lymphocytes (TIL), epithelial-to-mesenchymal transition/inflammation signature score, intestinal flora, microsatellite instability high/deficient mismatch repair, gene expression signatures and tumor-specific genotypes are potential biomarkers currently being explored (70,71). However, the identification of a ‘perfect’ biomarker is unlikely; therefore, assessing the suitability of patients for immunotherapy based on the comprehensive evaluation of multiple indicators is currently the most effective strategy. A study has shown that a combination of human leukocyte antigen (HLA) class I, CD8⁺ T cell infiltration, PD-L1 expression and tumor mutational load is a promising biomarker for predicting the efficacy of anti-PD-1 in the treatment of NSCLC (72). However, this combination still requires considerable clinical verification to confirm its predictive ability. Therefore, the focus of future studies should be not only on identification of novel biomarkers, but also the investigation of the most effective combination of various existing biomarkers.

With the development of immunotherapy, the global use of ICIs has increased; however, the development of drug resistance remains a considerable challenge (73). Resistance can be categorized as acquired or primary, with the probability of primary resistance at ~60% (74). Most patients develop resistance following immunotherapy, which reduces its anticancer effects. Therefore, overcoming resistance is important in improving immunotherapeutic efficacy. Existing literature suggest that the current understanding of resistance mechanisms is limited; the potential mechanisms of resistance (based on existing data) are summarized herein.

Firstly, tumor immunogenicity and TMB are associated with the mechanism of resistance. The necessary conditions for effective immunotherapy include tumor expression of the appropriate antigens and the generation of tumor antigen-specific T cells (75). Tumors with a high immunogenicity (NSCLC, renal cell carcinoma and human melanoma) are more sensitive to immunotherapy (76,77). However, tumors with a low TMB produce fewer tumor antigens and have poor immunogenicity, which negatively impacts the activation of effector T cells, further leading to resistance (78). Prostate cancer has poor immunogenicity and low expression of PD-L1, which is the primary reason for drug resistance (79). Therefore, tumor immunogenicity and TMB are closely associated with the mechanism of resistance. Secondly, immunotherapy restores T cell function by blocking immune checkpoints. However, if tumor cells cannot be identified by T cells, drug resistance ensues. The expression of tumor cell MHC-I is necessary for identification of tumor cells by T cells, and MHC-I loss or downregulation results in tumor immune escape. β₂ m is an integral part of the HLA class I molecule, and is necessary for antigen presentation. Mutations in β₂ m limit the recognition of CD8⁺ T cells, which results in T cell failure (80). Therefore, a promising solution is to generate synthetic long peptides from tumor-associated antigens, as well as DNA and RNA, in order to develop novel vaccines. The associated mechanism is that the vaccine is transported to MHC-I and MHC-II molecules of antigen-presenting cells, thus promoting CD8 and CD4 T cell responses to overcome resistance (81). In addition, radiotherapy can lead to the independent upregulation of MHC-I, recover antigen presentation and overcome resistance (82). Finally, the pathway of immunosuppression is not only associated with PD-1/PD-L1, but also other pathways with similar functions, such as those of TIM-3, LAG-3, CTLA-4 and B- and T-lymphocyte attenuator (BTLA). In the event of the combined action of these pathways, blockade of one may not be sufficient, as the others pathways will likely compensate for the loss of immunosuppressive signals. Therefore, the combination of multiple ICIs may improve their therapeutic effects (83). In clinical and mouse models, the expression of TIM-3 was increased following anti-PD-1 resistance, and the combination of anti-TIM-3 and anti-PD-1 was found to improve survival rates (84,85). Therefore, the combination of multiple ICIs may improve tumor control and overcome resistance.

PD-1/PD-L1 is one of the signaling pathways of several immune checkpoints, but various other immune checkpoints perform similar functions. The anti-tumor effect can be improved using a combination of multiple ICIs. The combination of anti-PD-1 and anti-CTLA-4 has been widely studied and shown to yield positive results. CheckMate 012 showed that the efficacy of nivolumab + ipilimumab as a first-line treatment was superior to that of nivolumab alone (86). The phase III clinical study CheckMate 227 confirmed that the efficacy of nivolumab + ipilimumab as a first-line treatment was superior to that of traditional chemotherapy. It was also shown that, in patients with a high TMB, the median PFS of the nivolumab + ipilimumab group was longer than that of the chemotherapy group (7.2 vs. 5.5 months) (87). Furthermore, CheckMate 568 revealed an association between the efficacy of nivolumab + ipilimumab as a first-line treatment, and the expression of PD-L1 and TMB. The results indicated that patients with a TMB of >10 mut/Mb had a higher ORR and longer PFS, regardless of PD-L1 expression, providing evidence for the use of TMB as a potential biomarker (5). In addition to the combination of anti-PD-1 and anti-CTLA-4, a number of novel immunotherapy combinations are in the experimental stages.

Chemotherapy is a traditional treatment for advanced cancer, which can increase tumor antigen presentation, enhance the activity of effector T cells, and increase the expression of PD-L1 in tumors (88). In addition, the regulatory effect of a variety of chemotherapeutic drugs on immune function has been reported. For example, cisplatin upregulates PD-L1 expression in tumor cells through the PI3K/AKT signaling pathway (53), and pemetrexed can promote the activation of NK cells, increasing the production of IFN-γ (89). Also, docetaxel selectively reduces the number of Tregs and prevents immune suppression (90). Therefore, chemotherapy and immunotherapy have a synergistic effect, suggesting that combination therapy has a superior effect to monotherapy.

A number of studies have shown that as a first-line treatment, the therapeutic benefits of immunotherapy combined with chemotherapy are greater than those of chemotherapy alone. KEYNOTE-021 studied the difference in efficacy between pembrolizumab + pemetrexed-carboplatin (PC) and PC alone. The results showed that the ORR of the combination group was 56.7%, while that of the PC group was 30.2%. Compared with PC alone, PFS and overall survival (OS) following combination therapy also improved significantly (91). Patients were divided into three groups based on first-line treatment: Atezolizumab + carboplatin + paclitaxel, atezolizumab + bevacizumab + carboplatin + paclitaxel (ABCP) and bevacizumab + carboplatin + paclitaxel (BCP) as. In the ITT-WT population (patients with EGFR or ALK alterations were excluded), the mPFS of the ABCP group was significantly higher than that of the BCP group (8.3 vs. 6.8 months). In the WT population which had high expression of an effector T cell (Teff) gene signature in the tumor (Teff-high WT) cases, the mPFS of the two groups was 11.3 and 6.8 months. The results showed that adding atezolizumab to bevacizumab + chemotherapy for non-squamous metastatic NSCLC improved OS and PFS, regardless of PD-L1 expression. In December 2018, the FDA approved atezolizumab + carboplatin + paclitaxel + bevacizumab as a first-line treatment option for patients with advanced metastatic NSCLC without EGFR and ALK mutations (92).

For patients with advanced NSCLC, radiotherapy is one of the most conventional and effective treatments, and improves survival rate by increasing the control of primary tumour (93). Radiotherapy can increase tumor antigen release, enhance antigen presentation and promote T-cell infiltration into the tumor tissue, thus enhancing the systemic anti-tumor immune response and altering the tumor microenvironment (94). Radiotherapy can induce immunogenic cell death, enhance the TIL repertoire and upregulate MHC and PD-L1 expression (95,96). In immunotherapy, the expression of PD-L1 plays an important role in predicting the therapeutic effect. A previous study revealed that radiotherapy increases the expression of PD-L1 in tumor cells by activating the PI3K/AKT and STAT3 signaling pathways. In addition, radiotherapy is less toxic, and thus a more favorable option for combined immunotherapy (97).

At present, there are few NSCLC clinical studies on radiotherapy combined with ICIs. PEMBRO-RT is a recent multi-center phase II randomized clinical trial, the purpose of which was to determine whether the use of stereotactic body radiotherapy before pembrolizumab treatment increases the anti-tumor response of patients with metastatic NSCLC. The patients in the experimental arm were treated with pembrolizumab following stereotactic body radiotherapy at a single tumor site, while the control arm were treated with pembrolizumab only. The results showed that the ORR at 12 weeks in the experimental arm was significantly higher than that of the control arm (36 vs. 18%). The mPFS of the experimental arm was 6.6 months, and that of the control arm was 1.9 months. The median OS was 7.6 in the control arm, and 15.9 months in the experimental arm. Although the experimental results showed that the ORR following combination therapy was significantly higher than that of the control arm, the results did not reach the expected standard. In addition, different patient PD-L1 expression levels may also have effeced the experimental results. Therefore, these findings require further experimental confirmation (98). However, based on the existing experimental outcomes, immunotherapy combined with radiotherapy is superior to radiotherapy alone, and does not increase the number of adverse reactions.

In patients with NSCLC, combined immunotherapy can compensate for the deficiencies of immunotherapy alone, allowing increased benefit to a greater number of patients. Combined immunotherapy appears promising, but the current thinking cannot be limited by the existing programs when investigating this research area. TCM has a long history of use in China (99). During the novel coronavirus outbreak in 2020, the majority of hospitals in China used a combination of western and TCM to treat patients, with successful results (100). China has also achieved encouraging results in providing TCM treatment for patients in other countries, which has become an important player in global disease treatment (101). The tumor environment is an important consideration when treating tumors, as changes therein may directly affect tumor cell proliferation rate, and subsequently, the effects of immunotherapy and occurrence of resistance (102). For the treatment of cancer, TCM has the potential to improve the tumor environment for increased treatment efficacy. Most TCM agents are derived from plants, and thus, have the advantages of low toxicity and multiple targets(diversity of compounds). In addition, TCM includes a specific class of drugs for the improvement of self-immunity, which improve the tumor environment; as such, the primary purpose of systematic treatment for patients with metastatic NSCLC is to reduce the burden of cancer symptoms, and to improve survival, while also maintaining patient quality of life (103). Currently, traditional treatments and immunotherapy cannot achieve satisfactory results in patients with metastatic NSCLC. Therefore, the use of TCM combined with these other treatment types is a promising option for patients with NSCLC, and research into the role of TCM in immunotherapy is ongoing.

Astragalus polysaccharide (PG2) is the active component of the dried roots of Astragalus membranaceus. A study showed that PG2 increased the M1/M2 macrophage polarization ratio of H441 and H1299 cells. It also increased the T cell-mediated antitumor immune response by promoting the functional maturity of DCs (104). Adding PG2 to cisplatin, a conventional chemotherapeutic drug, can synergistically increase the antitumor effect (105). According to the above theory, the effective results of immunotherapy combined with PG2 in NSCLC were expected (106). Phytolacca acinosa polysaccharides I (PAP-1) is a compound of Phytolacca, which has been proven to affect immune functions in mice. It can also enhance the production of IL-2 and NK cytotoxic factor, and exerts antitumor activity (107). It has also been reported that total glucosides of paeony can regulate the expression of PD-1 and PD-L1 in peripheral blood monocytes (108). Thus to conclude, TCM has the potential to positively influence immunotherapy. In addition, various other TCM agents can regulate immunity and antitumor effects. At present, research on the antitumor effects of combining TCM with immunotherapy is being conducted, the findings of which may provide patients with further treatment options and superior benefits.