Immunotherapy of hepatocellular carcinoma: strategies for combinatorial intervention

Targeting glypican 3 by immunotoxins: the promise of immunotherapy in hepatocellular carcinoma

INTRODUCTION

Tumor cell's resistance, high recurrence rate, and low overall survival rate have made hepatocellular carcinoma (HCC) a major health concern. The combination of advanced targeted therapies such as immunotherapy, with conventional treatments has gained traction for application on HCC. Immunotoxins (ITs) represent a category of biomolecules that combine the targeted affinity of antibodies with the cytotoxic properties of toxins.

AREAS COVERED

This study highlights Glypican3 (GPC3) as a potential candidate for targeted therapeutic interventions using ITs. It presents a comprehensive overview of the advantages and challenges associated with these modalities, and their promising outcomes in HCC treatment. A systematic literature review was conducted using PubMed, Web of Science and Scopus from 2015 to 2024.

EXPERT OPINION

Despite potential applicability, many concerns should be addressed before the employment of GPC3-based ITs. These include improving efficient penetration of ITs into the solid tumors, considering neutralizing antibodies against the drugs, and enhancing serum half-life of ITs. Furthermore, the ITs potential in eliminating cancer stem cells (CSCs) and residual tumor cells is discussed. The ability to target CSCs can significantly reduce the likelihood of recurrence and improve overall survival rate. This could make ITs a pivotal component in the future of HCC treatment.

One-day examination of triple nuclear medicine imaging and application in evaluating transarterial embolization

A diagnosis based on multiple nuclear medicine imaging (NMI) was more comprehensive in approaching the nature of pathological changes. In this research, a method to realize triple NMIs within one day was developed based on the reasonable arrangements of 68Ga-RGD PET/CT specialized on neovascularization, 99mTc-HL-91 SPECT/CT specialized on hypoxia and 18F-FDG PET/CT specialized on tumor metabolism. Feasibility was verified in evaluating the therapeutic effects of transarterial embolization (TAE) performed on rabbit models with VX2 tumor. Radiation dosimetry was carried out to record the radiation exposure from multiple injections of radiopharmaceuticals. In results, the one-day examination of triple NMIs manifested the diversity of the postoperative histological changes, including the local neovascularization induced by embolization, hypoxic state of embolized tissues, and suppression of tumor metabolism. More importantly, radiation dosage from radiopharmaceuticals was limited below 5.70 ± 0.90 mSv. In conclusion, the strong timeliness and complementarity of one-day examination of triple nuclear medicine imaging made it clinically operative and worthy of popularizing. There was flexibility in combining distinct NMIs according to the clinical demands, so as to provide comprehensive information for diagnosis.

Drug co-administration in the tumor immune microenvironment of Hepatocellular carcinoma

The etiology and exact molecular mechanisms of primary hepatocellular carcinoma (HCC) remain unclear, and its incidence has continued to increase in recent years. Despite tremendous advances in systemic therapies such as molecularly targeted drugs, HCC has some of the worst prognoses owing to drug resistance, frequent recurrence, and metastasis. Hepatocellular carcinoma is a widespread disease and its progression is regulated by the immune system. Traditional Chinese medicine (TCM) has been gradually theorized and systematized to have a holistic regulatory role for use in the prevention and treatment of tumors. Although half of the patients with HCC receive systemic therapy, traditionally sorafenib or lenvatinib are used as first-line treatment modalities. TCM is also widely used in the treatment of HCC, and the same immune checkpoint inhibitors (ICIs) such as PD-L1 have also received much focus in the field of continuously changing cancer treatment. Owing to the high probability of resistance to specific drugs and unsatisfactory efficacy due to administration of chemotherapy in single doses, the combination of drugs is the newest therapeutic option for patients with tumors and has become increasingly prominent for treatment. In this article, the research progress on combination therapy in the immunology of HCC is reviewed and the unique advantages of synergistic anti-tumor therapy with combination drugs are highlighted to provide new solutions for the clinical treatment of tumors.

Graphical abstract:

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Immunotherapeutic approaches in Hepatocellular carcinoma: Building blocks of hope in near future

Hepatocellular carcinoma (HCC) is the most common type of primary hepatic cancer and is among the major causes of mortality due to cancer. Due to the lack of efficient conventional therapeutic options for this cancer, particularly in advanced cases, novel treatments including immunotherapy have been considered. However, despite the encouraging clinical outcomes after implementing these innovative approaches, such as oncolytic viruses (OVs), adoptive cell therapies (ACT), immune checkpoint blockades (ICBs), and cancer vaccines, several factors have restricted their therapeutic effect. The main concern is the existence of an immunosuppressive tumor microenvironment (TME). Combination of different ICBs or ICBs plus tyrosine kinase inhibitors have shown promising results in overcoming these limiting factors to some extent. Combination of programmed cell death ligand-1 (PD-L1) antibody Atezolizumab and vascular endothelial growth factor (VEGF) antibody Bevacizumab has become the standard of care in the first-line therapy for untestable HCC, approved by regulatory agencies. This paper highlighted a wide overview of the direct and indirect immunotherapeutic strategies proposed for the treatment of HCC patients and the common challenges that have hindered their further clinical applications.

Co-delivery of Interleukin-12 and Doxorubicin Loaded Nano-delivery System for Enhanced Immunotherapy with Polarization toward M1-type Macrophages

Chemo-immunotherapy has gained increasing attention as one of the most promising combination therapy strategies to battle cancer. In this study, the therapeutic nanoparticles (TNPs) co-delivering doxorubicin (DOX) and IL-12 (IL-12) were developed for chemo-immunotherapy combination therapy on liver cancer. TNPs were synthesized based on the ionic interactions between cationic chitosan (Ch) and anionic poly-(glutamic acid) (PGA). DOX and IL-12 loaded in TNPs presented prolonged circulation in blood, efficient accumulation in tumors, and internalization in tumor cells. After that, DOX and IL-12 were co-released in the tumor microenvironment. The locally responsive property of TNPs could subsequently re-educate macrophages. More significantly, TNPs with no obvious side effects can remarkably inhibit the H22 tumor growth in vivo. A low dosage of loaded IL-12 in TNPs can effectively polarize macrophages toward the M1 phenotype to reduce tumor burden, further enhancing the antitumor efficacy. Our results suggest that the self-stabilized TNPs could be a secure and effective drug carrier for intravenous administration when deprived of protective agents.

A stratification model of hepatocellular carcinoma based on expression profiles of cells in the tumor microenvironment

Background

A malignancy of the liver, hepatocellular carcinoma (HCC) is among the most common and second-leading causes of cancer-related deaths worldwide. A reliable prognosis model for guidance in choosing HCC therapies has yet to be established.

Methods

A consensus clustering approach was used to determine the number of immune clusters in the Cancer Genome Atlas and Liver Cancer-RIKEN, JP (LIRI_JP) datasets. The differentially expressed genes (DEGs) among these groups were identified based on RNA sequencing data. Then, to identify hub genes among signature genes, a co-expression network was constructed. The prognostic value and clinical characteristics of the immune clusters were also explored. Finally, the potential key genes for the immune clusters were determined.

Results

After conducting survival and correlation analyses of the DEGs, three immune clusters (C1, C2, and C3) were identified. Patients in C2 showed the longest survival time with the greatest abundance of tumor microenvironment (TME) cell populations. MGene mutations in Ffibroblast growth factor-19 (FGF19) and catenin (cadherin-associated protein),β1(CTNNB1) were mostly observed in C2 and C3, respectively. The signature genes of C1, C2, and C3 were primarily enriched in 5, 23, and 26 pathways, respectively.

Conclusions

This study sought to construct an immune-stratification model for the prognosis of HCC by dividing the expression profiles of patients from public datasets into three clusters and discovering the unique molecular characteristics of each. This stratification model provides insights into the immune and clinical characteristics of HCC subtypes, which is beneficial for the prognosis of HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09647-5.

Cleavage Stimulation Factor Subunit 2: Function Across Cancers and Potential Target for Chemotherapeutic Drugs

The cleavage stimulation factor subunit complex is involved in the cleavage and polyadenylation of 3′-end pre-mRNAs that regulate mRNA formation and processing. However, cleavage stimulation factor subunit 2 (CSTF2) was found to play a more critical regulatory role across cancers. General cancer data sets from The Cancer Genome Atlas and Genotype-Tissue Expression project were thus downloaded for differential analysis, and the possible functions and mechanisms of CSTF2 in general cancer were analyzed using the Compartments database, cBioPortal database, Tumor Immune Single-cell Hub database, and Comparative Toxigenomics database using gene set enrichment analysis and R software. The results showed that CSTF2 could affect DNA repair and methylation in tumor cells. In addition, CSTF2 was associated with multiple tumor immune infiltrates in a wide range of cancers, and its high expression was associated with multiple immune checkpoints; therefore, it could serve as a potential target for many drug molecules. We also proved that CSTF2 promotes oral cell proliferation and migration. The high diagnostic efficacy of CSTF2 suggested that this gene may act as a new biomarker and personalized therapeutic target for a variety of tumors.

Impacts and mechanisms of alternative mRNA splicing in cancer metabolism, immune response, and therapeutics

Alternative pre-mRNA splicing (AS) provides the potential to produce diversity at RNA and protein levels. Disruptions in the regulation of pre-mRNA splicing can lead to diseases. With the development of transcriptome and genome sequencing technology, increasing diseases have been identified to be associated with abnormal splicing of mRNAs. In tumors, abnormal alternative splicing frequently plays critical roles in cancer pathogenesis and may be considered as new biomarkers and therapeutic targets for cancer intervention. Metabolic abnormalities and immune disorders are important hallmarks of cancer. AS produces multiple different isoforms and diversifies protein expression, which is utilized by the immune and metabolic reprogramming systems to expand gene functions. The abnormal splicing events contributed to tumor progression, partially due to effects on immune response and metabolic reprogramming. Herein, we reviewed the vital role of alternative splicing in regulating cancer metabolism and immune response. We discussed how alternative splicing regulates metabolic reprogramming of cancer cells and antitumor immune response, and the possible strategies to targeting alternative splicing pathways or splicing-regulated metabolic pathway in the context of anticancer immunotherapy. Further, we highlighted the challenges and discuss the perspectives for RNA-based strategies for the treatment of cancer with abnormally alternative splicing isoforms.

Camrelizumab Combined with FOLFOX4 Regimen as First-Line Therapy for Advanced Hepatocellular Carcinomas: A Sub-Cohort of a Multicenter Phase Ib/II Study

BACKGROUND

Immune checkpoint inhibitors and chemotherapy can synergistically increase efficacy in a variety of malignancies. We conducted this phase Ib/II study to assess the safety and efficacy of anti-PD-1 antibody camrelizumab in combination with FOLFOX4 for treatment-naive advanced hepatocellular carcinoma (aHCC).

METHODS

This open-label, multicenter phase Ib/II study (NCT03092895) enrolled patients with aHCC and without prior systemic treatment for treatment with camrelizumab (3 mg/kg) and FOLFOX4 every two weeks. First, six patients were enrolled, followed by an additional 28 patients after dose-limiting toxicity cases were determined to be <33% of patients. The primary endpoint was tolerability and safety of treatment.

RESULTS

A total of 34 aHCC patients were enrolled and received study treatment. No dose-limiting toxicity were observed in the first six patients enrolled. Twenty-nine (85.3%) of the total 34 patients had grade ≥3 treatment-related adverse events (TRAEs), with the most common ones being decreased neutrophil count (55.9%) and decreased white blood cell count (38.2%). No TRAEs-related deaths occurred. The objective response and disease control rate were 29.4% (95% CI, 15.1-47.5) and 79.4% (95% CI, 62.1-91.3), respectively. The median duration of response, progression-free survival, and overall survival was 6.9 months (range, 3.3-11.5), 7.4 months (95% CI, 3.9-9.2), and 11.7 months (95% CI, 8.2-22.0), respectively.

CONCLUSION

Camrelizumab combined with FOLFOX4 for first-line treatment of patients with aHCC showed good safety and tolerability, with promising preliminary antitumor activity.

Biomarkers in Immunotherapy-Based Precision Treatments of Digestive System Tumors

Immunotherapy, represented by immune checkpoint inhibitors (mainly referring to programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) blockades), derives durable remission and survival benefits for multiple tumor types including digestive system tumors [gastric cancer (GC), colorectal cancer (CRC), and hepatocellular carcinoma (HCC)], particularly those with metastatic or recurrent lesions. Even so, not all patients would respond well to anti-programmed death-1/programmed death-ligand 1 agents (anti-PD-1/PD-L1) in gastrointestinal malignancies, suggesting the need for biomarkers to identify the responders and non-responders, as well as to predict the clinical outcomes. PD-L1expression has increasingly emerged as a potential biomarker when predicting the immunotherapy-based efficacy; but regrettably, PD-L1 alone is not sufficient to differentiate patients. Other molecules, such as tumor mutational burden (TMB), microsatellite instability (MSI), and circulating tumor DNA (ctDNA) as well, are involved in further explorations. Overall, there are not still no perfect or well-established biomarkers in immunotherapy for digestive system tumors at present as a result of the inherent limitations, especially for HCC. Standardizing and harmonizing the assessments of existing biomarkers, and meanwhile, switching to other novel biomarkers are presumably wise and feasible.

Identification of Tumor Mutation Burden and Immune Infiltrates in Hepatocellular Carcinoma Based on Multi-Omics Analysis

We aimed to explore the tumor mutational burden (TMB) and immune infiltration in HCC and investigate new biomarkers for immunotherapy. Transcriptome and gene mutation data were downloaded from the GDC portal, including 374 HCC samples and 50 matched normal samples. Furthermore, we divided the samples into high and low TMB groups, and analyzed the differential genes between them with GO, KEGG, and GSEA. Cibersort was used to assess the immune cell infiltration in the samples. Finally, univariate and multivariate Cox regression analyses were performed to identify differential genes related to TMB and immune infiltration, and a risk prediction model was constructed. We found 10 frequently mutated genes, including TP53, TTN, CTNNB1, MUC16, ALB, PCLO, MUC, APOB, RYR2, and ABCA. Pathway analysis indicated that these TMB-related differential genes were mainly enriched in PI3K-AKT. Cibersort analysis showed that memory B cells (p = 0.02), CD8+ T cells (p = 0.09), CD4+ memory activated T cells (p = 0.07), and neutrophils (p = 0.06) demonstrated a difference in immune infiltration between high and low TMB groups. On multivariate analysis, GABRA3 (p = 0.05), CECR7 (p < 0.001), TRIM16 (p = 0.003), and IL7R (p = 0.04) were associated with TMB and immune infiltration. The risk prediction model had an area under the curve (AUC) of 0.69, suggesting that patients with low risk had better survival outcomes. Our study demonstrated for the first time that CECR7, GABRA3, IL7R, and TRIM16L were associated with TMB and promoted antitumor immunity in HCC.

Improving the Efficacy of Liver Cancer Immunotherapy: The Power of Combined Preclinical and Clinical Studies

Hepatocellular carcinoma (HCC) is a most deadly malignant disease worldwide, with no effective mechanism-based therapy available. Therefore, following the "miracle" outcomes seen in a few patients at the advanced stages of melanoma or lung cancer, the immune checkpoint inhibitors (ICIs) immediately entered clinical trials for advanced HCC patients without pre-clinical studies. Emerging data of clinical studies showed manageable toxicity and safety but limited therapeutic benefit to HCC patients, suggesting low response rate. Thus, one urgent issue is how to convert the liver tumors from cold to hot and responsive, which may rely on in-depth mechanistic studies in animal models and large scale data analysis in human patients. One ongoing approach is to design combinatorial treatment of different ICIs with other reagents and modalities. Indeed, a phase 3 clinical trial showed that combination of atezolizumab and bevacizumab achieved better overall and progression-free survival rates than sorafenib in unresectable HCC. This review highlights the value of animal models and the power of combining pre-clinical and clinical studies in efforts to improve HCC immunotherapy.

Identification and analysis of immune-related subtypes of hepatocellular carcinoma

Hepatocellular carcinoma is a malignance that remains difficult to cure. Immunotherapy has shown its potential application in a variety of refractory malignancies. Due to the complexity of immune microenvironment of hepatocellular carcinoma, the efficacy of immunotherapy for hepatocellular carcinoma is not as effective as expected. Expression data of hepatocellular carcinoma from the TCGA and ICGC databases were used for classification and verification of hepatocellular carcinoma subtypes. The immune-related functions and pathways were identified via gene set enrichment analysis, while the sections denoting the subsets of the immune cells were estimated using the CIBERSORT algorithm. Immunity low (Immunity_L), immunity medium (Immunity_M), and immunity high (Immunity_H) were specified as the three immune-related subtypes of hepatocellular carcinoma. The quantity of stromal and immune cells was the most substantial in Immunity_H, compared to the other subtypes. Interestingly, the proportion of M0 macrophages decreased from Immunity_L to Immunity_H, while the proportion of CD8 T cells increased. Furthermore, the HLA genes expression levels, as well as those of six immune checkpoint genes were substantially lower in Immunity_L than in Immunity_H. Functional analysis was performed for 1512 differentially expressed genes between Immunity_L and Immunity_H. Finally, the PPI network was constructed with 118 nodes. The highest connectivity degree nodes were B2M, HLA-DRA, and HLA-DRB1. The above results were verified in ICGC-JP and ICGC-FR databases with a consistent trend. In this study, we divided hepatocellular carcinoma into three subtypes and explored the immune-related characteristics of these subtypes. These results may provide new insights for immunotherapy of hepatocellular carcinoma.

Regulatory T cell and activated natural killer cell infiltration in hepatocellular carcinoma: immune cell profiling using the CIBERSORT

BACKGROUND

Hepatocellular carcinoma (HCC) is understood to be an immunogenic tumor caused by chronic liver disease. Emerging research has indicated close interaction between various immune cells and tumor cells. Immunophenotyping, which has shown potential predictive value for the prognosis of various human malignancies, might allow responsive and non-responsive patients to be identified based on the extent and distribution of immune cell infiltration. Several novel immunotherapeutic approaches have been trialed and have shown promising efficacy. However, the efficacy of immunotherapies in HCC is limited by several factors. This study aimed to investigate tumor-infiltrating immune cells in HCC.

METHODS

Cell-type Identification by Estimating Relative Subsets of RNA Transcripts (CIBERSORT) allows immune cell profiling analysis by deconvolution of gene expression microarray data. In this study, we analyzed the proportions of immune cells in 14 paired samples of HCC tissues obtained from GSE84402 in Gene Expression Omnibus (GEO) database.

RESULTS

In the 14 paired samples, HCC tissues showed significant infiltration by regulatory T cells (Tregs), activated natural killer (NK) cells, and M0 macrophages (P<0.001, P=0.007 and P=0.001, respectively), which were validated in CIBERSORT with the P value set at ≤0.05. In four paired samples identified from those selected by CIBERSORT, HCC tissues were found to have significant Treg and activated NK cell infiltration compared to non-tumor tissues (P=0.007 and P=0.015, respectively). Additionally, Pearson correlation analysis revealed Tregs to be positively correlated with activated NK cells (Correlation coefficient =0.41).

CONCLUSIONS

HCC tumor tissues were markedly infiltrated by Tregs and activated NK cells, which should be considered as candidate therapeutic targets in HCC multidisciplinary treatments.

Current progress and prospect of immune checkpoint inhibitors in hepatocellular carcinoma

In recent years, the incidence of liver cancer has increased and is currently the sixth most common tumor and the second leading cause of cancer-associated mortality worldwide. Most cases of liver cancer are hepatocellular carcinoma (HCC). Surgery, including liver transplantation or resection, and radiofrequency ablation therapies are all considered to be the curative treatment options for early-stage HCC. However, most patients have advanced HCC at the time of diagnosis, contributing to a poor prognosis. Therefore, improved treatment for late-stage HCC is needed. Immune checkpoint inhibitors (ICIs), among which programmed death receptor 1 (PD-1)/PD-ligand 1 and cytotoxic T lymphocyte-associated protein 4 are the representative immunological checkpoints, have shown great promise and progress for HCC treatment. The present review summarizes recent studies that have focused on ICIs and discusses the present limitations affecting the development of new therapeutic strategies.

Construction of chlorogenic acid-containing liposomes with prolonged antitumor immunity based on T cell regulation

As a potential cancer immunotherapeutic agent, chlorogenic acid (CHA) has entered phase II clinical trials in China as a lyophilized powder formulation for treating glioma. However, the in vivo instability of CHA necessitates daily intramuscular injections, resulting in patient noncompliance. In this study, CHA-phospholipid complex (PC)-containing PEGylated liposomes (CHA-PC PEG-Lipo, named as CPPL), with CHA-PC as the drug intermediate, were prepared to lower the administration frequency. CPPL demonstrated excellent physicochemical properties, enhanced tumor accumulation, and inhibited tumor growth even when the administration interval was prolonged to 4 days when compared to a CHA solution and CHA-PC loaded liposomes (CHA-PC Lipo, labeled as CPL), both of which only demonstrated antitumor efficacy with once-daily administration. Further evaluation of the in vivo antitumor immune mechanism suggested that the extended antitumor immune efficacy of CPPL could be attributed to its distinct immune-stimulating mechanism when compared with CHA solution and CPL, such as stimulating both CD4⁺ and CD8⁺ T cell infiltration, inhibiting myeloid-derived suppressor cell expression, reducing the expression of Th2 related factors, and notably, increasing the memory T cells in tumor tissues. This CHA-containing formulation could reduce the frequency of in vivo CHA administration during cancer treatment via T cells, especially memory T cell regulation.

Emerging role of metabolic reprogramming in tumor immune evasion and immunotherapy

Mounting evidence has revealed that the therapeutic efficacy of immunotherapies is restricted to a small portion of cancer patients. A deeper understanding of how metabolic reprogramming in the tumor microenvironment (TME) regulates immunity remains a major challenge to tumor eradication. It has been suggested that metabolic reprogramming in the TME may affect metabolism in immune cells and subsequently suppress immune function. Tumor cells compete with infiltrating immune cells for nutrients and metabolites. Notably, the immunosuppressive TME is characterized by catabolic and anabolic processes that are critical for immune cell function, and elevated inhibitory signals may favor cancer immune evasion. The major energy sources that supply different immune cell subtypes also undergo reprogramming. We herein summarize the metabolic remodeling in tumor cells and different immune cell subtypes and the latest advances underlying the use of metabolic checkpoints in antitumor immunotherapies. In this context, targeting both tumor and immune cell metabolic reprogramming may enhance therapeutic efficacy.

Comprehensive analysis of miRNA-gene regulatory network with clinical significance in human cancers

microRNAs (miRNAs), particularly the exosomal miRNAs have been widely used as biomarkers and promising therapeutic targets in cancer. However, a comprehensive analysis of miRNA-gene regulatory network with clinical significance remains scarce. The emergence of high-throughput multi-omics data over large, well-characterized patient cohorts provides an unprecedented opportunity to address this problem. Herein, we performed a clinic-centered analysis to identify cancer-associated miRNAs, miRNA-target axis. We first calculated the correlation among miRNA, mRNA and 75 unique clinico-pathological characteristics (CPCs) in 26 cancer types, and established an online resource (4CR). Interestingly, we found that the high expression of several DNA methylation-related enzymes was associated with adverse outcomes of cancer patients, and these genes were regulated by a cluster of miRNAs. Furthermore, by integrating exosomal miRNA and mRNA databases, we identified exosomal miRNA biomarkers for non-invasive cancer surveillance and therapy monitoring. Finally, we explored the role of CPC-related miRNAs for therapeutic effect prediction of drugs based on their shared targets. Our analysis pipeline illustrated the significance of clinic-centered analysis in miRNA-gene pair identification and provided helpful clues for future cancer studies.

Splicing dysregulation in cancer: from mechanistic understanding to a new class of therapeutic targets

RNA splicing dysregulation is widespread in cancer. Accumulating evidence demonstrates that splicing defects resulting from splicing dysregulation play critical roles in cancer pathogenesis and can serve as new biomarkers and therapeutic targets for cancer intervention. These findings have greatly deepened the mechanistic understandings of the regulation of alternative splicing in cancer cells, leading to rapidly growing interests in targeting cancer-related splicing defects as new therapies. Here we summarize the current research progress on splicing dysregulation in cancer and highlight the strategies available or under development for targeting RNA splicing defects in cancer.

Chimeric antigen receptor T cells in solid tumors: a war against the tumor microenvironment

Chimeric antigen receptor (CAR) T cell is a novel approach, which utilizes anti-tumor immunity for cancer treatment. As compared to the traditional cell-mediated immunity, CAR-T possesses the improved specificity of tumor antigens and independent cytotoxicity from major histocompatibility complex molecules through a monoclonal antibody in addition to the T-cell receptor. CAR-T cell has proven its effectiveness, primarily in hematological malignancies, specifically where the CD 19 CAR-T cells were used to treat B-cell acute lymphoblastic leukemia and B-cell lymphomas. Nevertheless, there is little progress in the treatment of solid tumors despite the fact that many CAR agents have been created to target tumor antigens such as CEA, EGFR/EGFRvIII, GD2, HER2, MSLN, MUC1, and other antigens. The main obstruction against the progress of research in solid tumors is the tumor microenvironment, in which several elements, such as poor locating ability, immunosuppressive cells, cytokines, chemokines, immunosuppressive checkpoints, inhibitory metabolic factors, tumor antigen loss, and antigen heterogeneity, could affect the potency of CAR-T cells. To overcome these hurdles, researchers have reconstructed the CAR-T cells in various ways. The purpose of this review is to summarize the current research in this field, analyze the mechanisms of the major barriers mentioned above, outline the main solutions, and discuss the outlook of this novel immunotherapeutic modality.