Research trends and hotspots in the immune microenvironment related to hepatocellular carcinoma: A bibliometric and visualization study

Abstract

BACKGROUND

The immune microenvironment (IME) in hepatocellular carcinoma (HCC) plays a pivotal role in determining patient outcomes and responses to treatment. This area is witnessing rapid growth in research interest. However, there is a lack of comprehensive bibliometric analyses that dissect trends and potential focal points in this field.

AIM

To explore the evolution of research on the IME in HCC from January 1, 2004, to December 31, 2023, using bibliometric methodologies.

METHODS

English articles and reviews concerning the IME of HCC were retrieved from the Web of Science Core Collection with a search date of December 31, 2023. The R package Bibliometrix was employed to compute basic bibliometric characteristics, illustrate collaborations among countries and authors, and create a three-field diagram illustrating the connections between authors, affiliations, and keywords. Analyses of country and institutional co-authorship, as well as keyword co-occurrence, were conducted using VOSviewer. Additionally, CiteSpace was utilized for the cite burst analysis of keywords and cited literature.

RESULTS

The study encompassed 3125 documents in the research areas related to HCC of IME, revealing a substantial and continuous increase in the annual publication trend over time. China and Fudan University emerged as leading contributors, with 2103 and 165 publications, respectively. Frontiers in immunology was the most prolific journal in this domain. Among the top ten researchers in the field, eight are based in China. Key research terms identified include tumour microenvironment, expression, immunotherapy, and prognosis.

CONCLUSION

The relationship between HCC and IME is receiving increasing attention, and related research is in a highly developed stage. Key focus areas, including IME and immune checkpoint inhibitors, immunotherapy are poised to be central to future research endeavors, offering promising pathways for further exploration.

Key Words: Immune microenvironment, Hepatocellular carcinoma, Bibliometrics, VOSviewer, Citespace, Frontiers

Core Tip: The current major research thrust in the field is to explore immune microenvironment (IME)-based immunotherapy related to hepatocellular carcinoma. Our researchers have taken an in-depth look at tumor IME and concluded that immune checkpoint-based immunotherapy, precision immunotherapy and immune cellular modalities are the frontiers and priorities for the coming years, which may provide valuable opportunities for further research.

INTRODUCTION

Hepatocellular carcinoma (HCC), the most prevalent liver cancer, ranks as the third highest cause of cancer mortality globally[1]. Risk factors for HCC include chronic hepatitis virus infection, alcohol abuse, metabolic syndrome, and certain monogenic diseases. Surgery emerges as the optimal treatment for the treatment of early HCC. However, the majority of patients are ineligible for surgical intervention due to advanced disease progression at the time of diagnosis. Alternative approaches, such as chemotherapy, radiotherapy, molecularly targeted agents, radiofrequency ablation and transarterial chemoembolisation, demonstrate limited efficacy in advanced HCC[2]. Despite a significant shift in the treatment of HCC, the application of immunotherapy remains constrained by the insufficient data available for predicting resistance and treatment response[3].

In recent years, the widespread utilization of immunotherapy across various types of tumors has underscored the significance of the tumor immune microenvironment (TIME). TIME stems from the broader concept of the tumor microenvironment (TME), which is defined as a heterogeneous mixture of cancer cells, different stromal cell types, and their associated parenchyma[4].

The liver functions as an immunoregulatory organ where T-cell responses are modulated by resident cells such as Kupffer cells (KCs), hepatic stellate cells (HSCs), dendritic cells (DCs) and regulatory T cells [5,6]. Conversely, the liver impedes immune surveillance and fosters tumor advancement[5,6]. Up to 80% of HCC cases are attributed to chronic inflammation, primarily induced by the infiltration of immune cells and activation of resident cells (e.g., KCs, HSCs, and liver sinusoidal cells)[7].

Recent studies have highlighted the importance of the hepatic IME in the development and progression of HCC and its potential treatment outcomes[8,9]. Research has focused on understanding the complex interactions between tumour cells, immune cells and liver tissue.

Existing bibliometric articles in the field of TIME provide comprehensive summaries and analyses of current research, offering valuable insights for future investigations. However, there has been no bibliometric study systematically examining trends and future research directions concerning the IME of HCC. This study attempts to explore the development patterns of research related to IME of HCC from 2004 to December 31, 2023, using a bibliometric approach.

MATERIALS AND METHODS

Process of gathering and retrieving data

We searched Web of Science Core Collection (WoSCC) database[10,11] for all articles related to the application of IME in HCC between January 1, 2004, and December 31, 2023, with the following search formula: ((((((((((((((((((TS=(Carcinoma, Hepatocellular)) OR TS=(Carcinomas, Hepatocellular)) OR TS=(Hepatocellular Carcinomas)) OR TS=(Liver Cell Carcinoma, Adult)) OR TS=(Liver Cancer, Adult)) OR TS=(Adult Liver Cancer)) OR TS=(Adult Liver Cancers)) OR TS=(Cancer, Adult Liver)) OR TS=(Cancers, Adult Liver)) OR TS=(Liver Cancers, Adult)) OR TS=(Liver Cancers, Adult)) OR TS=(Liver Cell Carcinoma)) OR TS=(Carcinoma, Liver Cell)) OR TS=(Carcinomas, Liver Cell)) OR TS=(Cell Carcinoma, Liver)) OR TS=(Cell Carcinomas, Liver)) OR TS=(Liver Cell Carcinomas)) OR TS=(Hepatocellular Carcinoma)) OR TS=(Hepatoma)) OR TS=(Hepatomas) AND TS=(immune microenvironment). The literature screening for this study adhered to the following inclusion criteria: (1) Availability of full-text publications related to the IME of HCC, and (2) Articles and review manuscripts written in English. Exclusion criteria included topics unrelated to IME of HCC and conference abstracts, news articles, or briefing papers. The WoS database was chosen for this study for several reasons. As it is used in academic literature research, WoSCC is one of the most influential academic databases containing more than 20000 journals covering a wide range of academic disciplines such as natural sciences, engineering and technology, and medicine. WoS is a widely accepted and commonly used database for conducting citation data analyses and for a variety of evaluation purposes and has been used in many bibliometric analyses, and is an analytical and essential resource for assessing scholarly publications[10,11]. In addition, although other databases such as Reference Citation Analysis, Scopus and PubMed have been applied in bibliometric studies[12-14], they all have their own limitations, such as biased search methods that prioritize the medical field.

Annual papers, national publication trends and proportions were analyzed and plotted using GraphPad prism v8.0.2. In addition, CtieSpace 6.1.6R (64-bit) Advanced Edition[15] and VOSviewer (version 1.6.18)[15,16] were used to analyze and visualize the scientific knowledge graph. The entire literature screening process is effectively depicted in Figure 1.

Figure 1  Diagram of the process and key steps of the study.

RESULTS

Publication growth trend

The analysis conducted from January 1, 2004, to December 31, 2023, revealed that the WoSCC database documented a total of 3125 publications related to IME of HCC. Among these, 2166 articles (69.3%) and 959 reviews (30.7%) were identified. The literature encompasses contributions from 74 countries and regions, 2598 institutions, and 13749 authors.

As shown in Figure 2, the annual number of published papers since 2004 displays a gradual increase, delineated into three distinct stages. From 2004 to 2017, there was a slow growth pattern with fewer than 100 articles per year, suggesting a lack of research attention to the field. In the years of 2018-2020, we have witnessed a gradual increase in the number of articles, indicating a growing interest among researchers. Subsequently, there was a rapid surge in publications after 2021, reaching a peak in 2023, indicating a significant rise in research activity and a heightened awareness of the field among researchers.

Figure 2 The annual publications between January 1, 2004 and December 31, 2023 and the cumulative articles. The annual number of published papers since 2004 displays a gradual increase, delineated into three distinct stages. From 2004 to 2017, there was a slow growth pattern with fewer than 100 articles per year. In the years 2018-2020, we have witnessed a gradual increase in the number of articles. There was a rapid surge in publications after 2021, reaching a peak in 2023.

Analysis of countries and institutions

The analysis of national publications is depicted in Supplementary Figure 1, and Supplementary Table 1. Research focusing on the application of IME in HCC has been conducted in 74 countries and regions. The leading countries in this field are China, the United States, Italy, Japan, and Germany. China, with 67.30% of the total publications, significantly surpasses other nations. Among the top ten countries/regions in terms of the number of papers published, China has 41168 citations (Supplementary Table 1), far exceeding all other countries/regions. However, its citation/publication ratio (19.58) in China ranks last among all countries/regions, suggesting generally lower quality of its published papers. The United States ranks second in the number of publications (527) and citations (29558) and third in the citation/publication ratio (56.09), indicating a high quality of published papers. The collaboration network illustrated in Supplementary Figure 2 revealed that China collaborates closely with the United States, while the United States collaborates closely with France, Italy, and the United Kingdom. China, on the other hand, collaborates even more closely with Japan, Korea, and Australia. Recently, the number of articles published by countries such as the United States and Australia has seen a rapid increase, possibly due to collaboration with China.

A total of 2598 institutions systematically published articles on IME of HCC. The top ten institutions in terms of publications were all from China (Table 1). Fudan University exhibited the highest publication output, with 165 papers and 4665 citations, averaging 28.27 citations per paper. Sun Yat-Sen University followed closely, ranking second with 149 papers and 4909 citations, averaging 32.95 citations per paper. Zhejiang University secured the third position with 138 papers and 2552 citations, averaging 18.49 citations per paper. Further analysis revealed a preference among both domestic and foreign institutions to collaborate with their domestic counterparts (Supplementary Figure 3). Hence, we advocate for enhancing collaboration between domestic and foreign institutions to break academic barriers and foster greater knowledge exchange.

Table 1 The top 20 most productive institutions in the field.

Rank	Institution	Country	Number of studies	Total citations	Average citation
1	Fudan University	China	165	4665	28.27
2	Sun Yat Sen University	China	149	4909	32.95
3	Zhejiang University	China	138	2552	18.49
4	Chinese Academy of Sciences	China	95	2124	22.36
5	Huazhong University of Science and Technology	China	95	1802	18.97
6	Chinese Academy of Medical Sciences-Peking Union Medical College	China	94	1116	11.87
7	Shanghai Jiao Tong University	China	88	1692	19.23
8	Southern Medical University-China	China	84	993	11.82
9	Zhengzhou University	China	81	1422	17.56
10	Nanjing Medical University	China	76	1577	20.75

Analysis of authors and academic journals

Table 2 presents the top 10 journals with the highest production and most citations in this field. Frontiers in immunology leads with 220 papers, accounting for 7.04% of the total publications, followed by Frontiers in Oncology (150 papers, 4.80%), Cancer (131 papers, 4.19%), and International Journal of Molecular Sciences (74 papers, 2.37%). Among these prolific journals, hepatology boasts the highest impact factor (IF) of 14.0. All journals are classified within the Q1 or Q2 zones.

Table 2 The top 10 most productive Journal in the field.

Rank	Journal	Article counts	Percentage (3125)	Impact factor	Quartile in category
1	Frontiers in Immunology	220	7.04	7.3	Q1
2	Frontiers in Oncology	150	4.80	4.7	Q2
3	Cancers	131	4.19	5.2	Q2
4	International Journal of Molecular Sciences	74	2.37	5.6	Q1
5	Frontiers in Genetics	72	2.30	3.7	Q2
6	Frontiers in Cell and Developmental biology	49	1.57	5.5	Q2
7	Aging-us	43	1.38	5.2	Q2
8	Hepatology	41	1.31	14.0	Q1
9	Journal of Hepatocellular Carcinoma	41	1.31	4.1	Q2
10	Journal for Immunotherapy of Cancer	40	1.28	10.9	Q1

According to Supplementary Figure 4 and Supplementary Table 2, the journal with the highest number of co-citations is Cancer (2122 times), followed by Hepatology (1964 times) and Nature (1897 times). Among the top 10 journals with the highest number of joint citations, Nature received 1897 citations, boasting the highest IF. All journals mentioned in the joint citations are classified in either the Q1 or Q2 category.

The thematic distribution of academic publications is illustrated through a double map overlay (Supplementary Figure 5). Coloured tracks indicate citation links, with citing journals displayed on the left and cited journals on the right. Our analysis revealed two primary colored citation paths: Studies published in Molecular Biology and Immunology Journals are predominantly cited by those in Molecular Biology and Genetics, while studies in Medical and Clinical Journals are primarily cited by publications in Molecular Biology and Genetics Journals.

Table 3 presents the top 10 authors based on the number of published papers in this field. Collectively, these authors contributed 196 papers, constituting 6.27% of all publications. Fan J holds the top position with 31 research papers, followed by Zhou J (30) and Zhang J (23). Further analysis reveals that eight of the top 10 authors are affiliated with institutions in China, one is from the United States, and the other is from Japan. The network among these authors is depicted through CiteSpace visualization (Supplementary Figure 6).

Table 3 Author's publications and co-citation.

Rank	Author	Count	Location	Rank	Co-cited author	Citation
1	Fan J	31	China	1	Llovet JM	880
2	Zhou J	30	China	2	Jemal A	491
3	Zhang J	23	China	3	Finn RS	443
4	Li J	18	China	4	EL-Khoueiry AB	432
5	Zheng LM	17	China	5	Sung H	407
6	Kudo M	16	Japan	6	Kudo M	366
7	Wang W	16	China	7	Zhu AX	361
8	Wang Y	16	China	8	Sangro B	337
9	Greten TF	15	United States	9	Hanahan D	331
10	Gao Q	14	China	10	Bruix J	313

Supplementary Figure 7 and Table 3 display the top 10 authors with the highest number of co-citations and citations, respectively. A total of 274 authors have received citations exceeding 50 times, indicating the significant reputation and influence of their research. The primary node is linked to the author, who has the highest number of co-references, including Llovet JM (880 citations), Jemal A (491 citations), and Finn RS (443 citations). Further analysis shows that Kudo M rank 6^th in terms of publication volume and citation count, indicating their status as key figures in this field.

Research hot spot analysis

Most cited publications: A one-year time slice within the 2000 to 2023 horizon revealed a co-cited reference network comprising 1615 nodes and 6997 links (Supplementary Figure 8). Among the top 10 most co-cited articles (Table 4), the article titled "Global Cancer Statistics 2020: GLOBOCAN Estimates of Incidence and Mortality Worldwide for 36 Cancers in 185 Countries" published in CA-A Cancer Journal for Clinicians (IF = 354.7) stood out as the most referenced, with Sung[17] identified as the first author of the article. This article provides an updated overview of the global cancer burden utilizing the GLOBOCAN 2020 estimates of cancer incidence and mortality issued by the International Agency for Research on Cancer[17]. It is estimated that by 2020, there will be approximately 19.3 million new cancer cases (18.1 million excluding non-melanoma skin cancers) and close to 10 million cancer-related deaths (9.9 million excluding non-melanoma skin cancers) worldwide[17]. The projected global cancer burden is expected to reach 28.4 million cases by 2040, marking a 47% increase from 2020. Therefore, it is imperative to establish sustainable infrastructure to improve the distribution of cancer prevention measures and the delivery of cancer care, which is essential for effective global cancer control.

Table 4 List of co cited literature.

Rank	Title	Journal IF (2021)	Author(s)	Total citations
1	Global cancer statistics 2020: GLOBOCAN estimates of incidence and mortality worldwide for 36 cancers in 185 countries	CA-A Cancer Journal for Clinicians (IF = 354.7)	Sung H	393
2	Nivolumab in patients with advanced HCC (CheckMate 040): an open-label, non-comparative, phase 1/2 dose escalation and expansion trial	Lancet (IF = 168.9)	El-Khoueiry AB	348
3	Atezolizumab plus Bevacizumab in Unresectable HCC	New England Journal of Medicine (IF = 158.5)	Finn RS	333
4	Pembrolizumab in patients with advanced HCC previously treated with sorafenib (KEYNOTE-224): a non-randomised, open-label phase 2 trial	Lancet Oncology (IF = 51.1)	Zhu AX	259
5	HCC	Nature Reviews Disease Primers (IF = 81.5)	Llovet JM	246
6	HCC	New England Journal of Medicine (IF = 158.5)	Villanueva A	228
7	Lenvatinib versus sorafenib in first-line treatment of patients with unresectable HCC: A randomised phase 3 non-inferiority trial	Lancet (IF = 168.9)	Kudo M	196
8	A global view of HCC: trends, risk, prevention and management	Nature Reviews Gastroenterology and Hepatology (IF = 65.1)	Yang JD	194
9	Pembrolizumab As Second-Line Therapy in Patients With Advanced HCC in KEYNOTE-240: A Randomized, Double-Blind, Phase III Trial	Journal of Clinical Oncology (IF = 45.4)	Finn RS	186
10	Molecular therapies and precision medicine for HCC	Nature Reviews Clinical Oncology (IF = 78.8)	Llovet JM	162

HCC: Hepatocellular carcinoma; IF: Impact factor.

Keyword analysis: Keyword analysis provides a rapid insight into the current status and future direction of a research field. Based on keyword co-occurrence analysis using VOSwitch, the most frequently identified keywords were tumor microenvironment (799), followed by expression (738), immunotherapy (694), and prognosis (500) (Figure 3, Supplementary Table 3). Irrelevant keywords were filtered out, resulting in a network comprising 175 keywords that appeared at minimum 26 times, forming 5 distinct clusters. The first cluster (in red) consists of 66 keywords, including tumor microenvironment, inflammation, hypoxia, macrophage, fibrosis, DCs, regulatory T-cells, tgf beta, myoid cells, poor diagnosis, immune suppression, autophagy, stem cells, antagonist immunity, exomes, and down regulation. The second group (in green) includes 53 keywords, such as expression, diagnosis, survival, immune, diagnosis, mechanisms, biomarkers, protein, mutations, apoptosis, promotion, growth, gene, nomogram, signature, classification, metastasis, and resistance. The third group contains 43 keywords (in blue), including immunotherapy, T-cells, therapy, pd-l1, blockade, melanoma, nanoparticles, efficacy, pembrolizumab, tumor infiltrating lymphocytes, immune response, and radiofrequency association. The fourth group contains 12 keywords (in yellow), including activation, B-cells, chemotherapy, differentiation, inhibition, mice, polarization, and receiver. The fifth group encompasses one keyword (in purple), including neutral. Additionally, a visual representation of the trends in research hotspots was created using CiteSpace, shown in the volcano map (Supplementary Figure 9).

Figure 3 High frequency keywords. The most frequently identified keywords were tumor microenvironment (799), followed by expression (738), immunotherapy (694), and prognosis (500)

Citation burst analysis: CiteSpace analysis identified the 50 most significant citation bursts in the IME in the HCC field. The 50 references identified were published between 2004 and 2023, reflecting their frequent citation over the past two decades (Supplementary Figure 10). Notably, five of these papers are currently experiencing peak citation periods (Supplementary Figure 11), suggesting sustained interest in IME in HCC research in the future.

Among the 521 key mutated keywords in the field, we focused on the top 50 keywords with the highest mutations (Supplementary Figure 12). These keywords signify the current research focal points in the field and offer potential directions for future research. One of the most cited references was published in the Lancet titled "Nivolumab in patients with advanced HCC (CheckMate 040): an open-label, non-comparative, phase 1/2 dose escalation and expansion trial", first authored by Dr Anthony[18]. While sorafenib remains the only globally approved drug for patients with advanced HCC, its efficacy is still limited. The study demonstrated the safety and efficacy of nivolumab, an immune checkpoint inhibitor targeting the programmed cell death protein-1 (PD-1), in patients with advanced HCC, regardless of the presence of chronic viral hepatitis[18].

DISCUSSION

This study highlights a growing trend in the volume of articles related to IME in HCC in recent decades, suggesting increased interest in the field. The keyword immunotherapy emerges as among the most commonly used keywords, underscoring a predominant focus on clinically oriented research within the context of IME in HCC. Furthermore, the clinical relevance of immunotherapy has notably escalated in recent years.

In our bibliometric study, we note that the leading countries in terms of publication volume (China, the United States, and Italy) account for over 85% of the IME of HCC-related publications. HCC is ranked as the sixth most common tumor and the third leading cause of cancer deaths[19]. Publications from China were the highest for HCC, reflecting the key role of Chinese researchers in the field. This phenomenon may be attributed to the higher prevalence of HCC in Asia compared to Europe and the increasing emphasis on cancer research in developing countries[17,20]. More than half of the world's confirmed cases of HCC occur in China[21]. In addition, in China, the evaluation system of academic work emphasizes quantity over quality[22]. As a result of this system, Chinese scholars focus too much on the number and speed of publications rather than the quality of their work. However, in recent years, with the increase in research funding and energy inputs, the research level of Chinese scholars is expected to develop rapidly, and it is feasible to keep pace with the level of certain developed countries. HCC is also the fastest growing cause of cancer deaths in United States [23] and a major public safety issue in the Asia-Pacific region[24]. This phenomenon may cause increasing emphasis on cancer research in developed countries[23,24]. The United States is in a leadership position and represents the global frontier. Despite its strong economic power, its achievements have been aided by significant investments in healthcare. The observed trend supports the notion that significant government financial investment in research correlates with academic productivity[25]. For example, United States has the highest annual health expenditure of $10202 per resident in 2020, more than any other country[26], with the second highest published figure.

Despite China producing nearly four times as many publications as the United States, Chinese publications only rank 10^th in citation frequency, significantly trailing behind the United States, indicating that the overall quality of papers from China is relatively low. The study of authors and institutions revealed that Chinese authors and institutions demonstrated the highest levels of productivity. Significantly, Chinese researchers have made substantial contributions in terms of effort and output to the IME field in HCC. However, there remains considerable scope for enhancing the impact of this research.

The journal analysis is beneficial for researchers seeking to identify compelling research topics and choose suitable journals for submitting IME or HCC-related papers. The journals Frontiers in Immunology, Frontiers in Oncology, and Cancer emerged as the most productive journals, surpassing other journals in terms of the volume of articles published. Our analysis revealed no significant correlations among the number of publications, IF values, and journal citation reports values for the journals examined, suggesting distinct preference tendencies among them. This diversity in journal preferences provides researchers with an expanded range of options for selecting the most appropriate journals that align with the specific subtypes of their research.

TMEs and tumour immunotherapy have emerged as focal points in cancer research over the past few decades[19]. Recent studies have highlighted that targeting TMEs may represent a more effective approach than solely focusing on treating tumor cells, addressing the challenge of tumor heterogeneity and diversity[27]. Immune cells in the TIMEs of HCC provide new targets for advanced immunotherapy strategies[28]. Current standard treatments for HCC primarily rely on ICIs targeting CTLA-4 and PD-1/PD-L1, with emerging therapies like CAR-T cells, TCR-T cells, and therapeutic vaccines rapidly evolving[29,30]. Although the response rate to ICIs in HCC patients remains modest, advancements in our comprehension of the interplay between ICIs and both adaptive and innate immune responses are expected to lead to the emergence of novel therapeutic approaches. Numerous strategies have been devised to optimize the integration of ICIs with other primary treatment modalities, such as chemotherapy, localized treatments, and targeted therapies. These innovative therapies have demonstrated efficacy in inducing remission and extending the survival of HCC patients[29,30]. In the future, precision medicine holds the promise of revolutionizing the management of HCC.

CONCLUSION

Overall, the relationship between HCC and IME is receiving increasing attention, and related research is in a highly developed stage. China has emerged as the leading contributor in terms of productivity, boasting numerous prolific authors and institutions in the field. Conversely, the United States demonstrates the highest citation frequency, underscoring the impact of American research efforts. Notably, the most prolific journal articles published in this domain are Frontiers in Oncology and Frontiers in Immunology. Currently, the primary focus of research in this field centers around exploring IME-based immunotherapy for HCC. Our researchers have conducted an in-depth analysis of TIME and have identified immune checkpoint-based immunotherapy, precision immunotherapy, and immune cellular modalities as the forefront and key priorities for future research endeavors. These identified areas hold promise for valuable research opportunities in the coming years.