Global research status of the impact of neutrophil extracellular traps on tumor from 2004 to 2023: A bibliometric and visualized analysis

Abstract

Background:

Tumors are a major threat to human life and health. Neutrophil extracellular traps (NETs) have become a research focus in this context, especially regarding their role in tumor progression. Since the concept of NETs was introduced in 2004, its implications for tumor research have attracted significant scholarly attention. This study aims to explore research trends and cutting-edge hotspots in NETs and tumors through bibliometric analysis and provide new ideas for clinical applications.

Methods:

We searched for literature on NETs and tumors published between 2004 and 2023 using the Web of Science database. Microsoft Excel 2019 was used for statistical analysis of influential articles, journals, authors, organizations, countries, and co-cited references. VOSviewer (version 1.6.16) and CiteSpace (V.5.8.R3) were employed for visualizing research data.

Results:

The analysis covered 790 articles authored by 4768 individuals from 1134 organizations in 56 countries. China and the United States are the leading contributors. The mechanism of NETs in tumor occurrence and development is likely linked to coagulation, inflammation, and infection. Hot topics in research include dendritic cells and thrombosis, with a shift from laboratory studies to clinical applications, suggesting a growing focus on treatment over etiology.

Conclusion:

This study offers the most comprehensive bibliometric analysis of NETs and tumors to date. Future research may focus on developing targeted therapies that block the interaction between NETs and tumors, offering a new direction for cancer treatment.

1. Introduction

Neutrophils, are the most abundant leukocytes in peripheral blood, accounting for 50% to 70% of all human leukocyte.^[1,2] As forerunners of the innate immune system, neutrophils are involved in regulating multiple acute and chronic process of immunity,^[3] injury and repair,^[4] infection,^[5] inflammation,^[6] and tumor^[7] responses through multiple mechanisms, including phagocytosis,^[4] oxidative burst,^[8] degranulation,^[7] and neutrophil extracellular traps (NETs).^[5] As the body’s 1st line of defense against foreign invaders, neutrophils possess powerful phagocytosis capabilities, 1 is the direct phagocytosis of pathogens under normal physiological conditions of neutrophils, and the other is the formation of NETs after necrosis or apoptosis of neutrophils, which continue to carry out immune maintenance. Chromatin and proteins are the main components of NETs, and nuclear DNA or mitochondrial DNA fibers make up the reticular structure of NETs, decorated with enzymes and histones that have anti-microbial and cytotoxic effects and trap and kill pathogens by releasing these enzymes and proteins to the outside of the cell.^[9,10] However, this special immune regulatory effect is a double-edged sword in the fight against cancer. There is growing evidence that NETs promote cancer metastasis and development, including promoting chemokines to attract cancer cells,^[11] aggravating hypercoagulable state in the blood and promoting thrombosis,^[12] awakening dormant cancer cells,^[13] and inducing tumor epithelial–mesenchymal transition to enhance their invasive capacity,^[1] rather than simply acting as “traps” for them. In the traditional sense, the immune system mainly recognizes and eliminates cancerous cells through immune surveillance function. However, Schreiber and Dunn et al also discovered that the immune system reshaped the immunity of tumor cells in addition to playing an active role in tumor killing, allowing weaker immunogenic cells to grow, leading to the growth of tumor cells.^[14]

Since 2004, the concept of NETs has been proposed for 20 years. With the in-depth study of NETs, the investigation of their association with tumors has become a popular topic, but there are still some inconsistencies in the findings, and there is a lack of quantitative review of the existing literature. Despite the proliferation of reviews on NETs and tumors, the depth of discussion is relatively limited and the results of the discussion are inevitably influenced by the subjectivity of the investigator. Therefore, many scholars’ studies on NETs and tumors are generally limited to comprehensive readings of the literature and summaries of personal clinical experiences, lacking the necessary completeness and macroscopicity. Bibliometrics is a new method for quantifying comprehensive research results, providing a valuable overview and visual analysis of existing academic literature, identifying research hotspots by reviewing previous research, and quantitatively and objectively assessing development trends.^[15–18] With the proliferation of online databases and the development of analytical softwares, bibliometrics has gained widespread attention. However, bibliometric studies related to NETs and tumors are rarely addressed at present; therefore, it is necessary to assess the current status of NETs and tumors research.

Based on this, we systematically surveyed research results on NETs and tumors up to December 31, 2023, to reveal current research priorities and frontiers. Using a bibliometric approach, we have compiled a list of highly cited papers and references in this field; high productive individuals among authors, journals, countries, and organizations were enumerated by descriptive analysis; collaboration between authors, between countries, and between organizations were described by visual analysis; and a comprehensive analysis of keywords was conducted. The purpose of this study is to determine the research focus and core hotspots in this field, predict future development trends, bring reference ideas for studying the mechanisms of tumor occurrence, development, and cell apoptosis, and also provide new scientific basis for early clinical diagnosis and treatment.

2. Materials and methods

2.1. Sources of the data and search strategy

As of December 31, 2023, the papers included in this study were searched through Web of Science (WOS) core collection. WOS is the world’s leading platform for scientific research and citation data, it covers all disciplines and resources, from journals and books to patents and litigation, with more than 12,000 high-quality scientific journals that are very reliable.^[19,20] It has been accepted by many researchers as a high-quality digital literature resource database, and several studies have shown that the WOS database is the most suitable database for bibliometric analysis.^[21,22] In order to ensure the comprehensiveness and accuracy of the retrieved data, the index was selected as SCI-EXPANDED. We hired a librarian and was involved in the search terms and search strategy creation. At the same time, considering the many ways in which NETs and tumors are described, the final search strategy is as follows: #1:TS = (Extracellular Trap* OR Extracellular DNA Trap* OR Eosinophil Extracellular Trap* OR Eosinophil Extracellular DNA Trap* OR Neutrophil Extracellular Trap*). #2:TS = (Tumor* OR Neoplasm* OR Neoplasia* OR Cancer* OR Malignancy OR Malignancies OR Malignant Neoplasm* OR Benign Neoplasm*). Final dataset: #1 AND #2. To capture as many data sources as possible, the wildcard character (*) that could be substituted for any other characters and allows variable endings of keywords was used. For example, “Tumor*” would also return the terms of “Tumor” and “Tumors.”^[23] The publication date was from January 01, 2004 to December 31, 2023. The document types were restricted to original articles and reviews and only papers published in English were retrieved. According to the search strategy, we retrieved 1746 documents and ultimately retained 790 documents related to the research topic. Data were downloaded from WOS in “plain text” format with “full records and cited references.” In addition, since this study did not include any animals or experiments, no ethical consent was required. Two authors (Bangjie Chen and Yafen Wang) searched and screened independently. If there was disagreement among them, discussion or recourse was made to the 3rd author (Xinyi Wang). The literature screening process for this study were shown in Figure 1. Furthermore, as this study did not involve any clinical trials or animal experiments, no ethical approval was required.

Figure 1.

Flow diagram of screening process related to NETs and tumors. NETs = neutrophil extracellular traps.

2.2. Data analysis and bibliometric software

In descriptive analysis, we used Microsoft Excel 2019 (Microsoft Corporation, Redmond) to statistically analyze the top 10 influential articles, journals, authors, organizations, countries, and references in the research field, and plotted an annual publications and citations trend chart and fit a curve to them. In visual analysis, VOSviewer (version 1.6.16; Centre for Science and Technology Studies [CWTS], Leiden University, Leiden, The Netherlands) and CiteSpace (V.5.8. R3; Drexel University, Philadelphia) were utilized. We used VOSviewer software to screen and sort the data and extract the top ranked results. At the same time, VOSviewer can also provide cluster analysis, build a visual network through the connection between nodes, manufacture the network visualization map and overlay visualization map of the influential authors, countries, and organizations, as well as collaborative networks for clustering analysis of high-frequency keywords and highly cited references. Visual images are mainly composed of nodes of different colors and sizes and lines between nodes.^[24,25] Different nodes can represent authors, journals, organizations, and other elements; the lines between the nodes indicate the relationship between the elements, the thickness of the line represents the strength of linkages, the thicker the line, the closer the connection. The size of the node is related to the number of times the node appears, the more times it appears, the bigger it is; the color of the node is determined by the legend of the specific picture, which represents different meanings.

Co-cited authors and references literature, keywords co-occurrence analysis are visualized by VOSviewer. A co-citation network is 1 in which 2 entries appear simultaneously in the reference of a 3rd entry, while a co-occurrence network is 1 in which the relationship between entries is based on the number of publications in which they co-occur.

On the other hand, CiteSpace compared the research fields of documents and references through the dual-map overlay analysis of journals. Through time line and clustering view analysis and burst plot of keywords, it clarified the trend of research hotspots in specific time and research fields.

3. Results

3.1. General distribution characteristics analysis

The 790 papers used in this study were from 4768 authors from 1134 organizations in 56 countries, published in 317 source journals, and cited 46,832 literature from 3924 journals.

3.1.1. Analysis of growth trends of publications

Although we set the starting point for literature search to 2004, it was not until 2010 that articles that met the requirements appeared (Fig. 2). The number of articles published on the study of NETs and tumors has increased every year from 2010 to 2023. In 2014, 13 papers were published, which was 2.2 times more than the number of papers published in 2013. In 2023, the number of articles reached 190 articles. In terms of the growth rate, we can roughly divide it into 3 stages: 2010 to 2013 (early stage, 3.25 articles per year), 2014 to 2018 (development period, 30.8 articles per year), and 2019 to 2023 (blowout period, 124.6 articles per year). The overall upward trend in citations from 2014 to 2020 is related to the year-on-year increase in the number of articles issued. The highest number of citations appeared in 2018 with 6362 citations, and this was followed by 5534 in 2020 and 4769 in 2016. The decline in citations in recent years is mainly due to the fact that the impact of recently published literature is still low and needs time to improve. Table 1 shown top 10 cited documents for the study of NETs and cancers. The top 3 documents all have more than 1000 citations, and all 10 are all located in the Q1 division, except for the last 1.

Figure 2.

The annual number of published NETs and tumors studies, 2004 to 2023. NETs = neutrophil extracellular traps.

Table 1.

Top 10 cited documents for the study of NETs and cancers.

Rank	Title	Citations	Journal	IF*	Quartile in category	Author	Year
1	Neutrophils in the activation and regulation of innate and adaptive immunity	1967	Nature Reviews Immunology	100.3	Q1	Mantovani	2011
2	Neutrophil extracellular traps in immunity and disease	1494	Nature Reviews Immunology	100.3	Q1	Papayannopoulos	2018
3	Roles of the immune system in cancer: from tumor initiation to metastatic progression	1033	Genes & Development	10.5	Q1	Gonzalez	2018
4	Neutrophil extracellular traps sequester circulating tumor cells and promote metastasis	808	Journal of Clinical Investigation	15.9	Q1	Cools-Lartigue	2013
5	Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice	773	Science	56.9	Q1	Albrengues	2018
6	An emerging role for neutrophil extracellular traps in noninfectious disease	723	Nature Medicine	82.9	Q1	Jorch	2017
7	Cancers predispose neutrophils to release extracellular DNA traps that contribute to cancer-associated thrombosis	604	Proceedings of the National Academy of Sciences of the United States of America	11.1	Q1	Demers	2012
8	Neutrophil-mediated anticancer drug delivery for suppression of postoperative malignant glioma recurrence	583	Nature Nanotechnology	38.3	Q1	Xue	2017
9	Cancer cells induce metastasis-supporting neutrophil extracellular DNA traps	532	Science Translational Medicine	17.1	Q1	Park	2016
10	Tumor-Associated Macrophages and Neutrophils in Tumor Microenvironment	527	Mediators of Inflammation	4.6	Q2	Kim	2016

NETs = neutrophil extracellular traps.

^*The impact factors (IF) of journals were obtained from the 2022 Web of Science Journal Citation Reports (JCR).

3.1.2. Analysis of distribution of journals

Dual-map overlay analysis of journals can be used to reveal the distribution of citing and cited journals, the map on the left is the distribution of disciplines of citing journals, and the map on the right is the distribution of disciplines of cited journals, each point represents a journal, and each line represents a citation relationship, and the thicker the line, the greater the interconnection. The spatial relationship between citing and cited journals is that of a flow from the accumulation of basic knowledge on the right to the application of cutting-edge knowledge on the left. As can be seen in Figure 3, the citing journals for studies related to NETs and tumors are mainly related to medicine, clinical, molecular, biology, immunology, neurology, sports, and ophthalmology, and the cited articles are mainly published in journals in the fields of health, nursing, medicine, molecular, biology, genetics. Table 2 ranks the 10 most influential journals related to this field. The top 3 cited journals were Nature Reviews Immunology, Frontiers in Immunology and Cancers, respectively.

Figure 3.

The dual-map overlay of NETs and tumors research-related journals. NETs = neutrophil extracellular traps.

Table 2.

Top 10 most influential source journals for the study of NETs and cancer.

Rank	Journal	Documents	Citations	Country	IF*	Quartile in category	Average citation
1	Nature Reviews Immunology	4	3730	UK	100.3	Q1	932.5
2	Frontiers in Immunology	68	1853	Switzerland	7.3	Q1	27.3
3	Cancers	31	1067	Switzerland	5.2	Q2	34.4
4	Cancer Research	11	1059	USA	11.2	Q1	96.3
5	Genes & Development	1	1033	USA	10.5	Q1	1033
6	Journal of Clinical Investigation	2	1020	USA	15.9	Q1	510
7	Nature Reviews Cancer	3	978	UK	78.5	Q1	326
8	Blood	5	960	USA	20.3	Q1	192
9	Proceedings of the National Academy of Sciences of the United States of America	3	833	USA	11.1	Q1	277.7
10	Science Translational Medicine	3	800	USA	17.1	Q1	266.7

NETs = neutrophil extracellular traps.

^*The impact factors (IF) of journals were obtained from the 2022 Web of Science Journal Citation Reports (JCR).

3.2. Co-authorship analysis

3.2.1. Co-authorship analysis of the excellent authors

Fifty-nine authors had 5 or more publications in the field, and in the collaborative analysis, and 56 authors were connected to each other. The network visualization map shown Tsung, Allan and Wang, Yanming had the most coauthors (Fig. 4A). The overlay visualization map shown the temporal co-authorship of 56 authors (Fig. 4B). Among them, Li, Jiacheng, Wang, Yuji and Yang, Shifeng were the most interested scholar in this field in recent years. Meanwhile, we listed the top 10 most influential authors (Table 3). In terms of publication, Tsung, Allan have 14 documents, ranking 1st, followed by Mackman, Nigel and Huang, Hai, all of whom have published more than 10 articles.

Figure 4.

Co-authorship analysis of the influential authors in the field of NETs and tumors. (A) Network visualization map of collaborations among the 1st 197 authors. (B) Overlay visualization map of of collaborations among the 1st 197 authors. NETs = neutrophil extracellular traps.

Table 3.

Top 10 most influential authors for the study of NETs and cancer.

Rank	Author	Documents	Citations	Average citation
1	Tsung, Allan	14	1224	87.4
2	Mackman, Nigel	12	777	64.8
3	Huang, Hai	11	1145	104.1
4	Tohme, Samer	10	954	95.4
4	Thalin, Charlotte	10	649	64.9
6	Zhang, Hongji	9	716	79.6
6	Spicer, Jonathan D.	9	517	57.4
8	Wagner, Denisa D.	8	1586	198.3
8	Yazdani, Hamza O.	8	912	114
8	Hisada, Yohei	8	641	80.1
8	Wallen, Hakan	8	621	77.6
8	Boone, Brian A.	8	382	47.8
8	Varricchi, Gilda	8	320	40

NETs = neutrophil extracellular traps.

3.2.2. Co-authorship analysis of the active institutions

As shown in Figure 5A, there were 70 institutions that have reached the point of publishing 5 articles, 59 of which have collaborative relationships with each other. Harvard Medical School in the United States is at the center of both the number of articles published and institutional collaborations, followed by Mcgill University. Both of them are academically engaged with each other and forming their own influential institutions collaborations. Southern Medical University, Jiamusi University, and Chinese People’s Liberation Army General Hospital are the most popular institutions in recent years, all belonging to China (Fig. 5B). It is worth noting that in recent years, Chinese institutions have conducted more research on NETs and the field of cancer, while many institutions in the United States and Europe have mainly focused on the research boom in this field before 2021. Among numerous institutions, Harvard Medical School, McGill University, and the University of Pittsburgh all ranked 1st in terms of publication volume, with 21 articles each (Table 4).

Figure 5.

Co-authorship analysis of the influential institutions in the field of NETs and tumors. (A) Network visualization map of collaborations among the 1st 70 institutions. (B) Network visualization map of collaborations among the 1st 70 institutions. NETs = neutrophil extracellular traps.

Table 4.

Top 10 most influential organizations for the study of NETs and cancer.

Rank	Organization	Country	Documents	Citations	Average citation
1	Harvard Medical School	USA	21	3366	160.3
1	McGill University	Canada	21	1836	87.4
1	University of Pittsburgh	USA	21	1579	75.2
4	Fudan University	China	20	701	35.1
5	Harbin Medical University	China	18	730	40.6
6	Shanghai Jiao Tong University	China	16	552	34.5
6	Chinese Academy of Medical Sciences & Peking Union Medical College	China	16	159	9.9
8	Karolinska Institute	Sweden	15	1041	69.4
8	Sun Yat-sen University	China	15	1003	66.9
8	The Ohio State University	USA	15	601	40.1

NETs = neutrophil extracellular traps.

3.2.3. Co-authorship analysis of the influential countries

In total, VOSviewer identified 31 countries with more than 5 publications and have collaborative relationships with each other. As shown in Figure 6A, the United States is the most extensive collaborator with other countries, and China is the most prolific country. Germany also leading the way in terms of publications and collaboration. Chronologically, China, Saudi Arabia, Belgium, and Spain are the countries where research on NETs and tumors has been more connected to other countries in recent years (Fig. 6B). In earlier times, Canada, Sweden, and Israel have more external collaborations for research on this topic. Figure 6A and B shown more visually the distribution of documents and cooperation among countries from the temporal and spatial level, presenting a situation of regional concentration and overall decentralization on the flat map. In addition, we also counted the top 10 countries in terms of citations (Table 5). The top 3 are China, the United States and Germany in terms of publication.

Figure 6.

Co-authorship analysis of the influential countries in the field of NETs and tumors. (A) Network visualization map of collaborations among the 1st 21 countries. (B) Network visualization map of collaborations among the 1st 21 countries. NETs = neutrophil extracellular traps.

Table 5.

Top 10 most influential countries for the study of NETs and cancer.

Rank	Country	Documents	Citations	Average citation
1	China	261	7093	27.2
2	USA	216	14,428	66.8
3	Germany	65	3853	59.3
4	Italy	55	4958	90.1
5	Canada	45	4358	96.8
6	UK	36	4516	125.4
7	Japan	34	720	21.2
8	Sweden	28	2729	97.5
9	France	27	1489	55.1
10	Spain	24	1301	54.2

NETs = neutrophil extracellular traps.

3.3. Co-occurrence analysis of keywords

In the fields of NETs and cancer, we selected keywords with a frequency of 30 or more occurrences for clustering, and a total of 41 high-frequency keywords were selected and divided into 3 categories (Fig. 7A): Cluster 1 (red): cellular biochemistry, includes metastasis, cells, expression and netosis. Cluster 2 (green): tumor mechanisms, includes neutrophils, extracellular traps, tumor microenvironment, and suppressor cells. Cluster 3 (blue): NETs induced diseases, including NETs, cancer, inflammation and thrombosis. Time line and clustering view map of all of keywords is shown in Figure 7B, they can be roughly divided into 11 categories: innate immunity (#0), thrombosis (#1), dna traps (#2), breast cancer (#3), suppressor cells (#4), lung cancer (#5), dendritic cells (#6), myeloproliferative neoplasms (#7), cells(#8), extracellular trap formation(#9), liver metastasis(#10). Among them, research on innate immunity and dendritic cells has been last for several years; while research on lung cancer has only emerged in recent years. Figure 7C shown the 25 keywords with the strongest citation bursts from 2010 to 2023.

Figure 7.

The visualization analysis of co-occurrence keywords in the field of NETs and tumors. (A) Network visualization map of 101 high-frequency keywords. (B) Time line and clustering view map of all of keywords. (C) Twenty-five keywords with the strongest citation bursts. NETs = neutrophil extracellular traps.

3.4. Co-citation analysis of co-cited references

Figure 8 shown the 45 references with the highest number of citations, with citations greater than or equal to 70. In the network visualization map, it can be seen that most of the cited references are from top journals, mainly involving the disciplines of immunology and oncology. Subsequently, we listed the top 10 cited references (Table 6). Brinkmann et al neurophil extracellular traps kill bacteria, published in Science in 2004, was cited 429 times, ranking 1st.^[9] This article 1st introduces the concept of NETs and reveals the role in infection and immunity and provides the initial theoretical basis for the research on NETs and tumors in the next 20 years. The co-cited references to some extent also reflected the development trends and research hotspots of the research of NETs and tumor, revealing the popular high scoring articles in this field.^[26]

Figure 8.

The visualization analysis of co-cited references in the field of NETs and tumors. NETs = neutrophil extracellular traps.

Table 6.

Top 10 co-citation references for the study of NETs and cancer.

Rank	Title	Journal	IF*	Quartile in category	Author	Year	Citations
1	Neutrophil extracellular traps kill bacteria	Science	56.9	Q1	Brinkmann	2004	429
2	Neutrophil extracellular traps sequester circulating tumor cells and promote metastasis	Journal of Clinical Investigation	15.9	Q1	Cools-Lartigue	2013	303
3	Cancers predispose neutrophils to release extracellular DNA traps that contribute to cancer-associated thrombosis	Proceedings of the National Academy of Sciences of the United States of America	11.1	Q1	Demers	2012	236
4	Cancer cells induce metastasis-supporting neutrophil extracellular DNA traps	Science Translational Medicine	17.1	Q1	Park	2016	230
5	Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice	Science	56.9	Q1	Albrengues	2018	227
6	Neutrophil Extracellular Traps Promote the Development and Progression of Liver Metastases after Surgical Stress	Cancer Research	11.2	Q1	Tohme	2016	213
7	Neutrophil extracellular traps in immunity and disease	Nature Reviews Immunology	100.3	Q1	Papayannopoulos	2018	183
8	DNA of neutrophil extracellular traps promotes cancer metastasis via CCDC25	Nature	64.8	Q1	Yang	2020	172
9	Polarization of Tumor-Associated Neutrophil Phenotype by TGF-β: “N1” vs “N2” TAN	Cancer Cell	50.3	Q1	Fridlender	2009	160
10	Novel cell death program leads to neutrophil extracellular traps	Journal of Cell Biology	7.8	Q1	Fuchs	2007	160

NETs = neutrophil extracellular traps.

4. Discussion

Bibliometrics can help predict future research directions and trends, which is one of the widely used methods for researchers to deal with quantitative and qualitative data on the biomedical field. Considering that the concept of NETs was only formalized in 2004, we performed a bibliometric analysis of paper about NETs and tumor published over a 20-year period from 2004 to 2023 using the WOS database, and found out the collaborations, hotspot changes, and some breakthroughs by comparing and analyzing all documents in the related fields in the past 20 years. Although the relationship between NETs and tumors has been previously discussed in the relevant literature, none of it is sufficiently in-depth and objective and lacks the necessary completeness and macroscopicity. Our study provided theoretical support for the question of whether we can think about prevention, diagnosis, and treatment of malignant tumors in the future at the level of neutrophils and NETs, as well as helped to guide the clinicians to understand tumors from a new perspective.

4.1. Global contribution in the research

The research of NETs and tumors can be roughly divided into 3 stages starting from 2010, with a steady increase in the number of publications. It is not difficult to see from the dual-map overlay analysis of journals that, the research involves both basic research and clinical disciplines, and multidisciplinary management and interprofessional collaboration is the trend of the future. Besides, win–win cooperation has become a mainstream trend in current academic research. We found that the United States are the leading country in this field, with a much higher number of (average) citations and a large number of top journals and institutions. In 2016, Allan Tsung from the University of Pittsburgh published an article titled “Neutral Extracellular Traps Promote the Development and Progression of Life Metastases after Surgical Stress” in the Cancer Research,^[27] they noted that cancer recurrence after surgical removal of the tumor is associated with the formation of NETs, which may involve multiple mechanisms such as stress and inflammation. Allan Tsung has proposed in multiple articles that surgical stress is an important part of NETs promoting tumor development, and has elaborated on the immune changes in the body after malignant tumor surgery.^[28–31] In recent years, Yang Shifeng, who has been active in this field, deeply investigated the relationship between NETs and gastric cancer. His studies have confirmed that NETs can promote gastric cancer angiogenesis,^[32] increase the risk of tumor-associated thrombosis in gastric cancer patients^[33] as well as promote gastric cancer metastasis by initiating COX-2 via Toll-like receptor 2.^[34] From the connection between the nodes, the United States and China both attach great importance to academic exchanges, and the communication between authors, institutions and countries is very close, which explained to a certain extent why they have a greater quantitative output and higher quality. At the same time, the fact that all top 10 most influential journals and top 3 institutions for the study of NETs and cancer are from developed countries also showed that a stable social structure and an inclusive academic environment are more likely to breed innovative ideas, while sufficient funds and perfect equipment are the material basis to ensure the smooth progress of research.^[35] However, most of the current research is still conducted in smaller groups, emphasizing individual contributions, and has not yet formed a larger academic community. In order to break through the concentration and unevenness within the academic field, we should actively organize academic conferences, only communication and cooperation are powerful catalysts for change and innovation, and will lead to a faster renewal and flow of knowledge.

4.2. Focus in the research

The development of scientific research is always changing rapidly, and this is also true in the field of NETs and tumors. Figure 7C showed that circulating tumor cells and hepatocellular carcinoma have became hot topics since 2020, which means the research is further moving towards tumors. Dendritic cells, innate immunity, and deep vein thrombosis used to be hot topics (Fig. 7C), we can conclude that the research direction is gradually shifting towards clinical environments related to specific diseases, from mechanism research to more practical clinical issues. In other words, the real hot topic is currently developing targeted drugs that block the connection between NETs and tumors for tumor therapy in this field.

It is well known that neutrophils are an important part of the innate immune system, in a healthy, stable state, neutrophils have a short circulating half-life, typically 6 to 8 hours in humans and mice.^[36–38] For a long time, the role of neutrophils was limited to the traditional degranulation and phagocytosis to mediate antimicrobial activity. As early as 1996 Takei et al^[39] found for the 1st time that neutrophils stimulated by phorbol myristate acetate showed a special death morphology completely different from typical apoptosis or necrosis. It was not until 2004 that Brinkmann et al^[9] proposed a novel cell death mode: NETosis, which is the process of formation of NETs.

Although NETs are another way for neutrophils to resist foreign invasion, more and more evidence suggested that NETs are a double-edged sword, related not only to sterilization but also to tissue damage. There are times when MPO, neutrophil elastase, extracellular DNA, and other components of NETs lead to inflammatory responses and a variety of other diseases such as SLE,^[40] IBD,^[41] sepsis,^[42] thrombosis,^[43] and tumors.^[44] According to reports, the 2nd leading cause of death in cancer patients is cancer-related thrombosis.^[45] Tumors can cause blood to be in a hypercoagulable state, and this mechanism is not yet clear. Interestingly, NETs also linked to thrombosis-related diseases.^[46] The negative charge of NETs themselves can activate FXII factor and thus initiate the endogenous coagulation pathway^[47]; tissue factor (TF) on NETs can activate the exogenous coagulation pathway^[48]; and C3b attached to NETs can stimulate the aggregation of platelets,^[49] which can all promote the formation of thrombus. And as one of the components of coagulation initiating factors, TF not only plays an important role in coagulation, but also had been found the expression of TF was elevated in tumor cells and correlated with poor prognosis.^[50] In tumor patients, cancer cells can promote the formation of NETs by releasing cytokines such as IL-8 and P-selectin and activating platelets, even leading to excessive NETs formation.^[12] On the other hand, tumor development and progression is usually accompanied by an inflammatory response.^[51] Neutrophilia is an important marker of infection and a source of excess NETs. The formation of excessive NETs triggers a series of inflammatory responses, inducing a significant increase in pro-inflammatory cytokines such as IL-6, IL-8, and TNF-α; at the same time, these inflammatory mediators can regulate neutrophil activity and induce the expression of chemoattractants, and this positive regulation leads to uncontrollable progressive inflammation, which is characterized by cytokine storm.^[52] And in extremely immunocompromised patients with advanced tumors, cytokine storms leading to acute lung injury and acute respiratory distress syndrome are often the fatal final blow.^[53] Therefore, NETs may be a risk factor for tumors that lead to a very poor tumor prognosis. However, some scholars believe that NETs can induce T cells to form cytotoxic T cells through adaptive immunity, clearing tumor cells, and improving prognosis.^[54,55] It is worth noting that among the keyword bursts, “risk factor” was once the hottest topic during 2014 to 2016, which suggests to some extent that NETs may not be a protective factor for tumors.

4.3. Tendency in the research

As the relationship between NETs and cancer becomes clear, a new chapter of NETs as a therapeutic target for malignant tumors^[56] has been opened. Both methods have been shown to improve cancer prognosis, whether it is blocking the formation pathway of NETs (knocking out the PAD4 gene,^[27] using IL-8 and its receptor CXCR1 and CXCR2 inhibitors^[57]), or directly disrupting NETs (using DNase I or NETs related antibodies^[58]). However, NETs themselves also have the effect of killing bacteria and viruses,^[59] and changing the number of NETs may produce a series of cascading reactions that occur in vivo, with subsequent problems brought on by up-regulation and decline in cytokines that are uncontrollable. In comparison, disrupting the mediators of action between NETs and malignant tumors may be a more promising direction.

As one of the mediators, MMP-9 is closely related to the proliferation and metastasis of tumor cells. It can promote the proliferation of dormant tumor cells,^[13] degrade extracellular matrix to promote metastasis,^[60] inhibit the apoptosis of tumor cells,^[61] induce tumor angiogenesis^[62] and other mechanisms to improve the prognosis of cancer. As early as 2006, Hiroaki Nozawa et al found that 80% of tumor cells in animal models was killed by MMP9 inhibitors.^[63] Moreover, it is expressed on neutrophils specifically and has high tissue specificity. However, those developed MMP-9 inhibitors had low selectivity,^[64] so they did not play a role effectively in clinical trials. But it is encouraging that Qiu Ju Han et al^[65] have developed a special nanoparticle that can target drugs to cancer cells and bind specifically to MMP-9, which has worked well in mouse models.

In addition to the adverse effects on cancer, NETs are sometimes our “friendly forces.” Experiments have also shown that NETs can inhibit the proliferation of colon cancer cells,^[66] head and neck squamous cell carcinoma,^[67] and melanoma cells,^[68] which reminded us that we also need to focus on whether there are differences in the relationship between NETs and different cancers. Although various targeted therapies have achieved good results at present, animal trials are still the main ones. In the future, relevant mechanisms should be further clarified, clinical trials should be carried out to evaluate clinical effects, and the balance between the formation and destruction of NETs^[44] should be explored to minimize the negative impact on immune function and improve the safety of tumor drugs, so as to provide references for cancer diagnosis and treatment.

5. Strengths and limitations

There are some advantages of our study. First, this is a rare article that investigates the relationship between NETs and tumors through bibliometrics and analyzes future research trends, and collected the vast majority of articles from the past 20 years since the introduction of NETs, with comprehensive research and a large scale. Second, bibliometrics as a methodology is time-saving, efficient, and economical, avoiding the significant amount of manpower and resources required in normal experiments. In addition, the published literature has a higher degree of frankness and authenticity, and the credibility of the research results is greater.

Our study has several drawbacks as well. First, although WOS includes articles from the vast majority of publications, it is inevitable that some of the literature will be missing, and, as we selected only the English language literature in our screening, it will also be missing. Second, the literature itself is subject to publication bias, with positive results being more likely to be published than negative results that are not statistically significant. Third, the literature search in this study may not be comprehensive enough, because the search formula did not include the specific relationship between NETosis and tumor proliferation, such as “metastasis” and “proliferation.” At the same time, the search formula is not precise to all types of cancer types, whereas most of the existing research literature is specific to tumor types or even subtypes as well as specific cancer pathways, and thus may significantly bias the results of the literature search. In addition, some recently published breakthrough papers may currently have less impact than previous papers and will need to be tested over time.

6. Conclusion

This study provides a visual analysis of research about NETs and tumors, and it is the most detailed and comprehensive bibliometric study in this field to date. The United States has had the most significant impact in this field. Developing countries wishing to have a higher level of creativity need to further collaborate with advanced institutions and countries. As a whole, the effects of NETs on tumors are mostly to promote tumor hypercoagulability and exacerbate inflammatory responses, and are not protective factors for tumors. Blocking the link between NETs and tumors may be one of the ideas for future tumor treatment.

Author contributions

Conceptualization: Kangsheng Gu, Bangjie Chen, Yiwen Jia.

Data curation: Xinyi Wang, Yafen Wang, Yipin Yang.

Formal analysis: Xinyi Wang, Yafen Wang, Yipin Yang.

Funding acquisition: Kangsheng Gu, Bangjie Chen.

Investigation: Xinyi Wang, Yafen Wang, Yipin Yang.

Project administration: Kangsheng Gu, Bangjie Chen, Yiwen Jia.

Supervision: Kangsheng Gu, Bangjie Chen, Yiwen Jia.

Visualization: Chao Wu, Mengyao Yin.

Writing – original draft: Xinyi Wang, Yafen Wang, Yipin Yang.

Writing – review & editing: Chao Wu, Enba Zhuo, Mengyao Yin, Mengyuan Zhou, Kangsheng Gu, Bangjie Chen, Yiwen Jia.