We searched the Web of Science Core Collection (WoSCC) to find articles on ILCs in the brain, intestine, and lung fields over the past 20 years (from 2003-01-01 to 2023-12-12). The search formula used was TS=(“ILC” OR “ILCs” OR “Innate lymphoid cells” OR “natural killer cell” OR “NK cell” OR “lymphoid tissue inducer” OR “LTi”) and (“brain” OR “neurotransmitters” OR “Encephalon” OR “nerve” OR “nervous” OR “lung” OR “pulmonary” OR “gut” OR “Colon” OR “colorectal” OR “Rectum” OR “Intestine”). The search was completed on December 12, 2023, to ensure data accuracy. Articles written in English were specifically chosen, and the document types were limited to articles and reviews, excluding meeting abstracts, editorial materials, proceedings papers, and other document types. Manual literature screening was conducted to exclude irrelevant articles, such as those related to testicular interstitial cells (ILC), breast invasive lobular carcinoma (ILC), International Law Commission (ILC), etc. Additionally, content that was unrelated to the fields of brain, lung, and intestine, such as liver, uterus, pregnancy, blood system diseases, breast diseases and physical realm, was also excluded ( Figure 1 ). The search results were then exported in Plain Text file format, and the Full Record and Cited References of the documents were collected. This included information such as authors, countries, institutions, references, keywords, journals, and other relevant details. These data were utilized for bibliometric and visual analysis purposes.

In this study, we will utilize HistCite, VOSviewer, and CiteSpace for bibliometric and visual analysis. Additionally, we will employ Microsoft Excel Office to organize relevant data including the number of publications and citation frequency.

Histcite is a document analysis tool developed by Thomson Reuters in the United States (16). It is specifically designed to analyze documents (Plain Text files) exported from the WoS database. The tool effectively identifies and counts various elements such as keywords, authors, countries, institutions, journals, publication years, languages, document types, cited documents, TLCS, TGCS, and other relevant information in each document. Additionally, it constructs the citation relationships between documents, which aids in understanding the overall context and relationships within a particular field. TLCS, also known as the total local citation score, indicates the level of attention a document has received from peers. A higher TLCS score implies greater attention and makes the document more worthy of reading. On the other hand, TGCS, or the total global citation score, takes into account the attention the document has received from other fields or disciplines.

VOSviewer is a powerful document visualization tool developed by Leiden University. It facilitates multi-dimensional bibliometric and visual analysis (17), including network visualization, coverage visualization, and density visualization. Through the construction of network relationship maps between authors, keywords, citations, etc., it aids in uncovering cooperative relationships, research hotspots, and development trends in the field. Network visualization depicts connections between documents, keywords, and authors using a network diagram. The size of nodes represents the frequency of occurrence of the corresponding unit, and clusters are indicated by the same color. Overlay visualization adds supplementary information to the original network diagram, providing further insights. Density visualization illustrates the distribution of a specific attribute in the network diagram, enabling the visualization of heat distribution.

Citespace, a document visualization software developed by Professor Chen, is capable of analyzing massive documents from various databases such as Web of Science, Pubmed, and CNKI. It provides insights on topics, keywords, author units, collaborative networks, journals, publication time, and document citations (18). By performing spatiotemporal analysis, hot spot analysis, and cluster analysis on topics, Citespace can detect research hot spots and popularity in different periods through keyword burst, thus analyzing the development trends in the field (19). In our study, we utilized cluster analysis to identify key research areas and directions of ILCs in the brain, intestines, and lungs by clustering keywords and co-cited documents. Modularity Q and mean silhouette were used as important evaluation indicators in cluster analysis. It is commonly accepted that Modularity Q > 0.3 indicates a significant clustering structure, while Mean silhouette > 0.5 suggests convincing clustering results. Furthermore, we conducted burst analysis and timeline analysis of keywords, references, and more to explore the hot spots and development trends of ILCs in the fields of brain, intestine, and lung.

When conducting visual analysis using Citespace, we typically analyze the data with the system’s default parameters: time slice=1, link strength measured by cosine, g-index K=25, and top N=50. For clustering, we employ the LLR algorithm. The algorithm used for burst analysis is based on a conference paper titled ‘Bursty and Hierarchical Structure in Streams’ by Kleinberg J in 2002. The number of nodes within a cluster indicates the level of activity in a field, representing an emerging trend in research. During burst analysis, we also follow the system’s default parameters, which involve configuring the detection model with F(x)=ae^-ax, a₁/a₀ = 2.0, a_i/a_i-1 = 2.0, The Number of States=2, γ [0,1]=1.0, Minimum Duration=2.

We conducted a comprehensive search of academic articles and reviews on ILCs in the fields of brain, intestine, and lung using Web of Science. A total of 8411 articles were screened, including 6070 articles and 2341 reviews. During the search period, Lin et al. published the first article in this field, which revealed that the expression levels of ICAM-1 and L-selectin on peripheral blood NK cells decreased during acute attacks of childhood asthma. This reduction promoted the migration of NK cells to the lung inflammation site (20). NK cells, the first ILCs discovered in the 1970s, have been the subject of extensive research due to their crucial role in both innate and adaptive immunity. Figure 2A illustrates a fitting curve that demonstrates an overall upward trend in the number of publications on ILCs in the brain, intestine, and lung fields since 2003. The average number of publications is approximately 400, with the highest number recorded in 2021 (n=953). In Figure 2B , it is evident that the growth rate of published articles in 2020-2021 was the highest at 36%, while the number of published articles declined in the past two years. Over the past 20 years, the total number of local citations for these articles reached 36672, with a global citation count of 417918. The peak values for total local citations (TLCS) and total global citations (TGCS) were observed in 2014 (TLCS=4110, TGCS=36160). However, starting from 2021, there has been a significant decrease in TLCS and TGCS compared to the previous year, reaching the lowest point in 2023 (TGCS=631).

From 2003 to 2023, a total of 99 countries have published articles on ILCs in the fields of brain, intestine, and lung. The geographical distribution of these countries is shown in Figure 3A . Table 1 displays the top ten countries with the most publications. According to Figure 3B , the United States has the highest number of publications, with a total of 3044 articles. China (including Taiwan) follows with 1559 articles, and Germany ranks third with 788 articles. In terms of citations, the United States leads with a total of 210776 citations. Germany comes second with 55098 citations, followed by the United Kingdom with 50887 citations. China ranks fourth, as shown in Figure 3D . Israel has the highest average number of citations, with 115 publications, 11390 citations, and an average of 99.00 citations per article. France follows with 443 publications, 39235 citations, and an average of 88.57 citations per article. China’s average number of citations is 26.86. Figure 3E visually displays the national cooperation relationships in the research on ILCs in the fields of brain, intestine, and lung. Notably, there is strong international research cooperation between China and the United States, followed by the United States and Germany, the United Kingdom, Japan, Canada, and other countries. The annual publication trend chart by country ( Figure 3C ) reveals that research in the United States, the United Kingdom, Australia, and other countries began earlier compared to China. However, the number of publications in China has been steadily increasing since 2010, and in recent years, China has become the leading country in terms of publication output in this field.

A total of 45279 authors from 6585 institutions participated in the research on ILCs in the fields of brain, intestine, and lung. The top 10 institutions with the most research results are presented in Figure 4A . Harvard Medical School in the United States leads with the highest number of publications (n=140, TGCS =6997, TLCS=800), followed by Karolinska Institutet in Sweden (n=134, TGCS=6709, TLCS=860), and University of California in the United States (n=124, TGCS=13404, TLCS=1914). In terms of total global citations (TGCS), the University of Pennsylvania in the United States holds the top position with 14,778 citations, followed by the University of California in the United States (TGCS=13404) and Inserm in France (TGCS=13200) ( Figure 4B ). The social network diagram of research institutions illustrates the division of institutions into 7 clusters based on an occurrence frequency of ≥10 and cooperative relationships ( Figure 4C ). Notably, there is significant collaboration between institutions, both domestically and internationally, with the strongest partnership observed between France’s Inst Pasteur and Inserm. According to Figure 4D , articles published by Harvard University and NIAID in the United States were published earlier. In contrast, Chinese institutions such as Shanghai Jiao Tong University and Huazhong University of Science and Technology have published a higher number of articles around 2020.

The top three authors with the most published articles on innate lymphoid cells (ILCs) in the fields of brain, intestines, and lungs are David Artis from the University of Pennsylvania (n=40, TGCS=10327, TLCS=2857), Marco Colonna from Washington University (n=37, TGCS=6803, TLCS=1510), and Andrew NJ McKenzie from the MRC Laboratory of Molecular Biology (n=36, TGCS=6866, TLCS=2158) ( Table 2 ). David Artis is highly regarded as an authoritative expert in the fields of brain, intestines, and lungs, as evidenced by his extensive research publications on ILCs. His articles have received the highest number of citations in this field. Notably, he published the first review on this topic in 2010 and has since continued his uninterrupted research on ILCs. In October 2023, David Artis published a comprehensive review in the journal Immunology, providing an in-depth analysis of ILC2s and TH2 cells and their synergistic effect in the expulsion of intestinal parasites (21). After conducting visualization ( Figure 4E ), we observed a relatively close cooperation among the authors. We formed 18 clusters consisting of authors who had an occurrence frequency of ≥10 and a cooperative relationship. The strongest collaboration was found between Colonna and Gilfillan. In terms of publication timing, most authors published their papers after 2015, as depicted in Figure 4F .

All articles were published in 1350 magazines. The top 10 magazines with the most articles are shown in Table 3 . According to Figure 5A , The Journal of Immunology leads with 330 articles (n=330, TGCS=20331, TLCS=2247), followed by Frontiers in Immunology with 289 articles (n=289, TGCS=9135, TLCS=2), and Plos One with 199 articles (n=199, TGCS=6294, TLCS=0). However, these journals have relatively low citation rates in the field. The top three journals with the highest citation counts are Immunity (TGCS=22719), Journal of Immunology (TGCS=20331), and Nature Immunology (TGCS=17537). Figures 5B, C depict the journal time series diagram and heat map, respectively. Notably, Frontiers in Immunology has gained popularity for submissions in recent years. According to the double overlay diagram, there are three main citation paths. The published articles were predominantly found in journals related to molecular biology, immunology, and clinical medicine. On the other hand, the cited articles were mostly published in journals covering molecular biology, genetics, health, nursing, and medicine ( Figure 5D ).

Table 4 presents the top 10 most frequently cited articles. These articles provide comprehensive descriptions of the roles and mechanisms of various types of ILCs in inflammation, allergy, and infection. By conducting a co-citation analysis of the 278,967 references, it was discovered that there were 139 articles with more than 100 citations. The article with the highest number of citations, authored by Hergen Spits et al. and published in the journal Nature reviews Immunology in 2013. In their review, the authors proposed a classification of ILCs into three categories based on the cytokines produced by ILCs and the transcription factors that regulate their development and function. This classification has been widely accepted and is now considered the standard for categorizing ILCs. It has significantly advanced research on ILCs in the field of brain-gut-lung axis (3). We constructed a network visualization of co-cited documents ( Figure 6A ) and conducted cluster analysis, which led to the identification of 19 cluster modules ( Figure 6B ). The modularity Q value was determined to be 0.8900, and the mean silhouette value was 0.9055. These cluster modules encompassed various topics such as Innate lymphoid cells, Tumor immunity, Airway inflammation, Inflammatory bowel disease, Cytokine storm, and ILC2 activation. Additionally, we visualized the clustering timeline ( Figure 6C ) and observed frequent mentions of Tumor immunity, NK cells, Multiple sclerosis, Inflammatory bowel disease, Ligands, Airway hyperreactivity, etc. in the early research on ILCs in the brain, intestine, and lung fields. The inclusion of these references suggests that they are considered research hotspots in this particular domain. The current focus in this research field is on Innate lymphoid cells, Immunity, Airway inflammation, Cytokine storm, and Tissue migration. Upon closer examination of the clustering patterns, it was observed that ‘#0 Innate lymphoid cells’ mainly refers to ILC1, ILC2, and ILC3. These three types of cells have been progressively recognized since 2010. Notably, in 2010, Sawa et al. proposed in Science that RORγt(+) innate lymphocytes (ILC) play a beneficial role in maintaining intestinal bacteria, stabilizing the population, and promoting the production of intestinal defense cells (22). This groundbreaking study paved the way for further research on these three types of ILCs in various organs such as the brain, intestine, and lungs. An burst analysis was conducted on the co-cited documents ( Figure 6D ). The review by Vivier et al. in 2018 (4) had the highest burst strength. This review provided detailed information on the research conducted in the past ten years, emphasizing the importance of ILCs in environmental homeostasis, tissue metabolism, and cell repair. Currently, the focus of the cited literature is on an article published by Huang et al. in Nature in 2018. They discovered that interleukin-25 or helminth-induced inflammation ILC2s can facilitate the chemotaxis of sphingosine 1-phosphate (S1P), enabling migration from the intestinal lamina propria to various tissues through blood and lymph circulation (12). In 2017, Cardoso et al. found that neuromodulation of type 2 innate lymphocytes can be achieved through neuromedin U (23). Additionally, Klose et al (24). published an article in Nature in the same year, suggesting that the nervous system regulates ILCs and type 2 inflammatory cells through the neuropeptide neuromedin U to mediate intestinal immune protection. This research area has gained considerable attention, highlighting the significance of neural regulation in intestinal immunity in recent years.

A visual analysis was conducted on 21,439 keywords in this field ( Figure 7A ). Keywords such as ILCs, NK cells, inflammation, asthma, cancer, dendritic cell, and immunotherapy appeared frequently in this field, with a frequency of more than 500 times. The cluster analysis resulted in 20 clusters ( Figure 7B ) with a modularity Q of 0.816 and a mean silhouette value of 0.9363. Combining the clustering timeline results, Figure 7C demonstrates that cancer, immunity, airway inflammation, asthma, RORγT, immunotherapy, inflammatory bowel disease, allergic inflammation, and other related topics have been the primary areas of focus in the research on ILCs in the brain, intestine, and lung since 2003. These findings highlight the key research topics in this field. We conducted an burst analysis on keywords and identified the top 25 burst keywords. Figure 7D displays the keyword with the highest burst strength, which is ‘in vivo’, and it remains burst for a duration of 10 years. Additionally, our analysis revealed that lung adenocarcinoma and tumor microenvironment are prominent keywords in recent research. This suggests that ILCs may significantly impact the diagnosis and treatment of brain, intestinal, and lung tumors.

After analyzing the keywords and co-cited documents, recent studies have focused on investigating ILCs in the brain, intestine, and lung. These studies primarily explore various aspects including allergic diseases, intestinal inflammation, infection, tumors, and neuroendocrine regulation. Notably, ILCs have been found to play a significant role in the brain-gut axis and the lung-gut axis. However, the specific role of ILCs in the brain-lung axis remains unclear, and there is a lack of research in this area. Understanding the contribution of ILCs in the brain-lung axis is crucial for establishing a comprehensive research framework on the brain-gut-lung axis. Therefore, further research is needed to fill the existing gaps and advance our knowledge in this domain.

Based on the above search terms, we added the conditions “Tumor” OR “Neoplasm” OR “Neoplasia” OR”Cancer” OR “Malignan”, A total of 5597 articles were retrieved, and after screening, 2709 articles related to ILCs in brain, intestinal, and lung tumors were included. In terms of publications, the United States leads with the highest number (n=821), followed by China (n=740) and Japan (n=252) ( Figure 8A ). The United States is also the most cited country (TGCS=53717), followed by China (TGCS=18282). The most cited article is a 2009 article by Castriconi et al., they discovered that NK cells have the ability to identify and eliminate human glioblastoma cells that exhibit stem cell-like characteristics (25). Moving on to authors, Smyth has the highest number of publications in this field (n=23) ( Figure 8B ). Keyword clustering of the literature resulted in 17 clusters (Q=0.7532, S=0.8871) as shown in Figure 8C , Among the various types of ILCs, NK cells have received the most attention in research. Time visualization and burst analysis of the clusters reveal that recent research has focused on hot topics like IFN-gamma, mechanism, immunotherapy, tumor microenvironment, immune checkpoint, lung cancer, colorectal cancer, and PD-1 ( Figures 8D, E ). PD-1 is a crucial immunosuppressive molecule (26, 27). The combination of PD-1 and PD-L1 reduces T cell activity and suppresses the immune system’s attack on tumor cells. Blocking the PD-1 and PD-L1 combination restores T cell activity, leading to effective tumor cell killing and remarkable efficacy in various cancer treatments. PD-1/PD-L1 monoclonal antibodies are currently the most extensively studied immune checkpoint inhibitors in clinical research and application, making them a prominent topic in tumor immunotherapy in recent years.