Advances in immunosuppressive treatment have had a great impact on the progress and success of solid organ transplantation. The use of immunosuppressive drugs, mainly after azathioprine replacement by cyclosporine in 1979, is effective in preventing and treating graft rejection (Starzl and Fung, 2010). However, the main long-term goals of therapy are to reduce drug exposure while maintaining a well-functioning graft. In this way, the achievement of immune tolerance to an allogeneic donor, defined as the graft survival with normal function in absence of immunosuppression (IS), has been a field of intense research, fueled by a critical need to avoid IS-related side effects (particularly nephrotoxicity, cancer, and cardiovascular events) (Mastoridis et al., 2017). In this regard, liver is defined as an immune-privileged organ (Forrester et al., 2008). While the concept of tolerance to a strange body has been recognized since the early 1950s (Billingham et al., 1953), liver transplantation (LT) tolerance has been addressed in clinical studies since the 1990s (Browne and Kahan, 1994), and although clinical tolerance existence or benefit has been discussed from the beginning (Riordan and Williams, 1999), tens of clinical trials and hundreds of papers, including many reviews, have been published so far. Bibliometric studies are of great help in addressing topics with a huge bibliographic load, providing an overview of the field and helping identify trends and key contributors in the area of interest (Zupic and Cater, 2014). However, no bibliometrics or citation analysis in the field of operational tolerance (OT) in LT (OT-LT) is available worldwide.

The aim of the present study was to analyze the research on OT-LT through a new web resource, to evaluate the progress in this field and try to answer important research questions such as a) what is the current publication trend, b) which are the most influential articles or authors and c) what is the current state of collaboration in OT-LT. Moreover, we highlight the impediments that exist facing current research in this field, and some areas involving OT-LT that may lead the way for the future in this challenging field.

In the clinical setting, OT is defined as the absence of acute and chronic rejection, and graft survival with normal function and histology in an IS-free, fully immunocompetent host, usually as an end result of a successful attempt at IS withdrawal (Wieers et al., 2007). The first OT-LT study dates back to 1969 (Calne et al., 1969), but it was not until the 1980s that the bulk of OT-LT research began, mainly in rodent models (Houssin et al., 1980; Kamada et al., 1980). Thenceforth, OT-LT has been a field of intense research, fueled by a critical need to avoid IS-related side effects. The huge amount of life science literature and associated knowledge available online makes it challenging for researchers to assimilate all the useful information, more so outside of their disciplines. This has generated both the demand and opportunity for the development of advanced computer-based word-processing tools to support traditional information-processing systems (Hristovski et al., 2013). We have developed a new web resource to facilitate the search of scientific literature and the exploitation of all information associated with these publications. Therefore, we present a comprehensive analysis of OT-LT to evaluate the progress in the field and answer important research questions.

The continuous increase in the number of publications in the last two decades is perfectly suited to the sharp increase in the number of studies conducted on humans since the early 1990s, principally adults, and with the implementation of controlled protocols of IS withdrawal. OT-LT has attracted the attention of researchers, as indicated by contributions from the most important countries. Obviously, the top publishing institutions belong to nations where transplant programs are widely implemented (Trotter, 2017). However, the ranking did not fit perfectly; some countries with a high rate of LT per million inhabitants, had a small number of OT-LT research articles (e.g., Italy and France). China and Japan showed opposite findings, probably due to their overwhelming populations or even by the high levels of investment in research compared to other countries (OECD, 2023). OT-LT belongs to a broad area of research, and as expected, the most influential journals in OT-LT were specialized in transplantation, hepatology, and also immunology. It is noteworthy how prestigious journals such as Nature or The Lancet, rank the top positions in number of citations with only two or three publications, respectively. Furthermore, most of these journals have a high JCR index; therefore, OT-LT has received attention from some of the best journals in the field of transplantation, whereas the leading publications have citation rates similar to those of more prolific research areas.

Although several methods are available to measure the impact of a research publication, citation analysis is the most common (Ding and Cronin, 2011). However, this method may be biased, as older publications may be favored. Thus, the use of new metrics is necessary to understand the current trend and the most influential publications in OT-LT research. “Citation Velocity” is a weighted average of the publication's citations for the last 3 years and fewer for publications published in the last year or two, which indicates how popular and lasting the publication is (AI2, 2018). Likewise, “Influential Citation Score” is determined utilizing a machine-learning model analyzing a number of factors including the number of citations to a publication, and the surrounding context for each, identifying citations where the cited publication has a significant impact on the citing publication, making it easier to understand how publications build upon and relate to each other (AI2, 2018). In this context, we can observe that animal models have lost influence in OT-LT research. Although the rat-model LT is a well-established experimental model with hundreds of publications in the last 15 years, rodent models in OT-LT research are usually based on the spontaneous ability to tolerate a liver from a different haplotype (Yokota et al., 2016). This makes it difficult to use this model to emulate IS withdrawal protocols carried out with patients. On the other hand, the idea of chimerism, which began in the early 1990s, has lost its strength; moreover, some publications have pointed out that chimerism does not influence allograft tolerance in human LT after IS withdrawal (Pons et al., 2003). However, we can highlight as a current trend studies that attempt to explain the immunological causes leading to OT-LT (Crispe, 2003), IS withdrawal protocols (Benitez et al., 2013), as well as adoptive cell transfer trials (Todo et al., 2016). Strikingly, a publication on living donors and pediatric transplantation (Feng et al., 2012) appeared as one of the most influential publications, even though this topic represented a limited percentage of the publications in this area. However, it is clear that this is a subject of great interest to LT researchers.

Tregs and DCs have been implicated in OT-LT in the last decades (Baroja-Mazo et al., 2016a). Tregs promote a state of antigen-specific peripheral tolerance by suppressing the activation and expansion of reactive effector cells. In 1970, it was found that T-cells not only amplified but also dampened immune responses and that this downregulation was mediated by T-cells that were different from Th-cells (Gershon and Kondo, 1970). The term regulatory or suppressor cells was reintroduced in 1995 based on studies that reported the development of a lethal autoimmune disease in mice thymectomized in the neonatal period (Sakaguchi et al., 1995). Tregs are characterized by the CD25 (IL2RA) and the FOXP3 expression, two of the most commonly used terms in OT-LT. FOXP3 was initially identified as the gene responsible for an X-linked recessive inflammatory disease in scurfy mutant mice, as well as fatal autoimmune/inflammatory disease, immune dysregulation, polyendocrinopathy, enteropathy, or X-linked syndrome in humans (Brunkow et al., 2001). The essential role of FOXP3 underlines the crucial importance of naturally arising FOXP3⁺CD4⁺Tregs for self-tolerance and immune homeostasis (Haque et al., 2011). Although the specific mechanism of action of Tregs is still not completely known, several studies have shown, for example, that Tregs use different mechanisms to hamper effector T-cells (Teffs): the modulation of dendritic cell function, IL-2 deficit, direct cytotoxicity, or the secretion of inhibitory cytokines, such as IL-10, IL-35, or tumor growth factor (TGF)-β (Pons et al., 2013). DCs are also involved in establishing immune tolerance by deleting T-cell clones or inducing Tregs (Zhuang et al., 2020). Typical tolerogenic DCs secrete and express cell surface regulatory molecules that mediate tolerogenic effects on recipient T-cells. As a result, tolerance to the donor antigen can be achieved through induction of Tregs as well as via the induction of anergy and deletion of effector and memory T-cells (Ezzelarab and Thomson, 2011). Nevertheless, the fact that both cells continue to be among the most frequently mentioned terms seem to be due to the importance of the adoptive transfer trials that have been growing in recent years. For example, clinical trials have been conducted infusing in vitro expanded Tregs (Yu et al., 2020), DCs (Que et al., 2020), or MSCs (Vandermeulen et al., 2022) and replacing classic immunosuppressive drugs with rapamycin derivatives (Adams et al., 2015), or even based on the differential effect exerted by Inhibitors of mechanistic Target of Rapamycin (mTOR) on human Tregs and Teffs. These drugs are used in solid organ transplantation to induce IS while promote the expansion of Tregs, which could lead to immunological tolerance (Baroja-Mazo et al., 2016b). CNIs, particularly tacrolimus, are first-line immunosuppressive drugs in most LT centers, both in Europe and the US (Di Maira et al., 2020). One of the major adverse effects of CNIs is nephrotoxicity, which leads to renal replacement therapy and increases mortality risk (Ojo et al., 2003). Therefore, other immunosuppressive regimens have been implemented, such as sirolimus or everolimus, which are potent immunosuppressive drugs with comparable efficacies, and could also better optimize renal function and diminish side effects compared with CNIs (Baroja-Mazo et al., 2016b).

It is also interesting to note how the co-occurrence of words highlights a great similarity between results obtained in rat and human models. Rat LT is a well-established experimental model (Yagi et al., 2013) with hundreds of publications in the last 15 years. Moreover, preclinical studies with rodents have been accepted as a previous step in clinical trials (Bryda, 2013). Nevertheless, both species share 95% of their genes (Gibbs et al., 2004). Keyword co-occurrence occurs when two keywords appear together in an article, indicating that a relationship exists between both concepts. Keyword analysis is conducted because an author's keywords adequately characterize an article's content. Scientific researchers use co-occurrence analyses to measure performance and draw innovations and information flows (Wormell, 2000).

Collaboration among researchers is the most recognized method of intellectual association in scientific research (Cisneros et al., 2018). The convergence of individual perspectives leads to the evolution and development of ideas. Moreover, a paper published by multiple authors has a more improved quality because fewer mistakes are made and contributions are provided from diverse disciplines (Tahamtan et al., 2016). Principal authors in OT-LT form a network, which suggests that research concentrates on a few authors, and most of the nodes appear to form a network of two or three. The co-authorship network can, thus, be seen as a collection of few networks that is fairly closed and shows few interactions among themselves. However, some authors such as Sánchez-Fueyo, Feng, and Thomson play an important role in the network, acting as knowledge brokers among groups. Notably, several researchers have followed the path paved by the leader and have established their own lines of research in OT-LT. This can also be applied to clinical trials, where, unfortunately, there are no multi-center trials conducted in different countries, even in the same continent. Understanding this issue is challenging. In recent years, Europe has hosted almost 3,000 multinational trials annually (https://www.clinicaltrialsregister.eu/). The limited number of groups studying clinical LT-OT, the complexity of proposed trials involving strict IS weaning protocols or high-tech expanded cell infusion, as well as variations in legislation, could all influence the results we observe.

Nevertheless, there are some limitations that may be biasing this study. We use PubMed and Semantic Scholar as the main sources for article retrieval and citation, respectively. The use of other bibliographic and citation sources, could improve the results. Likewise, older articles do not have the abstract available, so keyword retrieval from those abstracts is not possible. Moreover, non-selection of articles in languages other than English, or studying OT in organs other than liver, may result in the loss of some relevant contribution. We did not discard self-citations, which may be related to a visibility strategy by journals or authors (Gonzalez-Sala et al., 2019), although the analysis of citation based in indicators for 15 fields in the sciences, social sciences and humanities confirmed that, at this level of aggregation, there was no need to revise the national indicators and the underlying journal citation measures in the context of excluding self-citations (Glänzel and Thijs, 2004). Furthermore, we are unable to distinguish whether publications categorized by country originate from Universities, research institutes, or simply Foundations or Associations headquartered there. This lack of distinction may lead to inflated numbers of certain countries.

While acknowledging its limitations, our study presents a coherent depiction of OT-LT research through a comprehensive analysis and structured literature review conducted using our proprietary program. This provides both seasoned experts and emerging researchers with insights into the evolution of the field and a visual roadmap for its future direction. Likewise, we are able to detect impediments facing current research in this field despite continuing progress. Collaboration among researchers is necessary to develop a field, and therefore more cross-country collaborations are needed. Moreover, global collaboration networks allow developing nations to engage in the knowledge creation process traditionally led by developed countries (Palacios-Callender and Roberts, 2018). Clinical trials are essential for the development of new treatments, and when well-designed, can benefit the participants, investigators, and the medical community (Novitzke, 2008). This includes the need for a higher number of multi-center clinical trials and collaboration among institutions from different countries and even from different continents. Although women represent up to 60% of students entering the medical profession in many countries in the world (Kilminster et al., 2007), only two women, Dr. Sandy Feng and Dr. Noriko Murase, featured among the most important authors in OT-LT. The participation of both men and women is essential in ventures to create a more humane environment for the training and practice of medicine (Flaherty et al., 2019). Additionally, numerous studies have investigated the genetic and phenotypic profiles of tolerant transplant patients. Moreover, different groups have defined several biomarkers to distinguish between potentially tolerant and non-tolerant patients. However, most of these biomarkers have not practically been reanalyzed in an independent and/or prospective way by other groups (Pérez-Sanz et al., 2019) or even established for clinical practice (Kurian et al., 2018). Thus, more inter-center validation studies will be necessary to overcome this gap between bench and patients.

Furthermore, from our perspective, future research directions in OT-LT may be more focused on an induction of tolerance. This could entail not only exploring regulatory cell infusion trials, incorporating novel CAR-Tregs (Kaljanac and Abken, 2023; Proics et al., 2023), but also using new therapeutic or pharmacological approaches (Baroja-Mazo et al., 2018) or even manipulating aspects such as diet (Wu et al., 2020).

In conclusion, OT-LT remains an exciting frontier within the captivating realm of transplantation, poised to sustain the interest of researchers across diverse disciplines, including surgery, hepatology, immunology, and molecular biology.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding authors.

ÁE-G: Conceptualization, Methodology, Software, Validation, Writing—original draft, Writing—review & editing. JM-G: Investigation, Writing—review & editing. DV-C: Formal analysis, Writing—review & editing. AM-M: Investigation, Writing—review & editing. PR: Conceptualization, Formal analysis, Supervision, Writing—review & editing. JP: Conceptualization, Formal analysis, Funding acquisition, Methodology, Supervision, Writing—review & editing. AB-M: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Supervision, Writing—original draft, Writing—review & editing.