Human islet xenotransplantation in rodents: A literature review of experimental model trends.

Among the innovations for the treatment of type 1 diabetes, islet transplantation is a less invasive method of treatment, although it is still in development. One of the greatest barriers to this technique is the low number of pancreas donors and the low number of pancreases that are available for transplantation. Rodent models have been chosen in most studies of islet rejection and type 1 diabetes prevention to evaluate the quality and function of isolated human islets and to identify alternative solutions to the problem of islet scarcity. The purpose of this study is to conduct a review of islet xenotransplantation experiments from humans to rodents, to organize and analyze the parameters of these experiments, to describe trends in experimental modeling and to assess the viability of this procedure. In this study, we reviewed recently published research regarding islet xenotransplantation from humans to rodents, and we summarized the findings and organized the relevant data. The included studies were recent reports that involved xenotransplantation using human islets in a rodent model. We excluded the studies that related to isotransplantation, autotransplantation and allotransplantation. A total of 34 studies that related to xenotransplantation were selected for review based on their relevance and current data. Advances in the use of different graft sites may overcome autoimmunity and rejection after transplantation, which may solve the problem of the scarcity of islet donors in patients with type 1 diabetes.


' INTRODUCTION
According to the International Diabetes Federation (IDF), diabetes mellitus currently affects 382 million people, with a projected increase to 592 million people by 2035 (1).
The etiology of type I diabetes mellitus is unknown; however, histopathological findings indicate an autoimmune destruction of -cells, an association with HLA alleles and environmental factors, such as exposure to bovine milk. Diabetes mellitus was historically considered a fatal disease that resulted in hyperglycemic coma. However, since the discovery of the therapeutic application of insulin in the 1920s, diabetes mellitus has become a chronic disease that causes many complications, including retinopathy, nephropathy, vasculopathy and neuropathy.
In 1894, the first case of islet transplantation as a treatment for diabetes was described by Dr. Watson Williams and Hareshant. Notably, this case occurred before the insulin isolation of Banting, Best and Collip in 1921. In the early twentieth century, Dr. W. Williams attempted to implant sheep pancreatic fragments in the subcutaneous tissue of a 15-year-old male with ketoacidosis. However, the xenograft was rejected because of a lack of immunosuppressive techniques. In 1972, Dr. P. Lacey demonstrated the reversibility of diabetes in rodents by using islet implantation (2).
The first successes in islet allografts in the surgical treatment of diabetes occurred in 1990 with Scharp et al., who achieved insulin independence in a patient with type 1 diabetes mellitus for one month. However, many technical difficulties were found during the reproduction of this experiment.
One of the greatest barriers to the development of islet transplantation is the low number of pancreas donors and the low number of pancreases that can be used for transplantation (3). According to the Network of Organ Procurement and Transplantation, fewer than 20% of the pancreases that are collected from a total of 8,000 donors are available for transplantation. In addition, many pancreas donors do not meet the selection criteria, and many islets are handled incorrectly, negatively affecting the transplant procedure. (4) Other inconveniences are the high cost of islet isolation, the poor durability of insulin independence, autoimmunity and rejection after transplantation (2,3,5,6).
To supply the scarcity of islets, animal donors, such as pigs, could provide an alternative source of cells for transplantation (7). However, xenotransplantation is challenged by the possible risk of infection from pathogens within the donor animal. Specifically, all pigs contain multiple copies of porcine endogenous retrovirus and at least three variants of pig endogenous retrovirus (PERV), which can infect human cells in vitro. Thus, there is a risk of PERV infection associated with the xenotransplantation of pig islets to immunosuppressed human patients (8,9).
In this context, to evaluate the quality and function of isolated human islets (10), the rodent has been chosen over other animals in most studies that involve islet rejection and the prevention of type 1 diabetes (3).
Manikandan et al. (11) studied the antioxidant effect of black tea on the regeneration of pancreatic -cells and observed a positive therapeutic effect in rodent studies. Recently, Gu et al. (12) described an alternative therapeutic strategy to treat type 1 diabetes, namely, treatment by nanoparticles, which sustainably promotes the self-regulation of glucosemediated insulin secretion. This effect is observed for a longer period of time than the insulin injections that are currently used for treatment.
Although there have been many positive results related to the xenotransplantation of human islets to rodents, researchers have rarely achieved a breakthrough in the clinical treatment of islet transplantation, perhaps because of the differences between the human immune system and the rodent model. These differences have stimulated the development of humanized rodent models, which allow the detailed study of human immune system cells and transplanted human islets in vivo (3).
The purpose of this study is to review islet xenotransplantation experimental attempts from humans to rodents, to organize the parameters of these experiments and to analyze the viability of these procedures.

' METHODOLOGY
We reviewed studies regarding islet xenotransplantation from humans to rodents. The relevant data from recently published studies from 2006 to 2016 were summarized and organized.

Eligibility Criteria
Types of Studies. The study designs of previous reviews and experimental studies were included.
Types of Participants. Donor participants were humans from whom islets were isolated and transplanted to rodents (recipient).
Types of Intervention. The interventions were islet xenotransplantation from humans to rodents. There were different graft sites and types of islet recipients. In the present review, only the studies that relate to human to rodent islet xenotransplantation were selected.
Types of Parameters Analyzed. Several parameters were considered, namely, strain, gender, age and weight of the recipient, xenotransplantation site, graft survival time (follow up), number of transplanted islets and diabetes induction method.

Exclusion Criteria
Articles discussing transplantation in porcine, tilapia and nonhuman primates (which are some of the more common species that are used for transplantation) were excluded from the review to focus on the articles that relate to islet xenotransplantation from humans to rodents. Studies using stem cells or that had an unclear methodology were excluded from our review.
Research letters, articles not published in English and articles for which the full text was unavailable were not considered in this review.
Following the PubMed search, we reviewed the references from the retrieved publications and obtained the entire text of the publications for potential inclusion in the review.

Literature Search
Using the Medline database, the literature was searched for English-language articles that were published from January 2006 to January 2016.
We performed a manual search of the references and contacted experts in the field.
Search Strategy. We searched for published articles by using the Medline database with the keywords "rodent islet transplantation".
We also selected the most recent works that were published from January 2006 to January 2016 by using the following search terms: Articles that were published before 2006 were not included in the analysis because of a lack of information, relevance and current data.
Data Extraction. The data from each study were independently extracted by 3 of the authors. Disagreements were resolved by consensus. If no consensus was achieved, a fourth author was consulted.

' RESULTS
A total of 1,819 articles from 2006 to 2016 were found, but only 225 articles were related to xenotransplantation and were thus selected based on their relevance and current information. We selected 91 articles and analyzed them; 34 of these articles were had good methodological quality, such as updated information that is necessary for this review and a description of all comparative parameters related to islet xenotransplantation from human donors to rodents.
The results are organized and displayed in Tables 1 and 2. ' DISCUSSION Islet transplantation is an innovation for type 1 diabetes treatment that is less invasive and that has a 20-fold lower morbidity rate than pancreas transplantation (2,4,6,16).
Some studies have reported an 80% rate of insulin independence during the first postoperative year in the patients who were treated with islet transplantation. However, graft survival rates remain low (2).
The islet transplantation technique has been developed to provide an adequate supply of insulin, which solves the problem of donor shortage for diabetic patients (17). From 1991 to 2000, 450 islet transplantation attempts were performed in patients with type 1 diabetes with only an 8% success rate.
We discuss the analyzed studies in more detail below.

Recipient characteristics
In this study, we reviewed the articles describing xenograft transplantation in rodents. The majority of the animals were between 9 and 16 weeks old and were male (32.4% male; 17.6% female; 50% N/A). See Table 1. Although more studies used C57BL/6 mice in the xenograft experiments (22%), followed by NOD-SCID and BALB/c mice (14% each), no significant difference was observed in the results that were obtained using other strains.

Diabetes induction method
The standard diabetes induction method was the use of streptozotocin. The median dose was 170 mg/kg (50-250 mg/kg).

Islet xenotransplantation site
The authors used different sites for the xenografts (Table 2), but the kidney capsule (91.2% of the studies) was the most frequently used site for transplantation. Other sites, such as the intraperitoneal space, liver (portal vein), subcutaneous space, submandibular gland and dorsal window model, were used in a small number of studies.
The highest graft survival time was more than 300 days, which was obtained by Brehm MA et al. (19). This study used the subrenal space as the site of xenograft transplantation. Other studies that used the kidney capsule as the xeno-  and it may thus be difficult to conclude whether these locations provide better graft survival rates than the kidney. It is important to note that in many studies, the recipients were sacrificed for histopathological analysis.
We identified many variables on the analyzed studies. The characteristics of the xenotransplantation site are factors that can possibly influence the obtained results. Based on our analysis, it is possible to reproduce some of these studies and to modify additional variables to obtain better graft survival times. Nevertheless, one relevant limitation is that many studies did not describe the data that are essential to reproduce the described experiments, such as the strain, age and gender of the recipient animal and the diabetes induction method.
Although immunosuppressive drugs may increase the survival rates of islet allotransplantation in rodents by reducing the side effects (17), few studies have used immunosuppressants. It was therefore not possible to perform an analysis of the immunosuppressive effect in islet xenotransplantation. Future studies with improved methodologies are necessary to improve the graft survival time and to advance type 1 diabetes treatment.
The viability of pancreatic islet transplantation could be determined in only a small number of studies because of a lack of the information that is necessary to perform this procedure.
The survival rates in allograft experiments have increased with the use of novel graft sites. Different methodologies to conserve islets may overcome autoimmunity and rejection after transplantation and solve the problem of the scarcity of islet donors for patients with type 1 diabetes.