Nonhuman Primate Models in Translational Surgery Open Access

Nonhuman primates (NHPs) are the closest animal model to humans regarding genetics, anatomy, physiology, and immunology. Therefore, NHPs play a unique role in translational research aimed at developing therapies designed to prevent, cure, or ameliorate human disease by bridging the gap between basic and clinical research. Research in Old World monkeys (mainly macaques, but also baboons and vervet monkeys) and to a lesser extent New World monkeys (especially marmosets, but also owl and squirrel monkeys) has been essential for medical advances in disease states as diverse as cancer, neurological disorders, diabetes, and those requiring organ transplantation. Recent progress in NHP models generated by engineering at the embryo state, showed even more unique advantages in mimicking clinical patients [1]. Due to their anatomical and physiological similarities to humans, NHPs have been widely used as models for surgical research.

The indications and techniques for surgery in NHPs mirror those used in humans and are common components of NHP protocols in biomedical research. They must be performed by qualified and skilled personnel using aseptic techniques under appropriate conditions. Presurgical and postsurgical care must conform to current established medical practice. New surgical techniques in NHPs are frequently developed for research purposes. These include not only investigations into new therapeutic or diagnostic systems aimed at curing diseases but also studies about disease models, aimed, on the contrary, at inducing a condition with high reliability and reproducibility. In this review, we provide a comprehensive outline of recent progresses in NHP surgical methods which are translatable to humans.

Comparative cardiovascular research particularly profit of NHP models due to their significantly higher similarities to humans in physiology, rheology, and immunology, in comparison to other species. An effective way to solve the current shortage of heart donors would be cardiac (xeno)transplantation. However, in this approach, the use of NHPs as heart or lung donors is limited. Baboons (Papio sp.) are mostly used as recipients of heart or lung grafts. Heart transplantation in NHPs is typically performed in a heterotopic fashion with the graft placed in the abdominal cavity. The graft can then be followed by palpation, ultrasound, or ECG. Swines are typically used as donors, owing to the ease of breeding and genetic manipulation, such as the Gal antigen knockout, which reduces the risk of rejection [2]. The immunological barriers to successful xenotransplantation relate to the presence of natural anti-pig antibodies in humans and NHPs that bind to antigens expressed on the transplanted pig organ and activate the complement cascade, which results in rapid destruction of the graft (hyperacute rejection). The success of these techniques relies on genetically modified animals not eliciting an immune response, the introduction of novel immunosuppressive agents that target the immune system, and the development of clinically applicable methods to induce donor-specific tolerance [3, 4]. Beyond heart and lung transplantation, cardiopulmonary bypass surgery for the surgical repair of complex cardiac lesions is commonly investigated in baboons [5].

Moreover, baboon models of myocardial ischemic infarction and stroke showed to be invaluable to investigations into pathophysiology and for neuroprotective drug screening prior to clinical trials [6]. Rhesus macaque (Macaca mulatta) and baboon models of aneurismal subarachnoid hemorrhage were successfully developed and used in laboratories worldwide to investigate this relatively rare form of hemorrhagic stroke [7].

Recently, research has been done to investigate the application of induced pluripotent stem cell-based remuscularization of injured rhesus macaque hearts [8]. This technique resulted in a substantial remuscularization of the injured hearts, and an amelioration of function was repeatedly demonstrated. These findings can be considered encouraging for the translational field and resulted in clinical translation. Currently, several clinical trials using this technique in heart failure patients are ongoing [9].

NHPs show several similarities to humans also with respect to the reproductive system. These include the environmental and neuroendocrine control of the reproductive axis, the cyclic function of the ovary, the anatomy of the reproductive tract, the establishment and control of the maternal-fetal-placental unit, the immune ontogeny in infants, and the reproductive aging from puberty through menopause. Thus, NHPs are a valuable model for reproductive health, reproductive disorders, and developmental toxicity [10, 11].

For instance, baboons are common models in fetal surgery research, in which maternal safety and future reproduction potential are paramount. These models typically aim to assess the feasibility of human fetal interventions for attempting correction of malformations in utero. Several experiments into the feasibility, reasonability, and effectiveness of using NHPs for direct or indirect fetal cardiac approach and fetal cardiac bypass have shown favorable results for treatment of some congenital heart defects in humans [12]. With respect to postnatal models, rhesus and cynomolgus (Macaca fascicularis) macaques commonly serve for pediatric anesthetic exposure investigations aiming at assessing the effects of general anesthesia on the developing brain [13].

Moreover, NHP models for emerging diseases such as Zika virus-induced fetal defects have been showed to be an important for research to discover the mechanisms of injury as well as for testing vaccines and therapeutics designed to prevent teratogenesis, fetal and neonatal injury, and adverse pregnancy outcomes [14, 15]. Importantly, primate models for assisted reproductive technologies are available. Several molecular foundations of various assisted fertilization techniques were tested in NHP models and later shown to be successful in human-assisted reproduction modalities such as intracytoplasmic sperm injection [16].

Due to close similarities to humans in terms of brain structure and cognitive function, marmoset (Callithrix sp.) and macaque models for brain and psychiatric disorders such as dementia, Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and epilepsy were developed [17, 18]. Additionally, sophisticated genetic NHP models for neurodevelopmental disorders, developed by embryo engineering, provide even more unique advantages in mimicking clinical conditions [19].

Recently developed neurosurgical approaches include deep brain stimulation (e.g., in Parkinson’s disease), which implies the use of brain implants to stimulated specific brain areas and rescue brain function or to assess the role of certain regions of the brain [18]. Additionally, optogenetics is applied in a promising new approach in NHP, enabling in vivo analyses of neurological networks [20].

Behavioral analysis of subjects with discrete brain lesions provides important information about the mechanisms of various brain functions. Therefore, the causal roles of specific brain areas in particular functions have been elucidated by studying the effects of lesions on behavior of Japanese macaques (Macaca fuscata) [21]. Nonreversible and reversible brain lesions can be created by using optical, pharmacological, electrical, optogenetic, chemogenetic, pathway selective and ultrasound perturbation approaches [22].

NHP models of spinal cord injury reflect human injury and provide a good foundation to evaluate potential treatments and functional outcomes. The development of a rhesus macaque and a gray mouse lemur (Microcebus murinus) model of spinal cord injury based on mechanical and computational modeling is described by several authors [23, 24]. This model expanded our knowledge of normal primate motor function, helped assess the impact of disease and injury on sensory and motor function, and assisted in evaluating the safety and, potentially, the efficacy of candidate translational therapies. Recently, an advanced method (i.e., a partial laminectomy) for accessing the spinal cord in order to study injury and repair mechanisms was reported in rhesus macaques [25].

A notable application of using robotics in NHPs to answer questions in the neurosciences was achieved by applying a brain-machine interface to alleviate gait deficits in macaques undergoing spinal cord injury [26]. This study also represents a perfect example of the relevance of NHP surgical models for advancing human health. In fact, the same research group later followed up with two studies translating the same robotic approach to human patients, who recovered locomotor deficits manifest by spinal cord injury or Parkinson’s disease [27, 28]. Taken together, these cases demonstrate the potential for the extensive application of robotics to NHP models for surgical protocols.

Osteoarthritis and bone loss are some of the most important consequences of aging in humans. To date, NHP models for skeletal and reproductive aging and osteoarthritis do not yet perfectly mimic human disease conditions [29]. However, recent progress has been made to develop a rhesus macaque model of knee osteoarthritis with a focal cartilage defect that closely resembles the pathophysiological processes of spontaneous knee osteoarthritis in humans [30].

In orthopedic, spinal implant is the most used NHP model [31]. A model of total disc arthroplasty in baboons serves as the next frontier in the surgical management of intervertebral discogenic pathology [32]. Recently, a new cervical subtotal discectomy prosthesis was successfully designed. Once constructed, it provided an excellent NHP implantation model for the evaluation of subtotal disc replacement arthroplasty. It was demonstrated that cervical subtotal discectomy prosthesis had outstanding safety, osseointegration capacity, and biomechanical stability in cynomolgus macaques, which may constitute a viable new option in the treatment of cervical disc diseases and potentially change future outcomes of degenerative cervical diseases in human medicine [33].

Intestinal biopsy is a basic diagnostic method for studying pathological changes in the intestinal tract. One limitation is the availability of biological material. Therefore, jejunal resection and anastomosis were longitudinally investigated in rhesus macaques. The sampled gut tissues were showed suitable for histological, physiological, pathological, and immunological detection, as well as mechanistic analysis at the cellular and molecular level. In addition, the surgery did not affect the ratio or number of immune cells in peripheral blood or the concentration of lipids, proteins, and vitamins in plasma. These results demonstrate the feasibility of jejunal resection and anastomosis and that immune homeostasis and intestinal barrier integrity are not altered by surgery [34].

Metabolic surgery remains a key treatment for type 2 diabetes associated with morbid obesity in humans. A pilot study showed the feasibility of a novel magnetic compression device to create a patent limited caliber side-to-side jejunoileal partial diversion in a rhesus macaque model of diet-induced insulin resistance [35].

Xenotransplantation of macroencapsulated islets for treatment of diabetes was successful performed and showed to be safe in rhesus macaques without any immune suppression [36]. This strategy may not only result in pioneering clinical trials with xenotransplantation for treatment of diabetes but also represent a novel approach to efficient cell replacement for a broad spectrum of endocrine disorders and other organ dysfunctions.

Obesity contributes to overall morbidity and mortality worldwide. Bariatric surgery, including gastric bypass, sleeve gastrectomy, gastric band, and duodenal switch, is the gold standard in surgical treatment for obesity and metabolic dysfunction. Vertical sleeve gastrectomy in cynomolgus macaques mimics the complexity and outcomes in humans. It also conferred weight loss and durable metabolic benefit [37].

NHPs offer the closest anatomical analogy with the human urinary tract, with two exceptions. The NHP left kidney is located lower in the abdomen than that of humans, and the NHP kidneys are unipapillar. Nevertheless, just a few experimental urinary models involving NHPs have been described, presumedly owing to financial constraints and ethical issues. Currently, renal transplantation is well tolerated by the animals and complications are limited, with most issues related to the ureter [38].

Renal xenotransplantation has recently made considerable progress thanks to preclinical pig-to-primate models. In fact, using the latest strains of genetically modified porcine donors combined with controlling the anti-porcine immune response and coagulation showed promising results in transplanting porcine kidney into different NHP models, including the cynomolgus macaque [39].

The usefulness of cynomolgus macaques as translatable surrogates for pathophysiological studies of urinary sphincter deficiency and testing novel therapies has been recognized [40]. For that model, an injury to the sphincter complex was created by cauterizing and then transecting its pudendal innervation.

In humans, partial cystectomy procedures for urinary bladder-related dysfunction involve long recovery periods, during which urodynamic studies intermittently assess lower urinary tract function. However, urodynamic studies are not patient-friendly, they exhibit user-to-user variability, and only provide snapshots in time, limiting the ability to collect continuous, longitudinal data. Therefore, a wireless, implantable bioelectronic system for monitoring urinary bladder function was tested in baboons. It demonstrated concordance of pressure measurements in NHPs. These results suggested that this was a suitable alternative to urodynamic studies for long-term postoperative bladder recovery monitoring [41]. One should be aware that anesthesia has shown to have profound effects on the bladder in rhesus macaques and careful interpretation of urodynamic data is suggested [42].

NHPs represent the best animal model for the human respiratory system with respect to anatomy, physiology, immunology, and pathophysiology of the major respiratory infectious etiologies [43]. Whole-body imaging and controlled pathogen exposure and monitoring may be conducted in NHPs. These properties allow for the evaluation of new vaccine candidates and facilitate treatment options. Studies defining the influence of inhaled microbes, pollutants, or allergens on the NHP lung have provided insight on disease pathogenesis, with the potential for elucidation of molecular targets leading to new treatment modalities. In looking to the future, NHP models of respiratory diseases will continue to be instrumental for translating biomedical research aimed at improving human health.

Bronchoalveolar lavage (BAL) is a minimally invasive procedure frequently used for clinical and nonclinical research, allowing studies of the respiratory system. Surprisingly, a study demonstrated that a nonbronchoscopic performed BAL was safe, cost-effective, resulted in higher cell yields per mL, improved animal welfare, and allowed for fewer missed time points, and thus constitutes a refinement over the bronchoscopic BAL in both rhesus and cynomolgus macaques [44].

The shortage of organ donors for transplantation is more pronounced for the lung than for any other solid organ. To address this problem, the feasibility of pulmonary (xeno)transplantation has been investigated in cynomolgus monkeys [45]. The major limitation to using NHPs for this purpose is the susceptibility of animal organs to hyperacute rejection.

Although many procedures have been described for the correction of subglottic stenosis in pediatric human patients, none has been universally satisfactory. Cricoid resection and thyrotracheal anastomosis have been successfully performed in adult humans, thus applying the same techniques to correct subglottic stenosis in infants and children has been considered. The operation has been showed to be feasible in infant rhesus monkeys and is now considered an important treatment option for severe subglottic stenoses in infants and children [46].

NHP models of noise injury and auditory rehabilitation showed to be pivotal to assess and implement innovative technologies for hearing aids before translation to humans. Rhesus macaques and marmosets were indispensable for the development and improvement of auditory cochlear and auditory brainstem implants [47‒50].

The science regarding long-term effects of neonatal/pediatric anesthesia and surgery is still ongoing. The appropriate application and evaluation of the macaque model in the study of general anesthetic-induced developmental neurotoxicity have played a key role in enhancing the understanding and awareness of the potential neurotoxicity associated with pediatric general anesthetics [13]. Beyond pediatric research, the standard anesthetic protocols for NHP surgery are safe and stable, and new research in this field is limited.

NHPs are allowed to be used in experiments only when other models are not appropriate. This is due to different aspects, including their high cost, supply shortage, and animal welfare/ethics issues. In addition, compared to rodents, NHPs models typically require longer time for disease development, making them less practical for researchers. Nevertheless, their inclusion in research is vital to the quest for continued improvements in surgical advancements in human medicine. Novel sophisticated protocols are constantly developed, especially in the field of neurosurgery. These include the use of robotics as they enhance surgical precision and reliability while reducing risks and postoperative complications. Importantly, this review highlights evidence that refined surgical methods benefit not only human medicine but also the animals used in experimentation. Applying these principles and refined techniques allow for an enhanced ability to provide a higher quality of care, leading to successful outcomes. This emphasizes the importance of a collaboration among healthcare staff and researchers for future progress.

Prof. Thomas Hubert was a member of the journal’s Editorial Board at the time of submission. The authors have no other conflicts of interest to declare.

This study was not supported by any sponsor or funder.

J.B., T.B., M.A.G., and T.V. did the bibliographic research. J.B., T.B., M.A.G., T.V., and T.H. wrote the article and contributed substantially to its revision. J.B. did the bibliometric analysis. TH coordinated the writing of the manuscript. All authors take responsibility for the manuscript as a whole.