Importance of the pig as a human biomedical model

Animal models are essential tools for researching human development and diseases, and for identifying effective therapeutics and vaccines. Pigs are similar to humans in anatomical size and structure, immunology, genome, and physiology, and thus preferable to rodent models for translational and clinical research applications (1–7). Multiple attributes place pigs at an advantage over primates and other livestock models including short generation times, large litters, and genomes that are readily editable.

Similarities between the anatomy and physiology of pigs and humans (Table 1) support the use of pigs as a human biomedical model. For example, the similar size and physiology of pigs allow for safe dosage ranges to be defined in drug development studies and toxicological testing (8–10). The ability to generate customized animals, using surgical or drug alterations as well as genome editing methods, makes pigs an important large animal model (11, 12). In recent years, pigs have become the preferred animal for human xenotransplantation because of their similar organ physiology and size, relatively low reproduction costs, and compatibility with genetic modifications for humanization (13–15).

As detailed below, systems such as the porcine respiratory tract share many anatomical characteristics with humans, such as the Waldeyer’s ring and nasal-associated lymphoid tissue, and thus are relevant for the study of respiratory diseases and vaccine development. On the basis of similarities to humans, fetal pigs have been used extensively to study embryonic development and as a clinical practice tool. The systemic and mucosal immune responses of pigs are comparable to humans; immunologic assays can be used to assess innate and acquired immunity for applications including disease and vaccine research (2, 4, 16). The annotation of the pig genome has provided a valuable resource (17), with the newest 11.1 genome build now available (18). Dawson et al. (19) reported a refined pig gene structure annotation on greater than 1000 genes involved in immunity, which contributed to a better characterization of the pig immunome. More recent work has provided further comparative immunome updates (20, 21).

However, there are important differences between pigs and humans, including locations of certain muscles, liver anatomy (humans have four lobes, whereas pigs have six), and placenta barrier type (hemomonochorial in humans versus epitheliochorial in pigs). The latter can be an advantage because gnotobiotic pigs can be raised without any maternal proteins, providing important models for human therapies. With greater scientific understanding and further development of technology, the pig model, especially the transformed and gene-edited (GE) pig model, will be an important tool for translational medical research.

Genome editing in animals has proven to be a very useful tool in advancing biomedical research. Genome editing in the context of this manuscript refers to animals produced through all types of genetic engineering: transgenic, gene targeted, and direct modification [e.g., CRISPR-Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein)]. The first GE pigs were generated in 1985 using pronuclear DNA microinjection in zygotes (22). Other species such as mice are GE and often used to first identify important gene functions, affirming the feasibility of the proposed genome editing before genetic modifications are used in the pig to provide preclinical evidence for human disease and to test treatment options. Thus, the pig complements other model systems. GE pigs have been used to study human cancers, cardiovascular diseases, cystic fibrosis (CF), and neurodegenerative disorders and have made xenotransplantation possible (Table 2) (13, 23). Recently, the National Institutes of Health (NIH) supported expanding the use of genome editing in pigs with creation of the Swine Large Animal Testing Center as part of the Somatic Cell Genome Editing Consortium. Dmochewitz and Wolf (24) provide an extensive overview of the major advancements in genetic engineering in pigs for biomedicine.

Somatic cell nuclear transfer (SCNT) applications provided a major technological advancement in swine genome editing efforts (25–27) and enabled targeted genetic modification (28, 29). Genome-edited donor cell production methods were based on homologous recombination, adenovirus-associated targeting vectors, and modified bacterial artificial chromosome vectors. Since 2011, programmable nuclease-based methods have been generated to enable site-specific modifications by causing double-strand breaks in the DNA that are repaired via nonhomologous end joining or homology-directed repair. The major programmable nucleases include zinc finger nuclease (ZFN), transcription activator-like effector nucleases (TALENs), and RNA-guide endonucleases using the CRISPR-Cas system programmable nucleases (30, 31); efforts were enhanced with proof of direct genome modification of embryos (23). Most of the established GE biomedical models were not generated using programmable nucleases but rather by conventional homologous recombination or addition of transgenes. This is rapidly changing with the availability of precise CRISPR-Cas modifications. Many genome editing approaches have been carried out in somatic cells followed by SCNT rather than in fertilized oocytes.

The advances enabled by CRISPR-Cas are now examples of simultaneous multigene knockouts in pigs, as used for xenotransplantation (discussed below). These advances have been aided by the high-quality pig reference genome (18) and resulted in precision edits and substantial time savings. A disadvantage of using CRISPR-Cas technology for genome editing is the potential for off-target modifications, if guide RNAs are not, or cannot be, ideally designed. Methodologies to perform simultaneous multitargeted edits using this technology are still being optimized; screening and maintaining GE pigs can be time-consuming and costly (32). Genome editing requires many ovaries, which, for pigs, are readily available from abattoirs. Although in vitro maturation methods are well developed for pigs, in vitro fertilization (IVF) methods from a single sperm have been highlighted as a rate-limiting step (33, 34).

Advances in genome editing technologies and targets have tremendous potential to improve current and future pig biomedical models, e.g., using humanized pig models for preclinical testing. Because the pig is also an important food animal, opportunities exist to use genome editing in the context of dual-purpose dual-benefit research. For example, the U.S. Food and Drug Administration recently approved GalSafe GE pigs (29, 35) that can be used for food consumption [providing a source of meat for people who develop tick bite–induced allergic reactions to the sugar, galactose, a condition known as galactose-α1,3-galactose (α-Gal) syndrome] and that also can serve as a source of potential therapeutics for xenotransplantation (noted below). Genome editing is a powerful tool for understanding various diseases in more subtle ways, such as targeting promoters/enhancers to modify gene expression to, for example, alter disease progression or immune system activation. Other GE pig biomedical models will be discussed throughout the remainder of the manuscript and are noted in Table 2.

The porcine eye shares many similarities with that of humans, as reflected in the presence of holangiotic retinal vasculature, cone photoreceptors in the outer retina, no tapetum, comparable scleral thickness, and corneal collagen fibrillar arrangement (36, 37). The pig conjunctiva with its evenly distributed goblet cells is preferable to the rodent, which has clustered goblet cells (38). For selection of best-fit animal model for eye studies, the pig is favored when compared to nonhuman primates (39).

The value of the porcine eye in comparative research has been verified in studies on retinitis pigmentosa, glaucoma, retinal detachment, and transscleral drug delivery [reviewed in (40)]. Radial optic neurotomy treatment of central retinal vein occlusion revealed histological similarities in neuroglial components of the porcine eye (41). The pig ocular surface has been a model for investigating the potential effects of the external environment on existing defense mechanisms (38). Recent therapeutic protease treatments were tested in porcine eyes as models to combat vitreomacular traction (42). A pig corneal defect model has proven efficient in assessing the wound healing effects of hepatocyte growth factor loaded into photoresponsive hydrogel to regenerate epithelial layers and address ocular surface disorders (43). The porcine eye provides a useful model to detail responses to bacterial and fungal infection, adding insight into alternative nonantibiotic treatments, and to visualize the mechanisms, and improve the speed, of wound-healing from such traumas as corneal scratches.

For studies of choroidal neovascularization, porcine ophthalmological tissues were used to analyze histological changes in the retina and choroid for the improvements in interpretation of fluorescein angiograms (44). The cornea, lens, vitreous humor, sclera, iris, choroids, and eyelid of the porcine eye have been characterized for optical parameters between wavelengths 400 to 1400 nm. A simulated complete eye model, created with special software, was used to understand the interaction of light within the eye, providing a benefit for future laser surgery research to assess refractive index, transmission, reflection, absorption coefficient, and scattering coefficients (45).

In cataract repair, the porcine eye model was used to evaluate the efficacy of perisurgical treatment during intraocular lens implantation (46). On the basis of morphological similarities, investigating dry eye development in pigs will help reveal environmental and mechanical stresses that contribute to its incidence in humans. The porcine eye has served as a biomedical model for contact lens research (47). The pig orbital cavity, which provides mechanical protection for the eye, is small in comparison to that of humans; however, it can be used to test materials that can assist in bone reconstruction after physical trauma (40). Pig GE ophthalmology models exist as well (Table 2). A GE model for cone rod dystrophy was generated by GUCY2D-dominant human transgenesis and exhibited functionally impaired vision along with abnormal retina morphology (48). A model for Stargardt-like macular dystrophy was created in pigs with mutant human ELOVL4 transgenes, which showed loss of photoreceptors and disorganized inner and outer segments with diminished responsiveness (49).

Because of its anatomical similarities, the pig is an appropriate model for human craniofacial research in areas including the salivary gland, orbit, nasal cavity, maxilla, mandible, and temporomandibular joint (TMJ). As reviewed by Štembírek et al. (50), the nasal cavity of the pig can be used to study the in vivo histology of bone formation in maxillary sinus augmentation to test noninvasive drug treatments and to treat fungal sinusitis. Investigations into the most common human palate deformity, cleft palate, can be performed using the pig because they have similar maxillary bone regeneration rates (51), thus aiding in developing procedures for correction of cleft palates and assessing postsurgery healing mechanisms. The feasibility of a laser-based, sutureless wound closure system was tested ex vivo on a porcine palate as a first step toward its clinical application in tissue repair and oral surgery (52).

There are some complications with the pig oral cavity model including its long, narrow shape and decreased range of jaw movement as compared to human, making certain dental procedures more difficult than with other models. The study of the blood supply in the mandibular cortex in pigs is critical to research into the molecular mechanisms of mandible reconstruction after jaw injury (50). Human TMJ disorders are often difficult to study in vivo. The pig has been proposed as the best nonprimate biomedical model for TMJ due to its structural similarity, including its internal structures and attachments, making the pig appropriate for testing arthroscopic surgery to correct TMJ disorders and alternate enhancements (50).

Pigs and humans are both diphyodonts, with two successive sets of teeth, a temporary followed by a permanent set, with different types of teeth including incisors, canines, and molars that have the same type of mineralization. In addition, porcine alveolar bone, the part of the jaw that holds the teeth, shows similar bone mineral characteristics to human alveolar bone (53). Periodontitis, a serious gum infection that can destroy the jawbone, is the most widespread gum disease; inflammatory processes in tissues of pigs with this disease are comparable to those seen in humans (50). These similarities make the pig a promising large animal model for dental tissue disease and tooth regeneration research.

Dental implants tested in the pig mandible have ~60% success rate for stability and healing. Stem cells, isolated from the soft tissue attached to the immature teeth embedded in the gum, have been successfully used to form the root/periodontal complex of artificial tooth crowns in the minipig (54). A pig model of periodontitis showed that a placement of an allogenic periodontal ligament stem cell sheet onto the gums of affected pigs resulted in heathier gum status with increased alveolar bone growth after 12 weeks (53). Further pig studies could advance investigations into the mechanisms of correction of teeth abnormalities including undeveloped teeth and extra tooth formation in humans. Additional research into the mechanisms and prevention of jaw osteonecrosis, gum diseases, alveolar bone loss and reformation, as well as tooth germ pulp xenotransplants are underway and should be beneficial.

Pigs have similar femoral bone cross-sectional diameter, area, lamellar bone structure, bone regeneration processes, bone mineral density, and concentration as humans (55). Pigs have been used to study skeletal growth effects on joint biomechanics and engineered replacements of joints and on tissues, knees, bone, cartilage, and ligaments (56). Evaluation of bone tissue, cartilage thickness, permeability, and relative dimensions of fibrous soft knee affirm the advantage of pigs when compared to other large animal models (56). The therapeutic efficacy of cartilage repair and tissue engineering efforts has depended on porcine chondrocytes, in addition to the development of new treatment options for articular defects (57). Pigs with COL2A1 mutations, established using CRISPR-Cas9 and SCNT, exhibited severe skeletal dysplasia, providing a model to investigate skeletal disease pathogenesis and therapeutics (58). Skeletal muscle proteomics of pigs parallel those of humans (59). Thus, pigs are the biomedical model of choice for bone anatomy, morphology, osteonecrosis, healing, and remodeling studies.

Genome editing tools have enabled laboratories to induce dystrophin mutations in several animal models including mice, cats, and dogs; importantly, the severity of the clinical signs rises with increasing body size across species (60). Thus, pigs better recapitulate human disease in preclinical studies of Duchenne muscular dystrophy (DMD) (23). Somatic cell genome editing by sequence-specific nucleases was reported to ameliorate skeletal and cardiac muscle failure in pig models and in a patient-derived induced pluripotent stem cell (iPSC) model (61). A minipig model for DMD verified that the DMD gene can be reverted from nonfunctional to wild type with great success in the germ line (62). A somatic cell GE pig model, lacking exon 52 of DMD (DMDΔ52), induced expression of a shortened dystrophin (DMDΔ51–52) and improved skeletal muscle function (61). A GE pig model for spinal muscular atrophy (SMA), generated through SMN1 knockdown, exhibited loss of motoneurons and motor axons, overt proximal weakness, fibrillations, and reduced compound muscle action potential (63).

Sarcopenia, the loss of skeletal muscle mass and function, is a major public health issue. Risk factors include age, gender, disease, and level of physical activity. A pig model could help develop therapies to prevent or reverse this condition.

To ensure health and safety, animal testing of skin medications is routinely performed before human use. The pig model has been commonly used to study drug toxicity, treatment modalities, burn and wound healing, plastic surgery techniques, and artificial skin grafts. Porcine skin has comparable epidermis, thickness, hair follicle density, and bacteriome to human skin (64–66). Pig GE TYR-null models exhibit typical albinism characteristics (67). Recently, porcine skin was used for ex vivo accumulation and permeation studies, wound healing research, and pharmacology and toxicology studies (68–71). Porcine skin dendritic cell subclasses correspond to the subclasses described in human skin, thus providing the basis for further immune research (72).

Burn-related injuries result in mortalities due to fluid loss, pH imbalances, organ failure, and infection (66). As a wound healing model, the pig allows researchers to test multiple treatment modalities and controls on the dorsal skin of individual animals. A transient use for pig skin also has important applications. Cryopreserved, porcine skin xenotransplants can provide complete epithelial coverage for wound sites and enable victims of severe burns, such as those with injuries on 20% or more of their body, similar therapeutic benefits as the current standard of care (66). The utility of the pig model could improve with the development of tissue-engineered skin for burn injuries (73).

Pig biomedical models for human reproduction have greatly advanced our understanding of the basic science of puberty, fertilization, pregnancy, and disease [reviewed in (74)]. Similarities between pigs and humans, including those related to reproduction, are detailed in Table 3. Some advantages of the pig include multiparous litters, wide availability, short generation intervals, and established protocols for manipulating the reproductive cycle of swine. For IVF, pig ovaries are easily available from slaughterhouses in high quantities for relatively low cost. Human and pig oocyte sizes are similar, and in vitro maturation of porcine oocytes is well established (34); humans and pigs have similar proteome expression patterns during different stages of oocyte maturation (75). Fertilization failure in both humans and pigs is often caused by oocyte cytoplasm insufficiency (76). Porcine fertilization models have been used to test drug toxicity effects on reproduction (77), assess spermicides (78), and optimize culture conditions for effective IVF by investigating proper -omic programming (79).

The pig has provided insight into human pregnancy in areas such as the effects of IVF on pregnancy and offspring outcome, nutrition during gestation, and disease. Although pigs and humans have more commonalities in reproductive characteristics, some important differences should be mentioned such as gestational length (humans 266 days versus pigs 115 days) and uterine structure (humans pyramid and pigs bicornuate). Compared to humans, pigs have a more complex cervical structure and higher ovulation rates, placenta barrier types differ (humans hemomonochorial and pigs epitheliochorial), ovulation rates differ (humans are mono-ovulatory, whereas pigs are multi-ovulatory), as well as number of fetuses/pregnancy (humans typically 1 and pigs 5 to 18) (80). There are important differences between human and porcine embryos, notably the time to maturation after fertilization from zygote to two cells (humans 30 hours and pigs 32 hours) and two-cell stage to hatched (humans 6 days and pigs 5 days) (77). Maternal recognition of pregnancy is also different between humans and pigs; the pregnancy recognition signal in humans is chorionic gonadotrophin, whereas conceptus secreted estrogens are the primary signal in pigs (81).

Multiparous litters are advantageous for the pig model because they provide the ability to isolate specific fetuses and generate varying degrees of intrauterine crowding. The placentation differences are an important aspect because many physiological processes are influenced by this, such as the transfer of some maternal antibodies to the fetus occurring in hemomonochorial, but not epitheliochorial, placentas. Because of its placentation, the pig is an excellent source of gnotobiotic offspring, providing models for testing maternal and nutritional effects on fetuses and newborns. The pig has been a fruitful model for understanding the impact of epigenetic modifications for both development [by comparing piglets derived from IVF to their cloned counterparts (82)] and the impact of sperm methylation profiles on fertility (83).

The pig has been used to model human maternal nutrition during pregnancy and effects of genotype, abiotic stress, and disease on fetal outcome (74) as well as diseases such as aneuploidy, polycystic ovarian syndrome (PCOS), and preeclampsia. Age-associated aneuploidy is considered the main cause of female infertility (84), where fertilized aneuploid oocytes often result in fetal death or severe deficiencies, e.g., trisomy 21 in Down syndrome. Aneuploidy in swine occurs naturally at a rate comparable to humans (85), allowing investigations into mechanisms, and drug development, to improve human outcomes. According to the Centers for Disease Control and Prevention (CDC), PCOS affects 6 to 12% of U.S. females of reproductive age, resulting in irregular menstruation and sometimes infertility. PCOS occurs naturally in pigs: the Ossabaw pig breed has quite similar endocrine and follicular characteristics to humans (86).

Fetal pigs provide models for a wide-ranging set of conditions that appear during the developmental processes of humans. Pregnant sows can be used to test pharmaceuticals to generate preclinical toxicological data for fetal drug therapies (3), such as the use of pregnant Yucatan minipigs to test maternal transmission of drugs for fetal antiarrhythmic therapies (87). Abnormalities such as intrauterine growth retardation (IUGR) occur spontaneously at ~8% in both human and pig neonates, resulting in predisposition to poor metabolic health later in life, making pigs useful models to understand IUGR mechanisms and treatments (88). Fetal pigs have advanced our understanding of hair growth and loss by the identification of the MAP2 gene association with hair follicle density (89).

Future studies with the pig will inform development of improved assisted reproductive technologies. They should provide a more detailed understanding of the complex interactions of environment (e.g., maternal stress, diet, and microbiome) with reproduction and fetal development.

Pigs share many similarities with humans with respect to gastrointestinal (GI) functionality and composition, making the pig an ideal nonprimate large animal model (90). The minimum nutrient requirements for pigs are comparable to the daily nutritional allowances for humans. Nutrient sensing systems are conserved between humans and pigs; pigs can taste acids (sour), carbohydrates (sweet), glutamic acid (umami), and fatty acids similarly to humans, and this affects their food intake (90). Similarities in nutrient tasting systems make pigs relevant for studies of dietary taste testing and appetite regulation. Pigs and humans both use the colon, rather than the cecum, as the main fermentation site of fibrous dietary components. This shared physiology results in similar digesta transit times and processes of nutrient absorption (90).

Porcine GI microbiota share 96% similarity in functional pathways with humans (91); the pig’s fecal microbiota is compositionally similar to humans, although with differences in butyrate producers (92). Genetically defined pigs varying in obesity susceptibility have been used to evaluate effects of gut microbial composition on diet-induced obesity and nonalcoholic steatohepatitis (93). Gnotobiotic piglet models have been used to establish human microbiota–associated piglets using inocula from infants, children, and adults. Bifidobacterium and Bacteroides, two predominant bacterial groups of the infant gut, were successfully established in piglets (94). These gnotobiotic pig models, with completely defined bacterial populations, can be used to investigate host-microbiota interactions and effects of nutritional and probiotic interventions (95). Although costly and logistically complex, these models successfully mimic the human gut environment and can be used to investigate certain harmful bacteria, including Helicobacter pylori, and the mechanism behind its role in causing gastritis and gastric ulcers (90). Comparable microbiota-gut-brain interactions mean that pigs can be used to characterize microbiome changes after neurological events (96), identify at-risk metabolic and neurocognitive phenotypes, test improvements in responses toward malnutrition and unforgiving environments, and combat eating disorders (90).

Newborn pigs are suitable models to investigate acute and chronic effects of formula supplementation versus breastmilk feeding on neonatal metabolism and immune-associated miRNAs (97). Necrotizing enterocolitis, which causes the wall of the bowel to be destroyed by invading bacteria, is the leading cause of mortality related to the GI tract in infants. A model of this disease was successfully established in neonatal pigs and is preferred over mouse models because of the pig’s larger size and ease of gavage feeding (98). Short bowel syndrome, caused by a surgical resectioning of the intestine, results in decreased nutrient absorption and imbalance and is often a complication of necrotizing enterocolitis in infants. The pig model has been used to study in vivo mechanisms and is superior to the mouse because repeated measurements, including stool output and plasma proteins, can be collected for tests of long-term effects and probes of control mechanisms (98).

Metabolic syndrome is a combination of conditions including obesity, dyslipidemia, hypertension, and insulin resistance. Successful models of this syndrome have been established in Ossabaw and Landrace pigs (99). A diet with increased polyunsaturated fatty acid consumption influenced pig intestinal endotoxin transport, which has been linked to obesity, inflammation, and metabolic diseases such as diabetes (100). Further dietary studies have shown that the pig colon, like that of humans, has two distinct resident stem cells (ASCL-2 and BMI-1) and that a high-calorie diet increases expansion of colonic proliferative and stem cell zones (101). Because a high-calorie diet was shown to increase stem cell populations in the colon, the pig model could be used to study colon cancer and other inflammation-promoted diseases (101).

Future studies could investigate the potential therapeutic effects of diet on the gut microbiome or, conversely, the effects of improper diet on dysbiosis, such as the negative impact of the microbiome in women with persistent genital infections. There is the potential to expand knowledge of how physical damage to the GI tract alters microbial content. An environmental application of the pig GI model could be assessing the effects of climate change on microbiome and gut health.

Similar to humans, pig brains are gyrencephalic; this refers to the amount of cortical folding and connectivity of neural fibers (Table 3) (102). In contrast, rodent brains are lissencephalic and do not have cortical folds. Furthermore, the pig brain is larger than the rodent (~180 g versus ~10 g, respectively), readily available, and much less expensive than nonhuman primate brains (103). Human and pig white matter composition is similar (60:40 ratio of white to gray matter). White matter includes cells (oligodendrocytes), which produce myelin, the loss of which leads to brain atrophy and impaired cognitive function, suggesting that the mechanism of brain injury and recovery in pigs and humans may be similar (104, 105). Functional magnetic resonance imaging (fMRI) protocols used to map the brain and reveal changes in blood flow that occur when not performing explicit tasks (known as resting-state networks) have been developed for use in swine. These revealed that the six resting-state networks in the pig brain (executive control, cerebellar, sensorimotor, visual, auditory, and default mode) resemble those of humans (103). Innovative MRI imaging methods have resulted in the development of brain atlases for young and adolescent pigs (106) that should advance understanding the effects of neurodegenerative diseases on neural networks.

MRI images of brains of Yucatan minipigs, from different time points after blast-induced traumatic brain injury, provide the first templates, generated using both linear and nonlinear registration, for future neuroimaging analyses (107). To monitor pain behaviors, a functional neuropathic pain model has been created on the basis of swine induced with a common peroneal nerve injury (108) and will help investigate the mechanisms of neuropathic pain.

Several studies have established functional models of Alzheimer’s disease (AD) in the pig (Table 2). The homeostatic distributions of metals in the pig’s body show potential as a diagnostic for neurodegenerative diseases such as AD (109). Transgenic Göttingen minipigs with insertion of a single gene (APP or PSEN1) showed transgene expression in the brain but exhibited no physical characteristics of AD. Transgenic pigs with the double gene insertion (APP and PSEN1) displayed increases in intraneuronal amyloid beta (Aβ) plaque formation, but they exhibited no physical characteristics of AD (23). Using SCNT with a multi-cistronic vector for three human genes (APP, TAU, and PS1), Jeju black pigs expressed transgenes, Aβ-40/42, total tau, and glial fibrillary acidic protein in brain versus other tissues (110). However, pregnancy and delivery rates from surrogates were low, perhaps suggesting that the AD mutant genes affected embryonic development (110). Mouse models of AD that express mutant APP exist but do not fully replicate all AD hallmarks including neuronal loss (23). Because the neonatal pig brain growth pattern is like that of humans (102), and a pig’s life span is much shorter than that of humans, future studies could closely monitor the effects of aging in the pig brain to assess the mechanism of Aβ plaque formation. This could lead to studies to evaluate whether some memory capacity could be restored in patients with AD or anxiety and fear-related behaviors altered.

Parkinson’s disease can be induced in Göttingen minipigs by administering N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, which resulted in clinical symptoms including muscle rigidity, hypokinesia, and impaired coordination (111). Analysis of the brain revealed that dihydroxyphenylalanine (DOPA) decarboxylase—responsible for the synthesis of dopamine, a regulator of neurodegenerative disease—may be up-regulated in the pig striatum (67). More recently, Göttingen minipigs were injected unilaterally in the nigrostriatal pathway with 6-hydroxydopamine to decrease dopamine concentrations at the site of injection and destroy catecholaminergic neurons. This model showed dopaminergic imbalance and rotational behavior similar to Parkinson’s disease in humans, symptoms that were verified and partially ameliorated using subthalamic deep brain stimulation (112). A GE pig model for Parkinson’s disease was generated and showed DJ-1 protein disruption, but piglets died within 2 days of birth (113). Pigs can now be used for neurotoxicology studies and for testing the potential of drugs for prevention and treatment of neurological diseases.

SMA is an autosomal recessive neurodegenerative disease caused by a deletion or mutation of the SMN1 gene. Lorson et al. (114) expressed the human SMN1 and SMN2 minigenes in porcine cells and reported the genetic knockout of SMN1. Later work reported the creation of the first large animal model of SMA and explored mechanisms to correct the SMA phenotype (63). Ataxia telangiectasia (AT) is a neurodegenerative disease usually found in children, resulting in motor impairment, immune disorders, and increased susceptibility to cancer. A pig model was created using SCNT and targeted exon 57 of the endogenous ATM to recreate the disruption in the protein, which is found in AT patients. These pigs presented all associated characterizations of AT including cerebellar lesions, loss of Purkinje cells, and stunted growth. Previous mouse models had been unable to recreate all hallmarks of the disease (115).

A pig model of neural degeneration after traumatic brain injury, by cortical impact, hemorrhagic shock, and/or polytrauma, showed a decrease in neural apoptosis, degeneration, and inflammation 30 days after dosing of valproic acid, thus verifying the potential of swine as treatment models (116). A model for amyotrophic lateral sclerosis (ALS) was generated via pigs with the SOD-1-G93A-human transgene and exhibited both hindlimb motor defects and motor neuron degeneration (117). A gene “knock-in” swine model of Huntington’s disease (HD), developed using CRISPR-Cas9 and SCNT to insert 150 CAG repeats into the HTT gene, similar to what is seen in humans with HD, provided the first evidence that misfolded proteins at the endogenous amounts cause neurodegeneration in large mammals (118).

Because pigs differ from rodents and have a longer stage of puberty, with well-established puberty induction protocols, they can be tested for effects of puberty on neurodevelopment in a controlled population over time. For neurodegenerative disease-susceptible brains, the differences in plasticity of young versus old animal brains can be analyzed (102). Future work could study pigs for behaviors related to brain function and the distribution of neurotransmitters during neurological disease progression.

Investigations into heart diseases have typically used mice, dogs, and sheep as experimental animal models. Because differences between animal models decrease as body/heart weight ratio increases, pigs are an advantageous model in which to study heart disease (8). Pigs show similarities to humans in heart organ size, immunology, physiology, and gross morphology—specifically, similarities in structure of the atriums, ventricles, and coronary circulation pattern. Pigs have contributed to the development of cardiac reperfusion procedures and coronary balloon catheters (8). Chronic studies of cardiac function and response can be performed in vivo, with echocardiographic measurements comparable to adult patients, supporting the physiological and clinical relevance of the pig model. With analogous cardiovascular growth to humans, swine are ideal models for the development and further improvement of surgical procedures.

Because of the early asymptomatic stages of atherosclerosis in humans, pigs overcome ethical limitations associated with the evaluation of arterial endothelial function and disease of human subjects (119). Pigs have been critical for establishment of a long-term model to study right ventricular dysfunction and desynchrony observed in patients with tetralogy of Fallot, following surgical repair related to congenital heart disease, and to explore improvements to reperfusion procedures for treatment of blockage-induced myocardial infarction (8). In heart failure syndrome, porcine models have provided integral insight into molecular changes associated with myocardial failure and deterioration otherwise unattainable in human patients (120).

Tissue from porcine hearts has been used to develop human heart valve and blood vessel replacements (121, 122). Porcine coronary stent models are hallmarks of preclinical design experiments designed to identify acute stent thrombosis risk factors. The use of pig organ scaffolds, after decellularization and recellularization with human cells, for human heart transplantation has had some success (123). However, tests are needed for scaling up cell preparations, delivering cells throughout the heart, and assuring bioactivity within the scaffold. Evaluation of the interactions of pig-derived heart valve scaffold transplants with host tissues has affirmed the importance of the pig in heart valve tissue engineering (124). Options exist to replace infarcted areas with functional constructs or regenerated progenitor cells for treating cardiovascular disease issues.

Imaging technologies typically used for humans can be applied to pigs because of their similarities in size and have aided in the validation of innovative cardiovascular imaging techniques and identification of biomarkers (125, 126). Pig models have resulted in improved efficacy of emergency procedures, such as development of the portable cardiopulmonary bypass system, extracorporeal membrane oxygenation system, ventricular fibrillation technique, and cardiopulmonary resuscitation (127, 128). Pig models of cardiovascular disease have been generated using genome editing as well (Table 2). The GE pig cardiac arrhythmia model generated via homologous recombination for generation of the SCN5A-E558X mutation, followed by SCNT, showed slowed conduction and increased susceptibility to ventricular arrhythmias (129). A GE pig model for cardiovascular disease, generated via targeted disruption of LDLR, showed hypercholesterolemia and atherosclerotic lesions (130). A hypertrophic cardiomyopathy GE model has been created through MYH7-R723G-knock in; neonates displayed mild myocyte disarray, malformed nuclei, and MYH7 overexpression but died within 24 hours of birth (131). Porcine adipose tissue has been used to refill the pericardium of three-dimensional (3D) decellularized heart scaffolds and to promote heart cell proliferation and differentiation (132). Pig models can be used in preclinical studies for drug feasibility and safety testing.

Pigs and humans have similar lung lobar and bronchial anatomy, making them useful for modeling respiratory diseases and therapeutic testing (133). One notable distinction between humans and pigs is the number of lobes on each side of the lungs: Humans have three and two lobes on the right and left, respectively, whereas pigs have four and two lobes on the right and left, respectively (134). In addition, the conserved architecture and topologic specificity of lung extracellular matrices support the use of pig lungs for tissue engineering human lungs (135, 136). Decellularized pig tissues show promise as scaffolds for growing human tissues with similar structural morphology; they can retain organ-specific extracellular matrix for regenerative medicine (136). Decellularized liver scaffolds maintain tissue-specific microarchitecture of matrix proteins, providing spatial cues for cell adhesion, growth, and differentiation. Porcine lung tissues have been used to investigate orthotopic transplantation techniques (135). Similarities in collagen retention, glycosaminoglycans, and conservation of human cell types observed in porcine lungs support their potential as models for human lungs (132). Opportunities exist for further investigation into pig models of rare respiratory diseases or as models for respiratory function and disease, e.g., in influenza research (discussed below).

The development of the GE CF pig was a major advancement; it was the first GE pig that replicated a human genetic disease (137, 138) and the first CF animal model to develop spontaneous CF lung disease (139). CF is an autosomal recessive disease caused by a mutation in CFTR,an anion transporter gene. Human and fetal pig sinuses and lungs are lined with similar cells and bacteria and share similar hallmarks of disease; treatments investigated in fetal pig sinuses may be beneficial for lung diseases associated with CF (140). The main cause of mortality in patients diagnosed with CF is airway obstruction caused by recurrent to chronic inflammation, infection, and mucus secretion (141). Genome editing has recapitulated the human CF phenotype in swine (as compared to mice). Three GE pig models (Table 2) exhibit exocrine pancreatic destruction, abnormalities in vas deferens, focal biliary cirrhosis, and lung disease with inflammation and infection (23). The CF pig models helped define the adverse effects of acidic pH in airway surface liquid that leads to increased susceptibility to infection (142) and sticky mucus (143, 144). The CF pig models have been used to study host defense (145) and alleviation of meconium ileus in fetal pigs (146). Using CF pigs, an association was discovered between FGF10 and CFTR during proximal airway development (147). These pig models have been used to test gene therapies and were successful in improving anion transport and reduction in bacterial infection frequency (148). The role of CFTR during lung development can now be explored for prevention and treatment of CF using fetal pigs to investigate interactions between cell signaling pathways and CFTR, for treatment during early developmental, as well as testing the effect of manipulation of CFTR production on sinus deformities at birth (140).

The morphology and development of the pig pancreas resemble that of humans (149); pig insulin secreted by the pancreas differs from human insulin by only one amino acid (150). Furthermore, the endocrine cells of the islets are dispersed throughout the exocrine pancreas, in differing cell proportions, whereas the mouse has a defined islet structure, making it a poorer model (151). A 50% decrease in the mass of β cells found in the islets of both pigs and humans correlated with reduced insulin secretion and impaired glucose tolerance in both species. In contrast, mice have a larger decrease in β cell mass that does not affect glucose tolerance (23).

Of the Bama, Wuzhishan, and Guizhou miniature pig breed diabetes models, Wuzhishan miniature pigs were the most sensitive to diabetes induction with high-dose streptozotocin, which causes chemotoxicity to pancreatic islet β cells, and appropriately modeled the pathological changes of human diabetes (150). Numerous GE pig models (Table 2) have been generated to study diabetes, as previously reviewed (152). One porcine diabetes model is known as the mutant INS gene-induced diabetes of youth (MIDY) model. In MIDY pigs, mutations in the INS/INS2 gene leads to misfolded insulin in the endoplasmic reticulum, which induces β cell apoptosis (153). The induced diabetes in these pigs is characterized by impaired insulin secretion, increased fasting glucose amounts, and decreased β cell mass (23).

Pigs play a prominent role in drug testing and alternative studies for treating diabetes. Because many areas of the body are affected in diabetes, research is needed into the cross-talk between organs and tissues that causes complications during this disease. The search for previously unidentified biomarkers to detect diabetes at earlier stages is essential (99). There is a potential role of the pig as a donor of pancreatic islets for xenotransplantation (discussed below) or, alternately, exploration into using the pig as a host for growing a fully tissue-engineered human pancreas (23).

Pigs are a powerful model to study genetic predisposition, progression, detection, and treatment of cancers (154). Our understanding of the molecular mechanisms underlying human cancers has been greatly aided by studies in mice; however, cancer research in mice has limitations for which the pig can help bridge the gap for translational medicine. Cancer progression is more accurately studied in pigs compared to mice because of their average life span (humans, 79 years; pigs, 15 to 20 years; and mice, 12 months). Surgery, diagnostic imaging, and therapeutic testing are easier to perform in pigs compared to mice because of similarity to humans in average body size (humans, 65 kg; miniature pigs, 80 kg; and mice, 20 g). Swine cancer models have been available for more than 20 years. Well-characterized spontaneously occurring melanoma models include the melanoblastoma-bearing Libechov (MeLiM) (155) and Sinclair miniature pigs (156) that both show skin abnormalities early in life but whose tumors spontaneously regress thereafter. Canines have been a fruitful model to study spontaneous cancer development, complementing pig studies by providing insight into therapies aimed at addressing the escape phase, when tumor cells breach the host’s immune defenses (157). Although most pig models are GE, chemically induced swine cancer models, for example, using liquid plastic injected into pig kidneys to study kidney tumors, have been developed; however, inconsistencies and poor tumorigenesis limit the usefulness of these models (158).

Swine GE cancer models are most often produced by targeting changes to DNA that affect the function of proto-oncogenes, tumor suppressor genes, and DNA repair genes (Table 2). These models are generated through germline modification, somatic genome editing, autologous graft, and xenograft (159). Advances in genome editing have greatly aided the development of new swine models of breast cancer, colorectal cancer, pancreatic cancer, osteosarcoma, and melanoma, among others (23). These cancer models vary in degree of characterization. For example, in the breast cancer model, piglets died within 18 days of birth (160), and the colorectal cancer pig models produce polyps but no tumorigenesis (161). A pig cancer model for neurofibromatosis type I was recently developed (162); a longitudinal study revealed highly similar phenotypic characteristics and disease progression compared to the disease in human children (163).

The well-characterized transgenic Oncopig encodes a Cre-inducible tumor suppressor (TP53) and oncogene (KRAS); injection of adenovirus encoding Cre into these pigs results in rapid development of tumors of mesenchymal origin (164). The Oncopig recapitulates various human cancer phenotypes of soft tissue sarcomas, pancreatic cancer, and hepatocellular carcinoma and permits profiling for early cancer detection (165) and investigation into tumor cell microenvironment signaling (166).

Autologous graft GE models are generated by removing cells/tissues from a host, editing the removed cells/tissues, and then reimplanting them back into the original host. The GE colorectal cancer model is generated by introducing a premature stop codon in APC; these GE pigs develop colon polyps (23). Polyps can be removed, gene-edited a second time to target tumor suppressor genes such as P53, and reimplanted in pigs to produce variable colorectal cancer disease phenotypes (159), enabling research into oncogenesis and therapeutics.

An example of a xenograft model for cancer is the severe combined immunodeficiency disease (SCID) pig, which is most often produced via genome editing but occurs naturally as well (Table 2). Recent advances in classifying SCID pigs has affirmed them as the most suitable model for human SCID (167, 168) because their immune system, X-linked heritability, typical natural killer (NK) cell phenotype, and anatomy are comparable to SCID humans (169). SCID pigs have been created using CRISPR-Cas9 with modification to IL2RG and RAG2 genes (170). Naturally occurring mutant SCID pigs have been used to grow human cells for transplantation (171). Humanized SCID pigs can be created by transplanting human hematopoietic stem cells, peripheral blood mononuclear cells (PBMCs), or fetal tissue cells into SCID pigs to create a human immune system (171). More recently, cultured human CD34⁺ selected cord blood cells were injected in utero into fetal ART^−/−IL2RG^−/Y SCID pigs; at birth, human CD45⁺CD3ε⁺ cells were detected in cord and peripheral blood (172). As the technologies improve, our ability to recapitulate human cancers in SCID pigs will facilitate our understanding of disease progression and immune control, as well as provide models for preclinical testing of cancer therapeutics, helping to accelerate the field of cancer biology.

Although numerous cancer models have been developed, more efforts are needed to produce additional GE pig lines that better recapitulate human cancer phenotypes and enable exploration of mechanisms of cancer progression, alternative assays for preclinical testing, and identification of innovative drugs and biotherapeutics. Opportunities exist to study the complexities of environment by genetic interactions caused/modulated by drug exposure, microbiome composition, and diet.

Studies of infectious diseases in pigs not only benefit the swine industry but also serve as models for human medicine, help limit zoonotic diseases, and improve disease control by probing for drugs, therapeutics, and vaccines. Pigs have been a source of zoonotic disease for humans (and vice versa), from bacterial (salmonellosis) to viral (influenza) to parasitic (toxoplasmosis) diseases. Correlates of protection that developed in one species have informed treatments in other species, the “One Health” concept. Understanding basic viral disease biological mechanisms could benefit development of human therapies, as has been pointed out for coronavirus-induced diseases and related therapies and vaccines (173). Pigs have advantages over the mouse model for studying human infectious diseases because of their physiology, high genomic and protein similarity, and reports of >80% immune parameter similarity (2).

GE and humanized SCID pigs can provide alternate models to generate preclinical data on therapeutics for relevant diseases, such as immune mechanisms associated with influenza protection and recovery in a natural host (174). Using genome editing to generate pigs that respond more effectively to infectious diseases can enhance our understanding of mechanisms of disease resistance. However, most genome editing in pigs has focused on germline modification to generate resistance for diseases of pigs, such as porcine reproductive and respiratory syndrome (PRRS) and African swine fever (175, 176).

Using pigs to study salmonellosis can not only address the disease in pigs but also provide an important resource for research of human salmonellosis, informing development of vaccines and drugs for both species. Zhang et al. (177) demonstrated that the pig intestine can be colonized with human fecal microbiomes to generate a realistic model of the human GI tract. In addition, research using 3D intestinal epithelium models has been generated (178). Pharmacokinetics of danofloxacin and amoxicillin in pig models infected with Salmonella have been reported (179, 180), providing a reference for drug testing for human salmonellosis. This is especially important considering increasing evidence of Salmonella resistance to antibiotics. Alternative methods, such as use of probiotics and lactic acid, need to be explored for controlling Salmonella infections and for potential use as adjuvants for vaccination (181, 182).

Meurens and colleagues (2, 183) noted that pigs are an ideal model for vaccine research, enabling a more accurate assessment of the efficacy of potential human vaccines, particularly for respiratory diseases. Some progress has been made in the study of pig Salmonella vaccines (184), but there have been few reports of pigs as an animal model for evaluating human Salmonella vaccines. Considering the current need, pigs provide an excellent opportunity for development of such a vaccine.

Influenza is an important public health problem, causing morbidity and mortality worldwide (185). Existing vaccines are not fully protective; research into vaccines and drugs are continuously under development. Pigs are a promising animal model with substantial human translational value to provide important insights into influenza virus infection, pathogenesis, and immunity (186). Compared with other animal models, pigs have several advantages: (i) The influenza viruses that infect humans and pigs have the same subtype (H1N1, H1N2, and H3N2) and similar epidemiology (2). (ii) Influenza virus polymerases have the same adaptation process in humans and pigs (187). (iii) The virus has a similar infection mechanism in humans and pigs (188). Optimization of the pig biomedical model can better reveal the mechanism of human influenza virus infection (189) and explore therapies for inflammatory reactions caused by infection (190), the development of influenza vaccines and adjuvants (191), and the evaluation of vaccines (192). Given the current limitations of anti-influenza virus drugs (193), it may be possible to carry out research on new drugs or alternatives in pig models. Preventing vaccine-associated enhanced respiratory disease (VAERD), considered an important obstacle in vaccine development, could also be studied.

To reduce the incidence of tuberculosis (TB) in children, the World Health Organization (WHO) recommends additional research to elucidate host-pathogen interactions and neonatal and infant immune responses to Mycobacterium tuberculosis infection. Ramos et al. (194) have pointed out the critical role that the neonatal pig can play for exploring TB control and vaccine development. Such studies will provide new insights for human anti-TB immunity and accelerate vaccine research for neonatal TB control.

Models are needed to study the efficacy of vaccine candidates against genital human Chlamydia trachomatis infection, the most frequent bacterial sexually transmitted disease that leads to ectopic pregnancy and infertility (195). The pig has already been used to study genital Chlamydia infection (196) and shown similarities with humans in disease pathology and mucosal immune system responses. The structure of the porcine cervix allows for independent study of infection in the upper and lower reproductive tracts. Access to reproductive tracts from the slaughterhouse also allows for frequent low-cost isolation of primary epithelial cells. Amaral et al. (197) showed that for Chlamydia suis, closely related to C. trachomatis, vaccination of preexposed pigs effectively boosted the immune response, decreased C. suis burden, and induced CD4⁺ effector memory T cells. This pig model will be applied to explore vaccines and treatments for human Chlamydia infections. Other reproductively transmitted diseases such as PRRS, the virus for which can be transmitted in semen or congenitally from the mother, both resulting in reproductive failure (198), provide potential models for congenital viral transmission. The study of pig models of reproductive diseases will help promote the development and utilization of drugs and therapeutics for control of human reproductive infections.

Currently, more than 100,000 patients are on the U.S. organ transplant waitlist, whereas only about 28,000 transplants are performed using organs from deceased donors. This deficit could be improved using xenotransplantation (cross-species: pigs to humans) with the potential to offer an abundant supply of organs (199). Xenotransplantation of pig heart valves to humans has been happening for more than 50 years (200). Here, we focus on the most recent advances in xenotransplantation using CRISPR-Cas9 technology in the pig, its challenges, and future opportunities (Table 2).

Great strides have been realized in xenotransplantation because of genome editing in the pig, as has been reviewed elsewhere (201). Because of the physiological similarity with humans and the high demand in human patients, pig corneas, hearts, kidneys, livers, lungs, neuronal cells, and pancreatic islets have been well studied as candidates for xenotransplantation. Transplants from GE pigs have been tested in nonhuman primates for each organ and have had some success (202). Heart transplantation from GE pigs to nonhuman primates achieved preclinical efficacy with 75% survival rate for a prolonged period (>3 months after transplantation) (203). Careful screening of porcine cytomegalovirus in donor pigs was advised, as the two nonsurviving animals had succumbed to this infection. Although the standard ischemic cardioplegia methods have been successful in human allotransplantation, these techniques result in multiorgan failure for pig hearts transplanted to baboons (204). Promising new techniques have been developed to increase survival in pig-to-baboon (and future pig to human) cardiac xenotransplantation using cold non-ischemic heart preservation with continual perfusion (204). Advances in kidney transplantation from GE pigs, along with immunosuppressive therapy, have greatly increased survival time (up to 1 year in the rhesus model) (205).

The major barrier for using pigs as human organ donors is transplantation rejection, due to the phylogenomic distance of the two species, which triggers a hyperacute immune rejection response in the host. Xenotransplantation from a non-GE pig to a human results in rapid hyperacute rejection mediated by the humoral system with antibodies in the human that bind to epitopes on the porcine cells and activate the complement system (206). The α-Gal epitope is the main mediator of this hyperacute rejection (207): It is present in most mammals, including pig, but not in humans. The production of GE pigs lacking α-Gal, the GalSafe GE pigs (29, 35), was tested with optimism. However, acute rejection of non-Gal antigens hampered successful xenotransplantation (208). To solve this barrier, two important non-Gal epitopes have been identified: Neu5Gc (209) and B4GALNT2 (210) have helped make progress toward successful xenotransplantation. There are many examples of GE pigs that targeted removal of these or other epitopes, as well as additions of important human genes (202). Rejection mechanisms, such as acute humoral xenograft rejection, thrombotic microangiopathy, and coagulation dysregulation, must be resolved, and proper immunosuppressive regimens must be developed to increase host and organ survival. More research is needed to find immune-related strategies for extending the life and function of porcine tissues such as islets for potential human treatments (23). As genome editing technology improves, producing pigs with multiple edits with high efficiency will advance xenotransplantation.

In addition to host transplantation rejection, concerns of zoonotic disease transmission from pig to human have slowed successful adoption of xenotransplantation. Most zoonotic diseases of pigs can be eliminated by rearing animals in a highly sterile, pathogen-free environment. However, some endogenous retroviruses that integrate into the host genome can infect non-host cells. In vitro studies showed that porcine endogenous retrovirus (PERV) can pass from pig cells and integrate into the genome of certain human cells and thus could theoretically cause tumors in recipients (211). Pigs deficient in PERVs were generated via genome-wide silencing of all 25 PERV integration sites using CRISPR (212). As the technology continues to evolve and become more efficient, the possibility of creating multiple herds of pigs, with numerous gene edits targeted for each organ to be transplanted, is becoming a reality (150). If successful, these animals could be used to transplant any cell, tissue, or organ to humans.

Pig islet cells have been used in xenotransplantation to reduce the impact of diabetes. Pig islet cells that were encapsulated with barium alginate, which protected them from attack by the host’s immune system, successfully normalized glucose regulation in diabetic rats (213). After these encapsulated islets were removed, 60% of the rats returned to a hypoglycemic state, supporting that the xenotransplanted cells were responsible for the insulin regulation. Similar success has been achieved in transplantation of pig islets into nonhuman primate models (214). Encapsulated pig islets have been safety tested through clinical trials in New Zealand (215). However, isolated adult pig islets often secrete about 10-fold less insulin than human islets in response to glucose alone (216).

One exciting potential use of pigs is for stem cell therapy. Studies have demonstrated the successful isolation of mesenchymal stem cells from multiple origins such as bone marrow, skin, umbilical cord blood, amniotic fluid, peripheral blood, endometrium, and Wharton’s jelly (217). Porcine stem cells are predicted to be beneficial for treatment of Parkinson’s disease, cardiac ischemia, and hepatic failure (217). A caveat is that some observed anatomical differences between human and porcine tissues will yield challenges to the investigation of anesthetic response, age effects, and dosage comparisons across tissues (56).

An emerging area in xenotransplantation is patient-derived xenograft (PDX) where diseased human tissues are transplanted into GE pigs to improve our understanding of disease progression and enhance development of therapeutics. SCID pigs offer a model to study PDX among other interesting questions (170). Another emerging area in transplantation is the human-pig chimera. Chimeras resulting from GE pig embryos lacking lineages of cell types (e.g., cardiac) are transplanted with human iPSCs for eventual reimplantation into humans (218). This technology is relatively new and has ethical considerations as well as technical obstacles, e.g., preventing iPSCs from generating other cell lineages, matching developmental stages for transplantation, and improving survival of donor cells. Testing expansion of human iPSCs in pig decellularized organ scaffolds will help address some of these issues.

This review summarizes and updates the advantages, recent applications, and substantial potential for the use of pigs as human biomedical models as summarized in Fig. 1. Applications include heart and reproductive studies, growth and development, disease mechanisms and drug testing, vaccine design, and xenotransplantation options. Recent progress in genetic engineering of the pig and in humanization options has further enriched the variety of pig models available and, with increased knowledge of the pig genome and immune system, will enhance the potential of this species model. There are some limitations to using pigs as biomedical models when compared to small animal models: Pigs require more space, feed, and specific management protocols and are more expensive. Despite these limitations, the pig model should be embraced more frequently as the preferred model for human disease and translational medicine research. The continued development of basic science and applied technologies will enhance research pursuits using pigs.

We thank K. Summers, B. Rosenthal, J. Driver, and A. Pasternak, as well as the Science Translational Medicine reviewers, for their thorough reviews and suggestions for improving this manuscript.

Funding: This work was supported by USDA ARS project 8042-32000-102. C.D. was supported by Yangzhou University International Academic Exchange Fund YZUIAEF201901005. T.H.’s salary was supported by funding from USDA-NIFA AFRI grant no. 2019-67015-29815.

Competing interests: The authors declare that they have no competing interests, consulting agreements, or patents associated with this area.