Type 1 Diabetes Mellitus

How similar are human and animal endocrine systems?

 

This is not an exhaustive list of species-specific differences, nor can one be made given their unknown full extent, but rather an example of how these differences impact the face, construct, predictive, and intrinsic validity of animal models.

 

Not all species-specific differences can be accounted for in animal models, as there are hundreds of them, their relevance to Type 1 diabetes mellitus (T1DM) is unclear, and their interaction with other organ systems in the animal model makes it difficult to predict how they would have behaved within the human system.

 

Endocrine systems of human and non-human animals differ at every level, from molecular to organism level, contributing to poor translation of animal studies to humans.

 

​

Species-specific differences in structure and function of pancreatic islets

Inter-species differences in cytoarchitecture of pancreatic islets of Langerhans have consequences on the function of islets, susceptibility to diabetes, and disease pathophysiology.

In the islets of Langerhans, alpha, beta, delta, gamma and epsilon cells secrete hormones insulin, glucagon, somatostatin, pancreatic polypeptide and ghrelin, that by regulating, stimulating and inhibiting each other through paracrine and autocrine communication, maintain balanced blood glucose levels and metabolic homeostasis.

 

In human islets, most beta, alpha, and delta cells are aligned along blood vessels with no particular order, indicating that islet microcirculation likely does not determine the order of paracrine interactions Bosco et al., Diabetes, May 2010.

 

Imaging of cytoplasmic free Ca2+ concentration, [Ca2+]i, has shown that beta cell oscillatory activity was not coordinated throughout the human islet as it was in mouse islets, suggesting inter-species differences in beta-cell network dynamics and insulin release patterns.

Mathematical modelling of Ca2+ activity in human and mouse islets showed that Ca2+ activity in human islets was more vulnerable to inhibition of highly connected beta cells, termed hubs, than in mouse islets Lei et al., Islets, Aug 2018. In contrast to human islets, mouse islets have a more uniform beta cell distribution, making them less dependent on hubs. This observation suggests that humans may be more susceptible than mice to loss of hubs, that can occur through autoimmune attack (T1DM) or toxicity/inflammation (T2DM).

 

Furthermore, human islets responded with an increase in [Ca2+]i when lowering the glucose concentration, which could be explained by the fact that human islets contain proportionally fewer beta cells (that produce and secrete insulin, which lowers blood glucose) and more alpha cells (that produce and secrete glucagon, which raises blood glucose) than do mouse islets Cabrera et al., PNAS, Feb 2006, pointing to inter-species differences in glucose regulation.

 

The neurotransmitter acetylcholine has a major role in the function of the insulin-secreting pancreatic beta cells. Cholinergic input to exocrine acini ducts and endocrine islets of Langerhans stimulates secretion of digestive enzymes and hormones.

In contrast to mouse islets, cholinergic innervation of human islets is sparse. Instead, the alpha cells of human islets provide paracrine cholinergic input that primes the beta cell to respond optimally to increases in glucose concentration. Cholinergic signaling pathways within human islets could potentially be a therapeutic target in diabetes Rodriguez-Diaz et al., Nature Med., Jun 2011. However, because of inter-species differences in pancreatic islets structure and function, this therapeutic lead, and probably many other leads that have the potential to benefit patients, are likely to have failed in animal models of diabetes.

 

​

Species-specific differences in gene expression

As it is the case for the overwhelming majority of species, humans have a single insulin gene. Rodents, however, have two non-allelic insulin genes that, despite their differences in structure, are both equally expressed. It is unclear why rodents have two insulin genes and whether they might have different functions, which limits extrapolation to the single insulin gene system in humans Hay & Docherty, Diabetes, Dec 2006, Irwin, Front. Endocrinol., Apr 2021.

The effects of insulin gene duplication in rodents may be further amplified, since insulin transcriptionally regulates the expression of more than 150 genes in various tissues.

 

Differences in sequences of insulin genes between humans and animals affect the regulation of expression of insulin genes, as well as the structure and function of encoded proteins Chandrasekera & Pippin, Altex, May 2014.

Overall homology of the insulin promoter between humans and rodents is only around 45 to 48%. Many features of cis-regulatory elements and trans-acting factors are differentially regulated in human and mouse islets Hay & Docherty, Diabetes, Dec 2006,

There are also marked inter-species divergences in DNA binding between mouse and human glucose regulatory transcription factors. As much as 41%-89% of the orthologous promoters bound by a protein in one species were not bound by the same protein in the second species.

Species-specific differences in cis regulatory elements (transcription factor binding motifs, epigenetic marks) and trans regulatory elements (expression levels, post-translational modifications) affect how in each species the glucose related-genes respond to insulin.

Subsequently, the auto-regulatory mechanisms by which genes involved in glucose metabolism regulate their own expression and expression of other related genes via cis and trans inputs, diverge between humans and other species.

For instance, while the gene expression of gluconeogenic enzyme phosphoenolpyruvate carboxykinase is highly regulated by insulin in rodents, this insulin-mediated repression is less pronounced in humans, suggesting alternative regulatory pathways Cherrington et al., Endocrinology, Jun 2011.

​

The inter-species differences in insulin protein sequence and post-translational modifications are likely to manifest in inter-species differences in insulin function and proteolytic processing.

For example, in guinea pigs, that have highly divergent sequences of insulin genes, the biological activities of these insulins differ, acting more as a growth factor than as a metabolic hormone Irwin, Front. Endocrinol., Apr 2021.

Similarly, as a consequence of specific changes in sequences of insulin genes, some species of New World monkeys have insulin hormones with lower potency, despite sharing 85.5% identity with the human insulin precursor.

Of particular relevance for T1DM, inter-species differences in insulin protein sequence can influence how the immune system recognizes insulin and targets insulin-producing beta cells. In case of T2DM, they may impact how insulin binds to its receptor and thereby produce inter-species divergences in pathophysiology of insulin resistance. The response of T1DM animal models in preclinical trials to gene therapies via introduction  the human insulin gene may yield results that are not translatable to humans.

​

 

Species-specific differences in glucose regulation

Even though insulin is essential for maintaining optimum blood glucose levels in all vertebrate species, its function has evolved in species-specific manners.

The two most frequently used species to model T1DM/T2DM, mice and rats, differ significantly from humans at gene, protein, pathway, cellular, tissue, organ and organism levels of glucose regulation Chandrasekera & Pippin, Altex, May 2014.

  

Notably, inter-species differences between rodents and humans were showed in intracellular signal transduction and glucose metabolic pathways.

For example, glucose metabolism via anaplerotic enzymes pyruvate carboxylase and ATP citrate lyase was reported to be lower in human islets compared to rodent islets. Compared to rodent islets, human islets appeared to be less dependent on PC for insulin secretion MacDonald et al., J. Biol. Chem., Mar 2011. Such differences in glucose metabolic pathways are critical for understanding the regulation of human glucose homeostasis under normal and diabetic conditions.

 

The principal glucose transporter present in rodent beta cells is glucose transporter 2 (GLUT2), and greatly reduced GLUT2 expression levels have been shown to correlate with elements of T2DM in various diabetic rodent models. However, human islets predominantly express GLUT1 and GLUT3, and GLUT2 expression levels do not correlate with human T2DM van den Bunt & Gloyn, Diabet., Jun 2012.

 

Inter-species differences at the cellular level may also be responsible for the difficulties encountered in modeling irreversible beta cells degeneration in animal models of T1DM.

In the murine beta cell G1/S proteome, the E2F2 transcription factor is abundantly expressed, whereas human islets lack E2F2, but contain E2F3 and E2F7 transcription factors that are absent in murine islets. In humans, E2F3 and E2F7 are involved in regulation of cell cycle progression, proliferation, and metabolic processes, and their dysregulation may contribute to beta cell failure.

The cyclin family members, that regulate the cell cycle progression from G1 to S phase by activating cyclin-dependent kinases, play an essential role in survival and proliferation of beta cells.

The existing inter-species divergences in structure and function of cyclins Fiaschi-Taesch et al., Diab., Jun 2013 are therefore also likely to play a part in the lack of ability of animal models of T1DM to recapitulate the immune-mediated destruction of beta cells.

 

The stimulus-secretion coupling in human beta cell differs greatly from other species with respect to ion channel composition and function.

The ATP-sensitive potassium (KATP) channel plays an important role in insulin secretion, by linking glucose metabolism-related ATP production to membrane depolarisation. In humans, polymorphisms in the KATP channel subunit genes have been associated with increased risk of T2DM. However, the loss of function in subunits that compose KATP channels does not cause dysregulation of insulin secretion in mice, probably due to unknown species-specific compensatory mechanisms Aguilar-Bryan & Nakazaki, Rec. Prog. Hor. Res., 2011, Rodriguez-Rivera & Barrera-Oviedo, Int. J. Mol. Sci., Apr 2024.

 

There are also inter-species differences in functional importance of calcium channels that modulate the kinetics of insulin release and glucose-mediated suppression of glucagon secretion Fridlyand et al., Islets, Jan 2013.

The expression of voltage-gated delayed rectifier potassium channels that regulate exocytosis and delayed phase ionic current in humans also differs in rodents, as well as the expression and function of plasma membrane receptor and intracellular ion channel complements facilitating voltage-independent insulin release.

In contrast to mice, human beta cells carry voltage-gated Na+ current via Nav1.6 isoform and Nav1.7, that are important for action potential generation, nociception, and glucose-mediated insulin secretion. This means that Nav1.7 antagonists developed for use as analgesics could have serious implications for human patients who face potential impairment of insulin secretion as a side effect. Such adverse effects of drugs would not be predictable in mice since they lack functional beta cell Nav1.7.

 

At the tissue level, in humans, skeletal muscle is the primary site of glucose clearance, accounting for 50% to 90% of glucose uptake, making it the primary insulin-sensitive tissue and the primary site of dysregulation in human peripheral insulin resistance. By contrast, liver is the primary site of glucose clearance in rodents, with 5 to 10-fold higher glycogen storage in the liver than in muscle, versus 10-fold more glycogen storage in muscle than in liver in humans. This inter-species difference has functional implications since various aspects of glucose regulation differ between human skeletal muscle and rodent liver Chandrasekera & Pippin, Altex, May 2014.

 

Glucose homeostasis relies on multiple glucose-sensing cells in the body that monitor blood glucose levels and respond to adjust its glycemia Yoon & Diano, Diabetologia, May 2021.

Hepatocytes express glucokinase that acts as a glucose sensor and can switch between gluconeogenesis and glycolysis based on glucose availability.

Every animal species has a signature blood glucose level or glycemic set point, possibly reflecting evolutionary adaptation. Normoglycemia of one species would be life threatening for another. For example, mouse normoglycemia can be considered diabetic for humans Rodriguez-Diaz, Cell Metab., Mar 2018.

 

As a consequence of reliance on animal research for insights on glucose regulation, several erroneous concepts were established. For example, in contrast to rodent islets, glucagon input from the alpha cell to the insulin-secreting beta cell is necessary to fine-tune the distinctive human set point.

This and many other inter-species differences have major implications for safety of patients and success of therapies to treat diabetes. For instance, treatments that inhibit glucagon pathways might also eliminate their crucial input to beta cells in humans with diabetes.

 

​

Species-specific immunological and genetic differences in T1DM

Species-specific features of innate and adaptive immune systems are also likely to contribute to poor predictive validity of animal models of diabetes.

For example, treatment of T1DM by antithymocyte globulin, that targets multiple T cell antigens, had shown promising results in animal models of T1DM. However, after receiving the same treatment in clinical trials, patients with T1DM had suffered cytokine release syndrome accompanied by a depletion of regulatory T (Treg) cells Gitelman et al., Lancet Diab. & Endocrin., Dec 2013.

 

The lack of understanding of the precise role of individual components of the immune system in T1DM, combined with existence of species-specific divergencies in the immune system, makes it difficult to know to whether immune-mediated responses in animal models of T1DM recapitulate immune-mediated responses in T1DM patients.

 

In T1DM, autoreactive CD4+ and CD8+ T cells destroy beta cells, driven by autoantigens like GAD65 enzymes and insulin that are presented by MHC class II on the surface of antigen-presenting cells.

In humans, the proinflammatory cytokines IFN-α, IFN-β, IFN-γ, and IFN-λ, secreted by T cells, macrophage, NK cells and other immune cells, play a major role in mounting of innate and adaptive immune responses to islet antigens Coomans de Brachene et al., Diabet., Feb 2024.

While in humans these cytokines activate the transcription factor STAT4, that plays a pivotal role in insulitis, beta cells apoptosis, and cytotoxicity, the activation of STAT4 is much more restricted in mice Agnello et al., J. Clin. Immun., May 2003, leading to underestimation of its role in T1DM mouse models.

 

The gold standard non-obese diabetic (NOD) mouse model of T1DM mimics the autoimmune beta cell destruction and STAT4 responses characteristic of human T1DM. Nonetheless, the immune response in NOD mice diverges from that in human T1DM. For example, NOD mice exhibit a stronger Th1 response, leading to direct cytotoxic CD8+ T-cell-mediated beta-cell destruction, whereas in humans, Th17 plays a more significant role, contributing to chronic islet inflammation and beta cell dysfunction, rather than immediate cytotoxicity. The fact that T1DM mouse models may underrepresent Th17-mediated mechanisms is likely to limit their translational relevance.

In addition, insulitis - the inflammation of the islets of Langerhans - shows both qualitative and quantitative species-specific differences between humans with T1DM and animal models of T1DM. In humans, insulitis is less intense and is found in less than 10% of islets. In contrast, the NOD mouse model of T1DM exhibit a more aggressive form of insulitis that touches over 50% of the islets. As a result, reliance on NOD mice may lead to underestimation of the role of Treg cells and environmental modulators that have the potential to slow disease progression in humans.

 

As part of innate immunity, complement system enhances inflammation, opsonization, and cell lysis. Human studies showed elevated complement components in pancreatic islets of T1DM patients with insulitis. In humans, the complement system is tightly regulated by inhibitors, such as complement factor H (CFH) and factor-H related proteins (CFHR), and their dysregulation could contribute to T1DM Ajjan & Schroeder, Mol. Immun., Oct 2019. However, as genomic studies have revealed, mice lack certain CFHR genes present in humans, such as CFHR-C, Hellwage et al., Immunogen., Oct 2006.

Although the impact of species-specific divergencies in CFH is currently unknown, it could potentially produce inter-species divergencies in complement system regulation, immune response, and severity of insulitis.

 

The anti-inflammatory cytokine IL-10 suppresses immune responses and promotes Treg function, thereby mitigating beta cell destruction. While in humans, IL-10 is produced during both Th1 and Th2 responses, in mice, IL-10 is much less prominent in Th1 than in Th2 Mestas & Hughes, J. Immunol., Mar 2004.

This inter-species difference in IL-10 production has significant implications for T1DM mouse models, including for the Th1-biased NOD mouse, in which limited Il-10 production leads to a more aggressive disease phenotype than in humans, more rapid onset of T1DM symptoms than in humans, under-represented immune regulatory mechanisms compared to humans and under-estimated human therapeutic responses.

In NOD mice, the binding pocket of the MHC class II allele H2-Ag7 protein, responsible for binding peptides and presenting them on the surface of antigen-presenting cells to T cells, is similar to the binding pocket of the analogous human HLA-DQ. Yet, NOD mice do not express the class II MHC molecule I-E, meaning that they rely solely on I-Ag7 which has a unique peptide binding groove.

The class II human MHC alleles associated with T1DM have a greater degree of polymorphism than the NOD MHC genes, which allows for a more diverse range of antigen presentation Robinson et al., Nucl. Ac. Res., Oct 2019. Unlike in NOD mice, the HLA complex in humans includes several loci that independently confer risk of T1DM Corper et al., Science, Apr 2000.

The efficacy of antigen-specific treatments might differ depending on the MHC alleles involved and the way in which peptides bind in the MHC groove. For example, a single amino acid difference in the B chain of human insulin seems to affect the ability of porcine insulin, but not human insulin, to prevent development of T1DM in NOD mice.

 

Cluster of differentiation 28 (CD28) is involved in T1DM autoimmune response as a co-stimulatory receptor on T cells that binds to CD on antigen-presenting cells and thereby provides signals for T-cell activation. In T1DM patients, the expression of CD28 infiltrating T cells is elevated.

In mice, CD28 is expressed on all CD4+ and CD8+ T cells, whereas in humans it is expressed on all CD4+ but only about 50% of CD8+ T cells Vuddamalay & van Meerwijk, Front. Immunol., Jul 2017.

Because of species-specific differences in types of CD and surface expression of CD types, the effects on human patients of drugs that target CD are likely to be underestimated or overestimated.

For example, the CD28 antagonist Abatacept showed efficacy in preclinical studies, but not in T1DM patients Orban et al., Lancet, Jul 2011.

In the same manner, the CD28 agonist Theralizumab, also known as TGN1412, caused catastrophic cytokine storm with systemic organ failures in its first human trial, despite being approved as safe in mice and in primates Suntharalingam et al., NEJM, Sep 2006.

 

Administration of low-dose IL-2, alone or combined with rapamycin, prevented hyperglycemia in NOD mice. Additionally, low-dose IL-2 was reported to cure recent-onset T1DM in NOD mice Grinberg-Bleyer et al., JEM, Aug 2010. However in humans, the same treatment had significant detrimental effects on beta cell function and survival Long et al., Diabetes, Aug 2012. 

Natural killer (NK) cells from NOD mice exhibit a defect in functional maturation and have impaired cytotoxic functions, which may render these mice resistant to the toxic effects of IL-2 treatment, leading to failure to predict toxicity to humans.

 

While a single dose of expanded antigen-specific Tregs  could reverse the T1DM phenotype in NOD mice, the attempts to reproduce the same results in T1DM patients have repeatedly failed Bender et al., Sci. Transl. Med., May 2024.

 

As the above examples show, the reliance on the ability of animal models of T1DM to faithfully recapitulate human insulitis pathology has deleterious repercussions for understanding underlying disease mechanisms and for developing safe and effective therapies for T1DM. Decades of massive failure in clinical trials of hundreds of immune interventions approved in animal models of T1DM have prompted numerous calls for a more human-centred approach Roep & Atkinson, Diabetologia, Oct 2004, Veld, Sem. Immunopath., Jul 2014.

 

It was suggested that using animal models with a humanized immune system might improve translatability to humans, however, such an approach would face persistent, insurmountable challenges: the role of the human immune system in T1DM is complex and not fully understood, the equivalence of humanized animals to the human immune system was never demonstrated by objective measures Davis, Cell, Dec 2008, and the cross-talk between the human immune system and the rest of human organ systems cannot be recapitulated in animals.

 

Additionally, another aspect that cannot be recapitulated in humanized animal models, is the development and adaptation of the immune system in response to the environment, including the microbiome, diet and housing. This missing aspect in animal models is crucial, since as twin studies show, the human immune system is more shaped by the environment than by genes Brodin et al., Cell, Jan 2015.

 

 

Species-specific differences in tropism for T1DM-associated viruses

Several viruses have been associated with the development of T1DM, particularly through their potential to trigger autoimmune responses that damage insulin-producing beta cells Hober & Sauter, Nature Rev. Endocrinol., Mar 2010, Rothenburg & Brennan, Cell Trends Microbiol., Jan 2020, Altindis et al., PNAS, Feb 2018.

 

Species-specific genetic variations in host's cell surface receptors and in the immune system have important repercussions for species’ and individuals' responses to viral infections.

For example, humans possess human-specific single nucleotide polymorphisms (SNPs) in IFIH1 gene, that are absent in non-human Ifih1 gene, and that confer human-specific vulnerability (gain-of-function variants) or protection (loss-of-function variants) to T1DM.

The IFIH1 gene encodes the melanoma differentiation-associated protein 5 (MDA5), a cytosolic pattern recognition receptor (PRR), expressed in immune cells and in pancreatic beta cells, that can detect dsRNA from viral infections and trigger an innate immune response.

Human-based genetic and population studies have shown that increased activity of IFIH1/MDA5 in genetically susceptible individuals can promote the development of T1DM, by increasing type I interferon production, beta cells apoptosis, and innate-adaptive immune system crosstalk that subsequently amplifies autoimmune responses Smyth et al., Nature Gen., May 2006, Todd et al., Nature, Jun 2007.

None of the animal species used in T1DM animal research, including NOD/STZ/BB mice and NHP, possess these human-specific SNPs in IFIH1 gene, hindering the study of MDA5-mediated autoimmunity and development of MDA5-targeting therapies Fumagalli et al., PLOS Gen., Nov 2011.

This also raises the question of whether NHP, which do not naturally develop T1DM, possess other protective SNPs in orthologues of human genes involved in T1DM and that could jeopardize safe and effective translation of preclinical studies to T1DM patients.

 

In humans, coxsackie A virus, coxsackie B virus, echovirus, rubella virus, cytomegalovirus, mumps virus, rotavirus, HIV, SARS CoV2, Hepatitis C virus and others are known to have tropism for pancreatic beta cells and are therefore classified as diabetogenic viruses Rajsfus et al., Cell, Apr 2023.

To experimentally induce symptoms of T1DM in animals, encephalomyocarditis (EMCV), coxsackie, lymphocyte choriomeningitis (LMCV) viruses are typically used. However, there is no evidence that EMCV and LMCV cause diabetes in humans Oberste et al., J. Clin. Microbio., Mar 2000, Jaeckel et al., Ann. NY Acad. Sci., Jan 2006. These viruses are therefore likely to induce T1DM-like symptoms in animal models through mechanisms that are not relevant for human T1DM.

 

Moreover, in different animal species employed to model T1DM, different viruses may show varying degrees of tropism for pancreatic cells, leading to differences in disease outcomes that hinder translatability to humans Filippi & von Herrath, Diabetes, Nov 2008.

 

 

Species-specific differences in gut microbiome

The immune system homeostasis is modulated by the gut microbiome which varies significantly across different species, making findings from microbiological studies in animals not directly translatable to humans.

In diabetes, inflammatory responses can be triggered by an imbalance between beneficial and pathogenic bacteria (dysbiosis), by an increased intestinal permeability to bacterial endotoxins, by bacterial stimulation of pro-inflammatory cytokines, or by elevated levels in the blood of gram-negative bacteria cell walls-derived lipopolysaccharides. 

 

In humans with T1DM, it has been reported that the proinflammatory environment in the gut is associated with the combination of an increased relative abundance of Bacteroidetes and a decreased level of Firmicutes, regardless of geographical location Jamshidi et al., Gut Path., Oct 2019.

Studies from human cohorts suggest that dysbiosis contributes to T1DM primarily by promoting Th17/Th1 autoimmunity, gut barrier permeability, and beta cells destruction Kostic et al., Cell Ho. & Micr., Feb 2015, Vatanen et al., Nature, Oct 2018.

Dysbiosis of the gut microbiota is suggested to occur early in life, aggravating gut inflammation and influencing the immune system, before the onset of T1DM. The relationship between T1DM autoimmunity and gut microbiome dysbiosis is likely to be bidirectional and needs to be further investigated using human-based metagenomics and metaproteomics Durazzo et al., J. Clin. Med., Nov 2019.

 

The study of the microbiome has historically relied on mouse studies, based on coarse comparisons at higher taxonomy hierarchy level with a poorly characterized mouse gastrointestinal (GI) microbiota. However, advances in metagenomics, that have enabled a much more granular view of mouse microbiota at lower taxonomy level, have shed light on previously unknown profound species-specific differences in GI microbiome composition, quantity, and function.

 

A comparative survey of the phylogenetic composition of 16 human subjects and 3 often used mouse lines indicated that their microbiota show considerable similarity at the genus level but is quantitatively very different Hugenholtz & de Vos, Cell. Mol. Life Sci., Nov 2017.

Comparison of comprehensive mouse microbiota genome to microbiota from the human GI genomes shows an overlap of 62% at the genus but only 10% at the species level, demonstrating that human and mouse gut microbiota are largely distinct Kieser et al., Plos, Mar 2022.

What is more, mouse and human microbial strains of the same species can have divergent gene content and function, as is exemplified by the species Limosilactobacillus reuteri which has mouse-adapted and human-adapted strains, however, with very different functions.

Previously, DNA sequencing of fecal samples of mouse strains of diverse genetic, provider, housing and diet backgrounds and comparison to the human gastrointestinal genome has demonstrated that only 4% of mouse GI microbial genes were shared with human GI microbial genes Xiao et al., Nature Biotech., Sep 2015.

 

Rats tend to have a higher abundance of the phylum Bacteroidetes compared to mice. Mice, on the other hand, have a higher abundance of the family Muribaculaceae within the phylum Bacteroidetes Nagpal et al., Front. Microbiol., Nov 2018.

Even within closely related species, like striped hamsters and Djungarian hamsters, there can be differences in bacterial families and genera that reflect their distinct lifestyles and dietary habits Fan et al., Curr. Microbio., Mar 2023, De Jonge et al., Front Microbiol., Jun 2022.

 

Although the gut microbiota in humans was closer to NHP than to mice and rats, there are still significant differences between the two species.

For example, African green monkeys show an opposite microbial taxa and genes responses to a typical high-protein, high-fat, low-fiber Western diet compared to humans, suggesting that NHP are not a good model for studying the effect of the human gut microbiota on host metabolism and microbiome-associated human diseases Amato et al., Microbiome, Nov 2015.

 

Efforts have been made to humanize mice with human microbiota and to inter-cross inbred strains. However, it was found that the GI microbiome in mice varied immensely depending on the housing conditions, even more so than on the genetic background, and that the inter-individual variation in GI microbiome decreased after 10 generations Hugenholtz & de Vos, Cell. Mol. Life Sci., Nov 2017.

 

The so called human microbiota-associated (HMA) mouse models, that are designed to study the contribution of a dysbiotic microbiome to development of human diseases by comparing disease phenotypes of germ-free mice colonized with the fecal microbiota of patients to those of mice colonized with the microbiota of healthy controls, do not replicate patients' microbiome Arrieta et al., Cell Host Micr., May 2016 Engrafted HMA mice showed only a partial resemblance to the donor microbiota. It is believed that only the microbiome phylotypes that are adapted to the mouse intestinal environment, and that survive an ecological shift after engraftment to mice, will remain in the mouse gut.

Human-based research has shown that the gut microbiome plays a crucial role in shaping both the innate and adaptive immune system from infancy through adulthood through human host-specific interactions between human host-specific immune system and human host-specific microbiota.

Therefore it is not surprising that HMA mice have a low immune maturation with reduced numbers of adaptive and innate intestinal immune cells when compared with mice that harbor a murine microbiota.

That means that the engrafted human microbiota is not representative of the microbiome compositions in patients', neither does it reflect human pathologies-associated dysbiosis and immune responses.