Common and Not-So-Common Pathologic Findings of the Gastrointestinal Tract of Rhesus and Cynomolgus Macaques

Diversity of Campylobacter spp. circulating in a rhesus macaque (Macaca mulatta) breeding colony using culture and molecular methods

Campylobacter jejuni and Campylobacter coli represent the leading causes of bacterial gastroenteritis in humans, and infections can produce post-infectious irritable bowel syndrome (PI-IBS). Rhesus macaques (Macaca mulatta) (RM) are similarly susceptible to acute campylobacteriosis and represent a potential model of PI-IBS. We characterized the Campylobacter species circulating in an RM breeding colony using culture, qPCR, and whole genome sequencing (WGS). We also compared the C. jejuni and C. coli prevalence in RM as detected with qPCR versus culture and identified risk factors for bacteria presence and intestinal disease. Culture of 275 samples yielded C. coli (103) and C. jejuni (8), of which 21.6% were resistant to quinolones and 3.6% were resistant to macrolides. Multidrug-resistant isolates were obtained exclusively from animals exhibiting diarrhea or with histologically confirmed chronic enterocolitis. WGS revealed a non-clonal population of Campylobacter spp. Genotypic predictions of resistance were excellent except for aminoglycosides. All sequenced isolates contained genes for all subunits of cytolethal distending toxin. qPCR detected a prevalence of 45.9% for C. coli and 29.6% for C. jejuni. The quantity of either bacteria was significantly higher (P < 0.05) in animals with intestinal disease compared to healthy animals, though only young age was significantly associated with the presence of Campylobacter sp. or intestinal disease. Significantly more C. jejuni positive animals were detected with qPCR than with culture. These results provide a comprehensive characterization of Campylobacter spp. circulating in a breeding colony of RM in the United States and suggest that qPCR is superior for the detection of C. jejuni in RM.

IMPORTANCE

Gastrointestinal disease is one of the most common reasons for hospitalization in non-human primate colonies and accounts for over one-third of non-research related euthanasia. In rhesus macaques, this manifests as both acute diarrhea and chronic enterocolitis (CE), a syndrome of chronic diarrhea resulting in poor weight gain or weight loss which is minimally responsive to treatment. Campylobacter spp. are major causes of acute enterocolitis in rhesus macaques and may predispose individuals to the development of CE, similar to post-infectious irritable bowel syndrome in humans. Despite these concerns, there are few studies characterizing Campylobacter in rhesus macaque colonies, in particular utilizing whole genome sequencing and assessing findings with respect to the health status of the host. Our findings provide insight into Campylobacter strains circulating in rhesus macaque colonies, which can improve clinical monitoring, assist in treatment decisions, and provide new avenues of investigation into campylobacteriosis as a catalyst for CE.

Microbiome signature suggestive of lactose-intolerance in rhesus macaques (Macaca mulatta) with intermittent chronic diarrhea

BACKGROUND

Chronic diarrhea is a common cause of mortality and morbidity in captive rhesus macaques (Macaca mulatta). The exact etiology of chronic diarrhea in macaques remains unidentified. The occurrence of diarrhea is frequently linked to dysbiosis within the gut microbiome. Research into microbiome signatures correlated with diarrhea in macaques have predominantly been conducted with single sample collections. Our analysis was based on the metagenomic composition of longitudinally acquired fecal samples from rhesus macaques with chronic diarrhea and clinically healthy rhesus macaques that were obtained over the course of two years. We aimed to investigate potential relationships between the macaque gut microbiome, the presence of diarrhea and diet interventions with a selection of commercially available monkey diets.

RESULTS

The microbiome signature of macaques with intermittent chronic diarrhea showed a significant increase in lactate producing bacteria e.g. lactobacilli, and an increase in fermenters of lactate and succinate. Strikingly, two lactose free diets were associated with a lower incidence of diarrhea.

CONCLUSION

A lactose intolerance mechanism is suggested in these animals by the bloom of Lactobacillus in the presence of lactose resulting in an overproduction of intermediate fermentation products likely led to osmotically induced diarrhea. This study provides new insights into suspected microbiome-lactose intolerance relationship in rhesus macaques with intermittent chronic diarrhea. The integration of machine learning with metagenomic data analysis holds potential for developing targeted dietary interventions and therapeutic strategies and therefore ensuring a healthier and more resilient primate population.

Histopathological characterization and grading of chronic enterocolitis in Sulawesi crested macaques (Macaca nigra)

Sulawesi crested macaques (Macaca nigra) (SCMs) are critically endangered and frequently suffer from chronic intestinal disease in captivity. Often, despite routine diagnostic investigations and confirmation of intestinal inflammation, an aetiology cannot be identified, leading to a non-specific categorization as chronic enterocolitis rather than an aetiological diagnosis. This study evaluates the histological features of gastrointestinal tissues from 23 SCMs, comparing animals with a clinical history suggestive of chronic enterocolitis (n = 14) with those without gastrointestinal clinical signs (n = 9). Tissues were graded according to the Nancy index (NI), a scoring system used in human medicine to evaluate disease activity in ulcerative colitis, a common form of human inflammatory bowel disease (IBD). Additionally, inflammatory cells in the colonic lamina propria were visually identified by type, counted and subsequently compared between diseased and control animals. Moderate to severe lymphoplasmacytic inflammation and structural changes were most common in the colons of affected SCMs, whereas histopathological changes were absent or mild in all examined small intestine (n = 17) and stomach (n = 11) tissues. The colonic NI had a significant positive correlation with clinical disease severity and 57% (n = 8) of animals with clinical signs had a NI grade of ≥2, consistent with moderate to severe, active IBD. Half of SCMs with recurrent rectal prolapse (n = 6) had a NI grade of 0, suggesting that intestinal inflammation is not always part of this condition's pathogenesis. The numbers of colonic lymphocytes, plasma cells, neutrophils, macrophages and total leucocytes were significantly higher in diseased animals. This study validated the use of the NI in SCMs, enabling a more standardized histopathological evaluation of the colon in this species.

Maintenance of caecal homeostasis by diverse adaptive immune cells in the rhesus macaque

Objectives

The caecum bridges the small and large intestine and plays a front-line role in discriminating gastrointestinal antigens. Although dysregulated in acute and chronic conditions, the tissue is often overlooked immunologically.

Methods

To address this issue, we applied single-cell transcriptomic-V(D)J sequencing to FACS-isolated CD45+ caecal patch/lamina propria leukocytes from a healthy (5-year-old) female rhesus macaque ex vivo and coupled these data to VDJ deep sequencing reads from haematopoietic tissues.

Results

We found caecal NK cells and ILC3s to co-exist with a spectrum of effector T cells partially derived from SOX4 + recent thymic emigrants. Tolerogenic Vγ8Vδ1-T cells, plastic CD4+ T helper cells and GZMK + EOMES + and TMIGD2 + tissue-resident memory CD8+ T cells were present and differed metabolically. An IL13 + GATA3 + Th2 subset expressing eicosanoid pathway enzymes was accompanied by IL1RL1 + GATA3 + regulatory T cells and a minor proportion of IgE+ plasma cells (PCs), illustrating tightly regulated type 2 immunity devoid of ILC2s. In terms of B lymphocyte lineages, caecal patch antigen-presenting memory B cells sat alongside germinal centre cells undergoing somatic hypermutation and differentiation into IGF1 + PCs. Prototypic gene expression signatures decreased across PC clusters, and notably, expanded IgA clonotypes could be traced in VDJ deep sequencing reads from additional compartments, including the bone marrow, supporting that these cells contribute a steady stream of systemic antibodies.

Conclusions

The data advance our understanding of caecal immunological function, revealing processes involved in barrier maintenance and molecular networks relevant to disease.

Exploring rabbit as a nonrodent species for general toxicology studies

To avoid adverse events in humans, toxicity studies in nonclinical species have been the foundation of safety evaluation in the pharmaceutical industry. However, it is recognized that working with animals in research is a privilege, and conscientious use should always respect the 3Rs: replacement, reduction, and refinement. In the wake of the shortages in routine nonrodent species and considering that nonanimal methods are not yet sufficiently mature, the value of the rabbit as a nonrodent species is worth exploring. Historically used in vaccine, cosmetic, and medical device testing, the rabbit is seldom used today as a second species in pharmaceutical development, except for embryo-fetal development studies, ophthalmic therapeutics, some medical devices and implants, and vaccines. Although several factors affect the decision of species selection, including pharmacological relevance, pharmacokinetics, and ADME considerations, there are no perfect animal models. In this forum article, we bring together experts from veterinary medicine, industry, contract research organizations, and government to explore the pros and cons, residual concerns, and data gaps regarding the use of the rabbit for general toxicity testing.

Gastrointestinal tract pathology of the owl monkey (Aotus spp.)

Owl monkeys are small nocturnal new world primates in the genus Aotus that are most used in biomedical research for malaria. Cardiomyopathy and nephropathy are well-described common diseases contributing to their morbidity and mortality; less is known about lesions affecting the gastrointestinal tract. Records from a 14-year period (2008-2022) at the Keeling Center for Comparative Medicine and Research were queried to identify instances of spontaneous gastrointestinal disease that directly contributed to the cause of death from the 235 adult owl monkeys submitted for necropsy. Of the 235, 10.6% (25/235) had gastrointestinal disease listed as a significant factor that contributed to morbidity and mortality. Diagnoses included candidiasis (3/25), gastric bloat (4/25), and intestinal incarceration and ischemia secondary (11/25), which included intussusception (4/25), mesenteric rent (3/25), strangulating lipoma (2/25), intestinal torsion (1/25), and an inguinal hernia (1/25). Intestinal adenocarcinomas affecting the jejunum (4/25) were the most common neoplasia diagnosis. Oral squamous cell carcinoma (1/25) and intestinal lymphoma (2/25) were also diagnosed. This report provides evidence of spontaneous lesions in the species that contribute to morbidity and mortality.

Analysis of cell populations in the normal rhesus macaque (Macaca mulatta) lower intestinal tract and diagnostic thresholds for chronic enterocolitis

Rhesus macaques (Macaca mulatta) are used extensively in biomedical research, often with a focus on the gastrointestinal tract, and yet a full characterization of their normal resident intestinal cell populations has not been published. In addition, chronic enterocolitis (CE), also known as idiopathic chronic diarrhea, affects up to 25% of colony-housed rhesus macaques, often requiring euthanasia for welfare concerns and severely limiting their value as a breeding animal or research subject. We aimed to characterize subjective and objective variables in sections of the ileum, cecum, colon, and rectum in 16 healthy rhesus macaques and compare these results with a cohort of 37 animals euthanized for CE to produce relevant diagnostic thresholds and to improve case definitions for future studies. We found neutrophils to be an infrequent but expected component of the large intestinal leukocyte population. Animals with CE had significantly increased total leukocyte populations between crypts in the cecum, colon, and rectum; variable increases in specific cell populations across all levels of the distal intestinal tract; and significantly increased intraepithelial CD3+ T cells in the colon and rectum. Concentrations of enteroendocrine cells, enterochromaffin cells, and intestinal mast cells were not significantly different between healthy and affected individuals. This study characterizes individual leukocyte populations in the rhesus macaque lower intestinal tract, is the first to evaluate rhesus macaque intestinal mast cells, and provides key diagnostic thresholds for evaluating animals with potential CE.

Sarcina sp. as a presumptive cause of fatal acute gastric dilation and gastric emphysema in rhesus macaques

A 4-y-old female and 3-y-old male rhesus macaque (Macaca mulatta), both housed in the same facility, died unexpectedly within 2 wk. Postmortem examination revealed severe gastric dilation in both macaques and gastric emphysema in the female macaque. Histologically, bacteria consistent with Sarcina sp. were present in both macaques within the lungs and lumen of the trachea, esophagus, and gastrointestinal (GI) tract without associated inflammation. Additionally, in the female macaque, the bacteria were found in the gastric mucosa and associated with emphysematous spaces in the gastric wall without associated inflammation. PCR and Sanger sequencing of amplicons were subsequently performed on GI contents and non-alimentary tissues from the 2 affected monkeys and on comparative samples from unaffected rhesus monkeys in the same facility and an adjacent primate facility. The cases were compared using the 2-tailed Fisher exact test (p-value at 95% confidence). PCR identified Sarcina in GI contents of both affected and unaffected monkeys (p = 0.6084) and in non-alimentary tissues of affected monkeys only (p = 0.0083). These results suggest that the presence of Sarcina sp. in non-alimentary tissues is associated with gastric distension, gas accumulation, and unexpected death in nonhuman primates.

Special Issue on the Pathobiology of Laboratory Nonhuman Primates: A Review of Species, Substrain, Geographical Origin, Age, and Modality-Related Factors

Nonhuman primates (NHPs) are utilized in nonclinical safety testing due to their phylogenetic proximity to humans and similarity in physiology and anatomy. However, ethical considerations and the increased demand for NHPs, coupled with the current shortage in their supply, have increased the calls to minimize their use. In addition, the increased demand and supply shortage of NHPs have increased the use of animals sourced from different geographical origins, and animals of different ages, which can complicate the interpretation of study results. Coupled with the relative uniqueness of findings induced by novel therapeutic modalities, there is an increasing need for a deeper understanding of the systemic pathobiology of NHPs. Here we provide a brief preview of the two main themes discussed in this special issue, which include the influence of geographical origin, age, and sex on background pathology, clinical pathology reference values, other relevant toxicology endpoints, and organ system pathology.

The Influence of Geographical Origin, Age, Sex, and Animal Husbandry on the Spontaneous Histopathology of Laboratory Cynomolgus Macaques (Macaca Fascicularis): A Contemporary Global and Multisite Review of Historical Control Data

To investigate the influence of geographical origin, age, and sex on toxicologically relevant spontaneous histopathology findings in cynomolgus macaques (Macaca fascicularis), we performed a comparative analysis of historical control data (HCD) from 13 test sites that included 3351 animals (1645 females and 1706 males) sourced from Mauritius, China, Vietnam, and Cambodia, aged from 2 to 9.5 years, and from 446 toxicology studies evaluated between 2016 and 2021. The most common findings were mononuclear infiltrates in the kidney, liver, brain, and lung, which showed highest incidences in Mauritian macaques, and heart, salivary glands, and gastrointestinal tract (GIT), which showed highest incidences of mononuclear infiltrates in mainland Asian macaques. Developmental and degenerative findings were more common in Mauritian macaques, while lymphoid hyperplasia and lung pigment showed higher incidences in Asian macaques. Various sex and age-related differences were also present. Despite origin-related differences, the similarities in the nature and distribution of background lesions indicate that macaques from all geographical regions are suitable for toxicity testing and show comparable lesion spectrum. However, in a toxicity study, it is strongly recommended to use animals from a single geographical origin and to follow published guidelines when using HCD to evaluate and interpretate commonly diagnosed spontaneous lesions.