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.