Acute Exposure to the Food-Borne Pathogen Listeria monocytogenes Does Not Induce alpha-Synuclein Pathology in the Colonic ENS of Nonhuman Primates

Introduction: Gastrointestinal (GI) inflammation elicited by environmental factors is proposed to trigger Parkinson's disease (PD) by stimulating accumulation of pathological alpha synuclein (alpha-syn) in the enteric nervous system (ENS), which then propagates to the central nervous system via the vagus nerve. The goal of this study was to model, in nonhuman primates, an acute exposure to a common food-borne pathogen in order to assess whether the related acute GI inflammation could initiate persistent alpha-syn pathology in the ENS, ultimately leading to PD. Methods: Adult female cynomolgus macaques were inoculated by oral gavage with 1x108 colony-forming units (CFUs) Listeria monocytogenes (LM, n=10) or vehicle (mock, n=3) and euthanized 2 weeks later. Evaluations included clinical monitoring, blood and fecal shedding of LM, and postmortem pathological analysis of colonic and cecal tissues. Results: LM inoculation of healthy adult cynomolgus macaques induced minimal to mild clinical signs of infection; LM shedding in feces was not seen in any of the animals nor was bacteremia detected. Colitis varied from none to moderate in LM-treated subjects and none to minimal in mock-treated subjects. Expression of inflammatory markers (HLA-DR, CD3, CD20), oxidative stress (8-OHDG), alpha-syn, and phosphorylated-alpha-syn in the enteric ganglia was not significantly different between treatment groups. Discussion: Our results demonstrate that cynomolgus macaques orally inoculated with LM present with a clinical response that resembles human LM exposure. They also suggest that acute exposure to food-borne pathogens is not sufficient to induce significant and persistent alpha-syn changes in healthy adult female subjects. Based on the results of this limited experimental setting, we propose that, if LM has a role in PD pathology, other underlying factors or conditions, such as male sex, inflammatory bowel disease, exposure to toxins, dysbiosis, and/ or aging, are needed to be present.