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Introduction: Bacterial infections with a manifestation in the central nervous system (CNS) represent an important cause of morbidity and mortality in neonates. However, mechanisms of host susceptibility, route of infection and underlying mechanisms of inflammation in the CNS remain ill-defined.
Objective: The aim of this study was to establish a model of neonatal CNS infection with Listeria monocytogenes following mucosal challenge in order to investigate the cellular and molecular mechanisms of bacterial tissue tropism and innate immune responses.
Materials and Methods: Neonatal C57BL/6 mice were infected intranasally with L. monocytogenes. To determine bacterial dissemination, pups were sacrificed at various time points and organs were obtained for replica plating. Tissue tropism and immune responses were analyzed by immunohistochemistry, electron microscopy, flow cytometry and qRT-PCR.
Result: In contrast to the established gastrointestinal tropism of L. monocytogenes, bacteria were mainly reisolated from the brain, particularly from the olfactory bulb and the cerebrum. Only very few Listeria were found in the cerebellum and the brain stem as well as in the blood, suggesting a non-hematogenous dissemination from the nasal cavity to the CNS. Once inside the cranium, Listeria induced a multifocal meningo-encephalitis as evaluated by histopathological examination. Interestingly, mucosal invasion was restricted to the olfactory epithelium and completely independent of the two major listerial invasins InlA and InlB. Nevertheless, electron microscopic examination clearly showed that, during early time points, Listeria resided in non-myeloid supporting cells. Later, wild-type bacteria were found to be associated with axon bundles projecting from the olfactory cavity to the CNS. In contrast, an isogenic Listeria mutant lacking ActA, which facilitates intracellular motility and cell-to-cell spread, was still able to induce internalization into the olfactory epithelium but was entirely restricted to the olfactory mucosa and could not overcome the cribriform plate. Once inside the brain, wild-type Listeria were targeted by various immune cells. Flow cytometric and immunehistochemical analyses showed an accumulation of bacteria with a concomitant recruitment of CD45+CD11b+ microglia/macrophages and a significant increase of infiltrating Ly6C+ monocytes/macrophages. As expected, mRNA of key cytokines mediating intracranial inflammation and monocyte attraction, such as Tnfα, Cxcl2, Ccl2, and Ccl7, was highly upregulated.
Conclusion: The possibility to study neonatal CNS infection by L. monocytogenes following the natural route is essential for understanding onset and progression of disease. This robust and standardized in vivo model reflecting the pathogenesis of neonatal Listeria infections may, thus, be crucial for discovering new therapeutic approaches.