Citrobacter koseri is a Gram-negative bacterium that can cause a highly aggressive form of neonatal meningitis, which often progresses to establish multi-focal brain abscesses. C. koseri harbors multiple pathogenic motifs that can be recognized via host Toll-like receptors (TLRs). Considering the predilection of C. koseri for the brain parenchyma and the immune functions of resident parenchymal glial cells, we investigated the importance of the lipopolysaccharide receptor TLR4 and MyD88, an adaptor molecule involved in the activation of the majority of TLRS (with the exception of TLR3), for their roles in regulating glial activation in response to C. koseri. The results demonstrated that microglia respond to C. koseri with robust expression of proinflammatory molecules, which is dictated, in part, by TLR4- and MyD88-dependent signals. The residual proinflammatory mediator expression observed in TLR4 mutant and MyD88 KO microglia following C. koseri exposure indicated a contribution of TLR4- and MyD88-independent pathway(s) for maximal pathogen recognition. Interestingly, C. koseri was capable of surviving intracellularly in primary microglia, suggesting that these cells may serve as reservoirs for the pathogen during CNS infections. Enriched astrocytes expressed TLR4 constitutively and responded to LPS with moderate production of CCL2 in a TLR4-dependent manner. However, this TLR4-dependent signal was dispensable for chemokine production by enriched astrocytes in response to intact C. koseri. In the circumstance of brain parenchymal infection with C. koseri, TLR4-dependent signals exerted minimal effects on proinflammatory mediator production; although there was a slight trend towards elevated brain bacterial burdens in TLR4 mutant mice at both days 3 and 7 post-infection. In contrast, the lack of MyD88-dependent signals led to a severe impairment in C. koseri containment as revealed by significantly elevated bacterial burdens at 24 h post-infection. However, MyD88-independent signaling was evidenced by the fact that numerous inflammatory mediators were significantly elevated in MyD88 KO compared to WT mice at 24 h following C. koseri exposure.
Collectively, these findings implicate the critical involvement of multiple TLRs and MyD88-dependent signals for resident glial cells to elicit maximal proinflammatory responses and bacterial containment during C. koseri brain parenchymal infection.
|Advisor:||Kielian, Tammy Z.|
|School:||University of Arkansas for Medical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 71/10, Dissertation Abstracts International|
|Subjects:||Neurosciences, Pathology, Immunology|
|Keywords:||Astrocytes, Central nervous system, Citrobacter koseri, Immune responses, Pathogenesis, Toll-like receptors|
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