MyD88 Expression by CNS-Resident Cells is Pivotal for Eliciting Protective Immunity in Brain Abscesses

Abstract

MyD88 KO (knockout) mice are exquisitely sensitive to CNS (central nervous system) infection with Staphylococcus aureus, a common aetiological agent of brain abscess, exhibiting global defects in innate immunity and exacerbated tissue damage. However, since brain abscesses are typified by the involvement of both activated CNS-resident and infiltrating immune cells, in our previous studies it has been impossible to determine the relative contribution of MyD88-dependent signalling in the CNS compared with the peripheral immune cell compartments. In the present study we addressed this by examining the course of S. aureus infection in MyD88 bone marrow chimaera mice. Interestingly, chimaeras where MyD88 was present in the CNS, but not bone marrow-derived cells, mounted pro-inflammatory mediator expression profiles and neutrophil recruitment equivalent to or exceeding that detected in WT (wild-type) mice. These results implicate CNS MyD88 as essential in eliciting the initial wave of inflammation during the acute response to parenchymal infection. Microarray analysis of infected MyD88 KO compared with WT mice revealed a preponderance of differentially regulated genes involved in apoptotic pathways, suggesting that the extensive tissue damage characteristic of brain abscesses from MyD88 KO mice could result from dysregulated apoptosis. Collectively, the findings of the present study highlight a novel mechanism for CNS-resident cells in initiating a protective innate immune response in the infected brain and, in the absence of MyD88 in this compartment, immunity is compromised.

Central Nervous System Fibrosis Is Associated with Fibrocyte-Like Infiltrates

“…Only animals that displayed chimerism of greater than 90% were evaluated at approximately 10 to 12 weeks following bone marrow transfer, a period that we have established to be sufficient for establishing chimerism. 11 Importantly, in our studies, parenchymal microglia were not repopulated with GFP ϩ cells as has been described by others, 16 although GFP ϩ cells were associated with blood vessels, representing perivascular macrophages (see Supplemental Figure S1 at http:// ajp.amjpathol.org).…”

“…11 This paradigm allowed for the direct visualization of infiltrating GFP ϩ bone marrow-derived cells and an assessment of fibrotic molecule expression by signal colocalization following immunofluorescent staining. In addition, B6/SJL mice that are congenic for the CD45 allele (CD45.1) on a C57BL/6 background were purchased from The Jackson Laboratory and used to generate bone marrow chimeras with C57BL/6 mice (CD45.2) for some fluorescence activated cell-sorting (FACS) studies.…”

“…11,17 Briefly, GFP bone marrow chimera mice were perfused to remove circulating leukocytes from the vasculature, whereupon brain abscess tissues were minced and filtered through a 70-m nylon mesh cell strainer. The resulting homogenate was subjected to collagenase/DNase I digestion (both from Sigma-Aldrich) for 30 minutes, and after enzyme neutralization, cells were applied to a Percoll gradient (1.03 to 1.088 g/mL).…”

“…10 A pivotal role for microglia in initial bacterial recognition events in the CNS was demonstrated by a recent study from our group in which MyD88, an essential adaptor for Toll-like receptor and IL-1R/IL-18 signaling, was essential within the CNS compartment for the genesis of protective immunity during early brain abscess development. 11 In addition to orchestrating antimicrobial responses, macrophages and microglia can also facilitate inflammation resolution and wound healing processes. In particular, studies have demonstrated that macrophages, and more recently microglia, can be programmed as classically activated M1 or, alternatively activated M2 cells, depending on the local cytokine milieu.…”

Toll-Like Receptor Stimulation Enhances Phagocytosis and Intracellular Killing of Nonencapsulated and EncapsulatedStreptococcus pneumoniaeby Murine Microglia

“…Here, we hypothesized that activation of the innate immune response in microglia could increase the resistance of the brain tissue against CNS pneumococcal infections (14). This may be of particular interest in immunocompromised patients, whose outcome after S. pneumoniae meningitis is worse than that of immunocompetent individuals (9,44).…”