The role of microglia in the initiation and resolution of central inflammation; implications for obesity
Creators: name: Malik, Sajida
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320903
subject: Central nervous system

description: Microglia, major immune cells of the brain are acutely activated in response to infection and injury. As lipopolysaccharide (LPS) mimics bacterial infection to induce an immune response, LPS stimulation can be used to induce inflammation for understanding of role of resident and repopulated microglia in immune response initiation and resolution. High fat diet (HFD) and obesity are triggers that cause widespread and lasting brain inflammation and this effect is mediated by activation of microglia. Ageing promotes microglial activation induced neuroinflammation and together with HFD, can even worsen the affects. This neuroinflammation leads to cognitive dysfunction. Therefore, we hypothesized here that HFD and ageing in combination are more detrimental to cognition than either alone and that microglia play a role in this. 

We therefore aimed to investigate the role of microglia in behavioural and systemic responses to LPS-induced immune challenge, and to test if we could improve cognitive dysfunction in the aged and HFD fed animals by strategically suppressing microglial hyper-activity. To address these aims, we used CRISPR/Cas9 genome editing to develop a Cx3cr1-Dtr transgenic Wistar rat with a diphtheria toxin receptor (Dtr) gene expressed in the promoter for the fractalkine receptor (Cx3cr1), which is highly expressed on microglia and monocytes. Upon administration of diphtheria toxin (DT), microglia and monocytes are transiently depleted in Cx3cr1-Dtr, but not wild type rats. This allows us to specifically investigate the role of microglia/monocytes in normal and disease states including LPS-induced immune challenge and in ageing- and HFD-related cognitive decline. To understand role of microglia in sickness behaviour, we injected LPS in the absence of microglia and examined weight changes, fever generation and pro-inflammatory cytokine concentrations at 2 and 24 hr after LPS. To understand the mechanism of immune response in the absence of microglia and monocytes, we measured neutrophil count in blood and prostaglandin E2 levels in blood and in hypothalamus. To observe, how immune response occurs when microglia are repopulating after 7 days of microglial ablation, we injected LPS 7 days after DT injections. To directly assess the role of microglia in ageing-related vulnerability to HFD, we fed aged (and young) wild type and Cx3cr1-Dtr rats HFD for 8 weeks and assessed cognitive function with and without microglial ablation. 

Our data suggest that microglia and monocyte ablation did not abrogate sickness behaviour in rats followed by LPS administration. In contrast, absence of these cells caused more pronounced immune response to LPS. We found exacerbated cytokine storm in periphery and in brain, reduced locomotor activity, anxiety, and febrile response to LPS in the absence of microglia and monocytes. We also found that neutrophil infiltration was significantly increased in groups with microglia ablated even without LPS administration suggesting that neutrophils replace monocyte role in periphery and were primed to hyper respond when exposed to LPS.
 
We also found that immune response initiated and resolved normally in the presence of repopulated microglia in contrast to disrupted resolution of immune response in the absence of microglia and monocytes. 

In our ageing study, we found that HFD significantly increased weight gain in young and in ageing rats, and this was even accelerated in ageing rats accompanied by increased fat mass and reduced lean mass. We also report inflammatory condition was induced by HFD consumption, but ageing did not induce inflammation. However, we found reduced recognition ability in ageing and HFD fed rats. Microglial presence or deletion did not affect this. 

Together, our findings suggest that immune response initiation and development of sickness behaviour can take place independent of microglia and monocytes and that neutrophils being innate immune cells are driving the immune response in the absence of monocytes in periphery. Microglia and monocytes may play role in regulating immune response, but they are not sole drivers. Similarly, microglia might be involved in HFD-induced neuroinflammation and ageing related cognitive deficits, but they are not the key players. Further understanding of immune mechanism and how microglia interact with neurons would be worth looking at in future.