title: Neuroinflammation: exploring brain cell responses and leukocyte infiltration
Creators: name: Suessmilch, Maria
Publisher: name: University of Glasgow
Year: 2020

description: In recent years, the relationship between inflammation and psychiatric disorders has gained substantial research interest. Multiple studies have demonstrated that psychiatric disorders are associated with a dysregulation of the immune response, and changes in inflammatory factors are being explored as possible disease biomarkers (Gibney and Drexhage, 2013, Yuan et al., 2019). The Cavanagh group has studied the influence of peripheral inflammation on central nervous system (CNS) responses and behaviour for a number of years using animal models including the Aldara model of psoriasis-like skin inflammation. The active ingredient, imiquimod (IMQ), is a synthetic toll-like receptor-7 (TLR-7) agonist. Topical application of Aldara triggers a strong pro-inflammatory response in the periphery and the brain characterised by increased cytokine and chemokine transcription, immune cell entry into the brain and a decrease in motivated behaviour (McColl et al., 2016, Nerurkar et al., 2017). After initially regarding this as a model of peripheral inflammation, mass spectrometry analysis revealed that IMQ enters the brain rapidly after topical application, raising questions about whether this stimulus can act on neural cells directly (Nerurkar et al., 2017). This thesis explores whether neural cells can respond to IMQ directly and how immune cells enter the brain parenchyma in response to Aldara treatment. To investigate the reaction of neural cells to IMQ, we treated newly optimised primary mouse brain cell cultures in vitro. Results showed that primary brain cells (neurons, microglia, astrocytes and oligodendrocytes) upregulate the same pro-inflammatory cytokines and chemokines involved in the IMQ response in vivo. We also examined the cellular source of some of these factors and found astrocytes and oligodendrocytes to be majorly involved in their production. As interferons (IFNs) are the main mediators of the TLR-7 response, we also examined the influence of type I and type II IFNs on brain cells and found that they too trigger chemokine and cytokine transcription, albeit to different extents. The second part of this thesis focuses on leukocyte entry into the CNS within the Aldara model. Tracer dye experiments showed mild signs of blood-brain barrier (BBB) integrity loss, although genes involved in upholding BBB integrity were not downregulated. Ifnar1 knock out (KO) experiments highlighted the importance of type I IFNs (IFN-α and IFN-β) for T cell and monocyte recruitment following Aldara treatment. Inflammatory chemokine receptor (iCCR) KOs revealed iCCR importance in monocyte recruitment to the CNS, with CCR2 being the main iCCR expressed on infiltrating monocytes. Notably, both KO experiments showed a small monocyte population that continued to enter the CNS, despite the absence of either Ifnar1 or iCCRs. Overall, this thesis demonstrates that IMQ can act on neural cells directly and trigger a strong pro-inflammatory response in the absence of peripheral immune cell influence. In vivo, Aldara treatment causes mild BBB disruption and immune cell recruitment is reliant on type I IFNs as well as iCCRs, albeit to different extents across different leukocyte subsets. Further experiments are needed to understand immune cell entry mechanisms and examine the cellular sources of inflammatory mediators in the CNS. Overall, the Aldara model produces symptoms of mild encephalitis which makes it an easy, well controlled alternative to virus-based encephalitis models.