The impact of centrally and systemically administered TNFa on CNS inflamation and function

Abstract

Inflammation has long been recognised as a driver of dementia. However, the pathways by which inflammation alters neuronal function and causes neuronal death remain under investigation. Microglia are primed by chronic neurodegeneration such that toll-like receptor agonists, such as lipopolysaccharide (LPS), drive exaggerated pro-inflammatory cytokine responses on this background. The data presented in this thesis show that endogenous cytokines can also activate primed microglia. Furthermore this thesis describes the novel finding of primed astrocytes in the ME7 prion disease model of neurodegeneration and the APP/PS1 amyloid aggregation model. The astrocyte population showed exaggerated production of the chemokines CXCL1 and CCL2 in response to exogenous LPS or endogenous IL-1β or TNFα challenges in ME7 animals compared to NBH animals. In the ME7 model we examined downstream effects of exaggerated chemokine expression, which resulted in markedly increased neutrophil, T-cell and monocyte infiltration and astrocyte proliferation in the diseased brain. These data have significant implications for acute sterile inflammatory insults such as stroke and traumatic brain injury occurring on a background of aging or neurodegeneration. Systemic infection has been shown to exacerbate progression of neurodegeneration in AD cohorts, this was found to be significantly correlated with levels of systemic circulating TNFα. In mice a single acute dose of systemic TNFα (250μg/kg) exacerbated features of sickness behaviour; including core body temperature changes, open field activity and body weight, in ME7 compared to NBH. A lower dose (50μg/kg) can induce a transient cognitive deficit in a hippocampal dependent task in ME7 animals but not in NBH animals. However these changes are independent of any changes in features of pathology examined. While there was no TNFα effect we discovered a novel feature of pathology; a robust aggregation of APP varicosities in the posterior (PO) and Ventroposteromedial (VPM) nuclei of the thalamus, overlapping with significant neuronal loss in the PO and significant synaptic loss in the VPM. Given systemic TNFα administration did not significantly alter ME7 pathology we utilised systemic administration of XPRO1595 a dominant negative TNFα inhibitor prior to systemic administration of LPS. This showed that the exacerbation of ME7 pathology by LPS was independent of TNFα actions. The timepoint used in the ME7 model is one with severe and widespread pathology, the mild inflammation caused by systemic TNFα in comparison to systemic LPS was not sufficient to alter this severe pathology. It would be of great interest to examine the effects of TNFα on a less severe model of pathology. Elevated systemic TNFα has been shown to be associated with significantly more rapid cognitive decline across 6 months in an AD patient cohort. It is not clear whether this might constitute a causal relationship. A repeated systemic TNFα challenge model was used to examine consequential behaviour and pathology in female APP/PS1 mice. This showed that repeated TNFα did not significantly alter learning of the hippocampal dependent Y maze. Repeated systemic TNFα induced significant deficits in rearing behaviour in W+TNFα but not APP/PS1+TNFα animals. Progressive weight loss was observed in the WT mice but this was not apparent in the APP/PS1 mice. A significant decrease was observed in the hippocampal Amyloid β burden with a concurrent increase in aggregations of APP in both the hippocampus and thalamus. Microglia were increased in APP/PS1, however there was no alteration following repeated systemic TNFα. There were elevated numbers of T cells in the APP/PS1 animals. Following repeated TNFα there was a further increase in the T cells in APP/PS1s this did not occur in the WT+TNFα group. Repeated TNFα in APP/PS1 animals also diminished the levels of ongoing cell proliferation, a similar decrease did not occur in the WT+TNFα group. These results indicate that repeated systemic TNFα administration has differential effects in aged APP/PS1 and WT animals, suggesting important implications for cognitive and functional decline with systemic inflammation. The data presented here significantly contributes to the study of inflammatory processes in neurodegeneration. The importance of chronic sterile inflammatory co-morbidities is highlighted by this data and perhaps lends evidence to an alternative hypothesis of neurodegeneration, rather than the amyloid beta focused hypothesis that remains prevalent in the field.