Dealing with Danger in the CNS: The Response of the Immune System to Injury

Fighting pathogens and maintaining tissue homeostasis are prerequisites for survival. Both of these functions are upheld by the immune system, though the latter is often overlooked in the context of the CNS. The mere presence of immune cells in the CNS was long considered a hallmark of pathology, but this view has been recently challenged by studies demonstrating that immunological signaling can confer pivotal neuroprotective effects on the injured CNS. In this review, we describe the temporal sequence of immunological events that follow CNS injury. Beginning with immediate changes at the injury site, including death of neural cells and release of damage-associated molecular patterns (DAMPs), and progressing through innate and adaptive immune responses, we describe the cascade of inflammatory mediators and the implications of their post-injury effects. We conclude by proposing a revised interpretation of immune privilege in the brain, which takes beneficial neuro-immune communications into account

Unexpected role of interferon-gamma in regulating neuronal connectivity and social behaviour

Immune dysfunction is commonly associated with several neurological and mental disorders. Although the mechanisms by which peripheral immunity may influence neuronal function are largely unknown, recent findings implicate meningeal immunity influencing behaviour, such as spatial learning and memory. Here we show that meningeal immunity is also critical for social behaviour; mice deficient in adaptive immunity exhibit social deficits and hyper-connectivity of fronto-cortical brain regions. Associations between rodent transcriptomes from brain and cellular transcriptomes in response to T-cell-derived cytokines suggest a strong interaction between social behaviour and interferon-gamma (IFN-gamma)-driven responses. Concordantly, we demonstrate that inhibitory neurons respond to IFN-gamma and increase GABAergic (gamma-aminobutyric-acid) currents in projection neurons, suggesting that IFN-gamma is a molecular link between meningeal immunity and neural circuits recruited for social behaviour. Meta-analysis of the transcriptomes of a range of organisms reveals that rodents, fish, and flies elevate IFN-gamma/JAK-STAT-dependent gene signatures in a social context, suggesting that the IFN-gamma signalling pathway could mediate a co-evolutionary link between social/aggregation behaviour and an efficient anti-pathogen response. This study implicates adaptive immune dysfunction, in particular IFN-gamma, in disorders characterized by social dysfunction and suggests a co-evolutionary link between social behaviour and an anti-pathogen immune response driven by IFN-gamma signalling

Pre-existing neurological conditions and COVID-19 co-infection: Data from systematic reviews, meta-analyses, and scoping reviews

BackgroundPre-existing neurological diseases have been identified as risk factors for severe COVID-19 infection and death. There is a lack of comprehensive literature review assessing the relationship between pre-existing neurological conditions and COVID-19 outcomes. Identification of high risk groups is critical for optimal treatment and care.MethodsA literature review was conducted for systematic reviews, meta-analyses, and scoping reviews published between January 1, 2020 and January 1, 2023. Literature assessing individuals with pre-existing neurological diseases and COVID-19 infection was included. Information regarding infection severity was extracted, and potential limitations were identified.ResultsThirty-nine articles met inclusion criteria, with data assessing >3 million patients from 51 countries. 26/51 (50.9%) of countries analyzed were classified as high income, while the remaining represented middle-low income countries (25/51; 49.0%). A majority of evidence focused on the impact of cerebrovascular disease (17/39; 43.5%) and dementia (5/39; 12.8%) on COVID-19 severity and mortality. 92.3% of the articles (36/39) suggested a significant association between neurological conditions and increased risk of severe COVID-19 and mortality. Cerebrovascular disease, dementia, Parkinson's disease, and epilepsy were associated with increased COVID severity and mortality.ConclusionPre-existing neurological diseases including cerebrovascular disease, Alzheimer's disease and other dementias, epilepsy, and Parkinson's disease are significant risk factors for severity of COVID-19 infection and mortality in the acute infectious period. Given that 61.5% (24/39) of the current evidence only includes data from 2020, further updated literature is crucial to identify the relationship between chronic neurological conditions and clinical characteristics of COVID-19 variants