Microglia react distinctively during inflammation

2018 - 11 - 27

LIH researchers conducted a pioneering study to unravel the genetic programs that are triggered upon inflammation in microglia, specialised resident immune cells of the central nervous system. Using state-of-the-art single-cell transcriptomics, they uncovered distinct gene expression profiles under inflammatory conditions that greatly differ from the profiles observed in steady state conditions and in the context of neurodegenerative diseases. The findings may facilitate future studies on potential therapeutic approaches to restore normal microglia function in neurological disorders.

The NorLux Neuro-Oncology Laboratory at LIH’s Department of Oncology conducts research on brain diseases, with a special emphasis on glioma biology, drug resistance and systems approaches. Within this research unit, Dr Alessandro Michelucci focuses on the role of glial cells and inflammatory responses. Jointly with team member Dr Carole Sousa and collaborating research groups from LIH and the University of Luxembourg, major findings were published in the November 2018 issue of the renowned journal EMBO reports. Featured on the journal cover, their scientific report titled “Single‐cell transcriptomics reveals distinct inflammation‐induced microglia signatures” stands out as the most read article since its online release in September.

Studying the effect of inflammation

The brain is a unique organ with its own “tailored” immune cells and mechanisms, distinct from those of the rest of the body. The central nervous system (CNS) contains specialised parenchymal-resident phagocytes, termed microglia, that survey and modulate the neural environment and respond against infections, toxins or contaminants thereby promoting neuronal health and ensuring normal brain function. Microglia can sense homeostatic perturbations and coordinate immune responses between the periphery and the CNS. Dysfunctional microglia have been observed in chronic neurological disorders such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis as well as brain cancer and are thought to worsen their outcome.

The activity of microglia during acute neuro-inflammatory processes as those caused by infection remains largely elusive. Acute inflammation represents the early phase of what could result in chronic inflammation and/or neurodegenerative processes. Therefore, microglial responses at this very early phase of perturbation should provide important insights into the cells’ role and adaptive capacities. The aim of the present study was to uncover the heterogeneity of microglial responses under early acute inflammatory conditions to elucidate potential beneficial signatures of subpopulations that could contribute to resolving inflammation and avoiding to enter into a chronic phase causing disease.

To study the cells’ activation, the researchers from LIH isolated microglia from mice injected with lipopolysaccharide (LPS), a bacterial component mimicking an acute infection and triggering inflammation signals in the brain. The use of this model combined with modern single-cell sequencing and multicolour flow cytometry allowed for an in-depth profiling of microglia activation at the transcriptomics level.

Distinct inflammation-induced signatures revealed

The researchers observed a marked global downregulation of the typical microglial homeostatic signature and simultaneously an up-regulation of genes classically activated by inflammation. ‘When investigating further and comparing to published data, we could show that when being under acute systemic inflammation, microglia presented a highly activated state that is clearly distinct from neurodegenerative disease-associated profiles’, states Dr Sousa, who performed most of the experimental work.

Importantly, the researchers also noticed unforeseen heterogeneity among the activated cells. They hypothesised that a subset of reactive microglia may be less sensitive to the inflammatory stimulus caused by LPS or partly recovered from the activated state.

‘Our findings reveal that microglia responses in inflammatory conditions are heterogeneous and clearly distinct from the responses described in the context of neurodegenerative diseases’, underlines Dr. Michelucci, who initiated and led the project. ‘We hope that these results obtained from single-cell transcriptomic profiling of microglia under inflammatory conditions will contribute to the establishment of new resources that will clarify the specific responses to brain disorders. This should boost the development of novel therapeutic strategies against CNS diseases with an immunological component.’

Research funding and collaborations

This research work is a major part of Dr Carole Sousa’s PhD thesis which she successfully defended in March 2018. The project was led by Dr Alessandro Michelucci, an expert in glial cells and inflammation, and involved a fruitful collaboration with scientists from the Proteome and Genome Research Unit at LIH as well as with teams from the Luxembourg Centre for Systems Biomedicine (LCSB) at the University of Luxembourg, in particular with Dr Alexander Skupin. The work was essentially supported by an AFR PhD grant to Dr Carole Sousa from the Luxembourg National Research Fund (FNR) and by funding from “Fondation du Pélican de Mie et Pierre Hippert‐Faber”, a foundation under the aegis of “Fondation de Luxembourg”. The research team was supported by further research grants from the FNR and the National Institutes of Health (NIH).

Link to publication: Sousa et al., 2018, Single‐cell transcriptomics reveals distinct inflammation‐induced microglia signatures