Identity and functions of dendritic cell subsets in ischaemia-induced neuroinflammation

Cerebral ischaemia induces several inflammatory processes in the brain. Among them, the infiltration of immune cells is a hallmark of the pathology. Dendritic cells (DCs) are usually present in low numbers in the meninges and the choroid plexus, but rarely in the parenchyma. Upon ischaemia, the number of DCs increases, and the cells infiltrate the brain tissue, where they carry out different functions. In an experimental murine model of stroke, we set out to investigate the infiltration of several subsets of DCs to the brain and their functional role. Early after stroke, we show a rapid and significant influx of DCs, especially of conventional type 2 DCs (cDC2), which are the most abundant subset at all time points analysed. Twenty- four hours after stroke, these cells were the major source of IL-23, which was able to stimulate its receptor on γδ T cells, inducing their production of IL-17. In turn, IL-17 is responsible for the stimulation of the production of Cxcl1 by astrocytes, ultimately leading to the infiltration of neutrophils to the ischaemic brain and to the exacerbation of the tissue damage. We demonstrate that the interruption of the IL- 23/IL-17 axis decreases the infarct size and improves the neurological outcome of stroke in mice, suggesting that cDC2 may play a detrimental role in the early phase of the immune response to stroke. The analysis of the infiltration of DCs to the brain in inflammatory conditions has historically been difficult for the absence of univocal markers and for the similarity of their phenotype with other brain cells, especially microglia. The knowledge about the origin, phenotype and functions of brain DCs is therefore underdeveloped. One of the most commonly used markers for the study of DCs is CD11c, which is also expressed by a subset of microglia. The population of CD11c+ cells present in the brain increases after stroke, and we show that CD11c+ cells include proliferating microglia and infiltrating DCs. Despite their similarities, we demonstrate by RNA- Seq analysis that these two cell types exhibit a differential transcriptional profile, with interesting peculiarities in pattern recognition receptor and chemokine receptor expression. DCs extracted from the ischaemic brain outclass microglia in antigen presentation capacity, indicating a functional specialisation. We show that microglia are responsible for the production of chemokines that attract DCs to the brain, especially conventional type 1 DCs (cDC1). This specific subpopulation of DCs appears to have beneficial functions, reducing the infarct size and improving the functional outcome of ischaemic stroke. Altogether, the studies presented in this thesis shed light on the features discriminating DCs from microglia and uncover previously unknown roles of diverse subpopulations of infiltrating DCs in the outcome of ischaemic stroke.