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Interleukin-25 (IL-25) is the only anti-inflammatory cytokine of the IL-17 family, and it has been shown to be efficacious in inhibiting neuroinflammation. Known for its effects on cells of the adaptive immune system, it has been more recently described to be effective also on cells of the innate immune system, namely macrophages. We used a lentiviral-mediated gene therapy approach to deliver IL-25 to the central nervous system (CNS) in two mouse models of neuroinflammation, entorhinal cortex lesion and experimental autoimmune encephalomyelitis. In both, we found that IL-25 gene therapy was able to modulate CNS myeloid cells, either infiltrating macrophages or resident microglia, towards an anti-inflammatory, tissue-protective phenotype, as testified by the increase in markers such as Arginase-1 (Arg1), Mannose receptor 1 (CD206) and Chitinase 3-like 3 (Ym1). As a consequence, neuroinflammation was partly inhibited and the CNS protected from immune-mediated damage. To our knowledge, this is the first example of M2 shift (alternative activation) induced in vivo on CNS-resident myeloid cells by gene therapy, and may constitute a promising strategy to investigate the potential role of protective microglia in neurological disorders.

Objective: Microvesicles (MVs) have been indicated as important mediators of intercellular communication and are emerging as new biomarkers of tissue damage. Our previous data indicate that reactive microglia/macrophages release MVs in vitro. The aim of the study was to evaluate whether MVs are released by microglia/macrophages in vivo and whether their number varies in brain inflammatory conditions, such as multiple sclerosis (MS). Methods: Electron and fluorescence microscopy and flow cytometry were used to detect myeloid MVs in the cerebrospinal fluid (CSF) of healthy controls, MS patients, and rodents affected by experimental autoimmune encephalomyelitis (EAE), the animal model of MS. Results: Myeloid MVs were detected in CSF of healthy controls. In relapsing and remitting EAE mice, the concentration of myeloid MVs in the CSF was significantly increased and closely associated with disease course. Analysis of MVs in the CSF of 28 relapsing patients and 28 patients with clinical isolated syndrome from 2 independent cohorts revealed higher levels of myeloid MVs than in 13 age-matched controls, indicating a clinical value of MVs as a companion tool to capture disease activity. Myeloid MVs were found to spread inflammatory signals both in vitro and in vivo at the site of administration; mice impaired in MV shedding were protected from EAE, suggesting a pathogenic role for MVs in the disease. Finally, FTY720, the first approved oral MS drug, significantly reduced the amount of MVs in the CSF of EAE-treated mice. Interpretation: These findings identify myeloid MVs as a marker and therapeutic target of brain inflammation. ANN NEUROL 2012;72:610–624

CD4+Foxp3+ regulatory T cells (Tregs) have been considered crucial in controlling immune system homeostasis, and their derangement is often associated to autoimmunity. Tregs identification is, however, difficult because most markers, including CD25 and Foxp3, are shared by recently activated T cells. We show in this paper that CD4+Foxp3+ T cells are generated in peripheral lymphoid organs on immunization and readily accumulate in the target organ of an autoimmune reaction, together with classical inflammatory cells, constituting up to 50% of infiltrating CD4+ T cells. Most CD4+Foxp3+ T cells are, however, CD25− and express proinflammatory cytokines such as IL-17 and IFN-γ, questioning their suppressive nature. Moreover, in vitro CD4+ T lymphocytes from naive and autoimmune mice, stimulated to differentiate into Th1, Th2, Th17, and induced Tregs, display early mixed expression of lineage-specific markers. These results clearly point to an unprecedented plasticity of naive CD4+ T cells, that integrating inflammatory signals may change their fate from the initial lineage commitment to a different functional phenotype.

MVs are known vehicles of horizontal communication among cells, currently under scrutiny as powerful biomarkers in several pathological processes. The potential advantage of MVs relies on the assumption that their content reflects processes ongoing in pathologically relevant cell types. We have described that MVs of myeloid origin in the CSF are a marker of microglia/macrophage activation. Myeloid cells have different activation types, resulting in diverse functional phenotypes. Knowledge on the activation type of myeloid cells during disease would be of paramount importance for the understanding of ongoing pathogenic processes. We show here that macrophages activated in vitro in different ways all release increased amounts of MVs compared with NS cells. Moreover, we show that macrophage-derived MVs contain a repertoire of mRNAs that is not the result of casual sampling from the parental cells, as it is characterized by distinct mRNA enrichments and species. Nevertheless, mRNA content of MVs clearly allows identification in vivo of the activated phenotype of the cell of origin, indicating carryover of functional macrophage traits. We propose that detection of mRNAs in myeloid MVs permits identification of myeloid cell activation type during disease, allowing for further stratification of pathological processes.