An analysis of EBI expression

An analysis of EBI2 expression on dendritic cells (DCs) showed that EBI2 is highly expressed on CD4+ conventional cDCs. In line with this mice deficient for EBI2 largely lack this subset of cDCs in the spleen. Furthermore, the localization of the residual cells is disturbed [10,11]. It was found that EBI2 positive DCs follow a ligand gradient to marginal zone bridging channels and most likely receive survival signals via the lymphotoxin β receptor at this place. Localization of DCs at these bridging channels enables efficient uptake of blood borne particulate antigens and therefore fast T and B cell responses against the latter. Another important role for EBI2 was demonstrated for T follicular helper cells (Tfh cells). Those cells have a specific role to help B cells in antibody production. It was shown that Tfh cells need to migrate towards activated DCs in the outer T zones to receive specific signals via ICOS/ICOS-L under IL-2 depleted conditions to differentiate into Tfh cells [12]. Very recently, an important role of EBI2 expression by innate lymphoid cells type 3 (ILC3) for development of inflammation in the gut in a mouse colitis model was demonstrated [13]. It was shown that stromal cells in the gut produce the EBI2 ligand, which leads to migration of colonic ILC3s into cryptopatches and isolated lymphoid follicles. This fostered colitis induced in RAG-1 deficient mice by injection of anti-CD40 Cy5 RNA mg [13].
EBI2 in neuroinflammation The myelin sheet coating the axons of fast conducting neurons makes up largely for the so-called white matter of the CNS. The myelin sheet is basically a lipid-rich multilamellar membrane wrapped around the axon, which is needed to ensure fast saltatory electrical conduction. The myelin sheet contains very high concentrations of cholesterol, which is produced de novo inside the CNS. Due to the high amounts of those insulating sheets, the CNS is the organ containing the highest amount of cholesterol in the body [14]. In multiple sclerosis (MS) the myelin sheets are attacked by the patient's own immune system. T cells, B cells (with and without their antibodies) and macrophages play major parts in the pathology process leading to white matter lesions and neuronal cells death over time [[15], [16], [17]]. Cholesterol oxidation products in the serum or the cerebrospinal fluid (CSF) of MS patients (which would be predicted in an inflammatory destruction process of the myelin sheet) have been highly investigated [[18], [19], [20], [21], [22], [23], [24]] (Fig. 1). 24S-hydroxycholesterol is one of the major specific degradation products of cholesterol produced inside the CNS. In MS patients (as also in late stage neurodegenerative diseases) slightly less of this oxysterol is found in the plasma, which most likely reflects the loss of brain mass in these patients over time [19,25,26]. Conversely, increased levels of blood-derived 27-OHC can be found in the CSF of MS patients reflecting their blood brain barrier dysfunction [25,27]. A very recent systematic analysis of over 20 sterols in plasma and CSF revealed a decrease of 25-OHC levels in the plasma of relapsing-remitting MS patients but no significant changes of the EBI2 ligand 7α,25-OHC [24]. An important role for the precursor molecule of the main EBI2 ligand, 25-OHC, in the inhibition of inflammation was recently shown. 25-OHC was demonstrated to inhibit inflammasome induction in mouse macrophages [28]. Inflammasome assemblies are the sites of caspase 1 activation and lead to the generation and secretion of active IL-1 and IL-18 from their precursor forms. Since those cytokines play a major role in funneling the inflammatory process, 25-OHC's action inhibited inflammation. Furthermore, also the amount of Th17 cells (which are also expanded under the influence of IL-1β [29]) was lowered in immunized CH25H deficient mice [28]. The intriguing finding is that type I interferons such as IFN-β, which are largely used as treatment for MS patients, induce the enzyme CH25H responsible for generating 25-OHC from cholesterol [30] (Fig. 1). In line with this, CH25H deficient mice developed early onset EAE [28]. Nevertheless, contradictory data to the increased EAE [31] and to the effect of 25-OHC on inflammasome activity were reported by others [32]. Furthermore, a pro-inflammatory role of 25-OHC was recently shown by Vigne et al. [33]. They found that so-called type 1 regulatory T cells (TR1 cells), which are induced under the influence of IL-27 and which produce IL-10 and IFNγ, express CH25H and produce 25-OHC [33]. This was in contrast to other T cell subsets. TR1 cells depend on the transcription factor Blimp-1 and dampen immune responses via their secretion of IL-10 (and IFNγ in diseases such as EAE) [34]. Importantly, 25-OHC suppressed the production of both IL-10 and IFNγ of these cells. Suppression of TR1 cells via 25-OHC was associated with increased Liver X Receptor β (LXR) expression and decreased Blimp1 levels and thereby 25-OHC may act in a pro-inflammatory fashion in vivo [33] (Fig. 1).