B cell localization: regulation by EBI2 and its oxysterol ligand

Involvement of oxysterols in age-related diseases and ageing processes

Ageing is accompanied by increasing vulnerability to major pathologies (atherosclerosis, Alzheimer's disease, age-related macular degeneration, cataract, and osteoporosis) which can have similar underlying pathoetiologies. All of these diseases involve oxidative stress, inflammation and/or cell death processes, which are triggered by cholesterol oxide derivatives, also named oxysterols. These oxidized lipids result either from spontaneous and/or enzymatic oxidation of cholesterol on the steroid nucleus or on the side chain. The ability of oxysterols to induce severe dysfunctions in organelles (especially mitochondria) plays key roles in RedOx homeostasis, inflammatory status, lipid metabolism, and in the control of cell death induction, which may at least in part contribute to explain the potential participation of these molecules in ageing processes and in age related diseases. As no efficient treatments are currently available for most of these diseases, which are predicted to become more prevalent due to the increasing life expectancy and average age, a better knowledge of the biological activities of the different oxysterols is of interest, and constitutes an important step toward identification of pharmacological targets for the development of new therapeutic strategies.

25-Hydroxycholesterols in innate and adaptive immunity

Cholesterol and components of the cholesterol biosynthetic pathway have fundamental roles in all mammalian cells. Hydroxylated forms of cholesterol are now emerging as important regulators of immune function. This involves effects on the cholesterol biosynthetic pathway and cell membrane properties, which can have antiviral and anti-inflammatory influences. In addition, a dihydroxylated form of cholesterol functions as an immune cell guidance cue by engaging the G protein-coupled receptor EBI2, and it is required for mounting adaptive immune responses. In this Review, we summarize the current understanding of the closely related oxysterols 25-hydroxycholesterol and 7α,25-dihydroxycholesterol, and the growing evidence that they have wide-ranging influences on innate and adaptive immunity.

Specific fibroblastic niches in secondary lymphoid organs orchestrate distinct Notch-regulated immune responses

Fasnacht et al. now show that fibroblasts in secondary lymphoid organs are responsible for the production of Notch ligands regulating the differentiation of immune cells

Fibroblast-like cells of secondary lymphoid organs (SLO) are important for tissue architecture. In addition, they regulate lymphocyte compartmentalization through the secretion of chemokines, and participate in the orchestration of appropriate cell–cell interactions required for adaptive immunity. Here, we provide data demonstrating the functional importance of SLO fibroblasts during Notch-mediated lineage specification and immune response. Genetic ablation of the Notch ligand Delta-like (DL)1 identified splenic fibroblasts rather than hematopoietic or endothelial cells as niche cells, allowing Notch 2–driven differentiation of marginal zone B cells and of Esam⁺ dendritic cells. Moreover, conditional inactivation of DL4 in lymph node fibroblasts resulted in impaired follicular helper T cell differentiation and, consequently, in reduced numbers of germinal center B cells and absence of high-affinity antibodies. Our data demonstrate previously unknown roles for DL ligand-expressing fibroblasts in SLO niches as drivers of multiple Notch-mediated immune differentiation processes.

Access to follicular dendritic cells is a pivotal step in murine chronic lymphocytic leukemia B-cell activation and proliferation

In human chronic lymphocytic leukemia (CLL) pathogenesis, B-cell antigen receptor signaling seems important for leukemia B-cell ontogeny, whereas the microenvironment influences B-cell activation, tumor cell lodging, and provision of antigenic stimuli. Using the murine Eμ-Tcl1 CLL model, we demonstrate that CXCR5-controlled access to follicular dendritic cells confers proliferative stimuli to leukemia B cells. Intravital imaging revealed a marginal zone B cell-like leukemia cell trafficking route. Murine and human CLL cells reciprocally stimulated resident mesenchymal stromal cells through lymphotoxin-β-receptor activation, resulting in CXCL13 secretion and stromal compartment remodeling. Inhibition of lymphotoxin/lymphotoxin-β-receptor signaling or of CXCR5 signaling retards leukemia progression. Thus, CXCR5 activity links tumor cell homing, shaping a survival niche, and access to localized proliferation stimuli.

SIGNIFICANCE

CLL and other indolent lymphoma are not curable and usually relapse after treatment, a process in which the tumor microenvironment plays a pivotal role. We dissect the consecutive steps of CXCR5-dependent tumor cell lodging and LTβR-dependent stroma-leukemia cell interaction; moreover, we provide therapeutic solutions to interfere with this reciprocal tumor-stroma cross-talk.

Cognate interactions: extrafollicular IL-4 drives germinal-center reactions, a new role for an old cytokine

Over the past 25 years it has become clear that B and T lymphocytes go through a range of interactions and migratory events when B cells differentiate to become high-affinity, antibody-secreting cells. This B-cell differentiation is associated with multiple sequential cognate interactions. In this issue of the European Journal of Immunology, Turqueti-Neves et al. [Eur. J. Immunol. 2014. 44: 2130–2138] show that IL-4, a cytokine well known as a regulator of Ig class switch recombination, has another as-yet-unappreciated role. The authors show that IL-4 produced by T-helper cells outside germinal centers has a major effect on the early stages of germinal-center B-cell differentiation. This Commentary will summarize their findings and relate them to what we know on the sequence of cognate interactions and migratory events B cells undergo during T-dependent immune responses.

Oxysterol-EBI2 signaling in immune regulation and viral infection

The seven transmembrane G protein-coupled receptor Epstein-Barr virus (EBV) induced gene 2 (EBI2; also known as GPR183) was identified in 1993 on the basis of its substantial upregulation in EBV-infected cells. It is primarily expressed in lymphoid cells; most abundantly in B cells. EBI2 is central for the positioning of B cells within the lymphoid organs, a process that is regulated in part by a chemotactic gradient formed by the endogenous lipid agonists, and in part by a fine-tuned regulation of EBI2 cell surface expression. The most potent endogenous EBI2 agonist is 7α, 25-dihydroxyxcholesterol (7α,25-OHC), yet many structurally related oxysterols can bind to an EBI2 pocket that is defined by the upper parts of the transmembrane helices and extracellular receptor regions. EBI2 signals via Gαi, as well as via G protein-independent pathways like β-arrestin recruitment. The concerted action of these pathways leads to cell migration. By genetically interfering with its up- and downregulation, EBI2 was also recently shown to induce cell proliferation, an action that could be inhibited by small molecule antagonists. Here, we focus on the oxysterol-EBI2 axis in immune control, including its role in the EBV life cycle. We also summarize the structural and functional properties of EBI2 interaction with oxysterol agonists and small molecule antagonists and discuss EBI2 as therapeutic target for diseases of the immune system.

Follicular dendritic cells: origin, phenotype, and function in health and disease

Follicular dendritic cells (FDCs) were originally identified by their specific morphology and by their ability to trap immune-complexed antigen in B cell follicles. By virtue of the latter as well as the provision of chemokines, adhesion molecules, and trophic factors, FDCs participate in the shaping of B cell responses. Importantly, FDCs also supply tingible body macrophages (TBMs) with the eat-me-signaling molecule milk fat globule-EGF factor 8 (Mfge8), thereby enabling the disposal of apoptotic B cells. Recent studies have provided fundamental insights into the multiple functions of FDCs in both physiological and pathophysiological contexts and into their origin. Here we review these findings, and discuss current concepts related to FDC histogenesis both in lymphoid organs and in inflammatory lymphoneogenesis.