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  • The importance of EBI for B cell

    2019-12-03

    The importance of EBI2 for B cell function was first suggested by the dramatic upregulation of this receptor in EBV-transformed B cells and further inferred from its regulation in activated and GC B cells (Birkenbach et al., 1993, Glynne et al., 2000, Shaffer et al., 2000). An involvement of EBI2 in pathology has also been suggested by its dysregulated expression in B cell-associated autoimmune and neoplastic diseases (Aalto et al., 2001, Ye et al., 2003). However, the function of EBI2 and the significance and implications of its modulation have long awaited clarification. This study provides evidence for a biological function of EBI2 and indicates that this receptor provides an extra dimension to B cell migration and differentiation. Modulation of EBI2 expression is necessary for ensuring both the rapid and long-term antibody production that are required for optimal protection against pathogens. Identification of the putative ligand for EBI2 and elucidation of the molecular mechanisms by which it controls B cell migration and differentiation may prove valuable in designing new vaccine strategies and potential therapeutics for immune disorders.
    Experimental Procedures
    Acknowledgments We thank S. Tangye and A. Swarbrick for helpful discussions and critical reading of the manuscript; K. Wood, T. Camidge, and V. Turner for technical assistance; C. Brownlee for cell sorting; the staff of the Garvan Institute Biological Testing Facility for animal husbandry, and Ozgene for generation of mice. This work was funded by NHMRC Australia Program Grant 427620 to R.B, A.B., and C.R.M; D.G. and R.B. are supported by fellowships from the NHMRC Australia.
    Introduction Oxysterols are oxidized metabolites derived from cholesterol or cholesterol precursors either through radical processes or enzymatic reactions, bearing additional oxygen functionalities at the side chains and/or in the ring system of cholesterol [1]. In the past, oxysterols have been mainly considered as intermediates of bile SB 290157 trifluoroacetate salt and steroid hormone biosynthetic pathways. However, research in the last years emphasized their role as bioactive lipids involved in cholesterol, lipid and carbohydrate homeostasis, neuronal development and immune system regulation, but also implicated a contribution to the progression of multiple pathologies such as cancer, atherosclerosis, multiple sclerosis or Alzheimer’s disease (reviewed in [[2], [3], [4], [5], [6]]). Nevertheless, these insights predominantly focused on mono-hydroxylated oxysterols such as 25-hydroxycholesterol (25−OHC), 24S−OHC, (25R)-26-hydroxycholesterol (27-OHC), 22R−OHC, 7-ketocholesterol (7kC) or 7βOHC. Double substituted oxysterols gained consideration, particularly after the discovery of 7α,25-dihydroxycholesterol (7α25OHC) as the endogenous ligand for the G-protein-coupled receptor (GPCR) Epstein-Barr virus-induced gene 2 (EBI2) [7,8]. 7α25OHC, and to a lesser extent the corresponding β-isomer, 7β25OHC, were found to act as chemoattractants for immune cells expressing EBI2, thereby regulating immune cell migration. In order to unravel (patho-)physiological mechanisms involving oxysterols, it is crucial to elucidate the underlying formation and degradation of oxysterols and to uncover their intracellular and tissue-specific site of metabolism. Whereas the formation of 7α25OHC involves two hydroxylation steps from cholesterol by cholesterol 25-hydroxylase (CH25H) and cytochrome P450 7B1 (CYP7B1) [9], the metabolic pathways generating 7β25OHC and its oxidized metabolite 7k25OHC are not entirely clear. By catalyzing the local interconversion of glucocorticoids, 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) and 2 (11β-HSD2) exert important and well-known roles in the regulation of metabolic functions, electrolyte balance and the immune system [10,11]. Thereby, 11β-HSDs determine the tissue- and cell-specific availability of glucocorticoids, supplying either cortisol (11β-HSD1), the active ligand for the glucocorticoid receptor (GR), or inactivating cortisol to cortisone (11β-HSD2) and thus, ultimately control receptor activity [12]. However, several studies provided evidence for glucocorticoid-independent effects and therefore the presence of so far unexplored endogenous substrates of 11β-HSDs; 11β-HSD1 was found to efficiently catalyze the reduction of one of the major oxidized metabolites of cholesterol, 7kC to 7βOHC [[13], [14], [15]] or of the secondary bile acid 7-oxolithocholic acid (7oxoLCA) to chenodeoxycholic acid (CDCA) [[16], [17], [18]]. On the other hand, 11β-HSD2 was reported to catalyze the oxidation of 7βOHC to 7kC [19] as well as the conversion of the oxysterol, cholestane-3β,5α,6β-triol (CT) to the oncometabolite 6-oxo-cholestane-3β,5α-diol (OCDO) [20]. Considering the stereo-specific interconversion of 7kC and 7βOHC by 11β-HSDs, the present study aimed to investigate whether 11β-HSDs can catalyze the interconversion of 7k25OHC and 7β25OHC and thus regulate EBI2 activity.