Collectively our results indicate that GSK

Collectively, our results indicate that GSK682753A functions as a competitive antagonist and binds to the receptor in the same region as 7α,25-OHC. First, we observe linearity in the Schild plot analysis (Fig. 2B). Second, GSK682753A is highly dependent on F111 at position III:08/3.32 in TM-III (Fig. 3B). Although this is not the case for 7α,25-OHC (Fig. 3A), it has recently been shown that the TM-III residues Y112 at position III:09/3.33 (just next to F111) and Y116 at III:13/3.37 are crucial for agonist binding [9,10] indicating that the two ligands bind in the same region. Interestingly, the primary uHTS screen at Sanford-Burnham Center for Chemical Genomics identified 2946 compounds out of 364,168 tested that were able to suppress 7α,25-OHC-induced β-arrestin recruitment at 5 μM by more than 50% (PubChem BioAssay ID: 651636). Of note, the most efficacious of the active compounds are very similar in structure to GSK682753A. For instance, the best compound (CID: 5804570) also has two chlorine-substituted benzene-rings at each extremity, an enone moiety and a centrally-located nitrogen-containing ring (in this case a piperazine). Thus, it seems that these structural traits are important for efficacious inhibition of EBI2 activity. It should be noted that we first characterized GSK682753A as an inverse agonist [16]. However, new data indicate that a part of the constitutive activity of EBI2 may be a result of oxysterol contamination in the medium [9]. Thus, the inverse agonism of GSK682753A might have been antagonistic. It is possible that EBI2 is constitutively active; however, presently we cannot assess the magnitude of this (if any) with the tools available. As EBI2 is highly important for generation of an efficient T cell-dependent humoral immune response [[5], [6], [7]] it may be that aberrant expression or other dysregulation of this receptor contributes in B cell pathologies. In line with this, EBI2 expression is down-regulated in diffuse large B-cell lymphomas [12] and chronic lymphocytic leukemia [13] but up-regulated in PTLDs [14]. Interestingly, PTLDs are highly associated with Epstein–Barr virus (EBV) seropositivity which agrees well with the finding that EBV infection of Taurine australia in vitro results in transformation of these into highly proliferative lymphoblastoids [19]. In both cases, the expression of EBI2 is highly upregulated suggesting that this receptor could play a role in pathogenesis [2,15] putatively in combination with BILF1 [20]. This is also indicated by the observation that the proliferation of B cells over-expressing EBI2 is higher than wt counterparts [16]. Importantly, the proliferation could be blocked by GSK682753A suggesting that targeting EBI2 could be therapeutically beneficial. This may not only be limited to malignant diseases but may also extend to EBV-associated benign conditions such as mononucleosis for which no treatment is currently available.
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Introduction Dynamic changes in lymphocyte localization are fundamental to the rapid and efficient production of protective antibodies. Antibody responses are initiated by the relocalization of antigen-engaged B cells to the B zone-T zone (B-T) boundary where cognate interactions with T cells drive initial B cell proliferation (Kelsoe and Zheng, 1993, Okada and Cyster, 2006). Proliferating B cell blasts subsequently proceed down one of two independent pathways of migration and differentiation (Jacob et al., 1991, Liu et al., 1991). Responding B cells can migrate from the B-T boundary to extrafollicular areas where they are induced to rapidly expand and differentiate into plasmablasts and plasma cells (MacLennan et al., 2003). These transient antibody-secreting cells provide the most immediate source of antigen-specific antibodies. Alternatively, antigen-engaged B cells can localize in the central, follicular dendritic cell (FDC)-rich region of the follicle to form germinal centers (GCs) (MacLennan, 1994). B cells proliferating in GCs give rise to high-affinity clones and exit the GC as long-lived plasma cells and memory B cells (Manz et al., 2005, O\'Connor et al., 2003). This second GC-dependent pathway of B cell differentiation provides a sustained source of antibodies with enhanced antigen neutralization potential and mediates long-term immunity against reinfection. The early changes in positioning that recruit responding B cells to either the extrafollicular or the GC pathway of antibody production are therefore crucial for coordinating rapid versus long-term humoral responses.