Lymphocyte mobility and homing is modulated
Lymphocyte mobility and homing is modulated by the chemoattractant receptor subfamily of G protein-coupled receptors (GPCRs) (Campbell et al., 2003, Rot and von Andrian, 2004). B cell migration and position are controlled to a large extent by the lymphoid chemokines CXCL13, CXCL12, CCL19, and CCL21 and the regulated expression of their receptors CXCR5, CXCR4, and CCR7 (Allen et al., 2004, Forster et al., 1996, Forster et al., 1999, Hargreaves et al., 2001, Nie et al., 2004, Reif et al., 2002). Homing of Fmoc-Ser-OH to B cell follicles is dependent on their expression of CXCR5 (Forster et al., 1996), whereas their movement to the B-T boundary after antigen encounter is directed by the rapid upregulation of CCR7 (Reif et al., 2002). As activated B cells differentiate into plasma cells, they downregulate CXCR5 and CCR7 and upregulate CXCR4, which is critical for their localization in the splenic red pulp and subsequent accumulation in the bone marrow (Hargreaves et al., 2001). Expression of CXCR5 is retained on B cells seeding GCs and, together with CXCR4, mediates the organization of GCs (Allen et al., 2004). Although differential expression of these chemokine receptors on plasma cells and GC B cell plays an important role in the localization of these populations to their distinct microenviroments, it is unclear whether additional factors contribute to the migration of activated B cells to extrafollicular sites versus follicular GCs.
In an effort to discover additional GPCRs directing the migratory events of responding B cells, we identified Epstein-Barr virus (EBV)-induced gene 2 (EBI2) as a promising candidate. The gene encoding EBI2 (Ebi2, also known as Gpr183) was originally identified together with Ebi1 (Ccr7) as the most highly upregulated gene in EBV-infected Burkitt's lymphoma cells (Birkenbach et al., 1993). EBI2 was subsequently reported to be most homologous to members of the lipid and purine GPCR family (Rosenkilde et al., 2006, Surgand et al., 2006). Although lacking any close homology partner in the chemokine receptor family, EBI2 signals through the pertussis-sensitive Gαi protein, similarly to many chemokine receptors (Rosenkilde et al., 2006). This orphan GPCR is predominantly expressed in lymphoid tissues and high mRNA expression has been observed in naive B cells (Birkenbach et al., 1993, Rosenkilde et al., 2006). The already high constitutive expression of Gpr183 present in naive B cells is further increased by B cell receptor (BCR)-triggered NF-κB activation, although this upregulation is only transient (Glynne et al., 2000). In contrast, GC B cell differentiation is associated with the shut down of Gpr183 expression, which is controlled by the transcriptional repressor Bcl-6 (Shaffer et al., 2000). Despite this notable pattern of expression linked to B cell differentiation, the biological function of EBI2 remains undefined.
Discussion The early control of extrafollicular versus GC localization of responding B cells is controlled to a large extent by the activity of the chemokine receptors CXCR4, CXCR5, and CCR7. However, CXCR4-deficient plasma cells are not impacted in their access to extrafollicular sites, and GC clusters form, albeit with disorganized structure or orientation, in absence of CXCR4- or CXR5-mediated chemotaxis (Allen et al., 2004, Hargreaves et al., 2001). Thus, the existing experimental evidence points toward the involvement of an additional signal in the organization of early migratory events in T cell-dependent B cell responses. Our findings demonstrate that EBI2-driven positioning of B cells is a critical component of the mechanism that segregates activated B cells into the extrafollicular versus GC compartments. First, EBI2-dependent migration of responding B cells toward the bridging channels was required for the generation of the early extrafollicular plasmablast response, a process that was greatly enhanced by enforced expression of EBI2. Furthermore, the downregulation of EBI2 expression associated with GC B cell differentiation was sufficient to drive the accumulation of B cells in the deep FDC-rich regions of the follicle where GCs originate and enhanced the formation of GC responses. Although it is clear from the migration pattern of naive EBI2-deficient B cells that other activation-induced changes are typically required for B cells to enter GCs, our data indicate that the downregulation of Gpr183 by Bcl-6 is one of the critical activities of this transcription factor in mediating GC B cell differentiation (Shaffer et al., 2000).