GPR183 Is Dispensable for B1 Cell Accumulation and Function, but Affects B2 Cell Abundance, in the Omentum and Peritoneal Cavity.

B1 cells constitute a specialized subset of B cells, best characterized in mice, which is abundant in body cavities, including the peritoneal cavity. Through natural and antigen-induced antibody production, B1 cells participate in the early defense against bacteria. The G protein-coupled receptor 183 (GPR183), also known as Epstein-Barr virus-induced gene 2 (EBI2), is an oxysterol-activated chemotactic receptor that regulates migration of B cells. We investigated the role of GPR183 in B1 cells in the peritoneal cavity and omentum. B1 cells expressed GPR183 at the mRNA level and migrated towards the GPR183 ligand 7α,25-dihydroxycholesterol (7α,25-OHC). GPR183 knock-out (KO) mice had smaller omenta, but with normal numbers of B1 cells, whereas they had fewer B2 cells in the omentum and peritoneal cavity than wildtype (WT) mice. GPR183 was not responsible for B1 cell accumulation in the omentum in response to i.p. lipopolysaccharide (LPS)-injection, in spite of a massive increase in 7α,25-OHC levels. Lack of GPR183 also did not affect B1a- or B1b cell-specific antibody responses after vaccination. In conclusion, we found that GPR183 is non-essential for the accumulation and function of B1 cells in the omentum and peritoneal cavity, but that it influences the abundance of B2 cells in these compartments.

EBI2 overexpression in mice leads to B1 B-cell expansion and chronic lymphocytic leukemia-like B-cell malignancies.

Human and mouse chronic lymphocytic leukemia (CLL) develops from CD5+ B cells that in mice and macaques are known to define the distinct B1a B-cell lineage. B1a cells are characterized by lack of germinal center (GC) development, and the B1a cell population is increased in mice with reduced GC formation. As a major mediator of follicular B-cell migration, the G protein-coupled receptor Epstein-Barr virus-induced gene 2 (EBI2 or GPR183) directs B-cell migration in the lymphoid follicles in response to its endogenous ligands, oxysterols. Thus, upregulation of EBI2 drives the B cells toward the extrafollicular area, whereas downregulation is essential for GC formation. We therefore speculated whether increased expression of EBI2 would lead to an expanded B1 cell subset and, ultimately, progression to CLL. Here, we demonstrate that B-cell-targeted expression of human EBI2 (hEBI2) in mice reduces GC-dependent immune responses, reduces total immunoglobulin M (IgM) and IgG levels, and leads to increased proliferation and upregulation of cellular oncogenes. Furthermore, hEBI2 overexpression leads to an abnormally expanded CD5+ B1a B-cell subset (present as early as 4 days after birth), late-onset lymphoid cancer development, and premature death. These findings are highly similar to those observed in CLL patients and identify EBI2 as a promoter of B-cell malignancies.

Signaling via G proteins mediates tumorigenic effects of GPR87.

G protein-coupled receptors (GPCRs) constitute a large protein family of seven transmembrane (7TM) spanning proteins that regulate multiple physiological functions. GPR87 is overexpressed in several cancers and plays a role in tumor cell survival. Here, the basal activity of GPR87 was investigated in transiently transfected HEK293 cells, revealing ligand-independent coupling to Gαi, Gαq and Gα12/13. Furthermore, GPR87 showed a ligand-independent G protein-dependent activation of the downstream transcription factors CREB, NFκB, NFAT and SRE. In tetracycline-induced Flp-In T-Rex-293 cells, GPR87 induced cell clustering presumably through Gα12/13 coupling. In a foci formation assay using retrovirally transduced NIH3T3 cells, GPR87 showed a strong in vitro transforming potential, which correlated to the in vivo tumor induction in nude mice. Importantly, we demonstrate that the transforming potential of GPR87 was correlated to the receptor signaling, as the signaling-impaired mutant R139A (Arg in the conserved "DRY"-motif at the bottom of transmembrane helix 3 of GPR87 substituted to Ala) showed a lower in vitro cell transformation potential. Furthermore, R139A lost the ability to induce cell clustering. In summary, we show that GPR87 is active through several signaling pathways and that the signaling activity is linked to the receptor-induced cell transformation and clustering. The robust surface expression of GPR87 and general high druggability of GPCRs make GPR87 an attractive future anticancer target for drugs that - through inhibition of the receptor signaling - will inhibit its transforming properties.