title: CXCR4 expression on activated B cells is downregulated by CD63 and IL-21.
abstract: CXCR4 expression is critical for localization of centroblasts in the dark zone of germinal centers (GCs), and centrocytes downregulate CXCR4 and thus leave the dark zone to reside in the light zone. However, mechanisms governing CXCR4 downregulation on centrocytes are not known. In this study, we show that the amount of intracellular CXCR4 in centroblasts was similar to that in centrocytes, suggesting differential control of CXCR4 protein expression in these GC B cells. Restimulation of activated B cells with IL-21, which is a major cytokine produced by T follicular helper cells, accelerated CXCR4 internalization by inducing endocytosis-related GRK6 expression. Although CXCR4 expression was downregulated on GC B cells by IL-21 stimulation, CXCR4(low) centrocytes developed in the spleens of IL-21R-deficient mice, suggesting other mechanisms for downregulation. The level of CD63 (which recruits CXCR4 to late endosome in CD4 T cells) in centrocytes was more than that in centroblasts and was strikingly elevated in activated Bcl6-deficient B cells. Bcl6, a transcriptional repressor, was detected on the chromatin of the CD63 gene in resting B cells, therefore CD63 is a molecular target of Bcl6. Downregulation of CD63 mRNA in activated Bcl6-deficient B cells by small interfering RNA upregulated CXCR4 expression on the B cells. Furthermore, addition of Bcl6 inhibitor to activated B cell cultures increased CD63 mRNA expression in (and downregulated CXCR4 expression on) those activated B cells. Thus, CXCR4 can be downregulated on activated B cells by IL-21-induced endocytosis and CD63-mediated endosomal recruitment, and these mechanisms may contribute to downregulation of CXCR4 on centrocytes.
authors.affiliation: Department of Developmental Genetics, H2, Graduate School of Medicine, Chiba University, Chiba 260-8670, Japan.
authors.forename: Nobuya
authors.initials: N
authors.lastname: Yoshida
authors.forename: Daisuke
authors.initials: D
authors.lastname: Kitayama
authors.forename: Masafumi
authors.initials: M
authors.lastname: Arima
authors.forename: Akemi
authors.initials: A
authors.lastname: Sakamoto
authors.forename: Ayako
authors.initials: A
authors.lastname: Inamine
authors.forename: Haruko
authors.initials: H
authors.lastname: Watanabe-Takano
authors.forename: Masahiko
authors.initials: M
authors.lastname: Hatano
authors.forename: Takao
authors.initials: T
authors.lastname: Koike
authors.forename: Takeshi
authors.initials: T
authors.lastname: Tokuhisa
mesh_terms: D000818:Animals; D015703:Antigens, CD / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D016175:B-Lymphocyte Subsets / Q000166:cytology / Q000276:immunology* / Q000378:metabolism*; D002478:Cells, Cultured; D015536:Down-Regulation / Q000276:immunology*; D004705:Endocytosis / Q000276:immunology; D011992:Endosomes / Q000276:immunology / Q000378:metabolism; D007378:Interleukins / Q000502:physiology*; D008213:Lymphocyte Activation / Q000276:immunology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D010980:Platelet Membrane Glycoproteins / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D019718:Receptors, CXCR4 / Q000037:antagonists & inhibitors* / Q000096:biosynthesis* / Q000378:metabolism; D060149:Tetraspanin 30; D000098546:Interleukin-21
chemical_list: D015703:Antigens, CD; C514936:CXCR4 protein, mouse; C554587:Cd63 protein, mouse; D007378:Interleukins; D010980:Platelet Membrane Glycoproteins; D019718:Receptors, CXCR4; D060149:Tetraspanin 30; D000098546:Interleukin-21