PTMs of extracellular domains of membrane proteins can influence antibody binding and give rise to ambivalent results. Best proof of protein expression is the use of complementary methods to provide unequivocal evidence. CXCR7, a member of the atypical chemokine receptor family, mainly functions as scavenger for the chemokines CXCL12 and CXCL11. The expression of CXCR7 on nonhematopoietic cells and neoplasms is widely accepted, however, its expression on leukocytes was recently challenged. To solve the dissent, we thoroughly analyzed the expression of CXCR7 on human B cells. We validated the efficiency of different epitope-specific monoclonal antibodies to detect CXCR7 on transfected cells and primary human B cells. The specificity of the used antibodies was further confirmed by an experimentally independent double labeling approach. Examination of CXCR7-dependent scavenging of fluorescent-labeled CXCL12 revealed functional expression of the receptor on human B cells. Moreover, real-time PCR analysis of CXCR7 mRNA showed the presence of transcripts in human leukocytes. Finally, two CXCR7-specific peptides were identified by MS in immunoprecipitates from primary human B cells. Thus, we present a strategy based on combined proteomic and functional approaches that can be used to solve dissents on protein expression, i.e. demonstrating the expression of CXCR7 on human leukocytes.

The atypical chemokine receptor CXCR7 binds the chemokines CXCL12 and CXCL11. The receptor is widely expressed and was shown to tune CXCR12-induced responses of CXCR4. Here, the function of CXCR7 was examined at late stages of human B-cell maturation, when B cells differentiate into Ab-secreting plasmablasts. We identified two populations of CXCR7 + cells in tonsillar lymphocytes, one being presumably memory potential contribution of the scavenger receptor in final B-cell maturation. On in vitro differentiating B cells, we found a marked inverse correlation between CXCR7 and CXCR5 cell surface levels, whereas expression of CXCR4 remained almost constant. Migration assays performed with tonsillar mononuclear cells or in vitro differentiated cells revealed that inhibition of CXCR7 markedly increases chemotaxis toward CXCL12, especially at late stages of B-cell maturation. Chemotaxis was attenuated in the presence of CXCR4 antagonists, confirming that migration is CXCR4 mediated. Our findings unequivocally demonstrate a novel role for CXCR7 in regulating the migration of plasmablasts during B-cell maturation.Keywords: Atypical chemokine receptor r B cells r CXCR4 r CXCR5 r CXCR7Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionAfter birth, B lymphopoiesis occurs in the BM, where cells progress through pro-B cell and pre-B cell stages. By expression of a functional BCR, B cells acquire antigen specificity and enter the periphery as transitional immature B cells, eventually maturing into follicular or marginal zone B cells [1]. Follicular B cells circulateCorrespondence: Dr. Marcus Thelen e-mail: marcus.thelen@irb.usi.ch between lymphoid organs and form GCs within secondary lymphoid organs upon activation. The GC splits up into two histological distinct compartments, the dark zone, where B cells are tightly packed and proliferate, and the light zone. Within the GC, CXCL12 is preferentially expressed in the dark zone, whereas CXCL13, which is also expressed in the follicle, is mainly found in the light zone. B cells may modulate the surface expression of CXCR5 and move between the zones of the GC attracted by CXCL12 and CXCL13 [2]. Activated B cells can grow and differentiate into plasmablasts in extrafollicular foci or form GCs within follicles [3,4]. Within the GC, activated B cells can differentiate into memory or plasma C 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 694-705 Cellular immune response 695 cells [5]. Cells expressing a BCR with high avidity for an antigen differentiate preferentially along the plasma cell pathway and this process might be regulated by expression of factors such as the B-lymphocyte maturation-induced-protein-1, BCL-6, and X-box binding protein-1 [6][7][8] or cytokines such as IL-6 or APRIL [3]. Others showed that Bcl-2, the cytokine IL-21, and SWAP-70, which functions in activation, homing, and class switching of B cells, and is strongly upregulated in GC B cel...

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