BLR1 Ligand/ BCA-1/BLC Reinhold FoÈrster* Molecular Tumor Genetics and Immunogenetics, Max-Delbruck-Center for Molecular Medicine, Robert Rossle Street 10, Berlin, 13092, Germany * corresponding author tel: +49-30-9406-3330, fax: +49-30-9406-3884, e-mail:
[email protected] DOI: 10.1006/rwcy.2000.10011.
SUMMARY The CXC chemokine BLC/BCA-1 is constitutively expressed in lymphoid follicles of secondary lymphoid organs. It binds to the chemokine receptor CXCR5, which is found on mature B cells and a small subpopulation of T memory cells. By attracting B cells, BLC/BCA-1 essentially contributes to B cell homing and proper positioning of these cells within the microanatomic compartments of secondary lymphoid organs.
indicates that this chemokine is the major regulator of B cell migration to and within lymphoid follicles in most secondary lymphoid organs during normal immune surveillance.
Alternative names BLR1 ligand
GENE AND GENE REGULATION
BACKGROUND
Accession numbers
Discovery
GenBank/EMBL: Human BLC/BCA-1: AF044197, AJ002211 Mouse BLC/BCA-1: AF044196 EST DNA sequences used for cloning mouse BLC/ BCA-1: IMAGE Consortium Clone 596050 EST DNA sequences used for cloning human BLC/ BCA-1: T39765, AA298459, T55152; sequence-tagged side (STS) for chromosomal localization of human BLC/BCA-1: G14456
Two groups succeeded in identifying a novel member of the CXC chemokine family by searching the expressed sequence tag (EST) database for transcripts with sequence motifs characteristic for chemokines. Since it strongly attracts B lymphocytes, this chemokine has been termed B lymphocyte chemoattractant (BLC) (Gunn et al., 1998) and B cell-attracting chemokine (BCA) 1 (Legler et al., 1998). Both migration and calcium mobilization assays demonstrate that BLC/BCA-1 specifically binds to an orphan chemokine receptor formerly known as BLR1 (Dobner et al., 1992). In accordance with the current nomenclature, BLR1 was consequently termed CXCR5. BLC/BCA-1 is constitutively expressed in the B cellrich zone of most secondary lymphoid organs. This finding, together with the characteristic phenotype of CXCR5-gene targeted mice (FoÈrster et al., 1996),
Chromosome location The human BLC/BCA-1 gene is located on chromosome 4q21 and is thus in close proximity to most CXC chemokines including IL-8, GRO, and IP-10. The cDNA contains an open reading frame of 327 nucleotides encoding a putative protein of 109 amino acids with a predicted leader sequence consisting of
1126 Reinhold FoÈrster 21 and 22 amino acids in mice and humans respectively. In both species, the secreted protein contains four cysteines at positions characteristic for members of the CXC chemokine family. Expression of BLC/ BCA-1 has been identified in spleen, lymph nodes, Peyer's patches, and liver (Gunn et al., 1998; Legler et al., 1998).
Relevant linkages CXCR5.
PROTEIN
Accession numbers Mouse BLC/BCA-1: AF044196 Human BLC/BCA-1: AF044197, AJ002211
Sequence See Figure 1.
Description of protein Based on cDNA analysis, an open reading frame that encodes a putative protein of 109 amino acids has been identified in mice and humans. Murine BLC/ BCA-1 contains a predicted leader peptide of 21 amino acids, whereas in humans a predicted leader peptide of 22 amino acids has been found. Thus the predicted secreted protein consists of 88 (murine) and 87 (human) amino acids.
Important homologies The amino acid similarity between human and murine BLC/BCA-1 is 64% as compared with 24±34% for other CXC chemokines. Interestingly, a protein coded for by Mareks disease virus (MDV), Meq-sp, shows the highest similarity to human and murine BLC/ BCA-1. MDV, a lymphotropic herpesvirus, causes a lethal disease in chickens which is characterized by an initial lytic infection of B cells and the subsequent development of T cell lymphomas (Gunn et al., 1998; Legler et al., 1998).
CELLULAR SOURCES AND TISSUE EXPRESSION
Cellular sources that produce Northern blot analysis demonstrates that expression of BLC/BCA-1 is restricted to the liver and to secondary lymphoid organs such as spleen, lymph nodes, and appendix. In contrast, no expression could be observed in the thymus and the bone marrow or by any nonlymphoid organs (Gunn et al., 1998; Legler et al., 1998). In situ hybridization reveals that this chemokine is expressed in a reticular pattern. In the spleen it is expressed in the B cell follicles and at the marginal zone. In Peyer's patches, expression is greatest in the germinal centers, but also extends into the surrounding mantle zone. A similar expression pattern could be observed in lymph nodes but, interestingly, expression is variable and not seen in all follicles (Gunn et al., 1998). The reticular expression pattern and the follicular localization strongly suggest that follicular dendritic cells (FDCs) may be a source
Figure 1 Alignment of the protein sequence of murine and human BLC/BCA-1. The putative leader sequences are underlined. Identical amino acids are indicated by asterisks. Sequence Human BLC/BCA-1: SIVCVDPQAEWIQRMMEVLRKRSSSTLPVPVFKRKIP VLEVYYTSLRCRCVQESSVFIPRRFIDRIQILPRGNGCPRKEIIVWKKNK
BLR1 Ligand/BCA-1/BLC 1127
RECEPTOR UTILIZATION
constitutive expression of BLC/BCA-1 in lymphoid follicles and its marked chemotactic activity towards B cells, it seems likely that BLC/BCA-1 contributes to the development of B cell areas in some secondary lymphoid organs. This view is supported by the observed phenotype of gene-targeted mice lacking the chemokine receptor CXCR5. These animals show morphologically altered B cell follicles and impaired B cell migration in spleen and Peyer's patches (FoÈrster et al., 1996).
CXCR5 is the only receptor for BLC/BCA-1 identified so far.
References
of this chemokine. Expression of BLC/BCA-1 is strongly reduced in the spleen of RAG1-deficient mice. As these animals are deficient in lymphocytes, it seems likely that lymphocytes provide a signal that stimulates constitutive expression of BLC/BCA-1 in nonlymphoid cells of the spleen and other lymphoid organs.
IN VITRO ACTIVITIES
In vitro findings In vitro migration assays have shown that BLC/BCA1 attracts B lymphocytes with high efficacy. However, as high concentrations of the chemokine (1 mM) are needed to induce maximal migration, it has relatively low potency. Compared with the potent effect observed on B cells, BLC/BCA-1 induces a weak chemotactic response in T cells and macrophages, but is completely inactive on neutrophils (Gunn et al., 1998; Legler et al., 1998). The differential responsiveness of defined leukocyte subsets to stimulation with BLC/ BCA-1 correlates with the expression pattern of CXCR5. This chemokine receptor has been identified on mature B cells and small subsets of CD4+ and CD8+ T cells (Dobner et al., 1992; Kaiser et al., 1993; FoÈrster et al., 1994) and as a variant form on monocytes (Barella et al., 1995). Upon stimulation with BLC/BCA-1, CXCR5-transfected cells respond with a transient rise of intracellular (Ca2+). In contrast, peripheral blood B cells do not respond with intracellular Ca2+ mobilization when activated with this chemokine. This observation suggests that chemokine receptors might couple to different signaling pathways in B cells compared with other leukocytes.
IN VIVO BIOLOGICAL ACTIVITIES OF LIGANDS IN ANIMAL MODELS
Normal physiological roles The biological activities of this chemokine have not yet been studied in vivo. However, based on
Barella, L., Loetscher, M., Tobler, A., Baggiolini, M., and Moser, B. (1995). Sequence variation of a novel heptahelical leucocyte receptor through alternative transcript formation. Biochem. J. 309, 773±779. Dobner, T., Wolf, I., Emrich, T., and Lipp, M. (1992). Differentiation-specific expression of a novel G protein-coupled receptor from Burkitt's lymphoma. Eur. J. Immunol. 22, 2795± 2799. FoÈrster, R., Mattis, E. A., Kremmer, E., Wolf, E., Brem, G., and Lipp, M. (1996). A putative chemokine receptor, BLR1, directs B cell migration to defined lymphoid organs and specific anatomic compartments of the spleen. Cell 87, 1037±1047. FoÈrster, R., Emrich, T., Kremmer, E., and Lipp, M. (1994). Expression of the G-protein-coupled receptor BLR1 defines mature recirculating B cells and a subset of T memory helper cells. Blood 84, 830±840. Gunn, M. D., Ngo, V. N., Ansel, K. M., Ekland, E. H., Cyster, J. G., and Williams, L. T. (1998). A B-cell homing chemokine made in lymphoid follicles activates Burkitt's lymphoma receptor-1. Nature 391, 799±803. Kaiser, E., FoÈrster, R., Wolf, I., Ebensperger, C., Kuehl, W. M., and Lipp, M. (1993). The G protein-coupled receptor BLR1 is involved in murine B cell differentiation and is also expressed in neuronal tissues. Eur. J. Immunol. 23, 2532±2539. Legler, D. F., Loetscher, M., Roos, R. S., Clark-Lewis, I., Baggiolini, M., and Moser, B. (1998). B cell-attracting chemokine 1, a human CXC chemokine expressed in lymphoid tissues, selectively attracts B lymphocytes via BLR1/CXCR5. J. Exp. Med. 187, 655±660.
LICENSED PRODUCTS Recombinant proteins are available from R&D Systems and Peprotech.
