Overexpression of the CXCR5 Chemokine Receptor, and Its Ligand, B Cell-Activating Chemokine-1 (BCA-1/CXCL13) in B Cell Chronic Lymphocytic Leukemia.
Authors: Bürkle Andrea , Burger Jan A. 
Published: April 2020

Abstract
The chemokine B cell-activating chemokine-1 (BCA-1/CXCL13) is an important homing factor for lymphocytes to B cell zones of secondary lymphoid tissues. CXCL13 acts through its cognate receptor, CXCR5. Normal, mature B cells and a subset of memory T cells express CXCR5 chemokine receptors and migrate in response to BCA-1. However, BCA-1 displays a preferential chemotactic activity for B1 B cells when compared to “normal” B2 B cells. Because B lymphocytes from patients with Chronic Lymphocytic Leukemia (B-CLL) are in several aspects comparable to murine B1 cells, we hypothesized that the CXCR5-CXCL13 axis may be highly active in CLL. Initially, we noticed that CLL cells express functional CXCR5 receptors that induce actin polymerization, CXCR5 endocytosis, chemotaxis, and a prolonged activation of p44/42 MAP kinases. In addition, we examined CXCR5 surface expression in a series of CLL patients by flow cytometry and compared the results with normal B cells, or other leukemic B cell lymphoma. In CLL, leukemia B cells expressed significantly higher surface expression of CXCR5 (mean fluorescence intensity ratio/MFIR: 121 ± 9 (±SEM), n = 26) than circulating, CD19 positive B cells from healthy volunteers (CXCR5-MFIR: 69.9 ± 5.4, n = 11, p = 0.002). Neoplastic B cells from other leukemic B cell lymphomas displayed low surface CXCR5 expression (MFIR 19.7 ± 5.9, n = 11). Serum levels of CXCL13 were evaluated by ELISA. Sera from CLL patients displayed significantly higher levels of CXCL13 (mean ± SEM: 170.1 ± 21.5 pg/ml, n = 22) when compared to sera from healthy volunteers (mean ± SEM: 70.7 ± 5.2 pg/ml, n = 10, p = 0.004). Follicular dendritic cells (FDC) have been considered the main source of CXCL13 in secondary lymphoid tissues, thereby attracting T and B lymphocytes for cognate interactions. Surprisingly, we did not detect significant levels of CXCL13 in supernatants of HK follicular dendritic cells, that previously were demonstrated to protect CLL cells from apoptosis (Pedersen &Reed, Blood.2002;100:1795–801). In contrast, high levels of CXCL13 were detected in supernatants of CLL cell cultures in the presence of nurselike cells (NLC). In NLC cultures, CXCL13 levels were 610 ± 129.8 pg/ml (mean ± SEM, n = 4), whereas FDC supernatants contained 0.22 ± 0 pg/ml CXCL13 (mean ± SEM, n = 2). Because of these high CXCL13 levels in NLC cultures, we examined CXCR5 downregulation on CLL B cells in NLC co-cultures. When compared to freshly isolated CLL B cells, CLL cells from NLC cultures express significantly lower surface CXCR5. CXCR5 MFIR of CLL cells from NLC co-cultures was 7 ± 0.9, n = 4, compared to a CXCR5 MFIR of 91.6 ± 12 for freshly isolated CLL cells the same patients (mean ± SEM, n = 4, p = 0.000). These data indicate that high levels of bioactive CXCL13 are released in NLC cultures that stimulate cognate CXCR5 receptors on CLL B cells and induce signaling cascades, such as p44/42 MAPK, that induce prolonged survival. As such, this study provides a novel insight into interactions between CLL cells and their microenvironment within lymphoid tissues.