Importance of Crosstalk Between Chronic Lymphocytic Leukemia Cells and the Stromal Microenvironment: Direct Contact, Soluble Factors, and Extracellular Vesicles

Abstract

Chronic lymphocytic leukemia (CLL) is caused by the accumulation of malignant B cells due to a defect in apoptosis and the presence of small population of proliferating cells principally in the lymph nodes. The abnormal survival of CLL B cells is explained by a plethora of supportive stimuli produced by the surrounding cells of the microenvironment, including follicular dendritic cells (FDCs), and mesenchymal stromal cells (MSCs). This crosstalk between malignant cells and normal cells can take place directly by cell-to-cell contact (assisted by adhesion molecules such as VLA-4 or CD100), indirectly by soluble factors (chemokines such as CXCL12, CXCL13, or CCL2) interacting with their receptors or by the exchange of material (protein, microRNAs or long non-coding RNAs) via extracellular vesicles. These different communication methods lead to different activation pathways (including BCR and NFκB pathways), gene expression modifications (chemokines, antiapoptotic protein increase, prognostic biomarkers), chemotaxis, homing in lymphoid tissues and survival of leukemic cells. In addition, these interactions are bidirectional, and CLL cells can manipulate the normal surrounding stromal cells in different ways to establish a supportive microenvironment. Here, we review this complex crosstalk between CLL cells and stromal cells, focusing on the different types of interactions, activated pathways, treatment strategies to disrupt this bidirectional communication, and the prognostic impact of these induced modifications.

Keywords: chronic lymphocytic leukemia; extracellular vesicles; mesenchymal stromal cells; microenvironment; prognostic factor.

Figure 1

Crosstalk between CLL B-cells and FDCs via direct contact or soluble factors. In secondary lymphoid organs, CLL B cells interact with FDCs via different molecules, receptor/ligand couples including ICAM1/LFA-1 (8), VCAM1-VLA-4 (8), CD100/Plexin-B1 (9), immune complex/BCR (64), CXCR5/CXCL13 (10), BCMA/BAFF or TACI/BAFF (65), or by transpresentation of IL-15 from FDCs to germinal center B cells (6). These interactions could lead to a leukemic cell survival via a CD44-dependent mechanism involving up-regulation of Mcl-1 in CLL B cells (3), the activation NF-κB pathway (65), the migration, or the proliferation of leukemic cells. Figure created with BioRender.com.

Figure 2

Crosstalk between CLL B-cells and MSCs via direct contact. CLL B-cells interact with MSCs by direct contact by different ways. First via pseudopods that increase cell surface contact. Second by different receptors and ligands including Jagged1/Notch2 (95), LFA-1/VCAM1 (72, 73), VLA4/VCAM1 (75), CD100/Plexin B1 (9), BCR/vimentin (34). Interestingly the CXCR4/CXCL12 axis plays a central role by triggering VLA-4 and LFA-1 axis (74, 77). BCR stimulation also increase VLA-4 (79). All these interactions lead to the proliferation, the migration and/or the survival of leukemic cells by inducing the upregulation of several anti-apoptotic protein including Bcl-2 (96), Mcl-1 (67, 71, 98), or Bcl-XL (96, 98). Figure created with BioRender.com.

Figure 3

Crosstalk between CLL B-cells and MSCs via soluble factors. CLL B-cells interact with MSCs by several soluble factors including cytokines (115, 117), chemokines (42, 114), and growth factors (69, 127, 128). MSCs can produce CXCL10, CXCL13, CXCL12 that binds to their respective receptor on CLL B-cells CXCR3 (118), CXCR5 (112), CXCR4 (66, 69, 75). The triggering of CXCR4/CXCL12 axis lead to the activation of several pathways including PI3K (82), MAPK (106), or STAT3 (66) leading to the survival and the migration of the leukemic cells. Interestingly, BCR stimulation induces the downregulation of CXCR4 (107), the activation of BTK (109), and the increased secretion of some cytokines. MSCs in coculture also produce IL-1β while CLL B-cells produce TNFα suggesting that coculture creates an inflammatory context (115). Figure created with BioRender.com.

Figure 4

Crosstalk between CLL B-cells and MSCs via extracellular vesicles. Bi-directional communication exists between CLL B-cells and MSCs via the production of extracellular vesicles by both cell types. MSC EVs increase the migration, the survival of CLL B-cells and change their gene expression profile (138). CLL B-cells derived EVs can transfer microRNA (135, 144, 145) or protein (143) leading to the migration, the survival and the proliferation of MSCs (135). In addition, they induce an inflammatory phenotype in stromal cells resulting into a cancer-associated fibroblast (CAF)-phenotype (135). Interestingly BCR stimulation increases the production of CLL EVs (145). Figure created with BioRender.com.

Figure 5

Targeting the CLL/MSC crosstalk using CXCR4/CXCL12 and BCR signaling inhibitors. Several therapeutic strategies have been proposed to inhibit CLL/MSC interactions. (a) blocking of CXCR4 using AMD3100 (71) or using a CXCR4 antibody (148); (b) blocking CXCL12 using NOX-A12 (150); (c) CXCR4/CXCL12 and BCR pathways are interconnected, another way to interfere is to target BCR pathway using different inhibitors: ibrutinib that inhibits BTK and downstream events such as MAPK, PI3K, or NF-κB activation (151) but also reduces VLA-4 activation induced by CXCL12 binding (109) and cell adhesion (79). Acalabrutinib (155) that similarly acts on the migration, the homing and the mobilization of leukemic cells in the circulation. Idelalisib or duvelisib also reduced the ability of stromal cells to support CLL migration and adhesion (161); (d) finally, targeting the over-expression of BCL2 partially induced by the microenvironment has also been proposed (163, 164). Figure created with BioRender.com.