Of Lymph Nodes and CLL Cells: Deciphering the Role of CCR7 in the Pathogenesis of CLL and Understanding Its Potential as Therapeutic Target

Abstract

The lymph node (LN) is an essential tissue for achieving effective immune responses but it is also critical in the pathogenesis of chronic lymphocytic leukemia (CLL). Within the multitude of signaling pathways aberrantly regulated in CLL the homeostatic axis composed by the chemokine receptor CCR7 and its ligands is the main driver for directing immune cells to home into the LN. In this literature review, we address the roles of CCR7 in the pathophysiology of CLL, and how this chemokine receptor is of critical importance to develop more rational and effective therapies for this malignancy.

Keywords: CCR7; CLL (chronic lymphocytic leukemia); immunotherapy; lymph node; pathophysiology.

Figure 1

CCR7 and the reactive LN. In homeostasis, normal LN show three main cellular compartments: the cortex (B zone), the paracortex (T zone) and the medulla. Upon antigen stimulation, the primary follicles evolve to secondary follicles, made up of a germinal center (GC) and surrounding mantle zone. In reactive LN, CCR7 is necessary for the entry of naïve B cells, naïve T cells (T_N), regulatory T cells (T_REG, not shown), central memory T cells (T_CM, not shown), and dendritic cells (DCs). CCR7 guides lymphocyte homing through high endothelial venules (HEVs) in the paracortex [1] whereas DCs preferentially use afferent lymphatics [2]. CCR7 also drives interstitial migration of these immune subsets in the T zone facilitating, for instance, the interaction of T_N with antigen presenting cells such as B cells and DCs [3]. Upon activation, T cells are directed to the medulla following CCL19 gradients. CCL19 signaling also induces CCR7 internalization and the up-regulation of the egressing receptor S1P1. The balance between CCR7 and S1P1 is needed for the movement of activated T cells from the T zone to the medulla [4]. Similarly, a fine-tuned balance between CCR7 and CXCR5 allows the migration of activated B cells through the T zone and the follicle. In a first step, CCR7 is down-modulated while a concomitant up-regulation of CXCR5 allows activated B cells to enter into the follicle [5]. In reactive follicles, fully developed GC are polarized into two regions clearly differentiated: the dark zone (DZ) and the light zone (LZ). Although GC B cells re-express CCR7, migration of GC B cells between both regions relies on the CXCR5-CXCL13 axis [6]. In the DZ, GC B cells (centroblasts) interact with stromal cells, proliferate (clonal expansion) and undergo somatic hypermutation on the immunoglobulin genes. In the LZ, hypermutated resting GC B cells (centrocytes) interact with a dense network of CXCL13^hi follicular dendritic cells (FDCs) and CXCR5^hi follicular helper T cells (T_FH). FDCs display antigen and secrete cytokines and chemokines (CXCL13) that attract B cells and T_FH to the GC. T_FH are specialized CD4⁺ PD-1⁺ T cells that express BCL-6 and secrete cytokines that promote B cell proliferation and differentiation. T_FH deliver survival signals to GC B cells through a number of different pathways, including CD40-CD40L, PD1-PDL1, and IL-21. The pro-survival signals from T_FH counteract pro-apoptotic signals from the FAS-FASL pathway. Crosstalk of centrocytes with FDCs and T_HF allows the class-switch recombination and the selection of B cells. Centrocytes with the appropriate antigen affinity are selected to become memory B cells or antibody secreting plasma cells. The centrocytes that are not selected undergo apoptosis and are removed by tingible-body macrophages (TBM). Expression of CCR7 allows memory B cells to exit from follicles back to the T zone and, from there, to the medulla [7]. S1P1-expressing T cells and B cells move towards the efferent vessels following S1P gradients [8]. Notation: this scheme shows the main cell types in a reactive LN and in the GC, however in these complex tissues participate additional subtypes not listed here that can be further reviewed elsewhere (1, 30, 31, 41).

Figure 2

CCR7 and the lymph node in CLL. The figure shows the different ways in which CCR7 contributes to CLL pathobiology in the LN tissue. This receptor directs leukemic and accessory cells into the LN following CCL21 gradients that allow cells to across high endothelial venules (HEV) [1]. It is also likely that CCR7 might promote entry through a different gate, the afferent lymphatic vessels [2], although this last situation has not been reported yet in CLL cells. Both entry points are also used by accessory cells such as T cells and dendritic cells (DCs). When CLL cells get access through HEV, binding of CCL21 and subsequent CCR7 signaling promotes a more invasive phenotype, featured by enhanced production of matrix metalloproteases (MMP-2 and MMP-9) that degrade the extracellular matrix (ECM) [3]. This process facilitates trans-endothelial migration and the following interstitial migration within the LN tissue following CCL19 and CCL21 gradients favoring the right positioning of CLL cells within niches where accessory cells, stroma components, or soluble factors (e.g. cytokines and chemokines) are available [4]. Accessory and stromal cells are the main producers of CCR7 ligands thus facilitating the creation of chemotactic routes towards these niches. Similarly, some CCR7-expressing accessory cells can be directed by CCR7 ligands to these environments. Once CLL cells are driven to protective niches, such as proliferation centers (PC), tumor cells have access to CCL19 and CCL21 (which are produced by stromal cells and DCs) which rescue CLL cells from spontaneous or drug-induced apoptosis [5]. CLL cells also have access to BCR signaling and CD40-CD40L signaling [6] which regulate both CCR7 expression and chemotaxis in CLL cells further contributing to interstitial movement within the LN tissue. In the protective niches, CCR7 signaling in CLL cells is also involved in the secretion of trophic factors needed by accessory cells thus creating a positive feedback loop to preserve these tumor niches. For example, CLL cells themselves might preserve PC by means of secretion of lymphotoxin β (Lβ) which binds to Lβ-receptor in stromal cells and induces their differentiation into pro-tumor cells [7] which secrete Indian hedgehog protein (Ihh) triggering survival in malignant cells. Similarly, CLL cells can modulate activity of anti-tumor immunity through the recruitment of pro-tumor regulatory cells such as T_REG and myeloid-derived suppressor cells (MDSC); both subtypes characterized by expression of CCR7 which orchestrates their homing into the LN [8]. These suppressor cells inhibit anti-tumor effector cells (CTLs, NK cells, B cells, etc) through cell-cell interactions or well by creating a tolerant milieu enriched in IL-10 and TGFβ. As a result of all these described activities [5–8], CCR7 directly or indirectly promotes tumor growth in the T cell zone of the LN [9], contributing to the typical obliterated enlarged structure in CLL nodes. Moreover, CCR7 up-regulation in CLL cells (as a consequence of an aberrantly rapid recycling rate of the receptor) leads to an impaired up-regulation of S1P1, the receptor guiding the egress of immune cells trough S1P gradients towards the efferent lymphatic vessels. Therefore, CCR7 signaling retains CLL cell within the LN, increasing the residence time in protective niches thus contributing in an additional way to bulky disease in the LN [10].