Editor’s summary
Dendritic cells (DCs) migrate over long distances to shuttle antigen from peripheral tissues to lymph nodes. Immune cell trafficking is mediated by chemotactic gradients, but whether DCs can modulate these gradients is unknown. Using a combination of live cell imaging and mathematical modeling, Alanko et al. identify a dual role for the GPCR CCR7 in controlling DC migration. In addition to sensing CCL19 through CCR7, DCs were able to modulate local chemokine concentration via Lfc-mediated endocytosis of CCR7, “sinking” CCL19 from the environment. This self-shaping of chemokine gradients facilitated the accurate migration of DCs, and these gradients could also be sensed by T cells. Together these findings demonstrate that CCR7 can function as both a sensor and a sink for CCL19, facilitating collective leukocyte migration. – Hannah Isles
Abstract
Immune responses rely on the rapid and coordinated migration of leukocytes. Whereas it is well established that single-cell migration is often guided by gradients of chemokines and other chemoattractants, it remains poorly understood how these gradients are generated, maintained, and modulated. By combining experimental data with theory on leukocyte chemotaxis guided by the G protein–coupled receptor (GPCR) CCR7, we demonstrate that in addition to its role as the sensory receptor that steers migration, CCR7 also acts as a generator and a modulator of chemotactic gradients. Upon exposure to the CCR7 ligand CCL19, dendritic cells (DCs) effectively internalize the receptor and ligand as part of the canonical GPCR desensitization response. We show that CCR7 internalization also acts as an effective sink for the chemoattractant, dynamically shaping the spatiotemporal distribution of the chemokine. This mechanism drives complex collective migration patterns, enabling DCs to create or sharpen chemotactic gradients. We further show that these self-generated gradients can sustain the long-range guidance of DCs, adapt collective migration patterns to the size and geometry of the environment, and provide a guidance cue for other comigrating cells. Such a dual role of CCR7 as a GPCR that both senses and consumes its ligand can thus provide a novel mode of cellular self-organization.
INTRODUCTION
How cells navigate through tissues is one of the fundamental questions in biology, with great relevance in areas such as immunology, development, and cancer metastasis. Leukocytes in particular need to be recruited within short time frames because immunological threats arise at unpredictable sites in the body. Moreover, adaptive immunity is generated in lymphatic organs, which are usually remote from the primary site of infection, requiring long-range cellular trafficking (1). A key concept derived from in vitro studies of cultured cells is chemotaxis, where cells migrate toward increasing concentrations of externally established, supposedly defined distributions of chemotactic cues (2, 3). In a metazoan setting, the relationship between chemoattractant distribution and cellular response is poorly understood. This is mainly because the spatiotemporal distribution of soluble chemotactic cues is challenging to measure in vivo, and the mechanisms that determine how gradients are generated and maintained in complex dynamic tissues require further exploration (4, 5).
The majority of chemotactic cues are sensed by G protein–coupled receptors (GPCRs), which respond to a wide range of exogenous and endogenous ligands. The most diversified group of chemotactic proteins in animals is the chemokine system, which comprises around 50 ligands that bind to around 20 receptors (6). Chemokines are the main orchestrators of leukocyte trafficking, and they act by triggering extravasation from the bloodstream, as well as determining the interstitial positioning and the directional guidance of inflammatory cells within tissues (7). Chemokines also guide cells during developmental processes like germ cell and neural crest migration, neurogenesis and vasculogenesis, and organ development (3, 8–10).
Chemokine receptors signal predominantly via Gαi protein signaling. Whereas this specifies the directional response of the cell, ligand binding also leads to receptor internalization, which desensitizes the cell toward ongoing ligand exposure. Desensitization can terminate the migratory response of the cell whenever the local chemokine concentration exceeds a threshold (2, 11). Alternatively, receptor internalization followed by recycling effectively cleans receptors from ligands and might thereby allow cells to adapt to broader ranges of concentrations, e.g., when moving through gradients (2, 12).
The few established experimental paradigms where functional chemokine gradients have been substantiated in animals mostly involve source-sink systems, where the chemokine is secreted by one cell type and consumed by another so that a spatial gradient arises between source and sink (8, 13). This gradient is then sensed by a third migratory cell type. The functional unit forming a chemokine sink is a cell type that expresses a so-called decoy or scavenger receptor. Whereas decoy GPCRs lost the capacity to activate G proteins, they effectively internalize ligands (8).
A variant of such source-sink models is self-generated chemokine gradients that have been demonstrated to drive the collective migration of cell clusters in the lateral line of the developing zebrafish. Here, cells positioned at the leading edge of this traveling primordial organ express a sensory receptor that guides collective migration in an initially homogenous field of the chemokine CXCL12 (14–17). Cells at the trailing edge of the cell cluster express a decoy receptor for CXCL12 and thereby establish the gradient. Furthermore, in some cancer cells and Dictyostelium amoeba, single cells carry cell surface enzymes that create self-generated gradients by degrading guidance cues and thereby organize collective locomotion (18–20).
Here, we use the paradigm of CCR7-mediated chemotaxis to investigate leukocyte migration in different chemokine fields. CCR7 is one of the most prominent GPCRs in the adaptive immune system and is particularly critical for dendritic cells (DCs), professional antigen-presenting cells whose migration after pathogen encounter from peripheral tissues to draining lymph nodes relies on CCR7 and its two chemokine ligands, CCL19 and CCL21 (21–24). Using CCL19 as a model of soluble chemoattractant, we show that chemokine sensing and internalization by a signaling GPCR also affect ligand distribution, thereby allowing a single receptor to both self-generate and sense gradients that guide collective leukocyte migration.
RESULTS
Dendritic cells exhibit persistent CCR7-dependent migration in a uniform CCL19 field
As a model for leukocyte chemotaxis, we used CCR7-guided migration of mature DCs. Upon exposure to pathogenic threats, these cells rely on CCR7 to navigate from peripheral tissues toward the center of draining lymph nodes (25). This process can be faithfully mimicked in vitro by incorporating DCs into three-dimensional (3D) collagen gels overlaid with a medium containing the soluble CCR7 ligand CCL19 (Fig. 1A). The cells migrated directionally toward the chemokine diffusing from the overlaid compartment into the collagen gel (Fig. 1B) and showed a continuous increase in speed and persistence that lasted for hours and spanned millimeter distances (Fig. 1B; fig. S1, A and B; and movie S1). When we measured how the chemoattractant gradient evolved over time using 10-kDa fluorescein isothiocyanate (FITC)–dextran as a molecular proxy of comparable size, we found that the gradient gradually changed from a steep near-exponential distribution toward a flat, almost linear shape within 1 to 2 hours (fig. S2A). Although this might suggest that DCs migrate more efficiently in response to flat and linear gradients (fig. S2B), this seemed unlikely based on previous findings that the chemotactic prowess of DCs is higher in exponential than in linear gradients (26). Furthermore, immature DCs (27) and other leukocytes such as neutrophil-like cells showed a very transient migratory response when exposed to chemoattractants in the same experimental setup (figs. S1, A and B, and S2C and movies S2 and S3). This response pattern was in line with the established model where cells can only effectively sense a gradient when the chemoattractant concentration is in the range of the Kd [dissociation constant (binding affinity)] of the receptor and when the concentration difference of the ligand across the cell body (Δc) is sufficiently high to allow for spatial discrimination (28, 29). This, together with the long-recognized notion that the activity range of diffusion-based gradients of morphogens is very unlikely to reach the millimeter range (13), led us to consider the option that DCs might modulate and re-shape the chemokine gradient as they migrate…
