Activation of DCs by PRR stimulation causes DC maturation [
54]. During the maturation process, DCs downregulate processes linked to their sentinel function. Macropinocytosis is decreased by downregulation of Cdc42 and a reduction of Arp2/3 levels within the cell [
82]. In response to decreased macropinocytosis, DC sensitivity to hydraulic resistance is increased, and thus, they begin to undergo barotaxis. Neutrophils have also been shown to exhibit barotaxis in confinement, favouring the path of least resistance [
84]. Barotactic movement enables the activated DCs to take the most direct route to the nearest lymph vessel, as they avoid long routes and dead ends which have a higher resistance [
78]. Furthermore, upon maturation, DCs increase the expression of cell surface molecules related to antigen presentation and directional migration (Table
2) [
85‐
88]. Differential expression of tetraspanins CD9 and CD81 on human pDCs (Table
1) defines subsets with different localisation and function [
89,
90]. However, it is currently unknown if DC activation and maturation changes tetraspanin expression, and as such controls DC migration.
Table 2
Key cell surface proteins and associated tetraspanins in dendritic cell immune function [
85‐
88,
91,
100,
126,
167‐
169]
Adhesion | Semaphorin 7A (Sema7A) | Stimulate moDC migration by reducing adhesion and promoting protrusion formation | Unknown |
| Lymphocyte function-associated antigen 1 (LFA1) | Integrin able to regulate the duration of contact between DCs and naïve T cells during antigen presentation | |
Antigen cross-presentation | Major histocompatibility complex I (MHC-I) | Allow presentation of intracellular protein-derived peptides to CD8+ T cells | |
| Major histocompatibility complex II (MHC-II) | Allow presentation of extracellular protein-derived peptides to CD4+ T cells | |
Co-stimulation | CD40 | Receptor involved in further DC activation | Unknown |
| CD80, CD83, CD86 | Co-stimulatory surface proteins needed for T-cell activation | |
Migration | Chemokine receptor 7 (CCR7) | Chemokine receptor required for DC migration to the LN | Unknown |
| C-type lectin-like receptor 2 (CLEC-2) | Interaction with podoplanin, a glycoprotein expressed on the surface of LECs and FRCs | |
Expression of the G-protein coupled chemokine receptor CCR7 is required for DC migration through the lymphatic system [
91]. The chemokines CCL19 and CCL21 are both ligands of CCR7, but CCL21 is thought to be the chemokine critical for DC migration [
92,
93], whereas CCL19 plays a less significant role [
94]. Lymphatic endothelial cells (LECs) are constitutively expressing CCL21 allowing chemotaxis of DCs during steady state [
92,
95]. Upon inflammation, CCL21 expression is upregulated on LECs following the detection of pro-inflammatory cytokines, like TNFα, which facilitates increased haptotaxis of DCs towards the nearest lymphatic vessel [
95]. Haptotaxis is a form of directed cell movement along immobilized gradients of adhesion cues or chemokines [
92,
96]. The highly positively charged C-terminus of CCL21 can bind to heparin sulphates present on cell surfaces and within the extracellular matrix, thus forming a long-lasting local gradient of CCL21 on LECs [
92]. The gradient starts approximately 90 μm from the lymphatic vessel, which coincides with the distance at which DCs shift from random to highly directional movement [
92]. Oligomerisation of CCR7 on the cell surface of DCs, induced by the inflammatory mediator prostaglandin E2 (PGE
2), has been postulated to play a role in efficient migration of some DC subsets towards CCL21 [
97]. CCR7 oligomerisation allows binding and activation of Src family kinases, initiating Src signalling pathways in addition to G-protein coupled receptor signalling from CCR7. Phosphorylation of oligomeric CCR7 by Src at a tyrosine residue creates a binding site for further signalling molecules containing SH2-domains, which is important for efficient cell migration towards CCL21 [
97]. The gap junction protein connexin43 (Cx43) expressed in cDCs has also been identified as a potential player in DC migration towards CCL21 [
98]. In vitro studies using bone marrow-derived DCs (BMDCs) from mice with reduced Cx43 expression revealed defective migration towards CCL21. Moreover, reduced cDC migration to the lymph node in vivo was observed in mice expressing a truncated form of Cx43 [
98]. Although there was no direct connection defined between Cx43 and the directional movement of DCs, it has previously been noted that connexin interacts with c-Src kinase involved in CCL21-directed movement [
97‐
99]. Human monocyte-derived mature DCs highly express the GPI-anchored protein semaphorin 7A (SEMA7A) which has been shown to promote chemokine-driven DC migration [
100]. Ex vivo assays with LPS-stimulated BMDCs from
Sema7A knock-out mice showed a reduced capacity to migrate towards CCL21, despite expressing similar surface levels of CCR7. However, when replicated in vivo, these results were not demonstrated to be significant, and it was suggested that this was due to the complicated multi-step migration that occurs in vivo. Interestingly, when using a collagen matrix to simulate the complex tissue environment, migration of mature DCs with reduced SEMA7A expression (SEMA7A-KD) was more significantly decreased. These SEMA7A-KD DCs also lacked the ability to efficiently form actin-rich protrusions causing a slower migration through the 3D environment and were more adhesive. This suggests that, SEMA7A at least partially, controls migration by reducing cell adhesion and promoting protrusion formation [
100].
DC maturation induces cytoskeletal changes, which optimise DC motility to permit fast migration [
82]. In contrast to immature DCs, the main location of F-actin in mature DCs is within the cell cortex at the rear of the cell [
82]. The Formin protein family member mDia1, activated by the small GTPase RhoA, was shown to be critical in maintaining F-actin at the cell rear, thereby ensuring fast migration. Moreover, experiments involving mDia1 knock-out DCs, suggested its involvement in facilitating chemotaxis of mature DCs towards CCL21 [
82].