Cell shape sensing licenses dendritic cells for homeostatic migration to lymph nodes.

Immune cells experience large cell shape changes during environmental patrolling because of the physical constraints that they encounter while migrating through tissues. These cells can adapt to such deformation events using dedicated shape-sensing pathways. However, how shape sensing affects immune cell function is mostly unknown. Here, we identify a shape-sensing mechanism that increases the expression of the chemokine receptor CCR7 and guides dendritic cell migration from peripheral tissues to lymph nodes at steady state. This mechanism relies on the lipid metabolism enzyme cPLA2, requires nuclear envelope tensioning and is finely tuned by the ARP2/3 actin nucleation complex. We also show that this shape-sensing axis reprograms dendritic cell transcription by activating an IKKß-NF-κB-dependent pathway known to control their tolerogenic potential. These results indicate that cell shape changes experienced by immune cells can define their migratory behavior and immunoregulatory properties and reveal a contribution of the physical properties of tissues to adaptive immunity.

CD95-Mediated Calcium Signaling Promotes T Helper 17 Trafficking to Inflamed Organs in Lupus-Prone Mice.

CD95 ligand (CD95L) is expressed by immune cells and triggers apoptotic death. Metalloprotease-cleaved CD95L (cl-CD95L) is released into the bloodstream but does not trigger apoptotic signaling. Hence, the pathophysiological role of cl-CD95L remains unclear. We observed that skin-derived endothelial cells from systemic lupus erythematosus (SLE) patients expressed CD95L and that after cleavage, cl-CD95L promoted T helper 17 (Th17) lymphocyte transmigration across the endothelial barrier at the expense of T regulatory cells. T cell migration relied on a direct interaction between the CD95 domain called calcium-inducing domain (CID) and the Src homology 3 domain of phospholipase Cγ1. Th17 cells stimulated with cl-CD95L produced sphingosine-1-phosphate (S1P), which promoted endothelial transmigration by activating the S1P receptor 3. We generated a cell-penetrating CID peptide that prevented Th17 cell transmigration and alleviated clinical symptoms in lupus mice. Therefore, neutralizing the CD95 non-apoptotic signaling pathway could be an attractive therapeutic approach for SLE treatment.

Tumor suppressor Hippo/MST1 kinase mediates chemotaxis by regulating spreading and adhesion.

Chemotaxis depends on a network of parallel pathways that coordinate cytoskeletal events to bias cell movement along a chemoattractant gradient. Using a forward genetic screen in Dictyostelium discoideum, we identified the Ste20 kinase KrsB, a homolog of tumor suppressors Hippo and MST1/2, as a negative regulator of cell spreading and substrate attachment. The excessive adhesion of krsB(-) cells reduced directional movement and prolonged the streaming phase of multicellular aggregation. These phenotypes depended on an intact kinase domain and phosphorylation of a conserved threonine (T176) within the activation loop. Chemoattractants triggered a rapid, transient autophosphorylation of T176 in a heterotrimeric G protein-dependent and PI3K- and TorC2-independent manner. The active phosphorylated form of KrsB acts to decrease adhesion to the substrate. Taken together these studies suggest that cycling between active and inactive forms of KrsB may provide the dynamic regulation of cell adhesion needed for proper cell migration and chemotaxis. KrsB interacts genetically with another D. discoideum Hippo/MST homolog, KrsA, but the two genes are not functionally redundant. These studies show that Hippo/MST proteins, like the tumor suppressor PTEN and oncogenes Ras and PI3K, play a key role in cell morphological events in addition to their role in regulating cell growth.

The WASp L272P gain-of-function mutation alters dendritic cell coordination of actin dynamics for migration and adhesion.

Dendritic cells (DCs) devoid of the actin regulator Wiskott-Aldrich syndrome protein (WASp) show reduced directed migration and decreased formation of podosome adhesion structures. We examined DCs expressing a gain-of-function mutation in WASp, WASp L272P, identified in X-linked neutropenia patients. Analysis of WASp L272P DCs was compared to WASp-deficient DCs to examine how WASp activity influences DC migratory responses. In confined space, WASp-deficient DCs had increased migration speed whereas WASp L272P DCs had similar average speed but increased speed fluctuations, reduced displacement, and atypical rounded morphology, compared to wild-type (WT) DCs. Using an ear inflammation model and flow cytometry analysis, WT, WASp-deficient, and WASp L272P DCs were found to migrate in comparable numbers to the draining lymph nodes (LNs). However, histology analysis revealed that migratory DCs of WASp deficient and WASp L272P mice were mainly located in the collagenous capsule of the LN whereas WT DCs were located inside the LN. Analysis of ultrastructural features revealed that WASp L272P DCs had reduced cell area but formed larger podosome structures when compared to WT DCs. Together, our data suggest that WASp activity regulates DC migration and that loss-of-function and gain-of-function in WASp activity lead to different and phenotype-specific DC migratory behavior.

Proteostasis in dendritic cells is controlled by the PERK signaling axis independently of ATF4.

In stressed cells, phosphorylation of eukaryotic initiation factor 2α (eIF2α) controls transcriptome-wide changes in mRNA translation and gene expression known as the integrated stress response. We show here that DCs are characterized by high eIF2α phosphorylation, mostly caused by the activation of the ER kinase PERK (EIF2AK3). Despite high p-eIF2α levels, DCs display active protein synthesis and no signs of a chronic integrated stress response. This biochemical specificity prevents translation arrest and expression of the transcription factor ATF4 during ER-stress induction by the subtilase cytotoxin (SubAB). PERK inactivation, increases globally protein synthesis levels and regulates IFN-ß expression, while impairing LPS-stimulated DC migration. Although the loss of PERK activity does not impact DC development, the cross talk existing between actin cytoskeleton dynamics; PERK and eIF2α phosphorylation is likely important to adapt DC homeostasis to the variations imposed by the immune contexts.

Macropinocytosis Overcomes Directional Bias in Dendritic Cells Due to Hydraulic Resistance and Facilitates Space Exploration.

The migration of immune cells can be guided by physical cues imposed by the environment, such as geometry, rigidity, or hydraulic resistance (HR). Neutrophils preferentially follow paths of least HR in vitro, a phenomenon known as barotaxis. The mechanisms and physiological relevance of barotaxis remain unclear. We show that barotaxis results from the amplification of a small force imbalance by the actomyosin cytoskeleton, resulting in biased directional choices. In immature dendritic cells (DCs), actomyosin is recruited to the cell front to build macropinosomes. These cells are therefore insensitive to HR, as macropinocytosis allows fluid transport across these cells. This may enhance their space exploration capacity in vivo. Conversely, mature DCs down-regulate macropinocytosis and are thus barotactic. Modeling suggests that HR may help guide these cells to lymph nodes where they initiate immune responses. Hence, DCs can either overcome or capitalize on the physical obstacles they encounter, helping their immune-surveillance function.

Lysosome signaling controls the migration of dendritic cells.

Dendritic cells (DCs) patrol their environment by linking antigen acquisition by macropinocytosis to cell locomotion. DC activation upon bacterial sensing inhibits macropinocytosis and increases DC migration, thus promoting the arrival of DCs to lymph nodes for antigen presentation to T cells. The signaling events that trigger such changes are not fully understood. We show that lysosome signaling plays a critical role in this process. Upon bacterial sensing, lysosomal calcium is released by the ionic channel TRPML1 (transient receptor potential cation channel, mucolipin subfamily, member 1), which activates the actin-based motor protein myosin II at the cell rear, promoting fast and directional migration. Lysosomal calcium further induces the activation of the transcription factor EB (TFEB), which translocates to the nucleus to maintain TRPML1 expression. We found that the TRPML1-TFEB axis results from the down-regulation of macropinocytosis after bacterial sensing by DCs. Lysosomal signaling therefore emerges as a hitherto unexpected link between macropinocytosis, actomyosin cytoskeleton organization, and DC migration.