A mobile endocytic network connects clathrin-independent receptor endocytosis to recycling and promotes T cell activation

Abstract

Endocytosis of surface receptors and their polarized recycling back to the plasma membrane are central to many cellular processes, such as cell migration, cytokinesis, basolateral polarity of epithelial cells and T cell activation. Little is known about the mechanisms that control the organization of recycling endosomes and how they connect to receptor endocytosis. Here, we follow the endocytic journey of the T cell receptor (TCR), from internalization at the plasma membrane to recycling back to the immunological synapse. We show that TCR triggering leads to its rapid uptake through a clathrin-independent pathway. Immediately after internalization, TCR is incorporated into a mobile and long-lived endocytic network demarked by the membrane-organizing proteins flotillins. Although flotillins are not required for TCR internalization, they are necessary for its recycling to the immunological synapse. We further show that flotillins are essential for T cell activation, supporting TCR nanoscale organization and signaling.

In vivo function of the lipid raft protein Flotillin 1 during CD8+ T cell-mediated host surveillance

“…Flotillins are involved in TCR recycling and signaling at the IS (28,30). Yet, in our system using a transgenic TCR (OT-I) and cognate peptide expressed by replication-competent viruses, we did not observe a defect in expansion and memory formation of Flot1 -/-CD8 + T cells.…”

“…Flotillin-rich lipid rafts have been implicated in TCR signaling at the IS, contributing to T cell activation in vitro (28)(29)(30). In contrast, activation and memory formation of Flot1 -/-CD8 + T cells has not been systematically analyzed in vivo to date.…”

“…In vitro experiments suggest that organization of membrane microdomains by flotillins is required for optimal T cell migration (27). Furthermore, flotillin-containing lipid rafts assemble at immunological synapses (IS) and have been proposed to serve as scaffold for the TCR signaling machinery (28)(29)(30). Yet, there is no evidence to date on how flotillins affect CD8 + T cell-mediated organ surveillance in vivo, nor how they impact T cell activation during adaptive immune responses.…”

Antiviral Response by Plasmacytoid Dendritic Cells via Interferogenic Synapse with Infected Cells

“…Cell polarity of the vesicular trafficking is a hallmark of other synapses, primarily defined for secretory pathways, e.g., local secretion of synaptic vesicles or lytic granules at the neurological and immunological "lytic/NK" synapses, respectively. Additionally, the endocytosis machinery also polarizes at the level of the immunological synapse and is essential for T cell activation via the internalization and recycling of the T cell receptor (TCR) (Compeer et al, 2018;Das et al, 2004). Importantly, the feed-forward regulation by TLR7-induced signaling is reminiscent of the stabilization of the cell polarity and contact described for immunological and neurological synapses, referred to as inside-out signaling and induced by the response/activation in recipient cells (Dustin and Choudhuri, 2016;Hogg et al, 2011;Park and Goda, 2016).…”

Human FCHO1 deficiency reveals role for clathrin-mediated endocytosis in development and function of T cells

“…The precise localisation of the TCR that is continuously internalised and recycled back to the plasma membrane is of critical importance for the quality of signal transduction. The TCR can be internalised via clathrin-dependent [19][20][21]45 or clathrin-independent mechanisms 46 . This process may be ligand-dependent or ligandindependent 19,20,22,44,47,48 .…”