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.

Legend has it that when Gordias, a peasant farmer, rode his oxcart into the capital he was proclaimed the new king of Phrygia by an oracle. In gratitude, he tied his oxcart to a post and dedicated it as an offering to Zeus. The knot was so intricate that it resisted all attempts to untie it, and the oracle prophesied that whoever could untie it would rule over all of Asia. In 333 BC, Alexander the Great came upon the knot and, realizing it had no free ends, took his sword and cut it free. Since their first description in 2004+ T follicular regulatory (Tfr) cells have also resisted all attempts to disentangle them from T follicular helper (Tfh) cells and regulatory T (Treg) cells (Fig. 1). In subsequent landmark studies, Tfr cells were defined as a subset of thymic-derived, FOXP3 + natural Treg cells that are dependent on BCL6 and expression of the chemokine receptor CXCR5 to localize in the germinal center (GC) of the B-cell follicle [2-4]. However, while there was initial consensus on the origin and phenotype of these cells, there was some disagreement on their actual role in the antibody response. This discrepancy may have arisen because these studies