Final Report Summary - LYMPHATICS-HOMING (Lymph node homing of immune cells via afferent lymphatics – mechanisms and immune response)
Secondary lymphoid organs such as lymph nodes (LNs) are primary sites for the induction of adaptive immune responses against pathogens. Likewise, they are also essential for the induction of peripheral immune tolerance to harmless environmental antigens such as plant pollen or food components. Lymphocytes and antigen-presenting cells continually enter LNs. There, T and B lymphocytes scan their environment for their cognate antigen. In case they are not activated they get replenished by incoming cells after a few hours. This permanent recirculation allows fast and effective immune responses when necessary. LNs possess two ports of entry: From the blood via specialized endothelial cells (high endothelial venules, HEV) and from the lymphatic vasculature via afferent lymphatics. While homing of immune cells via HEV had been studied in detail, little is known about the molecules and mechanisms mediating their homing via lymphatic vessels. The primary objective of the EU-funded LYMPHATICS-HOMING (Lymph node homing of immune cells via afferent lymphatics – mechanisms and immune response) project was to investigate how lymph-derived cells home to LNs and how they contribute to immune responses. The investigators used photo-conversion of fluorescent proteins through the intact skin applying a novel Dendra-2 transgenic mouse model and found that recently activated T cells enter downstream lymph nodes via afferent lymphatics at high frequencies. Applying a combination of intra-lymphatic immune cell transfer and live imaging by epifluorescence as well as 2-photon microscopy it was shown that, independent of adhesion molecules, activated lymph-derived immune cells come to an instantaneous arrest mediated by the mechanical 3D-sieve barrier of the LN subcapsular sinus (SCS). Arrested cells subsequently start to randomly migrate on the sinus floor. Upon encounter of preformed pores they translocate through the SCS floor and display, for approximately 100 µm, a highly directional migration towards the T cell zone. Migration within the SCS is independent of both chemokines and integrins. However, chemokine receptors are imperative for guiding cells out of the SCS and their subsequent translocation towards the T cell zone. The chemokine receptor CCR7 expressed on incoming immune cells and its ligands, the chemokines CCL19 and CCL21, present in the LN parenchyma were identified as major contributors of immune cell homing. The investigators also found that the atypical chemokine receptor ACKR4 that is expressed on endothelial cells lining the ceiling but not the floor of the SCS scavenges CCR7-ligands from the sinus lumen. This formed chemokine gradients across the sinus floor and enabled the emigration of lymph derived immune cells. Furthermore it was shown that cytotoxic killer T cells that arrive at the SCS efficiently kill virus-infected cells at this location and substantially contribute to the control of virus-spread. In addition metastasizing tumor cells that also arrive at LNs via afferent lymphatics induce a rapid change in the cellular composition of the lymph node strongly suppressing adaptive immune responses. Overall, LYMPHATICS-HOMING provided mechanistic, functional and molecular insights in the processes that allow the homing of cells to LNs that arrive via afferent lymphatics. On the long term, targeted application of immune response-modulating cells by intra lymphatic delivery might offer considerable therapeutic potential.