Reassessing the adrenomedullin scavenging function of ACKR3 in lymphatic endothelial cells.

Atypical chemokine receptor 3 (ACKR3) is a scavenger of the chemokines CXCL11 and CXCL12 and of several opioid peptides. Additional evidence indicates that ACKR3 binds two other non-chemokine ligands, namely the peptide hormone adrenomedullin (AM) and derivatives of the proadrenomedullin N-terminal 20 peptide (PAMP). AM exhibits multiple functions in the cardiovascular system and is essential for embryonic lymphangiogenesis in mice. Interestingly, AM-overexpressing and ACKR3-deficient mouse embryos both display lymphatic hyperplasia. Moreover, in vitro evidence suggested that lymphatic endothelial cells (LECs), which express ACKR3, scavenge AM and thereby reduce AM-induced lymphangiogenic responses. Together, these observations have led to the conclusion that ACKR3-mediated AM scavenging by LECs serves to prevent overshooting AM-induced lymphangiogenesis and lymphatic hyperplasia. Here, we further investigated AM scavenging by ACKR3 in HEK293 cells and in human primary dermal LECs obtained from three different sources in vitro. LECs efficiently bound and scavenged fluorescent CXCL12 or a CXCL11/12 chimeric chemokine in an ACKR3-dependent manner. Conversely, addition of AM induced LEC proliferation but AM internalization was found to be independent of ACKR3. Similarly, ectopic expression of ACKR3 in HEK293 cells did not result in AM internalization, but the latter was avidly induced upon co-transfecting HEK293 cells with the canonical AM receptors, consisting of calcitonin receptor-like receptor (CALCRL) and receptor activity-modifying protein (RAMP)2 or RAMP3. Together, these findings indicate that ACKR3-dependent scavenging of AM by human LECs does not occur at ligand concentrations sufficient to trigger AM-induced responses mediated by canonical AM receptors.

Leukocyte Trafficking in Lymphatic Vessels.

To ensure proper immune function, most leukocytes constantly move within tissues or between them using the blood and lymphatic vessels as transport routes. While afferent lymphatic vessels transfer leukocytes from peripheral tissues to draining lymph nodes (dLNs), efferent lymphatics return lymphocytes from LNs back into the blood vascular circulation. Over the last decades, great progress has been made in our understanding of leukocyte migration into and within the lymphatic compartment, leading to the approval of new drugs targeting this process. In this review, we first introduce the anatomy of the lymphatic vasculature and the main cell types migrating through lymphatics. We primarily focus on dendritic cells (DCs) and T cells, the most prominent lymph-borne cell types, and discuss the functional significance as well as the main molecules and steps involved in their migration. Additionally, we provide an overview of the different techniques used to study lymphatic trafficking.