Clonal Proliferation and Stochastic Pruning Orchestrate Lymph Node Vasculature Remodeling.
Lymph node (LN) expansion during an immune response relies on the transient remodeling of its vasculature. Although the mechanisms driving LN endothelial cell division are beginning to be understood, a comprehensive view of LN endothelial cell dynamics at the single-cell level is lacking. Here, we used multicolored fluorescent fate-mapping models to track the behavior of blood endothelial cells during LN expansion upon inflammation and subsequent return to homeostasis. We found that expansion of the LN vasculature relied on the sequential assembly of endothelial cell proliferative units. This segmented growth was sustained by the clonal proliferation of high endothelial venule (HEV) cells, which act as local progenitors to create capillaries and HEV neo-vessels at the periphery of the LN. Return to homeostasis was accompanied by the stochastic death of pre-existing and neo-synthesized LN endothelial cells. Thus, our fate-mapping studies unravel-at a single-cell level-the complex dynamics of vascular-tree remodeling during LN expansion and contraction.
T Cell Zone Resident Macrophages Silently Dispose of Apoptotic Cells in the Lymph Node.
In lymph nodes (LNs), dendritic cells (DCs) are thought to dispose of apoptotic cells, a function pertaining to macrophages in other tissues. We found that a population of CX3CR1+ MERTK+ cells located in the T cell zone of LNs, previously identified as DCs, are efferocytic macrophages. Lineage-tracing experiments and shield chimeras indicated that these T zone macrophages (TZM) are long-lived macrophages seeded in utero and slowly replaced by blood monocytes after birth. Imaging the LNs of mice in which TZM and DCs express different fluorescent proteins revealed that TZM-and not DCs-act as the only professional scavengers, clearing apoptotic cells in the LN T cell zone in a CX3CR1-dependent manner. Furthermore, similar to other macrophages, TZM appear inefficient in priming CD4 T cells. Thus, efferocytosis and T cell activation in the LN are uncoupled processes designated to macrophages and DCs, respectively, with implications to the maintenance of immune homeostasis.
Hemogenic endothelial fate mapping reveals dual developmental origin of mast cells.
Hematopoiesis occurs in distinct waves. ‘Definitive’ hematopoietic stem cells (HSC) with the potential for all blood lineages emerge in the aorta-gonado-mesonephros, while ‘primitive’ progenitors, whose potential is thought to be limited to erythrocytes, megakaryocytes and macrophages (MΦ), arise earlier in the yolk sac (YS). Here, we questioned whether other YS lineages exist that have not been identified, partially owing to limitations of current lineage tracing models. We established the use of Cdh5-CreERT2 for hematopoietic fate mapping, which revealed the YS origin of mast cells (MC). YS derived MC are replaced by definitive MC, which maintain themselves independently from the bone marrow in the adult. Replacement occurs with tissue specific kinetics. MC in the embryonic skin, but not other organs, remain largely YS derived prenatally and are phenotypically and transcriptomically distinct from definite adult MC. We conclude that within myeloid lineages, dual hematopoietic origin is shared between MΦ and MC.
