3138 – DEFINITIVE HEMATOPOIETIC STEM AND PROGENITOR CELLS MINIMALLY CONTRIBUTE TO EMBRYONIC LYMPHOMYELOPOIESIS

Definitive hematopoietic stem and progenitor cells (HSPCs) are rare cells that arise embryonically and sustain adult hematopoiesis. Although the functional potential of newly-born HSPCs has been demonstrated by transplantation experiments, their native function during development is unknown. Here, we utilized a blood-specific and inducible injury model to assess hematopoietic regeneration in situ during zebrafish development. Using this system, depletion of draculin (drl)-expressing hematopoietic cells at two days post-fertilization (dpf) decreased runx1+23:mCherry HSPCs as early as one day post injury with recovery within four days. In contrast, depletion of drl-expressing cells had minimal impact on myeloid and lymphoid lineages during the same time window. Therefore, we demonstrated that hematopoietic injury triggered HSPCs self-renewal but had little impact on myeloid and lymphoid cell levels. We next employed a tamoxifen-inducible Cre-loxP lineage-tracing approach in which drl:CRE-ERT2-expressing cells undergo a GFP-to-mCherry fluorescent switch. Using this injury-independent lineage tracing approach, we measured steady-state hematopoietic differentiation and determined that embryonic progenitors, not HSPCs, sustain embryonic lymphomyelopoiesis during early larval development. Combined, our data demonstrate that while newly-born HSPCs possess self-renewal capacity, this is initially uncoupled from their ability to differentiate into mature blood cells. This finding suggests a greater complexity in the layering of hematopoietic ontogeny that has important implications for harnessing HSPCs for regenerative medicine.