Embryonic macrophages: monitoring hematopoietic stem cell quality determines hematopoietic clonality

summary

Tissue-specific stem cells can survive a lifetime, can continue to differentiate to maintain homeostasis, or transform into cancer-causing cells
.
Despite their importance, for newly formed stem cells, their quality assurance mechanisms
have not been described.
The researchers observed close and specific interactions between macrophages and neonatal hematopoietic stem cells in zebrafish embryos
.
This interaction often leads to the clearance of cytoplasmic material and stem cell division, or complete phagocytosis and stem cell death
.
Stress stem cells are labeled by surface calreticulin, which stimulates macrophage interactions
.
The researchers found that calcaretin knockout or embryonic macrophage depletion reduced the number of
stem cell clones that establish adult hematopoiesis.
The findings support the assumption that embryonic macrophages determine hematopoietic clonality
by monitoring stem cell quality.

The authors studied the blood development of zebrafish embryos and found that hematopoietic stem cells and progenitor cells (HSPCs) emerge from the abdominal wall of the dorsal aorta (VDA), enter the circulatory system, and stay in the embryonic niche, a vascular plexus
called caudal hematopoietic tissue (CHT).
Hematopoietic stem cells/progenitor cells rapidly expand in caudal hematopoietic tissue for 3 to 4 days and then migrate to the renal bone marrow, the adult hematopoietic niche.

In these two niches, HSPCs interact with a variety of cell types, including vascular endothelial cells, mesenchymal stromal cells, and macrophages
.
In vivo cloning markers indicate that 20 to 30 hematopoietic stem cell (HSC) clones produced in the dorsal aorta eventually produce an adult blood system
.
Are these new hematopoietic stem cells quality assured before establishing adult hematopoietic function? Here, using real-time imaging and cell barcoding labeling, the authors discovered discrete interactions between stem cells and embryonic macrophages that regulate the number
of blood-producing long-lived HSC clones in adulthood.

Macrophages help maintain homeostasis
by modulating inflammation, producing cytokines, and patrolling to remove dead, stressful, or aging cells.
The authors focused on the function of macrophages in niches
.
They performed high-resolution real-time imaging
using green fluorescent protein zebrafish embryos with mCherry + macrophages and enhanced green fluorescent protein-positive HSPC.
Shortly after entering the caudal hematopoietic tissue CHT, the HSPC comes into contact
with nearby macrophages.
These interactions can sometimes lead to macrophages uptaking fluorescent HSPC substances
.
From 56 to 106 hours (hpf) after fertilization, approximately 20% to 30% of HSPCs are joined
by macrophages at any point in time.
These interactions are specific
to HSPC.
The binding rate of macrophages to red blood cells and endothelial cells is quite low
.
Macrophages come into contact with HSPC for up to 45 minutes and sometimes absorb fluorescent HSPC substances
.
Interactions can be divided into three types: prolonged cell-cell contact; "Modification", during which the HSPC remains intact, but a small portion of the cellular material is absorbed by macrophages; or "doom", during which HSPC is completely engulfed and destroyed
by macrophages.
To check for macrophage-HSPC interactions in mammals, the authors studied embryonic day 14.
5 (E14.
5) mouse fetal liver sections by immunofluorescence and found that 33% of c-Kit+ hematopoietic cells were in contact
with F4/80+ macrophages.
This includes c-Kit+ cells squeezed or completely engulfed by macrophages, similar to what
was observed in zebrafish.
Overall, these data identified previously uncharacterized macrophage-HSPC interactions in embryonic hematopoietic nichemis
.

The results determined that the stress levels of stem cells during development are quality assured, which affects clones
that contribute to blood formation in adulthood.
Calcaretin acts as a "eat me" signal, initiating macrophage-HSPC interactions and leading to programmed cell clearance or stem cell expansion
.
Immediate homologs of CD47 and SIRPα, the "don't eat me" signal, have not been found in zebrafish, but other primitive signals may affect the behavior of
macrophages.
This quality assurance mechanism may also respond to environmental stresses in adulthood, such as during a bone marrow transplant or in
clonal stem cell diseases including myelodysplastic abnormalities and leukemia.
Macrophages can selectively expand or remove tissue-specific stem cell clones
from other systems.
This may occur
through selective proliferation of certain clones.
Manipulating this quality assurance mechanism may have important therapeutic implications
for stem cell disease and tissue regeneration.