Transcripts of repetitive DNA elements signal to block phagocytosis of hematopoietic stem cells

Area of Science:

• Hematology
• Immunology
• Stem Cell Biology

Background:

• Macrophages preserve hematopoietic stem cell (HSC) function using cell surface Calreticulin (Calr) as an "eat-me" signal.
• Reactive oxygen species (ROS) induce Calr expression, signaling macrophages to clear aged or damaged HSCs.

Purpose of the Study:

• To identify novel regulators of HSC-macrophage interactions.
• To understand the molecular mechanisms governing HSC quality control and proliferation.

Main Methods:

• Zebrafish genetics to identify key regulatory molecules.
• Chemical screening for compounds that modulate HSC surface markers.
• Whole-genome CRISPR-Cas9 screening to identify genes regulating Beta-2-microglobulin (B2m) expression.
• Analysis of repetitive element (RE) transcripts and Toll-like receptor 3 (Tlr3) signaling.

Main Results:

• Beta-2-microglobulin (B2m) was identified as a critical "don't eat-me" signal on HSCs.
• Chemical screen identified inducers of Calr that promote HSC proliferation without increasing ROS or macrophage clearance.
• Toll-like receptor 3 (Tlr3) signaling was found to regulate B2m expression.
• Targeting B2m or Tlr3 reduced HSC clonality, and elevated B2m correlated with high RE transcript expression.

Conclusions:

• The balance between Calreticulin (Calr) and Beta-2-microglobulin (B2m) on HSCs regulates macrophage interactions.
• Repetitive element-associated double-stranded RNA may interact with TLR3 to upregulate B2m on hematopoietic stem and progenitor cells.
• These findings provide insights into the mechanisms defining hematopoietic clonality and HSC quality control.