PLD3 and PLD4 are single-stranded acid exonucleases that regulate endosomal nucleic-acid sensing

Area of Science:

• Immunology
• Molecular Biology
• Genetics

Background:

• Leukocyte sensing of microbial DNA can trigger beneficial inflammation, but dysregulation leads to pathogenesis.
• Genome-wide association studies link phospholipase D3 (PLD3) and PLD4 genes to Alzheimer's, rheumatoid arthritis, and systemic sclerosis.
• The precise functions of endolysosomal proteins PLD3 and PLD4 remain largely unknown.

Purpose of the Study:

• To investigate the function of PLD3 and PLD4 in immune responses.
• To determine the role of PLD3 and PLD4 in the context of TLR9 signaling.
• To elucidate the enzymatic activity of PLD3 and PLD4.

Main Methods:

• Analysis of PLD4-deficient mice exhibiting inflammatory phenotypes.
• Assessment of dendritic cell and macrophage responsiveness to TLR9 ligands in PLD3/PLD4 deficient models.
• Enzymatic assays to determine the activity of PLD3 and PLD4.

Main Results:

• PLD4 deficiency in mice resulted in an inflammatory disease with elevated interferon-γ (IFN-γ) and splenomegaly.
• PLD4-deficient dendritic cells showed altered responses to single-stranded DNA sensor TLR9 ligands.
• Macrophages from PLD3-deficient mice exhibited exaggerated TLR9 responses.
• PLD3 and PLD4 were identified as 5' exonucleases that degrade TLR9 ligands, not phospholipases.
• Mice deficient in both PLD3 and PLD4 developed lethal early-life liver inflammation.

Conclusions:

• PLD3 and PLD4 function as 5' exonucleases critical for degrading nucleic acid ligands of TLR9.
• These enzymes are essential for regulating inflammatory cytokine production and preventing autoimmune pathology.
• Dysregulation of PLD3 and PLD4 contributes to severe inflammatory conditions, underscoring their importance in immune homeostasis.