Regulation of steady-state neutrophil homeostasis by macrophages

Area of Science:

• Immunology
• Hematology
• Cell Biology

Background:

• Macrophages play a key role in clearing apoptotic neutrophils at inflammation sites.
• The function of macrophages in maintaining neutrophil homeostasis during steady-state conditions is not well understood.
• Cellular FLICE-like inhibitory protein (C-FLIP) is an antiapoptotic gene critical for cell survival.

Purpose of the Study:

• To investigate the role of macrophages in maintaining neutrophil homeostasis under steady-state conditions.
• To explore the function of C-FLIP in myeloid cells regarding neutrophil clearance and homeostasis.
• To elucidate the mechanisms underlying neutrophilia resulting from impaired apoptotic neutrophil clearance.

Main Methods:

• Generation of a novel mouse model by conditionally deleting the C-FLIP gene in myeloid cells (c-FLIP(f/f) LysM-Cre mice).
• Analysis of hematological parameters, organ weights, body weight, and cytokine production (G-CSF, IL-1β, IL-17) in the generated mouse model.
• Assessment of neutrophil clearance rates in circulating blood.
• In vivo experiments involving blocking G-CSF or IL-1R signaling to evaluate their effect on neutrophilia.

Main Results:

• Mice with deleted C-FLIP in myeloid cells exhibited severe neutrophilia, splenomegaly, extramedullary hematopoiesis, and decreased body weight.
• Increased production of granulocyte colony-stimulating factor (G-CSF) and IL-1β was observed, but not IL-17.
• These mice showed delayed clearance of circulating neutrophils, indicating impaired apoptotic neutrophil removal.
• Blocking G-CSF signaling, but not IL-1R signaling, rescued the observed neutrophilia.

Conclusions:

• Impaired clearance of apoptotic neutrophils by macrophages leads to cytokine production that drives excessive granulopoiesis.
• A G-CSF-dependent, IL-1β-independent pathway is implicated in promoting neutrophil production when apoptotic cell clearance is defective.
• Macrophage-mediated clearance of apoptotic neutrophils is essential for maintaining neutrophil homeostasis under steady-state conditions.