Macrophages self-generate and refine chemotactic gradients during migration towards complement C5a

Area of Science:

• Immunology
• Cell Biology
• Biophysics

Background:

• Macrophages are crucial immune cells that navigate to sites of infection and tissue damage via chemotaxis.
• Self-generated gradients, where cells alter local attractant concentrations, enhance chemotactic accuracy.
• The complement component C5a is a key chemoattractant for macrophages.

Purpose of the Study:

• To investigate how macrophages utilize self-generated gradients to navigate towards C5a.
• To elucidate the molecular mechanisms of C5a depletion by macrophages in different species.
• To understand the impact of C5a concentration on macrophage migration dynamics.

Main Methods:

• Real-time visualization of fluorescent C5a dynamics during macrophage migration.
• Computational modeling to reproduce observed chemotactic behaviors.
• Investigation of C5a depletion mechanisms, including receptor-mediated endocytosis and enzymatic degradation.

Main Results:

• Mouse bone marrow-derived macrophages (BMDMs) actively deplete C5a through C5aR1-dependent endocytosis, creating guiding gradients.
• Different C5a concentrations elicit distinct temporal migration patterns, with optimal chemotaxis observed below 10 nM.
• Human macrophages primarily inactivate C5a via carboxypeptidase-mediated degradation, with optimal concentrations around 30 nM.
• Both species sharpen externally imposed C5a gradients, enhancing directional guidance.

Conclusions:

• Macrophages employ self-generated gradient formation as a conserved strategy for immune navigation, despite species-specific C5a inactivation mechanisms.
• C5a depletion is a critical process enabling macrophages to extract directional information from their environment.
• Understanding these mechanisms provides insights into immune cell migration and potential therapeutic targets.