Antigen presentation by macrophages is enhanced by the uptake of necrotic, but not apoptotic, cells

Area of Science:

• Immunology
• Cell Biology

Background:

• Macrophages play a crucial role in immune responses by presenting antigens to T cells.
• The process of phagocytosis, engulfment of cells by macrophages, can influence macrophage function.
• Distinguishing the effects of phagocytosing necrotic versus apoptotic cells on macrophage antigen presentation is important for understanding immune regulation.

Purpose of the Study:

• To investigate how the phagocytosis of necrotic or apoptotic cells impacts antigen presentation by murine bone marrow-derived macrophages.
• To determine the molecular mechanisms underlying any observed differences in macrophage function.

Main Methods:

• Murine bone marrow-derived macrophages were cultured and exposed to either necrotic or apoptotic neutrophils.
• T cell proliferation assays were performed in vitro using recall antigens and mitogens.
• Flow cytometry was used to analyze the expression of surface markers like CD40 on macrophages.
• Cytokine levels (transforming growth factor beta1 and tumour necrosis factor alpha) in macrophage cultures were measured.

Main Results:

• Phagocytosis of necrotic neutrophils significantly enhanced T cell proliferation compared to phagocytosis of apoptotic neutrophils.
• Macrophages that ingested necrotic cells showed increased CD40 expression within 4 hours.
• Macrophage cultures exposed to apoptotic neutrophils exhibited higher levels of transforming growth factor beta1 and lower levels of tumour necrosis factor alpha.

Conclusions:

• Macrophages efficiently co-stimulate T cells after engulfing necrotic neutrophils, primarily due to rapid CD40 up-regulation.
• Macrophages that engulf apoptotic cells are less effective at co-stimulation and secrete inhibitory cytokines, potentially suppressing immune responses.
• These findings highlight distinct functional outcomes of macrophage phagocytosis depending on the cell death pathway.