Neutrophil apoptosis during experimentally induced Staphylococcus aureus mastitis

Area of Science:

• Veterinary Immunology
• Bovine Mastitis Pathogenesis
• Innate Immune Response

Background:

• Bovine mastitis, an inflammation of the mammary gland, is often caused by bacterial infections like Staphylococcus aureus.
• Neutrophil apoptosis and subsequent efferocytosis by macrophages are critical for resolving inflammation and clearing pathogens.

Purpose of the Study:

• To investigate the modulation of neutrophil apoptosis and macrophage-mediated elimination by Staphylococcus aureus infection in the bovine mammary gland.
• To compare the inflammatory and resolution dynamics following S. aureus infection versus a control stimulus.

Main Methods:

• Experimental intramammary infection with S. aureus in virgin heifers.
• Collection of mammary gland lavage samples at multiple time points post-administration (24, 48, 72, 168 hours).
• Flow cytometry for neutrophil apoptosis (Annexin-V/PI), light microscopy for karyopyknosis, and cytochemistry for myeloperoxidase-positive (MPO+) macrophages.

Main Results:

• S. aureus instillation induced a persistent intramammary infection.
• Peak neutrophil apoptosis and karyopyknosis occurred at 24 hours post-administration for both S. aureus and control.
• Macrophage activity (MPO+ cells) peaked at 48 hours post-S. aureus and 72 hours post-control, with highest percentages at 72 hours in both groups.
• Inflammatory resolution dynamics were similar between S. aureus and control groups, yet infection persisted.

Conclusions:

• Early activation of acute inflammatory resolution mechanisms did not prevent chronic S. aureus mastitis.
• Despite robust neutrophil apoptosis and macrophage involvement, the mammary gland failed to clear the bacterial pathogen.
• The study highlights a paradoxical outcome where inflammation resolution processes may inadvertently contribute to persistent infection.