The role of neutrophil apoptosis during experimentally induced Streptococcus uberis mastitis

The object of the study was to determine if apoptosis of neutrophils and their subsequent elimination from the mammary gland by macrophages are modulated by an infection of Streptococcus uberis. The experiments were carried out in 5 clinically normal Holstein × Bohemian Red Pied crossbred heifers, aged 14 to 18 months. Before the experimental infection mammary glands were stimulated by PBS as a control. The samples of cell populations were obtained by lavages of the mammary glands in 4 intervals (24, 48, 72 and 168 h) after the PBS and after the experimental infection. Flow cytometry was used to determine the Annexin V positive and propidium jodide negative neutrophils (Annexin V+/PI–). The light microscopy was used to determine apoptotic neutrophils and myeloperoxidase (MPO) positive macrophages. After PBS and S. uberis administration the total number of both Annexin V+/PI– neutrophils and karyopycnotic neutrophils peaked at 24 hours. The highest percentages of Annexin V+/PI– neutrophils were detected at 72 h after PBS and S. uberis, respectively. The highest percentages of karyopycnotic neutrophils were detected at 72 h after PBS and 168 h after S. uberis, respectively. The total number of MPO+ macrophages was the highest at 24 h after PBS and 72 h after S. uberis. The percentage of MPO+ macrophages was the highest at 72 h after PBS and S. uberis. The results of this study demonstrate that during experimental infection of the mammary gland by S. uberis, the apoptosis of neutrophils is modulated. Apoptosis of neutrophils and the subsequent phagocytosis of apoptotic neutrophils by macrophages were delayed. This may cause the transition of the acute inflammatory reaction to a chronic state.