Human mononuclear macrophages and neutrophils: differences in killing ability and mechanism against Leptospira interrogans
Objective 
To understand the differences in killing ability and mechanism of human mononuclear macrophages and neutrophils against Leptospira interrogans. 
 
 
Methods 
Human THP-1 and HL-60 cell lines were respectively pretreated with PMA (phorbol 12-myristate 13-acetate) and ATRA (all-trans retinoic acid) to induce their differentiation into macrophages and neutrophils. Confocal microscopy was used to detect the changes in total ROS (reactive oxygen species) and NO (nitric oxide) levels as well as free Ca2+ concentration ([Ca2+ ]i) in THP-1 monocyte-derived macrophages and HL-60 cell-derived neutrophils after infection with Leptospira interrogans serogroup Icterohaemorrhagiae serovar Lai strain Lai. Fluorospectrophotometry was applied to analyze the differences in killing ability between THP-1 monocyte-derived macrophages and HL-60 cell-derived neutrophils against intracellular leptospires before and after treatment with total ROS and NO inhibitors and intracellular free Ca2+ chelator. 
 
 
Results 
The total ROS and NO levels and [Ca2+ ]i in THP-1 monocyte-derived macrophages and HL-60 cell-derived neutrophils were significantly increased after infection with the spirochete (P<0.05). Moreover, the total ROS and NO levels and [Ca2+ ]i in the former were significantly higher than those in the latter (P<0.05). The THP-1 monocyte-derived macrophages had stronger killing ability against intracellular leptospires than the HL-60 cell-derived neutrophils (P<0.05). Inhibiting total intracellular ROS, NO or free Ca2+ could result in decreased killing ability of THP-1 monocyte-derived macrophages against intracellular leptospires, but not affect the killing ability of HL-60 cell-derived neutrophils. 
 
 
Conclusion 
Mononuclear macrophages rather than neutrophils act as the main phagocytes eliminating Leptospira interrogans. High levels of total intracellular ROS, NO and free Ca2+ are closely associated with the ability of mononuclear macrophages to kill Leptospira interrogans. 
 
 
Key words: 
Leptospira interrogans; Mononuclear macrophage; Neutrophil; Reactive oxygen species; Nitric oxide; Intracellular free Ca2+