To understand the differences in engulfing ability and phagolysosome formation between mononuclear-macrophages and neutrophils during Leptospira interrogans infection.

Human THP-1 monocytes and HL-60 cells were pretreated with PMA (phorbol-12-myristate-13-acetate) and ATRA (all-trans retinoic acid) to differentiate them into mononuclear-macrophages and neutrophils, respectively. The phagocytosis of Leptospira interrogans in THP-1-PMA mononuclear-macrophages and HL-60-ATRA neutrophils was detected by confocal microscopy. The morphology of intracellular Leptospira was determined by transmission electron microscopy. The viability of phagocytized Leptospira and the percentages of dead Leptospira were analyzed by confocal microscopy and spectrofluorimetry, respectively. Confocal microscopy was used to measure the formation of phagolysosomes in different phagocytes.

Both THP-1-PMA mononuclear-macrophages and HL-60-ATRA neutrophils could phagocytize Leptospira interrogans, but the phagocytic ability of the former was notably stronger than that of the latter (P<0.05). Intracellular Leptospira were surrounded by phagocytic vesicles in both types of phagocytes. THP-1-PMA mononuclear-macrophages were better than HL-60-ATRA neutrophils in killing intracellular Leptospira (P<0.05). More phagolysosomes were formed in THP-1-PMA mononuclear-macrophages than in HL-60-ATRA neutrophils (P<0.05).

Human mononuclear-macrophages but not neutrophils act as major phagocytes that play an important role in phagocytizing and killing Leptospira during infection. Less fusion of the phagosomes with lysosomes may be responsible for the lower Leptospira-killing ability of neutrophils.

Leptospira interrogans; Mononuclear-macrophages; Neutrophils; Phagolysosome