Inhibition of the Spontaneous Apoptosis of Neutrophil Granulocytes by the Intracellular Parasite Leishmania major

Macrophages are the major target cell population of the obligate intracellular parasites Leishmania. Although polymorphonuclear neutrophil granulocytes (PMN) are able to internalize Leishmania promastigotes, these cells have not been considered to date as host cells for the parasites, primarily due to their short life span. In vitro coincubation experiments were conducted to investigate whether Leishmania can modify the spontaneous apoptosis of human PMN. Coincubation of PMN with Leishmania major promastigotes resulted in a significant decrease in the ratio of apoptotic neutrophils as detected by morphological analysis of cell nuclei, TUNEL assay, gel electrophoresis of low m.w. DNA fragments, and annexin V staining. The observed antiapoptotic effect was found to be associated with a significant reduction of caspase-3 activity in PMN. The inhibition of PMN apoptosis depended on viable parasites because killed Leishmania or a lysate of the parasites did not have antiapoptotic effect. L. major did not block, but rather delayed the programmed cell death of neutrophils by approximately 24 h. The antiapoptotic effect of the parasites could not be transferred by the supernatants, despite secretion of IL-8 by PMN upon coculture with L. major. In vivo, intact parasites were found intracellularly in PMN collected from the skin of mice 3 days after s.c. infection. This finding strongly suggests that infection with Leishmania prolongs the survival time of neutrophils also in vivo. These data indicate that Leishmania induce an increased survival of neutrophil granulocytes both in vitro and in vivo.