Neutrophil granulocytes as host cells and transport vehicles for intracellular pathogens: Apoptosis as infection-promoting factor

Polymorphonuclear neutrophil granulocytes (PMN) are primary antimicrobial effector cells of the innate immune system and serve to destroy invading pathogens. Although most ingested microorganisms are killed readily inside PMN, several obligate or facultative intracellular pathogens survive even in this hostile environment. Extension of the life span of neutrophils is a general escape mechanism of pathogens residing in PMN. However, after 2-4 days, even infected neutrophils become apoptotic and are phagocytosed by macrophages. Since microbes entering macrophages via the uptake of infected apoptotic PMN may survive and multiply in macrophages, apoptotic neutrophils can serve as "Trojan horses" for certain pathogens. Interfering with activating signaling pathways appears to be another potent mechanism by which intracellular microorganisms suppress cellular activation in neutrophils. In addition to provide a short overview of the topic, the present review aims to summarize our own findings regarding the interaction between human neutrophils and intracellular pathogens as well as regarding the disease promoting role of apoptotic cells after infection with Leishmania major.