The Role of Neutrophils in the Interaction with <i>Leishmania</i>: Far beyond a Simple Trojan Horse?

Neutrophils are the most numerous leukocyte in mammals and normally they are the first phagocyte observed in recently damaged or infected tissues. They play a key role in the innate immune responses to Leishmania and several other microorganisms, nonetheless an exacerbated neutrophils activity can generate a harmful response to the host, therefore its turnover rate is very important to maintain the homeostasis and averts the host tissue damage. Both apoptosis followed by phagocytosis by mononuclear phagocytes (eferocytosis) and reverse transmigration have been considered the main processes for the clearance of neutrophils from injured or infected tissues. However, the interaction with Leishmania and other microbes, as well as molecules produced by arthropod vectors such as sandflies saliva can modify the behavior of neutrophils, causing immediate lysis to prolong their life. In fact, as a result of a long course of coevolution, several microorganisms have developed skills to avoid neutrophil effector mechanisms and take advantage of neutrophil clearance pathways to promote their spread in the host’s body. Leishmania, Chlamydia pneumoniae and Yersinia pestis for example use the efferocytic Trojan horse process for their dissemination and immune protection, in a different way vaccinia Ankara virus and Toxoplasma gondii exploit the neutrophil reverse transmigration for the same reason. Here we present an overview of some characteristics of neutrophils and their different destinations after interaction with several microorganisms, with an emphasis on Leishmania species. It was also suggested the probable role of neutrophils reverse transmigration as another possible route for the spreading of Leishmania in the visceral leishmaniasis.