Enhanced survival of <i>Leishmania major</i> in neutrophil granulocytes in the presence of apoptotic cells

<div><p>Neutrophil granulocytes are the first leukocytes that encounter and phagocytose <i>Leishmania major</i> (<i>L</i>. <i>major</i>) parasites in the infected skin. The parasites can nonetheless survive within neutrophils. However, the mechanisms enabling the survival of <i>Leishmania</i> within neutrophils are still elusive. Previous findings indicated that human neutrophils can engulf apoptotic cells. Since apoptotic neutrophils are abundant in infected tissues, we hypothesized that the uptake of apoptotic cells results in diminished anti-leishmanial activity and, consequently, contributes to enhanced survival of the parasites at the site of infection. In the present study, we demonstrated that <i>L</i>. <i>major</i>-infected primary human neutrophils acquire enhanced capacity to engulf apoptotic cells. This was associated with increased expression of the complement receptors 1 and 3 involved in phagocytosis of apoptotic cells. Next, we showed that ingestion of apoptotic cells affects neutrophil antimicrobial functions. We observed that phagocytosis of apoptotic cells by neutrophils downregulates the phosphorylation of p38 MAPK and PKCδ, the kinases involved in activation of NADPH oxidase and hence reactive oxygen species (ROS) production. In line, uptake of apoptotic cells inhibits TNF- and <i>L</i>. <i>major</i>-induced ROS production by neutrophils. Importantly, we found that the survival of <i>Leishmania</i> in neutrophils is strongly enhanced in neutrophils exposed to apoptotic cells. Together, our findings reveal that apoptotic cells promote <i>L</i>. <i>major</i> survival within neutrophils by downregulating critical antimicrobial functions. This suggests that the induction of enhanced uptake of apoptotic cells represents a novel evasion mechanism of the parasites that facilitates their survival in neutrophil granulocytes.</p>