Nitric oxide controls proliferation of Leishmania major by inhibiting the recruitment of permissive host cells.

Nitric oxide (NO) is an important antimicrobial effector but also prevents unnecessary tissue damage by shutting down the recruitment of monocyte-derived phagocytes. Intracellular pathogens such as Leishmania major can hijack these cells as a niche for replication. Thus, NO might exert containment by restricting the availability of the cellular niche required for efficient pathogen proliferation. However, such indirect modes of action remain to be established. By combining mathematical modeling with intravital 2-photon biosensors of pathogen viability and proliferation, we show that low L. major proliferation results not from direct NO impact on the pathogen but from reduced availability of proliferation-permissive host cells. Although inhibiting NO production increases recruitment of these cells, and thus pathogen proliferation, blocking cell recruitment uncouples the NO effect from pathogen proliferation. Therefore, NO fulfills two distinct functions for L. major containment: permitting direct killing and restricting the supply of proliferation-permissive host cells.

CD11c-expressing Ly6C+CCR2+ monocytes constitute a reservoir for efficient Leishmania proliferation and cell-to-cell transmission.

The virulence of intracellular pathogens such as Leishmania major (L. major) relies largely on their ability to undergo cycles of replication within phagocytes, release, and uptake into new host cells. While all these steps are critical for successful establishment of infection, neither the cellular niche of efficient proliferation, nor the spread to new host cells have been characterized in vivo. Here, using a biosensor for measuring pathogen proliferation in the living tissue, we found that monocyte-derived Ly6C+CCR2+ phagocytes expressing CD11c constituted the main cell type harboring rapidly proliferating L. major in the ongoing infection. Synchronization of host cell recruitment and intravital 2-photon imaging showed that these high proliferating parasites preferentially underwent cell-to-cell spread. However, newly recruited host cells were infected irrespectively of their cell type or maturation state. We propose that among these cells, CD11c-expressing monocytes are most permissive for pathogen proliferation, and thus mainly fuel the cycle of intracellular proliferation and cell-to-cell transfer during the acute infection. Thus, besides the well-described function for priming and activating T cell effector functions against L. major, CD11c-expressing monocyte-derived cells provide a reservoir for rapidly proliferating parasites that disseminate at the site of infection.

Frontline Science: Leishmania mexicana amastigotes can replicate within neutrophils.

Cutaneous leishmaniasis is a neglected tropical disease, causing a spectrum of clinical manifestations varying from self-healing to unhealing lesions that may be very difficult to treat. Emerging evidence points to a detrimental role for neutrophils during the first hours following infection with many distinct Leishmania species (spp.) at a time when the parasite is in its nonreplicative promastigote form. Neutrophils have also been detected at later stages of infection in unhealing chronic cutaneous lesions. However, the interactions between these cells and the replicative intracellular amastigote form of the parasite have been poorly studied. Here, we show that Leishmaniamexicana amastigotes are efficiently internalized by neutrophils and that this process has only a low impact on neutrophil activation and apoptosis. In neutrophils, the amastigotes were found in acidified vesicles. Furthermore, within cutaneous unhealing lesions, heavily infected neutrophils were found with up to 6 parasites per cell. To investigate if the amastigotes could replicate within neutrophils, we generated photoconvertible fluorescent parasites. With the use of flow cytometry imaging and time-lapse microscopy, we could demonstrate that a subset of parasites replicated within neutrophils. Overall, our data reveal a novel role for neutrophils that can act as a niche for parasite replication during the chronic phase of infection, thereby contributing to disease pathology.