Migratory Dermal Dendritic Cells Act as Rapid Sensors of Protozoan Parasites

Abstract

Dendritic cells (DC), including those of the skin, act as sentinels for intruding microorganisms. In the epidermis, DC (termed Langerhans cells, LC) are sessile and screen their microenvironment through occasional movements of their dendrites. The spatio-temporal orchestration of antigen encounter by dermal DC (DDC) is not known. Since these cells are thought to be instrumental in the initiation of immune responses during infection, we investigated their behavior directly within their natural microenvironment using intravital two-photon microscopy. Surprisingly, we found that, under homeostatic conditions, DDC were highly motile, continuously crawling through the interstitial space in a Gα_i protein-coupled receptor–dependent manner. However, within minutes after intradermal delivery of the protozoan parasite Leishmania major, DDC became immobile and incorporated multiple parasites into cytosolic vacuoles. Parasite uptake occurred through the extension of long, highly dynamic pseudopods capable of tracking and engulfing parasites. This was then followed by rapid dendrite retraction towards the cell body. DDC were proficient at discriminating between parasites and inert particles, and parasite uptake was independent of the presence of neutrophils. Together, our study has visualized the dynamics and microenvironmental context of parasite encounter by an innate immune cell subset during the initiation of the immune response. Our results uncover a unique migratory tissue surveillance program of DDC that ensures the rapid detection of pathogens. Cutaneous Leishmaniasis is a difficult-to-treat disease affecting millions of people worldwide. Hence, there is high demand for the development of vaccines against Leishmania parasites, begging for a better understanding of immune responses against this pathogen. Dendritic cells, as part of the innate immune system, are thought to act as gatekeepers against intruding pathogens. However, their behavior in the context of intact tissues is incompletely understood. Here, we have used intravital two-photon microscopy to visualize the behavior of skin resident dendritic cells in real time, both in the steady-state and upon parasite encounter. We have found that migratory dermal dendritic cells are capable of rapidly sensing Leishmania parasites injected into the skin. This occurred through the formation of highly motile cellular processes capable of engulfing parasites, followed by parasite uptake into the cell. Together, our study provides a new vista of the orchestration of host cell–pathogen encounter in the three-dimensional context of intact tissues. Our results serve as the basis for a better understanding of the dynamic regulation of tissue surveillance by dendritic cells.