The role of dermis resident macrophages and their interaction with neutrophils in the early establishment of Leishmania major infection transmitted by sand fly bite

Abstract

There is substantial experimental evidence to indicate that Leishmania infections that are transmitted naturally by the bites of infected sand flies differ in fundamental ways from those initiated by needle inocula. We have used flow cytometry and intravital microscopy (IVM) to reveal the heterogeneity of sand fly transmission sites with respect to the subsets of phagocytes in the skin that harbor L. major within the first hours and days after infection. By flow cytometry analysis, dermis resident macrophages (TRMs) were on average the predominant infected cell type at 1 hr and 24 hr. By confocal IVM, the co-localization of L. major and neutrophils varied depending on the proximity of deposited parasites to the presumed site of vascular damage, defined by the highly localized swarming of neutrophils. Some of the dermal TRMs could be visualized acquiring their infections via transfer from or efferocytosis of parasitized neutrophils, providing direct evidence for the “Trojan Horse” model. The role of neutrophil engulfment by dermal TRMs and the involvement of the Tyro3/Axl/Mertk family of receptor tyrosine kinases in these interactions and in sustaining the anti-inflammatory program of dermal TRMs was supported by the effects observed in neutrophil depleted and in Axl-/-Mertk-/- mice. The Axl-/-Mertk-/- mice also displayed reduced parasite burdens but more severe pathology following L. major infection transmitted by sand fly bite.

Visualizing the In Vivo Dynamics of Anti-Leishmania Immunity: Discoveries and Challenges

“…Intravital studies will be instrumental in revealing immune players, such as eosinophils or regulatory T cells, that are responsible for generating and maintaining a permissive environment that promote long-term parasite survival. Visualizing complex behaviors such as neutrophil extravasation from blood and their swarming at the site of bite injury ( 17 ), L. major transmission between cells through efferocytosis ( 104 ), and dynamic capture of L. major promastigotes by migratory dermal DCs in the skin ( 90 ) can only be accomplished by IVM directly in living tissues, and helps illuminate dynamic host:parasite interactions in vivo that cannot be captured by conventional, static approaches. Integrating intravital microscopy with other novel approaches, such as spatial transcriptomics analysis, will help reveal how tissue heterogeneity and gene expression impacts cellular behaviors to describe mechanisms in place to ensure parasite survival.…”

“…While viable L. major parasites can be seen released by apoptotic neutrophils in the vicinity of surrounding macrophages ( 17 ), direct cell-to-cell transmission was also captured by IVM imaging. Using the non-healing L. major Ryan strain, infected neutrophils can transmit infection to dermal tissue resident macrophages via efferocytosis, or phagocytosis of apoptotic cells, in the first 24 hours of infection ( 104 ). In these studies, dermal tissue resident macrophages were initially infected after sand fly transmitted infection, while infection transitioned to myeloid cells including inflammatory monocytes and monocyte-derived dendritic cells by day 12 post-infection.…”

Leishmania donovani metacyclic promastigotes impair phagosome properties in inflammatory monocytes

“…Leishmania metacyclic promastigotes are deposited by infected sand flies into the dermis of mammals where they are internalized by various phagocyte populations (30, 47-49). These include inflammatory monocytes which, despite their potent antimicrobial capacities, are highly permissive to L. major replication (30).…”

The Impact of Neutrophil Recruitment to the Skin on the Pathology Induced by Leishmania Infection

“…Although most of the sand flies transmit a low dose of Leishmania parasites, needle injection of at least 10 5 Leishmania is required to promote rapid parasite-dependent recruitment of neutrophils ( 70 , 74 , 76 ), further revealing the importance of sand fly derived components in the recruitment of these cells. Recently, it has been reported that depending on the distance of parasite deposition to the bite site, the predominant cells to contain parasites can be either neutrophils or dermal macrophages ( 78 ).…”

“…This model was recently validated in vivo and few dermal macrophages were visualized acquiring parasites through phagocytosis of apoptotic and parasitized neutrophils. In the same context, depletion of neutrophils before the infection reduced the number of infected dermal macrophages ( 78 ).…”