We have identified a population of M2-like dermal macrophages that are present under steady state conditions and that are preferentially infected by a strain of Leishmania major isolated from a patient with chronic cutaneous lesions in a mannose receptor dependent fashion to promote non- healing cutaneous disease is conventionally resistant C57Bl/6 mice. The dermal macrophages are not replaced by blood precursors during infection, but are embryonic derived and locally maintained by IL-4 and IL-10 and retain M2 functionality despite the high levels of IFNg produced in the site. The IL-4 dependence of the dermis resident macrophages has prompted us to try to identify the cellular source of this cytokine in the skin. We show that eosinophils are a major source of localized IL-4 production in cutaneous lesions, in the absence of which both the numbers of dermal TRMs and disease progression are attenuated. Two-photon, intravital microscopy revealed a rapid and sustained eosinophilic infiltrate at the site of infection followed by the tight and stable interaction between dermal TRMs and eosinophils. Importantly, IL-4-stimulated dermal TRMs, in concert with IL-10, produced a large amount of CCL24 (eotaxin), which was required to amplify eosinophil influx and their intimate association with dermal TRMs. These observations were extended to an experimental helminth infection model in the peritoneum, using Ascaris suum eggs, in which eosinophil-derived IL-4 was again required to maintain the number of tissue macrophages in the site via a CCL24-mediated amplification loop. CCL24 secretion was confined to resident macrophages in other tissues, suggesting that eosinophil -TRM cooperative interactions can be generalized to different inflammatory settings and tissue pathologies. Leishmania infect phagocytic cells such as macrophages, neutrophils and dendritic cells in susceptible mammalian hosts. The M2-like, embryonic derived, dermis resident macrophage population described above provide a strong and persisting replicative niche for L. major in C57Bl/6 mice. Since tissue-resident macrophages are also specialized to engulf apoptotic cells, we hypothesized that dermal macrophages may become infected via capture of apoptotic neutrophils containing parasites. Our data confirm that sand fly transmission induces a massive neutrophil recruitment, and further show that neutrophils and dermal macrophages are the main cells infected within the first hours and days following transmission by bite. By two photon intravital microscopy, we demonstrate that some neutrophils carry L. major to dermal macrophages. Moreover, when we depleted neutrophils, dermal macrophage infection was decreased. We also observed that after L. major infection, dermal macrophages begin to express neutrophils markers such as myeloperoxidase (MPO). Thus, our data suggest that dermis resident macrophages and neutrophils are the main cells harboring L. major early after sand fly transmission, and that clearance of apoptotic neutrophils contributes to the parasitization of dermal macrophages and to the maintenance of their M2-like phenotype in a strong, pro-inflammatory environment. In contrast to cutaneous strains of Leishmania, strains such as L. infantum disseminate to and establish infection in the deep viscera. To better understand how L. infantum establishes infection in the liver, we have studied liver resident macrophages, Kupffer cells (KCs), and monocytes and monocyte derived cells by flow cytometry and by intravital imaging. By chlodronate depletion of KCs, we demonstrate that KCs are essential for the initial growth and establishment of infection. We show that the infection induced KCs to proliferate via a mechanism that is dependent on both apoptotic cells and IL-4/13, and that the proliferation is required for granuloma formation and maturation. We also demonstrate by intravital imaging that eosinophils accumulate around liver granulomas, and contribute to IL-4/13 production to induce KCs proliferation during infection. The findings elucidate the dual role of KCs in allowing parasite growth in the acute phase of the infection, but contributing to the effector mechanisms and killing of Leishmania during the chronic phase.
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