IMMUNE REGULATION AND VACCINE DEVELOPMENT IN LEISHMANIASIS

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

Tissue-resident macrophages (TRMs) are critical for tissue homeostasis/repair, and their development is governed by tissue niche-specific signals. Although the importance of these nurturing signals for TRMs to perform tissue-specific functions has been extensively studied, it remain poorly addressed how TRMs maintain their homeostatic properties during inflammation, in particular infection-driven inflammation in the skin. We previously showed the importance of eosinophil-TRM cooperative interactions for the maintenance of the M2-like properties of dermal TRMs within the strong pro-inflammatory environment of the L. major infected dermis; eosinophils provide IL-4 to dermal TRMs, and IL-4-stimulated dermal TRMs produce CCL24 (eotaxin) which in turn functions to recruit more eosinophils. In our current studies, we have identified two independent subsets of dermal TRMs, MHCII+ mannose receptor (MR) low and MHCII- MR hi populations. By single cell RNA seq, the MHCII- MR hi subset of dermal TRMs was shown to be the sole producer of CCL24 in the infected skin, and was also found to be the sole source of the alarmin thymic stromal lymphopoietin (TSLP) required to activate innate lymphoid cells 2 (ILC2), which produce IL-5 to amplify eosinophil-TRM interactions. Both selective depletion of IL-5+ ILC2 or genetic ablation of TSLP impaired the localized TH2 circuitries required to maintain the numbers and functionality of M2-like dermal TRMs, and disease progression was ameliorated. Thus, in the setting of the strong pro-inflammatory environment of the L. major infected dermis, dermal TRMs actively maintain themselves by producing CCL24 and TSLP, and any disruption of the localized TH2 circuitries will alter their number and activation states, and promote stronger resistance to infection. Visceral leishmaniasis (VL) is a tropical disease caused by Leishmania species within the L. donovani complex (L. donovani/L. infantum). Macrophages in the liver, spleen, and bone-marrow are the principal target cells for infection. Kupffer cells (KCs) are the embryonic-derived, liver resident macrophages, and are characterized by Clec4f and Tim4 expression. In homeostasis, KCs maintain their numbers via self-proliferation, but in some inflammatory settings they can die and be replaced by monocyte-derived cells (moKCs). In the murine VL model, KCs are important for both the initial growth of the parasite in the liver and for granuloma formation, which is associated with the eventual protective response in this tissue. KC death, their replacement by moKCs, and the functionality of KCs vs moKCs has never been investigated in VL. In C57BL/6 mice infected with L. infantum, at 42 days post-infection we found evidence of KC apoptosis, measured by cleaved caspase 3 staining by confocal microscopy, and ferroptosis, by lipid peroxidation detection by flow cytometry. Consistently, Clec4f and Tim4 expression in KCs was reduced, while monocyte markers such as Ly6C and CD11c were enhanced. As further evidence of their monocytic origin during chronic infection, KC numbers were lower in CCR2-/- mice when compared to WT mice, and in experiments involving congenic parabiotic mice, the KCs present within the granulomas expressed both congenic markers. Selective KC depletion and replacement by moKCs prior to infection using Clec4f-Cre-DTR mice, demonstrated that the predominance of moKCs resulted in lower parasite loads, indicating that moKCs are more effective in controlling infection. Thus, KCs death and their replacement by monocyte-derived cells appears to be central to the protective response in this organ.