γδ T cells control murine skin inflammation and subcutaneous adipose wasting during chronic Trypanosoma brucei infection

From fat storage to immune hubs: the emerging role of adipocytes in coordinating the immune response to infection

Adipose tissue is a rich source of diverse cell populations, including immune cells, adipocytes and stromal cells. Interactions between these different cell types are now appreciated to be critical for maintaining tissue structure and function, by governing processes such as adipogenesis, lipolysis and differentiation of white to beige adipocytes. Interactions between these cells also drive inflammation in obesity, leading to an expansion of adipose tissue immune cells, and the secretion of proinflammatory cytokines from immune cells and from adipocytes themselves. However, in evolutionary terms, obesity is a recent phenomenon, raising the question of why adipocytes evolved to express factors that influence the immune response. Studies of various pathogens indicate that adipocytes are highly responsive to infection, altering their metabolic profiles in a way that can be used to release nutrients and fuel the immune response. In the case of infection with the extracellular parasite Trypanosoma brucei, attenuating the ability of adipocytes to sense the cytokine IL-17 results in a loss of control of the local immune response and an increased pathogen load. Intriguingly, comparisons of the adipocyte response to infection suggest that the immune responses of these cells occur in a pathogen-dependent manner, further confirming their complexity. Here, with a focus on murine adipose tissue, we discuss the emerging concept that, in addition to their canonical function, adipocytes are immune signalling hubs that integrate and disseminate signals from the immune system to generate a local environment conducive to pathogen clearance.

IL-17A/IFN-γ producing γδ T cell functional dichotomy impacts cutaneous leishmaniasis in mice

γδ T cells play diverse roles in immune responses, producing either interleukin (IL)-17A or interferon γ (IFN-γ). Here, we investigated the impact of this functional dichotomy on cutaneous leishmaniasis. We demonstrate that in Sv129 mice susceptible to Leishmania amazonensis, Vγ4+ γδ T cells are the main source of IL-17A. In type 1 IFN receptor-deficient (A129) mice with heightened susceptibility, there is an increased frequency of IL-17A-producing γδ T cells. L. amazonensis' lipophosphoglycan induces these IL-17A-producing γδ T cells. Notably, C57BL/6 mice deficient in γδ T cells or IL-17 receptor exhibit smaller lesions, indicating a pathogenic role of IL-17A-producing γδ T cells in cutaneous leishmaniasis. Conversely, adoptive transfer of fluorescence-activated cell sorting (FACS)-sorted γδ T cells lead to an accumulation of IFN-γ-producing γδ T cells, associated with control of lesion development. On the other hand, adoptive transfer of FACS-sorted IFN-γ-deficient γδ T cells abolished the control of lesion development. These data demonstrate a pathophysiological dichotomy in which IL-17A-producing γδ T cells promote pathogenesis, while IFN-γ-producing γδ T cells offer therapeutic potential in cutaneous leishmaniasis.

Myeloid-derived IL-33 drives γδ T cell-dependent resistance against cutaneous infection by Strongyloides ratti

Interleukin 33 (IL-33) is a pleiotropic cytokine released from diverse cell types that regulate both pro- and anti-inflammatory responses during pathogen infection. However, it remains unclear whether IL-33 controls key aspects of cutaneous immunity against skin-penetrating parasites. In this study, mice percutaneously infected with the parasitic helminth Strongyloides ratti were investigated to understand mechanisms of anamnestic immunity at the skin barrier. Surprisingly, mice lacking the Type 2 transcription factor STAT6 (signal transducer and activator of transcription 6) had no defects in secondary resistance to infection, whereas IL-33 gene deficiency or local blockade of IL-33 receptor (ST2) signaling abrogated host resistance. Depletion of CD4+ T cells or type 2 innate lymphoid cells had only a moderate impact on protection, but the loss of γδ T cells completely ablated cutaneous immunity against rechallenge. We identified a CD62Lhi IL-33 receptor (ST2)-expressing γδ T cell population that accumulated in the skin of protected mice that was dependent upon IL-33 expression in myeloid lineage antigen-presenting cells. This work suggests a previously unrecognized mechanism wherein noncanonical type 2 immunity operates through myeloid antigen-presenting cells and skin γδ T cells to adaptively repel skin-penetrating helminth larvae.

Key actors in neuropathophysiology: The role of γδ T cells

The neuroimmune axis has been the focus of many studies, with special emphasis on the interactions between the central nervous system and the different immune cell subsets. T cells are namely recognized to play a critical role due to their interaction with nerves, by secreting cytokines and neurotrophins, which regulate the development, function, and survival of neurons. In this context, γδ T cells are particularly relevant, as they colonize specific tissues, namely the meninges, and have a wide variety of complex functions that balance physiological systems. Notably, γδ T cells are not only key components for maintaining brain homeostasis but are also responsible for triggering or preventing inflammatory responses in various pathologies, including neurodegenerative diseases as well as neuropsychiatric and developmental disorders. Here, we provide an overview of the current state of the art on the contribution of γδ T cells in neuropathophysiology and delve into the molecular mechanisms behind it. We aim to shed light on γδ T cell functions in the central nervous system while highlighting upcoming challenges in the field and providing new clues for potential therapeutic strategies.

Cholesterol Efflux Decreases TLR4-Target Gene Expression in Cultured Macrophages Exposed to T. brucei Ghosts

Trypanosoma brucei causes African trypanosomiasis in humans. Infection with T. brucei elicits a potent pro-inflammatory immune response within infected human hosts, and this response is thought to at least be partially due to Toll-like receptor (TLR) activation. In response to stimulation by lipopolysaccharide and other pathogen antigens, TLR4 translocates to lipid rafts, which induces the expression of pro-inflammatory genes. However, cholesterol efflux is acknowledged as anti-inflammatory due to promoting lipid raft disruption. In this study, we wanted to assess the impact of T. brucei "ghosts", which are non-viable T. brucei essentially devoid of intracellular contents, in stimulating macrophage TLR4 translocation to lipid rafts, and whether promoting cholesterol efflux in macrophages incubated with T. brucei ghosts attenuates TLR4-target gene expression. When cultured macrophages were exposed to T. brucei ghosts, we observed an increase in lipid raft TLR4 protein content, which suggests certain surface molecules of T. brucei serve as ligands for TLR4. However, pretreating macrophages with cholesterol acceptors before T. brucei ghost exposure decreased lipid raft TLR4 protein content and the expression of pro-inflammatory TLR4-target genes. Taken together, these results imply that macrophage cholesterol efflux weakens pro-inflammatory responses which occur from T. brucei infection via increasing macrophage lipid raft disruption.

A breath of fresh air: impact of insect-borne protozoan parasites on the respiratory system

The protozoan parasites Plasmodium, Leishmania, and Trypanosoma are transmitted by hematophagous insects and cause severe diseases in humans. These infections pose a global threat, particularly in low-resource settings, and are increasingly extending beyond the current endemic regions. Tropism of parasites is crucial for their development, and recent studies have revealed colonization of noncanonical tissues, aiding their survival and immune evasion. Despite receiving limited attention, cumulative evidence discloses the respiratory system as a significant interface for host-pathogen interactions, influencing the course of (co)infection and disease onset. Due to its pathophysiological and clinical implications, we emphasize that further research is needed to better understand the involvement of the respiratory system and its potential to improve prevention, diagnosis, treatment, and interruption of the chain of transmission.

Advances in the Immunology of the Host-Parasite Interactions in African Trypanosomosis, including Single-Cell Transcriptomics

Trypanosomes are single-celled extracellular parasites that infect mammals, including humans and livestock, causing global public health concerns and economic losses. These parasites cycle between insect vectors, such as tsetse flies and vertebrate hosts, undergoing morphological, cellular, and biochemical changes. They have remarkable immune evasion mechanisms to escape the host's innate and adaptive immune responses, such as surface coat antigenic variation and the induction of the loss of specificity and memory of antibody responses, enabling the prolongation of infection. Since trypanosomes circulate through the host body in blood and lymph fluid and invade various organs, understanding the interaction between trypanosomes and tissue niches is essential. Here, we present an up-to-date overview of host-parasite interactions and survival strategies for trypanosomes by introducing and discussing the latest studies investigating the transcriptomics of parasites according to life cycle stages, as well as host cells in various tissues and organs, using single-cell and spatial sequencing applications. In recent years, this information has improved our understanding of trypanosomosis by deciphering the diverse populations of parasites in the developmental process, as well as the highly heterogeneous immune and tissue-resident cells involved in anti-trypanosome responses. Ultimately, the goal of these approaches is to gain an in-depth understanding of parasite biology and host immunity, potentially leading to new vaccination and therapeutic strategies against trypanosomosis.

IL-17 signalling is critical for controlling subcutaneous adipose tissue dynamics and parasite burden during chronic murine Trypanosoma brucei infection

In the skin, Trypanosoma brucei colonises the subcutaneous white adipose tissue, and is proposed to be competent for forward transmission. The interaction between parasites, adipose tissue, and the local immune system is likely to drive the adipose tissue wasting and weight loss observed in cattle and humans infected with T. brucei. However, mechanistically, events leading to subcutaneous white adipose tissue wasting are not fully understood. Here, using several complementary approaches, including mass cytometry by time of flight, bulk and single cell transcriptomics, and in vivo genetic models, we show that T. brucei infection drives local expansion of several IL-17A-producing cells in the murine WAT, including TH17 and Vγ6+ cells. We also show that global IL-17 deficiency, or deletion of the adipocyte IL-17 receptor protect from infection-induced WAT wasting and weight loss. Unexpectedly, we find that abrogation of adipocyte IL-17 signalling results in a significant accumulation of Dpp4+ Pi16+ interstitial preadipocytes and increased extravascular parasites in the WAT, highlighting a critical role for IL-17 signalling in controlling preadipocyte fate, subcutaneous WAT dynamics, and local parasite burden. Taken together, our study highlights the central role of adipocyte IL-17 signalling in controlling WAT responses to infection, suggesting that adipocytes are critical coordinators of tissue dynamics and immune responses to T. brucei infection.