Cytokines and splenic remodelling during Leishmania donovani infection

An integrated analysis of the structural changes and gene expression of spleen in human visceral leishmaniasis with and without HIV coinfection

The spleen plays a pivotal role in the pathogenesis of visceral leishmaniasis. In severe forms of the disease, the spleen undergoes changes that can compromise its function in surveilling blood-circulating pathogens. In this study, we present an integrated analysis of the structural and gene expression alterations in the spleens of three patients with relapsing visceral leishmaniasis, two of whom were coinfected with HIV. Our findings reveal that the IL6 signaling pathway plays a significant role in the disorganization of the white pulp, while BCL10 and ICOSLG are associated with spleen organization. Patients coinfected with HIV and visceral leishmaniasis exhibited lower splenic CD4+ cell density and reduced expression of genes such as IL15. These effects may contribute to a compromised immune response against L. infantum in coinfected individuals, further impacting the structural organization of the spleen.

Transcriptional signatures in human macrophage-like cells infected by Leishmania infantum, Leishmania major and Leishmania tropica

BACKGROUND

In the Mediterranean basin, three Leishmania species have been identified: L. infantum, L. major and L. tropica, causing zoonotic visceral leishmaniasis (VL), zoonotic cutaneous leishmaniasis (CL) and anthroponotic CL, respectively. Despite animal models and genomic/transcriptomic studies provided important insights, the pathogenic determinants modulating the development of VL and CL are still poorly understood. This work aimed to identify host transcriptional signatures shared by cells infected with L. infantum, L. major, and L. tropica, as well as specific transcriptional signatures elicited by parasites causing VL (i.e., L. infantum) and parasites involved in CL (i.e., L. major, L. tropica).

METHODOLOGY/PRINCIPAL FINDINGS

U937 cells differentiated into macrophage-like cells were infected with L. infantum, L. major and L. tropica for 24h and 48h, and total RNA was extracted. RNA sequencing, performed on an Illumina NovaSeq 6000 platform, was used to evaluate the transcriptional signatures of infected cells with respect to non-infected cells at both time points. The EdgeR package was used to identify differentially expressed genes (fold change > 2 and FDR-adjusted p-values < 0.05). Then, functional enrichment analysis was employed to identify the enriched ontology terms in which these genes are involved. At 24h post-infection, a common signature of 463 dysregulated genes shared among all infection conditions was recognized, while at 48h post-infection the common signature was reduced to 120 genes. Aside from a common transcriptional response, we evidenced different upregulated functional pathways characterizing L. infantum-infected cells, such as VEGFA-VEGFR2 and NFE2L2-related pathways, indicating vascular remodeling and reduction of oxidative stress as potentially important factors for visceralization.

CONCLUSIONS

The identification of pathways elicited by parasites causing VL or CL could lead to new therapeutic strategies for leishmaniasis, combining the canonical anti-leishmania compounds with host-directed therapy.

Splenic stromal niches in homeostasis and immunity

The spleen is a gatekeeper of systemic immunity where immune responses against blood-borne pathogens are initiated and sustained. Non-haematopoietic stromal cells construct microanatomical niches in the spleen that make diverse contributions to physiological spleen functions and regulate the homeostasis of immune cells. Additional signals from spleen autonomic nerves also modify immune responses. Recent insight into the diversity of the splenic fibroblastic stromal cells has revised our understanding of how these cells help to orchestrate splenic responses to infection and contribute to immune responses. In this Review, we examine our current understanding of how stromal niches and neuroimmune circuits direct the immunological functions of the spleen, with a focus on T cell immunity.

Leishmania infection-derived extracellular vesicles drive transcription of genes involved in M2 polarization

Although it is known that the composition of extracellular vesicles (EVs) is determined by the characteristics of the cell and its environment, the effects of intracellular infection on EV composition and functions are not well understood. We had previously shown that cultured macrophages infected with Leishmania parasites release EVs (LiEVs) containing parasite-derived molecules. In this study we show that LdVash, a molecule previously identified in LiEVs from L. donovani infected RAW264.7 macrophages, is widely distributed in the liver of L. donovani infected mice. This result shows for the first time that parasite molecules are released in EVs and distributed in infected tissues where they can be endocytosed by cells in the liver, including macrophages that significantly increase numbers as the infection progresses. To evaluate the potential impact of LiEVs on macrophage functions, we show that primary peritoneal exudate macrophages (PECs) express transcripts of signature molecules of M2 macrophages such as arginase 1, IL-10, and IL-4R when incubated with LiEVs. In comparative studies that illustrate how intracellular pathogens control the composition and functions of EVs released from macrophages, we show that EVs from RAW264.7 macrophages infected with Salmonella Typhimurium activate PECs to express transcripts of signature molecules of M1 macrophages such as iNOS, TNF alpha, and IFN-gamma and not M2 signature molecules. Finally, in contrast to the polarized responses observed in in vitro studies of macrophages, both M1 and M2 signature molecules are detected in L. donovani infected livers, although they exhibit differences in their spatial distribution in infected tissues. In conclusion, EVs produced by macrophages during Leishmania infection lead to the gene expression consistent with M2 polarization. In contrast, the EVs produced during S. Typhimurium infection stimulated the transcription of genes associated with M1 polarization.

Regulation of immune cell metabolism in health and disease: Special focus on T and B cell subsets

Metabolism is a dynamic process and keeps changing from time to time according to the demand of a particular cell to meet its bio-energetic requirement. Different immune cells rely on distinct metabolic programs which allow the cell to balance its requirements for energy, molecular biosynthesis, and effector activity. In the aspect of infection and cancer immunology, effector T and B cells get exhausted and help tumor cells to evade immunosurveillance. On the other hand, T cells become hyperresponsive in the scenario of autoimmune diseases. In this article, we have explored the uniqueness and distinct metabolic features of key CD4⁺ T and B helper cell subsets, CD4⁺ T, B regulatory cell subsets and CD8⁺ T cells regarding health and disease. Th1 cells rely on glycolysis and glutaminolysis; inhibition of these metabolic pathways promotes Th1 cells in Treg population. However, Th2 cells are also dependent on glycolysis but an abundance of lactate within TME shifts their metabolic dependency to fatty acid metabolism. Th17 cells depend on HIF-1α mediated glycolysis, ablation of HIF-1α reduces Th17 cells but enhance Treg population. In contrast to effector T cells which are largely dependent on glycolysis for their differentiation and function, Treg cells mainly rely on FAO for their function. Therefore, it is of utmost importance to understand the metabolic fates of immune cells and how it facilitates their differentiation and function for different disease models. Targeting metabolic pathways to restore the functionality of immune cells in diseased conditions can lead to potent therapeutic measures.

Toll-Like Receptor- and Protein Kinase R-Induced Type I Interferon Sustains Infection of Leishmania donovani in Macrophages

Leishmania donovani is a protozoan parasite that causes visceral leishmaniasis, provoking liver and spleen tissue destruction that is lethal unless treated. The parasite replicates in macrophages and modulates host microbicidal responses. We have previously reported that neutrophil elastase (NE) is required to sustain L. donovani intracellular growth in macrophages through the induction of interferon beta (IFN-β). Here, we show that the gene expression of IFN-β by infected macrophages was reduced by half when TLR4 was blocked by pre-treatment with neutralizing antibodies or in macrophages from tlr2^-/- mice, while the levels in macrophages from myd88^-/- mice were comparable to those from wild-type C57BL/6 mice. The neutralization of TLR4 in tlr2^-/- macrophages completely abolished induction of IFN-β gene expression upon parasite infection, indicating an additive role for both TLRs. Induction of type I interferon (IFN-I), OASL2, SOD1, and IL10 gene expression by L. donovani was completely abolished in macrophages from NE knock-out mice (ela2^-/-) or from protein kinase R (PKR) knock-out mice (pkr^-/-), and in C57BL/6 macrophages infected with transgenic L. donovani expressing the inhibitor of serine peptidase 2 (ISP2). Parasite intracellular growth was impaired in pkr^-/- macrophages but was fully restored by the addition of exogenous IFN-β, and parasite burdens were reduced in the spleen of pkr^-/- mice at 7 days, as compared to the 129Sv/Ev background mice. Furthermore, parasites were unable to grow in macrophages lacking TLR3, which correlated with lack of IFN-I gene expression. Thus, L. donovani engages innate responses in infected macrophages via TLR2, TLR4, and TLR3, via downstream PKR, to induce the expression of pro-survival genes in the host cell, and guarantee parasite intracellular development.

Interferon-γ-Producing CD4+ T Cells Drive Monocyte Activation in the Bone Marrow During Experimental Leishmania donovani Infection

Ly6C^hi inflammatory monocytes develop in the bone marrow and migrate to the site of infection during inflammation. Upon recruitment, Ly6C^hi monocytes can differentiate into dendritic cells or macrophages. According to the tissue environment they can also acquire different functions. Several studies have described pre-activation of Ly6C^hi monocytes in the bone marrow during parasitic infection, but whether this process occurs during experimental visceral leishmaniasis and, if so, the mechanisms contributing to their activation are yet to be established. In wild type C57BL/6 (B6) mice infected with Leishmania donovani, the number of bone marrow Ly6C^hi monocytes increased over time. Ly6C^hi monocytes displayed a highly activated phenotype from 28 days to 5 months post infection (p.i), with >90% expressing MHCII and >20% expressing iNOS. In comparison, in B6.Rag2^-/- mice <10% of bone marrow monocytes were MHCII⁺ at day 28 p.i., an activation deficiency that was reversed by adoptive transfer of CD4⁺ T cells. Depletion of CD4⁺ T cells in B6 mice and the use of mixed bone marrow chimeras further indicated that monocyte activation was driven by IFNγ produced by CD4⁺ T cells. In B6.Il10^-/- mice, L. donovani infection induced a faster but transient activation of bone marrow monocytes, which correlated with the magnitude of CD4⁺ T cell production of IFNγ and resolution of the infection. Under all of the above conditions, monocyte activation was associated with greater control of parasite load in the bone marrow. Through reinfection studies in B6.Il10^-/- mice and drug (AmBisome^®) treatment of B6 mice, we also show the dependence of monocyte activation on parasite load. In summary, these data demonstrate that during L. donovani infection, Ly6C^hi monocytes are primed in the bone marrow in a process driven by CD4⁺ T cells and whereby IFNγ promotes and IL-10 limits monocyte activation and that the presence of parasites/parasite antigen plays a crucial role in maintaining bone marrow monocyte activation.

Effects of Visceralising Leishmania on the Spleen, Liver, and Bone Marrow: A Pathophysiological Perspective

The leishmaniases constitute a group of parasitic diseases caused by species of the protozoan genus Leishmania. In humans it can present different clinical manifestations and are usually classified as cutaneous, mucocutaneous, and visceral (VL). Although the full range of parasite—host interactions remains unclear, recent advances are improving our comprehension of VL pathophysiology. In this review we explore the differences in VL immunobiology between the liver and the spleen, leading to contrasting infection outcomes in the two organs, specifically clearance of the parasite in the liver and failure of the spleen to contain the infection. Based on parasite biology and the mammalian immune response, we describe how hypoxia-inducible factor 1 (HIF1) and the PI3K/Akt pathway function as major determinants of the observed immune failure. We also summarize existing knowledge on pancytopenia in VL, as a direct effect of the parasite on bone marrow health and regenerative capacity. Finally, we speculate on the possible effect that manipulation by the parasite of the PI3K/Akt/HIF1 axis may have on the myelodysplastic (MDS) features observed in VL.

Dissecting pathways to thrombocytopenia in a mouse model of visceral leishmaniasis

Visceral leishmaniasis is an important yet neglected parasitic disease caused by infection with Leishmania donovani or L infantum. Disease manifestations include fever, weight loss, hepatosplenomegaly, immune dysregulation, and extensive hematological complications. Thrombocytopenia is a dominant hematological feature seen in both humans and experimental models, but the mechanisms behind this infection-driven thrombocytopenia remain poorly understood. Using a murine model of experimental visceral leishmaniasis (EVL), we demonstrated a progressive decrease in platelets from day 14 after infection, culminating in severe thrombocytopenia by day 28. Plasma thrombopoietin (TPO) levels were reduced in infected mice, at least in part because of the alterations in the liver microenvironment associated with granulomatous inflammation. Bone marrow (BM) megakaryocyte cytoplasmic maturation was significantly reduced. In addition to a production deficit, we identified significant increases in platelet clearance. L donovani-infected splenectomized mice were protected from thrombocytopenia compared with sham operated infected mice and had a greater response to exogenous TPO. Furthermore, infection led to higher levels of platelet opsonization and desialylation, both associated with platelet clearance in spleen and liver, respectively. Critically, these changes could be reversed rapidly by drug treatment to reduce parasite load or by administration of TPO agonists. In summary, our findings demonstrate that the mechanisms underpinning thrombocytopenia in EVL are multifactorial and reversible, with no obvious residual damage to the BM microenvironment.