Fabian de Labastida Rivera

Immune-mediated mechanisms of parasite tissue sequestration during experimental cerebral malaria

Cerebral malaria is a severe complication of malaria. Sequestration of parasitized RBCs in brain microvasculature is associated with disease pathogenesis, but our understanding of this process is incomplete. In this study, we examined parasite tissue sequestration in an experimental model of cerebral malaria (ECM). We show that a rapid increase in parasite biomass is strongly associated with the induction of ECM, mediated by IFN-γ and lymphotoxin α, whereas TNF and IL-10 limit this process. Crucially, we discovered that host CD4+ and CD8+ T cells promote parasite accumulation in vital organs, including the brain. Modulation of CD4+ T cell responses by helminth coinfection amplified CD4+ T cell-mediated parasite sequestration, whereas vaccination could generate CD4+ T cells that reduced parasite biomass and prevented ECM. These findings provide novel insights into immune-mediated mechanisms of ECM pathogenesis and highlight the potential of T cells to both prevent and promote infectious diseases.

Type I interferons suppress CD4+ T-cell-dependent parasite control during blood-stage Plasmodium infection

During blood‐stage Plasmodium infection, large‐scale invasion of RBCs often occurs before the generation of cellular immune responses. In Plasmodium berghei ANKA (PbA)‐infected C57BL/6 mice, CD4+ T cells controlled parasite numbers poorly, instead providing early help to pathogenic CD8+ T cells. Expression analysis revealed that the transcriptional signature of CD4+ T cells from PbA‐infected mice was dominated by type I IFN (IFN‐I) and IFN‐γ‐signalling pathway‐related genes. A role for IFN‐I during blood‐stage Plasmodium infection had yet to be established. Here, we observed IFN‐α protein production in the spleen of PbA‐infected C57BL/6 mice over the first 2 days of infection. Mice deficient in IFN‐I signalling had reduced parasite burdens, and displayed none of the fatal neurological symptoms associated with PbA infection. IFN‐I substantially inhibited CD4+ T‐bet+ T‐cell‐derived IFN‐γ production, and prevented this emerging Th1 response from controlling parasites. Experiments using BM chimeric mice revealed that IFN‐I signalled predominantly via radio‐sensitive, haematopoietic cells, but did not suppress CD4+ T cells via direct signalling to this cell type. Finally, we found that IFN‐I suppressed IFN‐γ production, and hampered efficient control of parasitaemia in mice infected with non‐lethal Plasmodium chabaudi. Thus, we have elucidated a novel regulatory pathway in primary blood‐stage Plasmodium infection that suppresses CD4+ T‐cell‐mediated parasite control.

Type I IFN signaling in CD8– DCs impairs Th1-dependent malaria immunity

Many pathogens, including viruses, bacteria, and protozoan parasites, suppress cellular immune responses through activation of type I IFN signaling. Recent evidence suggests that immune suppression and susceptibility to the malaria parasite, Plasmodium, is mediated by type I IFN; however, it is unclear how type I IFN suppresses immunity to blood-stage Plasmodium parasites. During experimental severe malaria, CD4+ Th cell responses are suppressed, and conventional DC (cDC) function is curtailed through unknown mechanisms. Here, we tested the hypothesis that type I IFN signaling directly impairs cDC function during Plasmodium infection in mice. Using cDC-specific IFNAR1-deficient mice, and mixed BM chimeras, we found that type I IFN signaling directly affects cDC function, limiting the ability of cDCs to prime IFN-γ-producing Th1 cells. Although type I IFN signaling modulated all subsets of splenic cDCs, CD8- cDCs were especially susceptible, exhibiting reduced phagocytic and Th1-promoting properties in response to type I IFNs. Additionally, rapid and systemic IFN-α production in response to Plasmodium infection required type I IFN signaling in cDCs themselves, revealing their contribution to a feed-forward cytokine-signaling loop. Together, these data suggest abrogation of type I IFN signaling in CD8- splenic cDCs as an approach for enhancing Th1 responses against Plasmodium and other type I IFN-inducing pathogens.

Critical roles for LIGHT and its receptors in generating T cell-mediated immunity during Leishmania donovani infection.

LIGHT (TNFSF14) is a member of the TNF superfamily involved in inflammation and defence against infection. LIGHT signals via two cell-bound receptors; herpes virus entry mediator (HVEM) and lymphotoxin-beta receptor (LTβR). We found that LIGHT is critical for control of hepatic parasite growth in mice with visceral leishmaniasis (VL) caused by infection with the protozoan parasite Leishmania donovani. LIGHT-HVEM signalling is essential for early dendritic cell IL-12/IL-23p40 production, and the generation of IFNγ- and TNF-producing T cells that control hepatic infection. However, we also discovered that LIGHT-LTβR interactions suppress anti-parasitic immunity in the liver in the first 7 days of infection by mechanisms that restrict both CD4+ T cell function and TNF-dependent microbicidal mechanisms. Thus, we have identified distinct roles for LIGHT in infection, and show that manipulation of interactions between LIGHT and its receptors may be used for therapeutic advantage.

Bone marrow-derived and resident liver macrophages display unique transcriptomic signatures but similar biological functions

Graphical abstract

Blimp-1-Dependent IL-10 Production by Tr1 Cells Regulates TNF-Mediated Tissue Pathology

Tumor necrosis factor (TNF) is critical for controlling many intracellular infections, but can also contribute to inflammation. It can promote the destruction of important cell populations and trigger dramatic tissue remodeling following establishment of chronic disease. Therefore, a better understanding of TNF regulation is needed to allow pathogen control without causing or exacerbating disease. IL-10 is an important regulatory cytokine with broad activities, including the suppression of inflammation. IL-10 is produced by different immune cells; however, its regulation and function appears to be cell-specific and context-dependent. Recently, IL-10 produced by Th1 (Tr1) cells was shown to protect host tissues from inflammation induced following infection. Here, we identify a novel pathway of TNF regulation by IL-10 from Tr1 cells during parasitic infection. We report elevated Blimp-1 mRNA levels in CD4+ T cells from visceral leishmaniasis (VL) patients, and demonstrate IL-12 was essential for Blimp-1 expression and Tr1 cell development in experimental VL. Critically, we show Blimp-1-dependent IL-10 production by Tr1 cells prevents tissue damage caused by IFNγ-dependent TNF production. Therefore, we identify Blimp-1-dependent IL-10 produced by Tr1 cells as a key regulator of TNF-mediated pathology and identify Tr1 cells as potential therapeutic tools to control inflammation.

VCAM-1 and VLA-4 modulate dendritic cell IL-12p40 production in experimental visceral leishmaniasis.

Vascular cell adhesion molecule-1 (VCAM-1) interacts with its major ligand very late antigen-4 (VLA-4) to mediate cell adhesion and transendothelial migration of leukocytes. We report an important role for VCAM-1/VLA-4 interactions in the generation of immune responses during experimental visceral leishmaniasis caused by Leishmania donovani. Our studies demonstrate that these molecules play no direct role in the recruitment of leukocytes to the infected liver, but instead contribute to IL-12p40-production by splenic CD8+ dendritic cells (DC). Blockade of VCAM-1/VLA-4 interactions using whole antibody or anti-VCAM-1 Fab′ fragments reduced IL-12p40 mRNA accumulation by splenic DC 5 hours after L. donovani infection. This was associated with reduced anti-parasitic CD4+ T cell activation in the spleen and lowered hepatic IFNγ, TNF and nitric oxide production by 14 days post infection. Importantly, these effects were associated with enhanced parasite growth in the liver in studies with either anti-VCAM-1 or anti-VLA-4 antibodies. These data indicate a role for VCAM-1 and VLA-4 in DC activation during infectious disease.

Type I Interferons Regulate Immune Responses in Humans with Blood-Stage Plasmodium falciparum Infection

Summary The development of immunoregulatory networks is important to prevent disease. However, these same networks allow pathogens to persist and reduce vaccine efficacy. Here, we identify type I interferons (IFNs) as important regulators in developing anti-parasitic immunity in healthy volunteers infected for the first time with Plasmodium falciparum. Type I IFNs suppressed innate immune cell function and parasitic-specific CD4+ T cell IFNγ production, and they promoted the development of parasitic-specific IL-10-producing Th1 (Tr1) cells. Type I IFN-dependent, parasite-specific IL-10 production was also observed in P. falciparum malaria patients in the field following chemoprophylaxis. Parasite-induced IL-10 suppressed inflammatory cytokine production, and IL-10 levels after drug treatment were positively associated with parasite burdens before anti-parasitic drug administration. These findings have important implications for understanding the development of host immune responses following blood-stage P. falciparum infection, and they identify type I IFNs and related signaling pathways as potential targets for therapies or vaccine efficacy improvement.

Cutting Edge: Selective Blockade of LIGHT-Lymphotoxin β Receptor Signaling Protects Mice from Experimental Cerebral Malaria Caused by Plasmodium berghei ANKA

Studies in experimental cerebral malaria (ECM) in mice have identified T cells and TNF family members as critical mediators of pathology. In this study we report a role for LIGHT-lymphotoxin β Receptor (LTβR) signaling in the development of ECM and control of parasite growth. Specific blockade of LIGHT-LTβR, but not LIGHT-herpesvirus entry mediator interactions, abrogated the accumulation of parasites and the recruitment of pathogenic CD8+ T cells and monocytes to the brain during infection without affecting early activation of CD4+ T cells, CD8+ T cells, or NK cells. Importantly, blockade of LIGHT-LTβR signaling caused the expansion of splenic monocytes and an overall enhanced capacity to remove and process Ag during infection, as well as reduced systemic cytokine levels when control mice displayed severe ECM symptoms. In summary, we have discovered a novel pathogenic role for LIGHT and LTβR in ECM, identifying this TNF family receptor-ligand interaction as an important immune regulator during experimental malaria.

Therapeutic Glucocorticoid-Induced TNF Receptor-Mediated Amplification of CD4+ T Cell Responses Enhances Antiparasitic Immunity

Chronic infectious diseases and cancers are often associated with suboptimal effector T cell responses. Enhancement of T cell costimulatory signals has been extensively studied for cancer immunotherapy but not so for the treatment of infectious disease. The few previous attempts at this strategy using infection models have lacked cellular specificity, with major immunoregulatory mechanisms or innate immune cells also being targeted. In this study, we examined the potential of promoting T cell responses via the glucocorticoid-induced TNF receptor (GITR) family-related protein in a murine model of visceral leishmaniasis. GITR stimulation during established infection markedly improved antiparasitic immunity. This required CD4+ T cells, TNF, and IFN-γ, but crucially, was independent of regulatory T (Treg) cells. GITR stimulation enhanced CD4+ T cell expansion without modulating Treg cell function or protecting conventional CD4+ T cells from Treg cell suppression. GITR stimulation substantially improved the efficacy of a first-line visceral leishmaniasis drug against both acute hepatic infection and chronic infection in the spleen, demonstrating its potential to improve clinical outcomes. This study identifies a novel strategy to therapeutically enhance CD4+ T cell-mediated antiparasitic immunity and, importantly, achieves this goal without impairment of Treg cell function.