In the presence of Trypanosoma cruzi antigens, activated peripheral T lymphocytes retained in the liver induce a proinflammatory phenotypic and functional shift in intrahepatic T lymphocyte

The liver's dilemma: sensing real danger in a sea of PAMPs: the (arterial) sinusoidal segment theory

The liver is susceptible to viruses and bacterial infections, tumors, and sterile tissue damage, but immunological danger recognition in the liver is highly unconventional. When analyzing innate and adaptive immunity in the organ, the valid concepts that guide danger recognition and immune response in the periphery should be put aside. In the liver, the vascular anatomy is a game changer, as about 80% of the blood that percolates the organ arrives from the hepatic portal vein, draining blood rich in molecules from the intestinal flora. This 24/7 exposure to high amounts of pathogen-associated molecular pattern (PAMPs) molecules results in hepatic immunological tolerance. In the liver, dendritic, Kupffer (KC), liver sinusoidal endothelial cells (LSECs), and even hepatocytes express PD-L1, a T lymphocyte downregulatory molecule. Most cells express Fas-L, IL-10, TGF-β, low levels of co-stimulatory molecules, lack of or have low levels of MHC-I and/or MHC-II expression. Moreover, other negative regulators such as CTLA-4, IDO-1, and prostaglandin E2 (PGE2) are regularly expressed. Then, how can real danger be discerned and recognized in this sea of PAMPs? This is an open question. Here, we hypothesize that conventional immunological danger recognition can occur in the liver but in specific and minor arterial sinusoidal segments,. Then, in the portal triad, where the hepatic artery ramificates into the stroma and carries arterial blood with no gut-derived PAMPs, there is no evolutive or environmental pressure for immunosuppressive pathways, and conventional immunological danger recognition could occur. Therefore, in arterial sinusoidal segments with no sea of PAMPs, the liver could recognize real danger and support innate and adaptive immunity.

Trypanosoma cruzi infection increases atherosclerotic lesion in ApoE-deficient mice

Apolipoprotein E (ApoE) is the major ligand for the transporting and removal of chylomicrons and lipoproteins by the liver. Since the creation of the ApoE-knockout mice, it is well established that ApoE deficiency results in spontaneous atherosclerosis in aged animals. Atherosclerosis is also observed in animals infected with Trypanosoma cruzi, a protozoan that elicits a systemic inflammatory response in mammalian hosts, culminating in damage to cardiac, neuronal, and endothelial cells. Pro-atherogenic effects related to the experimental infection with T. cruzi may be induced by inflammatory components affecting the vascular wall. Herein, we evaluated whether infection with different strains of T. cruzi worsened the atherogenic lesions observed in aged ApoE^-/- mice. After four weeks of infection with Berenice-78 (Be-78) or Colombian (Col) strains of the parasite, mice presented increased CCL2 and CCL5 production and high migration of inflammatory cells to cardiac tissue. Although the infection with either strain did not affect the survival rate, only the infection with Col strain caused abundant parasite growth in blood and heart and increased aortic root lesions in ApoE^-/- mice. Our findings show, for the first time that ApoE exerts a protective anti-atherosclerotic role in the aortic root of mice in the acute phase of experimental infection with the Col strain of T. cruzi. Further studies should target ApoE and nutritional interventions to modulate susceptibility to cardiovascular disabilities after T. cruzi infection, minimizing the risk of death in both experimental animals and humans.

Immunological Tolerance in Liver Transplant Recipients: Putative Involvement of Neuroendocrine-Immune Interactions

The transplantation world changed significantly following the introduction of immunosuppressants, with millions of people saved. Several physicians have noted that liver recipients that do not take their medication for different reasons became tolerant regarding kidney, heart, and lung transplantations at higher frequencies. Most studies have attempted to explain this phenomenon through unique immunological mechanisms and the fact that the hepatic environment is continuously exposed to high levels of pathogen-associated molecular patterns (PAMPs) or non-pathogenic microorganism-associated molecular patterns (MAMPs) from commensal flora. These components are highly inflammatory in the periphery but tolerated in the liver as part of the normal components that arrive via the hepatic portal vein. These immunological mechanisms are discussed herein based on current evidence, although we hypothesize the participation of neuroendocrine-immune pathways, which have played a relevant role in autoimmune diseases. Cells found in the liver present receptors for several cytokines, hormones, peptides, and neurotransmitters that would allow for system crosstalk. Furthermore, the liver is innervated by the autonomic system and may, thus, be influenced by the parasympathetic and sympathetic systems. This review therefore seeks to discuss classical immunological hepatic tolerance mechanisms and hypothesizes the possible participation of the neuroendocrine-immune system based on the current literature.

In the Acute Phase of Trypanosoma cruzi Infection, Liver Lymphoid and Myeloid Cells Display an Ambiguous Phenotype Combining Pro- and Anti-Inflammatory Markers

Multiple cell populations, cellular biochemical pathways, and the autonomic nervous system contribute to maintaining the immunological tolerance in the liver. This tolerance is coherent because the organ is exposed to high levels of bacterial pathogen-associated molecular pattern (PAMP) molecules from the intestinal microbiota, such as lipopolysaccharide endotoxin (LPS). In the case of Trypanosoma cruzi infection, although there is a dramatic acute immune response in the liver, we observed intrahepatic cell populations combining pro- and anti-inflammatory markers. There was loss of fully mature Kupffer cells and an increase in other myeloid cells, which are likely to include monocytes. Among dendritic cells (DCs), the cDC1 population expanded relative to the others, and these cells lost both some macrophage markers (F4/80) and immunosuppressive cytokines (IL-10, TGF-β1). In parallel, a massive T cell response occured with loss of naïve cells and increase in several post-activation subsets. However, these activated T cells expressed both markers programmed cell death protein (PD-1) and cytokines consistent with immunosuppressive function (IL-10, TGF-β1). NK and NK-T cells broadly followed the pattern of T cell activation, while TCR-γδ cells appeared to be bystanders. While no data were obtained concerning IL-2, several cell populations also synthesized IFN-γ and TNF-α, which has been linked to host defense but also to tissue injury. It therefore appears that T. cruzi exerts control over liver immunity, causing T cell activation via cDC1 but subverting multiple populations of T cells into immunosuppressive pathways. In this way, T. cruzi engages a mechanism of hepatic T cell tolerance that is familiar from liver allograft tolerance, in which activation and proliferation are followed by T cell inactivation.

The Liver and the Hepatic Immune Response in Trypanosoma cruzi Infection, a Historical and Updated View

Chagas disease was described more than a century ago and, despite great efforts to understand the underlying mechanisms that lead to cardiac and digestive manifestations in chronic patients, much remains to be clarified. The disease is found beyond Latin America, including Japan, the USA, France, Spain, and Australia, and is caused by the protozoan Trypanosoma cruzi. Dr. Carlos Chagas described Chagas disease in 1909 in Brazil, and hepatomegaly was among the clinical signs observed. Currently, hepatomegaly is cited in most papers published which either study acutely infected patients or experimental models, and we know that the parasite can infect multiple cell types in the liver, especially Kupffer cells and dendritic cells. Moreover, liver damage is more pronounced in cases of oral infection, which is mainly found in the Amazon region. However, the importance of liver involvement, including the hepatic immune response, in disease progression does not receive much attention. In this review, we present the very first paper published approaching the liver's participation in the infection, as well as subsequent papers published in the last century, up to and including our recently published results. We propose that, after infection, activated peripheral T lymphocytes reach the liver and induce a shift to a pro-inflammatory ambient environment. Thus, there is an immunological integration and cooperation between peripheral and hepatic immunity, contributing to disease control.

New Biomarker in Chagas Disease: Extracellular Vesicles Isolated from Peripheral Blood in Chronic Chagas Disease Patients Modulate the Human Immune Response

Chagas disease, a neglected tropical disease (NTD) caused by the flagellated protozoan Trypanosoma cruzi (T. cruzi), is a major public health problem. It was initially restricted to Latin America, but it is now expanding globally. Host and pathogen interactions are crucial in the establishment of disease, and since 1970, it has been known that eukaryotic cells release extracellular vesicles (EVs), which in turn have an important role in intercellular communication in physiological and pathological conditions. Our study proposed to characterize and compare circulating EVs isolated from the plasma of chronic Chagas disease (CCD) patients and controls. For this, peripheral blood was collected from patients and controls, and mononuclear cells (PBMCs) were isolated and stimulated with parasite EVs, showing that patient cells released fewer EVs than control cells. Then, after plasma separation followed by EV total shedding enrichment, the samples were subjected to ultracentrifugation to isolate the circulating EVs, which then had their size and concentration characterized by nanoparticle tracking analysis (NTA). This showed that patients had a lower concentration of circulating EVs while there were no differences in size, corroborating the in vitro data. Additionally, circulating EVs were incubated with THP-1 cells (macrophages) that, after the interaction, had their supernatant analyzed by ELISA for cytokine detection. In relation to their ability to induce cytokine production, the CCD patient EVs were able to induce a differential production of IFN-γ and IL-17 in relation to controls, with differences being more evident in earlier/less severe stages of the disease. In summary, a decreased concentration of circulating EVs associated with differential activation of the immunological system in patients with CCD is related to parasite persistence and the establishment of chronic disease. It is also a potential biomarker for monitoring disease progression.

After Experimental Trypanosoma cruzi Infection, Dying Hepatic CD3+TCRαβ+B220+ T Lymphocytes Are Rescued from Death by Peripheral T Cells and Become Activated

The unusual phenotype of CD3⁺ T lymphocyte expressing B220, a marker originally attributed to B lymphocytes, was first observed in the liver of Fas/Fas-L-deficient mice as a marker of apoptotic T lymphocytes. However, other CD3⁺B220⁺ T lymphocyte populations were later described in the periphery as functional cytotoxic or regulatory cells, for example. Then, in this work, we studied whether hepatic CD3⁺B220⁺ T lymphocytes could play a role in experimental Trypanosoma cruzi infection. In control and infected mice, we observed two subpopulations that could be discerned based on CD117 expression, which were conventional apoptotic CD3⁺B220⁺(CD117^-) and thymus-independent CD3⁺B220⁺CD117⁺ T lymphocytes. Regardless of CD117 expression, most B220⁺ T lymphocytes were 7AAD⁺, confirming this molecule as a marker of dying T cells. However, after infection, we found that around 15% of the CD3⁺B220⁺CD117⁺ hepatic population became B220 and 7AAD negative, turned into CD90.2⁺, and upregulated the expression of CD44, CD49d, and CD11a, a phenotype consistent with activated T lymphocytes. Moreover, we observed that the hepatic CD3⁺B220⁺CD117⁺ population was rescued from death by previously activated peripheral T lymphocytes. Our results extend the comprehension of the hepatic CD3⁺B220⁺ T lymphocyte subpopulations and illustrate the complex interactions that occur in the liver.

In the presence of Trypanosoma cruzi antigens, activated peripheral T lymphocytes retained in the liver induce a proinflammatory phenotypic and functional shift in intrahepatic T lymphocyte

In secondary lymphoid organs, pathogen‐derived and endogenous danger molecules are recognized by pattern recognition receptors, leading to adaptive proinflammatory immune responses. This conceptual rule does not apply directly to the liver, as hepatic immune cells tolerate gut‐derived bacterial molecules from the flora. Therefore, the recognition of danger and proinflammatory stimuli differs between the periphery and the liver. However, the tolerant nature of the liver must be overcome in the case of infections or cancer, for example. The central paradigm is the basis for danger recognition and the balance between inflammation and tolerance in the liver. Here, we observed functional integration, with activated peripheral T lymphocytes playing a role in the induction of a proinflammatory environment in the liver in the presence of Trypanosoma cruzi antigens. When only parasite extract was orally administered, it led to the up‐regulation of hepatic tolerance markers, but oral treatment plus adoptively transferred activated splenic T lymphocytes led to a proinflammatory response. Moreover, treated/recipient mice showed increased levels of TNF, IFN‐γ, IL‐6, and CCL2 in the liver and increased numbers of effector and/or effector memory T lymphocytes and F4/80⁺ cells. There was a reduction in FoxP3⁺ Treg cells, NKT cells, and γδ T lymphocytes with increased liver damage in the presence of activated peripheral T cells. Our results show that the induction of a proinflammatory liver response against T. cruzi danger molecules is at least partially dependent on cooperation with activated peripheral T cells.