Fibronectin modulates thymocyte-thymic epithelial cell interactions following Trypanosoma cruzi infection.

Developing thymocytes interact with thymic epithelial cells (TECs) through cell-cell interactions, TEC-derived secretory moieties and extracellular matrix (ECM)-mediated interactions. These physiological interactions are crucial for normal thymocyte differentiation, but can be disrupted in pathological situations. Indeed, there is severe thymic atrophy in animals acutely infected with Trypanosoma cruzi due to CD4+CD8+ thymocyte depletion secondary to caspase-mediated apoptosis, together with changes in ECM deposition and thymocyte migration. We studied an in vitro model of TEC infection by T. cruzi and found that infected TEC cultures show a reduced number of cells, which was likely associated with decreased proliferative capacity, but not with increased cell death, as demonstrated by bromodeoxyuridine and annexin-V labelling. The infected TEC cultures exhibited increased expression of fibronectin (FN), laminin (LM) and type IV collagen. Importantly, treatment with FN increased the relative number of infected cells, whereas treatment with anti-FN or anti-LM antibodies resulted in lower infection rates. Consistent with these data, we observed increased thymocyte adhesion to infected TEC cultures. Overall, these results suggest that ECM molecules, particularly FN, facilitate infection of the thymic epithelium and that the consequent enhancement of ECM expression might be associated with changes in TEC-thymocyte interactions.

Oral Trypanosoma cruzi Acute Infection in Mice Targets Primary Lymphoid Organs and Triggers Extramedullary Hematopoiesis.

During the acute phase of Chagas disease, Trypanosoma cruzi circulation through the bloodstream leads to high tissue parasitism in the host. In primary lymphoid organs, progenitor cell reduction paralleled transient immunosuppression. Herein we showed that acute oral infection in mice promotes diffuse parasitism in bone marrow cells at 14 and 21 days post-infection (dpi), with perivascular regions, intravascular regions, and regions near the bone being target sites of parasite replication. Phenotypic analysis of hematopoietic differentiation in the bone marrow of infected mice showed that the cell number in the tissue is decreased (lineage-negative and lineage-positive cells). Interestingly, analysis of hematopoietic branching points showed that hematopoietic stem and progenitor cells (HSPCs) were significantly increased at 14 dpi. In addition, the pool of progenitors with stem plasticity (HSC-MPP3), as well as multipotent progenitors (MPPs) such as MPP4, also showed this pattern of increase. In contrast, subsequent progenitors that arise from MPPs, such as common lymphoid progenitors (CLPs), lymphoid-primed MPPs (LMPPs), and myeloid progenitors, were not enhanced; conversely, all presented numeric decline. Annexin V staining revealed that cell death increase in the initial hematopoietic branching point probably is not linked to CLPs and that myeloid progenitors decreased at 14 and 21 dpi. In parallel, our investigation provided clues that myeloid progenitor decrease could be associated with an atypical expression of Sca-1 in this population leading to a remarkable increase on LSK-like cells at 14 dpi within the HSPC compartment. Finally, these results led us to investigate HSPC presence in the spleen as a phenomenon triggered during emergency hematopoiesis due to mobilization or expansion of these cells in extramedullary sites. Splenocyte analysis showed a progressive increase in HSPCs between 14 and 21 dpi. Altogether, our study shows that the bone marrow is a target tissue in T. cruzi orally infected mice, leading to a hematopoietic disturbance with LSK-like cell bias accounting on HSPCs possibly affecting myeloid progenitor numbers. The LMPP and CLP reduction converges with defective thymocyte development. Lastly, it is tempting to speculate that the extramedullary hematopoiesis seen in the spleen is a mechanism involved in the hematological maintenance reported during the acute phase of oral T. cruzi infection.

Oral Route Driven Acute Trypanosoma cruzi Infection Unravels an IL-6 Dependent Hemostatic Derangement.

Oral transmission of Trypanosoma cruzi, the etiologic agent of Chagas disease, is presently the most important route of infection in Brazilian Amazon. Other South American countries have also reported outbreaks of acute Chagas disease associated with food consumption. A conspicuous feature of this route of transmission is presenting symptoms such as facial and lower limbs edema, in some cases bleeding manifestations and risk of thromboembolism are evident. Notwithstanding, studies that address this route of infection are largely lacking regarding its pathogenesis and, more specifically, the crosstalk between immune and hemostatic systems. Here, BALB/c mice were orally infected with metacyclic trypomastigotes of T. cruzi Tulahuén strain and used to evaluate the cytokine response, primary and secondary hemostasis during acute T. cruzi infection. When compared with control uninfected animals, orally infected mice presented higher pro-inflammatory cytokine (TNF-α, IFN-γ, and IL-6) serum levels. The highest concentrations were obtained concomitantly to the increase of parasitemia, between 14 and 28 days post-infection (dpi). Blood counts in the oral infected group revealed concomitant leukocytosis and thrombocytopenia, the latter resulting in increased bleeding at 21 dpi. Hematological changes paralleled with prolonged activated partial thromboplastin time, Factor VIII consumption and increased D-dimer levels, suggest that oral T. cruzi infection relies on disseminated intravascular coagulation. Remarkably, blockade of the IL-6 receptor blunted hematological abnormalities, revealing a critical role of IL-6 in the course of oral infection. These results unravel that acute T. cruzi oral infection results in significant alterations in the hemostatic system and indicates the relevance of the crosstalk between inflammation and hemostasis in this parasitic disease.

Atrophy of mesenteric lymph nodes in experimental Chagas' disease: differential role of Fas/Fas-L and TNFRI/TNF pathways.

It is currently accepted that experimental acute infection by Trypanosoma cruzi promotes changes in secondary lymphoid organs, with general T and B lymphocyte polyclonal activation. Here we show that mesenteric lymph nodes (MLN) of acutely infected mice show severe atrophy due to extensive lymphocyte apoptosis. Accordingly, clusters of apoptotic cells are detected in the initial phase of infection in MLN but not in subcutaneous nodes. Moreover, such atrophy is independent of the infection route, parasite load or the mouse strain used. Studies in Fas-L deficient (BALB gld/gld+/+) and in TNF type 1 receptor (p55-/-) knockout mice indicate that both molecules are involved in MLN atrophy: Fas-L participates in cell death of CD4+ as well as B lymphocytes, whereas the TNF type 1 receptor is important for the apoptosis of CD4+ and CD8+ T lymphocytes. In contrast, perforin does not play a role, as lymph nodes from perforin-deficient mice do not behave differently from the corresponding wild types. Our data support the concept that, even in a systemic infection, differential (even opposing) responses can be found in different lymph node chains.

Trypanosoma cruzi Infection through the Oral Route Promotes a Severe Infection in Mice: New Disease Form from an Old Infection?

Oral transmission of Chagas disease has been documented in Latin American countries. Nevertheless, significant studies on the pathophysiology of this form of infection are largely lacking. The few studies investigating oral route infection disregard that inoculation in the oral cavity (Oral infection, OI) or by gavage (Gastrointestinal infection, GI) represent different infection routes, yet both show clear-cut parasitemia and heart parasitism during the acute infection. Herein, BALB/c mice were subjected to acute OI or GI infection using 5x10(4) culture-derived Trypanosoma cruzi trypomastigotes. OI mice displayed higher parasitemia and mortality rates than their GI counterparts. Heart histopathology showed larger areas of infiltration in the GI mice, whereas liver lesions were more severe in the OI animals, accompanied by higher Alanine Transaminase and Aspartate Transaminase serum contents. A differential cytokine pattern was also observed because OI mice presented higher pro-inflammatory cytokine (IFN-γ, TNF) serum levels than GI animals. Real-time PCR confirmed a higher TNF, IFN-γ, as well as IL-10 expression in the cardiac tissue from the OI group compared with GI. Conversely, TGF-ß and IL-17 serum levels were greater in the GI animals. Immunolabeling revealed macrophages as the main tissue source of TNF in infected mice. The high mortality rate observed in the OI mice paralleled the TNF serum rise, with its inhibition by an anti-TNF treatment. Moreover, differences in susceptibility between GI versus OI mice were more clearly related to the host response than to the effect of gastric pH on parasites, since infection in magnesium hydroxide-treated mice showed similar results. Overall, the present study provides conclusive evidence that the initial site of parasite entrance critically affects host immune response and disease outcome. In light of the occurrence of oral Chagas disease outbreaks, our results raise important implications in terms of the current view of the natural disease course and host-parasite relationship.

Caspase-8 and caspase-9 mediate thymocyte apoptosis in Trypanosoma cruzi acutely infected mice.

Trypanosoma cruzi acute infection leads to thymic atrophy, largely as a result of death of immature DP T cells. In a second vein, the glucocorticoid hormone imbalance promotes DP T cell apoptosis in infected mice. Herein, we assessed the involvement of caspase signaling in thymocyte death during T. cruzi acute infection. BALB/c mice were infected i.p. with 10(2) trypomastigote forms of T. cruzi and analyzed from 7 to 19 dpi. Thymocyte apoptosis was observed in early stages of infection, increasing along with time postinfection. Immature DN and DP as well as CD4(+) and CD8(+) thymocytes from infected mice showed increased activation of caspase-8, -9, and -3. In vitro treatment of thymocytes from infected mice with a general caspase inhibitor or the combination of caspase-8- and caspase-9-specific inhibitors increased the number of living thymocytes. Intrathymic injection of the general caspase inhibitor, but not caspase-8 or -9 inhibitors individually, prevented thymic atrophy and thymocyte depletion in infected mice. Moreover, blockade of glucocorticoid receptor activity with RU486 prevented DP thymocyte apoptosis, together with caspase-8 and -9 activation. These findings indicate that DP T cell apoptosis following experimental T. cruzi acute infection is dependent on glucocorticoid stimulation, promoting caspase-8 and -9 activation.

Trypanosoma cruzi Entrance through Systemic or Mucosal Infection Sites Differentially Modulates Regional Immune Response Following Acute Infection in Mice.

Acute Chagas disease is characterized by a systemic infection that leads to the strong activation of the adaptive immune response. Outbreaks of oral contamination by the infective protozoan Trypanosoma cruzi are frequent in Brazil and other Latin American countries, and an increased severity of clinical manifestations and mortality is observed in infected patients. These findings have elicited questions about the specific responses triggered after T. cruzi entry via mucosal sites, possibly modulating local immune mechanisms, and further impacting regional and systemic immunity. Here, we provide evidence for the existence of differential lymphoid organ responses in experimental models of acute T. cruzi infection.

Trans-sialidase from Trypanosoma cruzi enhances the adhesion properties and fibronectin-driven migration of thymocytes.

In experimental Trypanosoma cruzi infections, severe thymic atrophy leads to release of activated CD4(+)CD8(+) double-positive (DP) T cells to the periphery. In humans, activated DP T cells are found in the blood in association with severe cardiac forms of human chronic Chagas disease. The mechanisms underlying the premature thymocyte release during the chagasic thymic atrophy remain elusive. We tested whether the migratory properties of intrathymic thymocytes are modulated by the parasite trans-sialidase (TS). We found that TS affected the dynamics of thymocytes undergoing intrathymic maturation, and these changes were accompanied by an increase in the number of recent DP thymic emigrants in the peripheral lymphoid organs. We demonstrated that increased percentages of blood DP T cell subsets were associated with augmented antibody titers against TS in chagasic patients with chronic cardiomyopathy. In vitro studies showed that TS was able to activate the MAPK pathway and actin filament mobilization in thymocytes. These effects were correlated with its ability to modulate the adhesion of thymocytes to thymic epithelial cells and their migration toward extracellular matrix. These findings point to effects of TS that could influence the escape of immature thymocytes in Chagas disease.

Chagasic thymic atrophy does not affect negative selection but results in the export of activated CD4+CD8+ T cells in severe forms of human disease.

Extrathymic CD4+CD8+ double-positive (DP) T cells are increased in some pathophysiological conditions, including infectious diseases. In the murine model of Chagas disease, it has been shown that the protozoan parasite Trypanosoma cruzi is able to target the thymus and induce alterations of the thymic microenvironment and the lymphoid compartment. In the acute phase, this results in a severe atrophy of the organ and early release of DP cells into the periphery. To date, the effect of the changes promoted by the parasite infection on thymic central tolerance has remained elusive. Herein we show that the intrathymic key elements that are necessary to promote the negative selection of thymocytes undergoing maturation during the thymopoiesis remains functional during the acute chagasic thymic atrophy. Intrathymic expression of the autoimmune regulator factor (Aire) and tissue-restricted antigen (TRA) genes is normal. In addition, the expression of the proapoptotic Bim protein in thymocytes was not changed, revealing that the parasite infection-induced thymus atrophy has no effect on these marker genes necessary to promote clonal deletion of T cells. In a chicken egg ovalbumin (OVA)-specific T-cell receptor (TCR) transgenic system, the administration of OVA peptide into infected mice with thymic atrophy promoted OVA-specific thymocyte apoptosis, further indicating normal negative selection process during the infection. Yet, although the intrathymic checkpoints necessary for thymic negative selection are present in the acute phase of Chagas disease, we found that the DP cells released into the periphery acquire an activated phenotype similar to what is described for activated effector or memory single-positive T cells. Most interestingly, we also demonstrate that increased percentages of peripheral blood subset of DP cells exhibiting an activated HLA-DR+ phenotype are associated with severe cardiac forms of human chronic Chagas disease. These cells may contribute to the immunopathological events seen in the Chagas disease.

Differential regional immune response in Chagas disease.

Following infection, lymphocytes expand exponentially and differentiate into effector cells to control infection and coordinate the multiple effector arms of the immune response. Soon after this expansion, the majority of antigen-specific lymphocytes die, thus keeping homeostasis, and a small pool of memory cells develops, providing long-term immunity to subsequent reinfection. The extent of infection and rate of pathogen clearance are thought to determine both the magnitude of cell expansion and the homeostatic contraction to a stable number of memory cells. This straight correlation between the kinetics of T cell response and the dynamics of lymphoid tissue cell numbers is a constant feature in acute infections yielded by pathogens that are cleared during the course of response. However, the regional dynamics of the immune response mounted against pathogens that are able to establish a persistent infection remain poorly understood. Herein we discuss the differential lymphocyte dynamics in distinct central and peripheral lymphoid organs following acute infection by Trypanosoma cruzi, the causative agent of Chagas disease. While the thymus and mesenteric lymph nodes undergo a severe atrophy with massive lymphocyte depletion, the spleen and subcutaneous lymph nodes expand due to T and B cell activation/proliferation. These events are regulated by cytokines, as well as parasite-derived moieties. In this regard, identifying the molecular mechanisms underlying regional lymphocyte dynamics secondary to T. cruzi infection may hopefully contribute to the design of novel immune intervention strategies to control pathology in this infection.

Fibronectin modulates thymocyte-thymic epithelial cell interactions following Trypanosoma cruzi infection

Developing thymocytes interact with thymic epithelial cells (TECs) through cell-cell interactions, TEC-derived secretory moieties and extracellular matrix (ECM)-mediated interactions. These physiological interactions are crucial for normal thymocyte differentiation, but can be disrupted in pathological situations. Indeed, there is severe thymic atrophy in animals acutely infected with Trypanosoma cruzi due to CD4+CD8+ thymocyte depletion secondary to caspase-mediated apoptosis, together with changes in ECM deposition and thymocyte migration. We studied an in vitro model of TEC infection by T. cruzi and found that infected TEC cultures show a reduced number of cells, which was likely associated with decreased proliferative capacity, but not with increased cell death, as demonstrated by bromodeoxyuridine and annexin-V labelling. The infected TEC cultures exhibited increased expression of fibronectin (FN), laminin (LM) and type IV collagen. Importantly, treatment with FN increased the relative number of infected cells, whereas treatment with anti-FN or anti-LM antibodies resulted in lower infection rates. Consistent with these data, we observed increased thymocyte adhesion to infected TEC cultures. Overall, these results suggest that ECM molecules, particularly FN, facilitate infection of the thymic epithelium and that the consequent enhancement of ECM expression might be associated with changes in TEC-thymocyte interactions.

Triiodothyronine modulates extracellular matrix-mediated interactions between thymocytes and thymic microenvironmental cells.

OBJECTIVES: Thyroid hormones exert immunomodulatory activities and the thymus is one of their target organs. We previously showed that triiodothyronine (T(3)) modulates thymic hormone production and extracellular matrix (ECM) expression by mouse thymic epithelial cells (TEC). This concept is enlarged herein by studying the effects of T(3) in human TEC preparations including primary cultures derived from thymic nurse cell complexes, as well as human and murine TEC lines. METHODS AND RESULTS: We observed that in all cases, ECM ligands and receptors (such as fibronectin, laminin, VLA-5 and VLA-6) are enhanced in vitro, as ascertained by immunocytochemistry, ELISA and cytofluorometry. Moreover, thymocyte adhesion to these TEC preparations is augmented by T(3). Interestingly, TEC-thymocyte adhesion is also upregulated when thymocytes from T(3)-treated mice adhere to untreated TEC cultures. Such an enhancing effect of T(3) upon TEC-thymocyte interactions is likely due to the increase in the expression of ECM ligands and receptors, since it is prevented when T(3)-treated TEC cultures are incubated with anti-ECM antibodies prior to the adhesion assay. We then tested whether T(3) could modulate interactions between thymocytes and nonepithelial microenvironmental cells, exemplified herein by the phagocytic cells of the mouse thymic reticulum. In fact, in vitro treatment of these cells with T(3) increases ECM ligands and receptors and augments their ability to adhere to thymocytes. Lastly, using immunochemistry-based assays, we showed the presence of the nuclear T(3) receptor in all thymic microenvironmental cell preparations. CONCLUSION: Our data show that T(3) upregulates ECM-mediated heterocellular interactions of thymocytes with distinct thymic microenvironmental cells, in both humans and mice.

Galectin-3 modulates carbohydrate-dependent thymocyte interactions with the thymic microenvironment.

The process of thymocyte differentiation occurs within the context of the thymic microenvironment, in which T cell precursors interact with thymic microenvironmental cells and extracellular matrix. Here we studied the expression of galectin-3, a beta-galactoside binding lectin, in the thymus of young adult mice. Galectin-3 was found mainly in the medulla and to a lesser extent in the cortex. We further showed that distinct microenvironmental elements, such as thymic epithelial cells, the epithelial component of thymic nurse complexes and phagocytic cells of the thymic reticulum produce, secrete and accumulate galectin-3 on the cell surface. Functionally, galectin-3-enriched medium inhibited in vitro thymocyte interactions with thymic microenvironmental cells, accelerated the release of thymocytes from thymic nurse cells and inhibited the reconstitution of these lymphoepithelial complexes. These effects were blocked by exogenous lactose (Galbeta1-4Glc), but not melibiose (Galalpha1-6Glc), and by a monospecific anti-galectin-3 antibody. Recombinant galectin-3 also inhibited thymocyte/thymic epithelial cell interactions. Our data indicate that intrathymically produced galectin-3 disrupts thymocyte/microenvironmental cell interactions, thus acting as a de-adhesion molecule.

Phagocytic cells of the thymic reticulum interact with thymocytes via extracellular matrix ligands and receptors.

We previously showed that, in the context of thymic epithelial cells, thymocyte migration is partially controlled by extracellular matrix (ECM)-mediated interactions. Herein we evaluated whether these interactions could be involved in cell migration related events in the context of non-epithelial cells of the thymic microenvironment, the phagocytic cells of the thymic reticulum (PTR). We first showed, by immunocytochemistry, cytofluorometry, and RT-PCR, that PTR produce ECM components, including fibronectin and laminin, and express the corresponding integrin-type receptors, VLA-4, VLA-5, and VLA-6. Thymocytes adhere onto PTR monolayers, with immature CD4(+)CD8(+) cells being predominant. Importantly, such an adhesion is partially mediated by ECM ligands and receptors, since it was impaired by anti-ECM or anti-ECM receptor antibodies. Conjointly, our data reveal that the ECM-dependence for thymocyte adhesion onto the thymic microenvironment is not restricted to the epithelial cells, being also seen when they encounter non-epithelial phagocytic cells.

Functional insulin-like growth factor-1/insulin-like growth factor-1 receptor-mediated circuit in human and murine thymic epithelial cells.

Interactions between thymocytes and thymic epithelial cell (TEC) can be modulated by growth hormone via insulin-like growth factor-1 (IGF-1). In this study, we showed IGF-1 and IGF-1 receptor mRNA expression by human and murine TEC and thymocytes. Functionally, IGF-1 stimulates extracellular matrix production by human TEC. Moreover, pretreatment of murine TEC with IGF-1 increases their adhesion to thymocytes. Interestingly, we observed an increase in the frequency of CD4-CD8-CD90+ T cells which adhered to pretreated TEC, supporting the concept that IGF-1 may also act indirectly on intrathymic T cell differentiation and migration through the thymic epithelium.