Changes in cell migration-related molecules expressed by thymic microenvironment during experimental Plasmodium berghei infection: consequences on thymocyte development

Thymic atrophy induced by Plasmodium berghei ANKA and Plasmodium yoelii 17XL infection

The thymus is the anatomical site where T cells undergo a complex process of differentiation, proliferation, selection, and elimination of autorreactive cells which involves molecular signals in different intrathymic environment. However, the immunological functions of the thymus can be compromised upon exposure to different infections, affecting thymocyte populations. In this work, we investigated the impact of malaria parasites on the thymus by using C57BL/6 mice infected with Plasmodium berghei ANKA and Plasmodium yoelii 17XL; these lethal infection models represent the most severe complications, cerebral malaria, and anemia respectively. Data showed a reduction in the thymic weight and cellularity involving different T cell maturation stages, mainly CD4-CD8- and CD4+CD8+ thymocytes, as well as an increased presence of apoptotic cells, leading to significant thymic cortex reduction. Thymus atrophy showed no association with elevated serum cytokines levels, although increased glucocorticoid levels did. The severity of thymic damage in both models reached the same extend although it occurs at different stages of infection, showing that thymic atrophy does not depend on parasitemia level but on the specific host-parasite interaction.

Thymus, undernutrition, and infection: Approaching cellular and molecular interactions

Undernutrition remains a major issue in global health. Low protein-energy consumption, results in stunting, wasting and/or underweight, three deleterious forms of malnutrition that affect roughly 200 million children under the age of five years. Undernutrition compromises the immune system with the generation of various degrees of immunodeficiency, which in turn, renders undernourished individuals more sensitive to acute infections. The severity of various infectious diseases including visceral leishmaniasis (VL), influenza, and tuberculosis is associated with undernutrition. Immunosuppression resulting from protein-energy undernutrition severely impacts primary and secondary lymphoid organs involved in the response to related pathogens. The thymus—a primary lymphoid organ responsible for the generation of T lymphocytes—is particularly compromised by both undernutrition and infectious diseases. In this respect, we will discuss herein various intrathymic cellular and molecular interactions seen in undernutrition alone or in combination with acute infections. Many examples illustrated in studies on humans and experimental animals clearly revealed that protein-related undernutrition causes thymic atrophy, with cortical thymocyte depletion. Moreover, the non-lymphoid microenvironmental compartment of the organ undergoes important changes in thymic epithelial cells, including their secretory products such as hormones and extracellular matrix proteins. Of note, deficiencies in vitamins and trace elements also induce thymic atrophy. Interestingly, among the molecular interactions involved in the control of undernutrition-induced thymic atrophy is a hormonal imbalance with a rise in glucocorticoids and a decrease in leptin serum levels. Undernutrition also yields a negative impact of acute infections upon the thymus, frequently with the intrathymic detection of pathogens or their antigens. For instance, undernourished mice infected with Leishmania infantum (that causes VL) undergo drastic thymic atrophy, with significant reduction in thymocyte numbers, and decreased levels of intrathymic chemokines and cytokines, indicating that both lymphoid and microenvironmental compartments of the organ are affected. Lastly, recent data revealed that some probiotic bacteria or probiotic fermented milks improve the thymus status in a model of malnutrition, thus raising a new field for investigation, namely the thymus-gut connection, indicating that probiotics can be envisioned as a further adjuvant therapy in the control of thymic changes in undernutrition accompanied or not by infection.

IFNγ and iNOS-Mediated Alterations in the Bone Marrow and Thymus and Its Impact on Mycobacterium avium-Induced Thymic Atrophy

Disseminated infection with the high virulence strain of Mycobacterium avium 25291 leads to progressive thymic atrophy. We previously showed that M. avium-induced thymic atrophy results from increased glucocorticoid levels that synergize with nitric oxide (NO) produced by interferon gamma (IFNγ) activated macrophages. Where and how these mediators act is not understood. We hypothesized that IFNγ and NO promote thymic atrophy through their effects on bone marrow (BM) T cell precursors and T cell differentiation in the thymus. We show that M. avium infection cause a reduction in the percentage and number of common lymphoid progenitors (CLP). Additionally, BM precursors from infected mice show an overall impaired ability to reconstitute thymi of RAGKO mice, in part due to IFNγ. Thymi from infected mice present an IFNγ and NO-driven inflammation. When transplanted under the kidney capsule of uninfected mice, thymi from infected mice are unable to sustain T cell differentiation. Finally, we observed increased thymocyte death via apoptosis after infection, independent of both IFNγ and iNOS; and a decrease on active caspase-3 positive thymocytes, which is not observed in the absence of iNOS expression. Together our data suggests that M. avium-induced thymic atrophy results from a combination of defects mediated by IFNγ and NO, including alterations in the BM T cell precursors, the thymic structure and the thymocyte differentiation.

Infection-Associated Thymic Atrophy

The thymus is a vital organ of the immune system that plays an essential role in thymocyte development and maturation. Thymic atrophy occurs with age (physiological thymic atrophy) or as a result of viral, bacterial, parasitic or fungal infection (pathological thymic atrophy). Thymic atrophy directly results in loss of thymocytes and/or destruction of the thymic architecture, and indirectly leads to a decrease in naïve T cells and limited T cell receptor diversity. Thus, it is important to recognize the causes and mechanisms that induce thymic atrophy. In this review, we highlight current progress in infection-associated pathogenic thymic atrophy and discuss its possible mechanisms. In addition, we discuss whether extracellular vesicles/exosomes could be potential carriers of pathogenic substances to the thymus, and potential drugs for the treatment of thymic atrophy. Having acknowledged that most current research is limited to serological aspects, we look forward to the possibility of extending future work regarding the impact of neural modulation on thymic atrophy.

Intravital microscopy: Imaging host-parasite interactions in lymphoid organs

Intravital microscopy allows imaging of biological phenomena within living animals, including host-parasite interactions. This has advanced our understanding of both, the function of lymphoid organs during parasitic infections, and the effect of parasites on such organs to allow their survival. In parasitic research, recent developments in this technique have been crucial for the direct study of host-parasite interactions within organs at depths, speeds and resolution previously difficult to achieve. Lymphoid organs have gained more attention as we start to understand their function during parasitic infections and the effect of parasites on them. In this review, we summarise technical and biological findings achieved by intravital microscopy with respect to the interaction of various parasites with host lymphoid organs, namely the bone marrow, thymus, lymph nodes, spleen and the mucosa-associated lymphoid tissue, and present a view into possible future applications.

Protein malnutrition promotes dysregulation of molecules involved in T cell migration in the thymus of mice infected with Leishmania infantum

Protein malnutrition, the most deleterious cause of malnutrition in developing countries, has been considered a primary risk factor for the development of clinical visceral leishmaniasis (VL). Protein malnutrition and infection with Leishmania infantum leads to lymphoid tissue disorganization, including changes in cellularity and lymphocyte subpopulations in the thymus and spleen. Here we report that protein malnutrition modifies thymic chemotactic factors by diminishing the CCL5, CXCL12, IGF1, CXCL9 and CXCL10 protein levels in infected animals. Nevertheless, T cells preserve their migratory capability, as they were able to migrate ex vivo in response to chemotactic stimuli, indicating that malnutrition may compromise the thymic microenvironment and alter in vivo thymocyte migration. Decrease in chemotactic factors protein levels was accompanied by an early increase in the parasite load of the spleen. These results suggest that the precondition of malnutrition is affecting the cell-mediated immune response to L. infantum by altering T cell migration and interfering with the capacity of protein-deprived animals to control parasite spreading and proliferation. Our data provide evidence for a disturbance of T lymphocyte migration involving both central and peripheral T-cells, which likely contribute to the pathophysiology of VL that occurs in malnourished individuals.

Laminin-Mediated Interactions in Thymocyte Migration and Development

Intrathymic T-cell differentiation is a key process for the development and maintenance of cell-mediated immunity, and occurs concomitantly to highly regulated migratory events. We have proposed a multivectorial model for describing intrathymic thymocyte migration. One of the individual vectors comprises interactions mediated by laminins (LMs), a heterotrimeric protein family of the extracellular matrix. Several LMs are expressed in the thymus, being produced by microenvironmental cells, particularly thymic epithelial cells (TECs). Also, thymocytes and epithelial cells express integrin-type LM receptors. Functionally, it has been reported that the dy/dy mutant mouse (lacking the LM isoform 211) exhibits defective thymocyte differentiation. Several data show haptotactic effects of LMs upon thymocytes, as well as their adhesion on TECs; both effects being prevented by anti-LM or anti-LM receptor antibodies. Interestingly, LM synergizes with chemokines to enhance thymocyte migration, whereas classe-3 semaphorins and B ephrins, which exhibit chemorepulsive effects in the thymus, downregulate LM-mediated migratory responses of thymocytes. More recently, we showed that knocking down the ITGA6 gene (which encodes the α6 integrin chain of LM receptors) in human TECs modulates a large number of cell migration-related genes and results in changes of adhesion pattern of thymocytes onto the thymic epithelium. Overall, LM-mediated interactions can be placed at the cross-road of the multivectorial process of thymocyte migration, with a direct influence per se, as well as by modulating other molecular interactions associated with the intrathymic-trafficking events.

Bacterial clearance reverses a skewed T-cell repertoire induced by Salmonella infection

Salmonella typhimurium invades the spleen, liver, and peripheral lymph nodes and has recently been detected in the bone marrow and thymus, resulting in a reduced thymic size and a decline in the total number of thymic cells. A specific deletion of the double-positive cell subset has been characterized, yet the export of mature T cells to the periphery remains normal. We analyzed Salmonella pathogenesis regarding thymic structure and the T-cell maturation process. We demonstrate that, despite alterations in the thymic structure, T-cell development is maintained during Salmonella infection, allowing the selection of single-positive T-cell clones expressing particular T-cell receptor beta chains (TCR-Vβ). Moreover, the treatment of infected mice with an antibiotic restored the normal thymic architecture and thymocyte subset distribution. Additionally, the frequency of TCR-Vβ usage after treatment was comparable to that in non-infected mice. However, bacteria were still recovered from the thymus after 1 month of treatment. Our data reveal that a skewed T-cell developmental process is present in the Salmonella-infected thymus that alters the TCR-Vβ usage frequency. Likewise, the post-treatment persistence of Salmonella reveals a novel function of the thymus as a potential reservoir for this infectious agent.

Exacerbation of autoimmune neuro-inflammation in mice cured from blood-stage Plasmodium berghei infection

The thymus plays an important role shaping the T cell repertoire in the periphery, partly, through the elimination of inflammatory auto-reactive cells. It has been shown that, during Plasmodium berghei infection, the thymus is rendered atrophic by the premature egress of CD4+CD8+ double-positive (DP) T cells to the periphery. To investigate whether autoimmune diseases are affected after Plasmodium berghei NK65 infection, we immunized C57BL/6 mice, which was previously infected with P. berghei NK65 and treated with chloroquine (CQ), with MOG35-55 peptide and the clinical course of Experimental Autoimmune Encephalomyelitis (EAE) was evaluated. Our results showed that NK65+CQ+EAE mice developed a more severe disease than control EAE mice. The same pattern of disease severity was observed in MOG35-55-immunized mice after adoptive transfer of P. berghei-elicited splenic DP-T cells. The higher frequency of IL-17+- and IFN-γ+-producing DP lymphocytes in the Central Nervous System of these mice suggests that immature lymphocytes contribute to disease worsening. To our knowledge, this is the first study to integrate the possible relationship between malaria and multiple sclerosis through the contribution of the thymus. Notwithstanding, further studies must be conducted to assert the relevance of malaria-induced thymic atrophy in the susceptibility and clinical course of other inflammatory autoimmune diseases.

Tolerance has its limits: how the thymus copes with infection

The thymus is required for T cell differentiation; a process that depends on which antigens are encountered by thymocytes, the environment surrounding the differentiating cells, and the thymic architecture. These features are altered by local infection of the thymus and by the inflammatory mediators that accompany systemic infection. Although once believed to be an immune privileged site, it is now known that antimicrobial responses are recruited to the thymus. Resolving infection in the thymus is important because chronic persistence of microbes impairs the differentiation of pathogen-specific T cells and diminishes resistance to infection. Understanding how these mechanisms contribute to disease susceptibility, particularly in infants with developing T cell repertoires, requires further investigation.

Thymic alterations induced by Plasmodium berghei: expression of matrix metalloproteinases and their tissue inhibitors

The thymus plays a crucial role in the generation of T-cells, and so our laboratory has been interested in the study of the intrathymic events that occur during infection diseases and may cause disruption in its functions. Previously, we showed that thymus from experimentally Plasmodium berghei-infected mice present histological alterations with high levels of apoptosis, changes in cell migration-related molecules, and premature egress of immature thymocytes to periphery. In addition, parasites were found inside the thymus. In this work we investigated alterations in the expression pattern and activity of matrix metalloproteinases MMP-2 and -9, and their tissue inhibitors, TIMP-1 and TIMP-2. Our results show enhanced expression and widespread distribution of these molecules in thymus from infected animals. Also, the presence of active MMP-2 was detected. These data are suggestive of MMPs and TIMPs importance in the earlier observed changes in the extracellular matrix during thymic alterations after plasmodium infection.

Thymus atrophy and double-positive escape are common features in infectious diseases

The thymus is a primary lymphoid organ in which bone marrow-derived T-cell precursors undergo differentiation, leading to migration of positively selected thymocytes to the T-cell-dependent areas of secondary lymphoid organs. This organ can undergo atrophy, caused by several endogenous and exogenous factors such as ageing, hormone fluctuations, and infectious agents. This paper will focus on emerging data on the thymic atrophy caused by infectious agents. We present data on the dynamics of thymus lymphocytes during acute Trypanosoma cruzi infection, showing that the resulting thymus atrophy comprises the abnormal release of thymic-derived T cells and may have an impact on host immune response.

Effects of Plasmodium berghei on thymus: high levels of apoptosis and premature egress of CD4(+)CD8(+) thymocytes in experimentally infected mice

We have previously showed alterations in the thymus during experimental infection with Plasmodium berghei, the causative agent of Malaria. Such alterations comprised histological changes with loss of delimitation between cortical and medullar regions, a profound atrophy with depletion of CD4(+)CD8(+) double-positive (DP) thymocytes, and severe changes in the expression of cell migration-related molecules, belonging to the extracellular matrix and chemokine protein families. Taken together, these considerations prompted us to evaluate if the acute thymic atrophy observed during Plasmodium infection was correlated with increased apoptotic levels of thymocytes or with their premature emigration to the periphery. Our results confirmed that the marked reduction of the thymus weight in infected animals was accompanied by histological alterations, which included a very large number of cells showing nuclear condensation and karyorrhectic changes surrounded by histiocytes suggesting increased levels of apoptosis. This was confirmed by immunohistochemistry and flow cytometry techniques. In order to verify if an accelerated emigration of thymic cells to the peripheral lymphoid organs was also occurring we analyzed the spleen and mesenteric lymph nodes from control and infected mice. No significant differences were found in the spleen, but were seen after 14 days of infection between control and infected mice in the mesenteric lymph nodes. The main alteration was the presence of double negative (CD4(-)CD8(-)) and double positive (CD4(+)CD8(+)) cells. We concluded that both apoptosis of thymocytes and premature egress of immature cells take place during infection. Additional studies will be necessary to verify how such alterations might influence the systemic immune response to the parasite.

Nutritional imbalances and infections affect the thymus: consequences on T-cell-mediated immune responses

The thymus gland, where T lymphocyte development occurs, is targeted in malnutrition secondary to protein energy deficiency. There is a severe thymic atrophy, resulting from massive thymocyte apoptosis (particularly affecting the immature CD4+CD8+ cell subset) and decrease in cell proliferation. The thymic microenvironment (the non-lymphoid compartment that drives intrathymic T-cell development) is also affected in malnutrition: morphological changes in thymic epithelial cells were found, together with a decrease of thymic hormone production, as well as an increase of intrathymic contents of extracellular proteins. Profound changes in the thymus can also be seen in deficiencies of vitamins and trace elements. Taking Zn deficiency as an example, there is a substantial thymic atrophy. Importantly, marginal Zn deficiency in AIDS subjects, children with diarrhoea and elderly persons, significantly impairs the host's immunity, resulting in an increased risk of opportunistic infections and mortality; effects that are reversed by Zn supplementation. Thymic changes also occur in acute infectious diseases, including a severe thymic atrophy, mainly due to the depletion of CD4+CD8+ thymocytes, decrease in thymocyte proliferation, in parallel to densification of the epithelial network and increase in the extracellular matrix contents, with consequent disturbances in thymocyte migration and export. In conclusion, the thymus is targeted in several conditions of malnutrition as well as in acute infections. These changes are related to the impaired peripheral immune response seen in malnourished and infected individuals. Thus, strategies inducing thymus replenishment should be considered as adjuvant therapeutics to improve immunity in malnutrition and/or acute infectious diseases.

The thymus microenvironment in regulating thymocyte differentiation

The thymus plays a crucial role in the development of T lymphocytes by providing an inductive microenvironment in which committed progenitors undergo proliferation, T-cell receptor gene rearrangements and thymocyte differentiate into mature T cells. The thymus microenvironment forms a complex network of interaction that comprises non lymphoid cells (e.g., thymic epithelial cells, TEC), cytokines, chemokines, extracellular matrix elements (ECM), matrix metalloproteinases and other soluble proteins. The thymic epithelial meshwork is the major component of the thymic microenvironment, both morphologically and phenotypically limiting heterogeneous regions in thymic lobules and fulfilling an important role during specific stages of T-cell maturation. The process starts when bone marrow-derived lymphocyte precursors arrive at the outer cortical region of the thymic gland and begin to mature into functional T lymphocytes that will finally exit the thymus and populate the peripheral lymphoid organs. During their journey inside the thymus, thymocytes must interact with stromal cells (and their soluble products) and extracellular matrix proteins to receive appropriate signals for survival, proliferation and differentiation. The crucial components of the thymus microenvironment, and their complex interactions during the T-cell maturation process are summarized here with the objective of contributing to a better understanding of the function of the thymus, as well as assisting in the search for new therapeutic approaches to improve the immune response in various pathological conditions.