Thymic function is maintained during Salmonella-induced atrophy and recovery

Primary and secondary defects of the thymus

The thymus is the primary site of T-cell development, enabling generation, and selection of a diverse repertoire of T cells that recognize non-self, whilst remaining tolerant to self- antigens. Severe congenital disorders of thymic development (athymia) can be fatal if left untreated due to infections, and thymic tissue implantation is the only cure. While newborn screening for severe combined immune deficiency has allowed improved detection at birth of congenital athymia, thymic disorders acquired later in life are still underrecognized and assessing the quality of thymic function in such conditions remains a challenge. The thymus is sensitive to injury elicited from a variety of endogenous and exogenous factors, and its self-renewal capacity decreases with age. Secondary and age-related forms of thymic dysfunction may lead to an increased risk of infections, malignancy, and autoimmunity. Promising results have been obtained in preclinical models and clinical trials upon administration of soluble factors promoting thymic regeneration, but to date no therapy is approved for clinical use. In this review we provide a background on thymus development, function, and age-related involution. We discuss disease mechanisms, diagnostic, and therapeutic approaches for primary and secondary thymic defects.

Immune Responses to Influenza D Virus in Calves Previously Infected with Bovine Viral Diarrhea Virus 2

Bovine viral diarrhea virus (BVDV) induces immunosuppression and thymus depletion in calves. This study explores the impact of prior BVDV-2 exposure on the subsequent immune response to influenza D virus (IDV). Twenty 3-week-old calves were divided into four groups. Calves in G1 and G3 were mock-treated on day 0, while calves in G2 and G4 received BVDV. Calves in G1 (mock) and G2 (BVDV) were necropsied on day 13 post-infection. IDV was inoculated on day 21 in G3 calves (mock + IDV) and G4 (BVDV + IDV) and necropsy was conducted on day 42. Pre-exposed BVDV calves exhibited prolonged and increased IDV shedding in nasal secretions. An approximate 50% reduction in the thymus was observed in acutely infected BVDV calves (G2) compared to controls (G1). On day 42, thymus depletion was observed in two calves in G4, while three had normal weight. BVDV-2-exposed calves had impaired CD8 T cell proliferation after IDV recall stimulation, and the α/β T cell impairment was particularly evident in those with persistent thymic atrophy. Conversely, no difference in antibody levels against IDV was noted. BVDV-induced thymus depletion varied from transient to persistent. Persistent thymus atrophy was correlated with weaker T cell proliferation, suggesting correlation between persistent thymus atrophy and impaired T cell immune response to subsequent infections.

Thymic expression of immune checkpoint molecules and their implication for response to immunotherapies

The thymus is responsible for generating a diverse T cell repertoire that is tolerant to self, but capable of responding to various immunologic insults, including cancer. Checkpoint blockade has changed the face of cancer treatment by targeting inhibitory molecules, which are known to regulate peripheral T cell responses. However, these inhibitory molecules and their ligands are expressed during T cell development in the thymus. In this review, we describe the underappreciated role of checkpoint molecule expression during the formation of the T cell repertoire and detail the importance of inhibitory molecules in regulating T cell lineage commitment. Understanding how these molecules function in the thymus may inform therapeutic strategies for better patient outcomes.

Dynamic changes in thymic sub-populations during acute and long-term infections with virulent and virulence-attenuated Salmonella Typhimurium strains in C57BL/6 and autoimmune-prone lpr mice

SALMONELLA

Typhimurium infection in mice results in drastic loss of immature CD4^- CD8^- double negative (DN) and CD4⁺ CD8⁺ double positive (DP) thymic subsets compared to mature single positive (SP) subsets. We investigated changes in thymocyte sub-populations post infection with a wild type (WT) virulent strain and ΔrpoS, a virulence-attenuated strain, of Salmonella Typhimurium in C57BL/6 (B6) and Fas-deficient autoimmune-prone lpr mice. The WT strain caused acute thymic atrophy with greater loss of thymocytes in lpr mice compared to B6 mice. Infection with ΔrpoS caused progressive thymic atrophy in B6 and lpr mice. Analysis of thymocyte subsets revealed that immature thymocytes including the DN, immature single positive (ISP), and DP thymocytes underwent extensive loss. SP thymocytes were more resistant to loss in WT-infected B6 mice, whereas WT-infected lpr and ΔrpoS-infected mice exhibited depletion of SP thymocytes. Overall, thymocyte sub-populations exhibited differential susceptibilities depending on bacterial virulence and the host background.

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.

Maternal organic selenium supplementation alleviates LPS induced inflammation, autophagy and ER stress in the thymus and spleen of offspring piglets by improving the expression of selenoproteins

The thymus and spleen are the main reservoir for T lymphocytes, which can regulate the innate immune response and provide protection against pathogens and tissue damage. Oxidative stress, excessive inflammation, abnormal autophagy and endoplasmic reticulum (ER) stress can all lead to dysfunction of the thymus and spleen. This study was conducted to investigate the effect of maternal 2-hydroxy-4-methylselenobutanoic acid (HMSeBA, an organic Se source) supplementation during pregnancy on the selenoprotein expression, inflammation, ER stress and autophagy of their young offspring's thymus and spleen. Thirty sows were randomly assigned to receive one of the following two diets during gestation: control diet (control, basal diet, n = 15) or HMSeBA supplemented diet (HMSeBA, basal diet +0.3 mg Se kg^-1 as HMSeBA, n = 15). Tissues of thymus and spleen were collected from the offspring at birth and weaning after the lipopolysaccharide challenge. Results showed that maternal HMSeBA supplementation significantly up-regulated the gene expression of selenoproteins in the thymus and spleen of newborn piglets compared with the basal diet (p < 0.05), as well as the protein abundance of GPX1 and GPX4 (p < 0.05). In addition, maternal HMSeBA supplementation effectively decreased the expression of inflammation and autophagy related proteins in the thymus and spleen of newborn piglets as compared with the control group (p < 0.05). In weaning piglets, maternal HMSeBA significantly increased the antioxidative capacity of thymus and spleen (p < 0.05), and reversed LPS induced MDA content as compared with the control group (p < 0.05). Furthermore, maternal HMSeBA supplementation during gestation reversed the activation of the MAPK/NF-κB pathway, ER stress and autophagy induced by the LPS challenge in the thymus and spleen of weaning piglets (p < 0.05). In conclusion, maternal HMSeBA supplementation during gestation could decrease the level of inflammation, autophagy and ER stress in the thymus and spleen of young offspring by improving the antioxidative capacity and selenoprotein expression in these tissues. Therefore, maternal HMSeBA supplementation during gestation might be beneficial for the immune function of their offspring by alleviating inflammation, autophagy and ER stress levels in the thymus and spleen. This study showed more evidence for the function of Se on mater-offspring integrated nutrition.

Dietary genistein supplementation alters mRNA expression profile and alternative splicing signature in the thymus of chicks with lipopolysaccharide challenge

Genistein is abundant in the soybean products, which exerts prominent effects on immune function. Little information is available about the effect of dietary genistein on thymic transcriptome, especially when suffering from lipopolysaccharide challenge. In this study, 180 one-day-old male broilers were randomly allocated to 3 groups: nonchallenged chicks given a basal diet (CON), and lipopolysaccharide-challenged chicks fed a basal diet (LPS), or lipopolysaccharide-challenged chicks fed a basal diet supplemented with 40 mg/kg genistein (GEN). Lipopolysaccharide injection induced thymocyte apoptosis and inflammatory reactions in the chicks. The results showed dietary genistein significantly reduced the percentage of CD3+ T lymphocytes by 10.04% and CD4+/CD8+ T lymphocyte ratio by 21.88% in the peripheral blood induced by lipopolysaccharide injection (P < 0.05). In addition, genistein significantly reduced the thymus index by 50% and apoptotic index by 12.34% induced by LPS challenge (P < 0.05). Transcriptomic analysis identified 1,926 DEGs (1,014 upregulated and 912 downregulated, P < 0.05) between GEN and LPS groups, which altered the mRNA expression profile and signaling pathways (Toll-like receptor, and NOD-like receptor signaling pathway) in the thymus. Furthermore, 5 splicing (AS) isoforms of the Drosophila Disabled-2 (DAB2) gene were detected, which were significantly upregulated in the GEN group compared with that in the LPS group. In summary, dietary genistein supplementation altered the RNA expression profile and AS signatures in the thymus, and alleviated immune response against lipopolysaccharide challenge.

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.

Bioluminescent Imaging Identifies Thymus, As Overlooked Colonized Organ, in a Chronic Model of Leishmania donovani Mouse Visceral Leishmaniasis

The search for new drugs against neglected parasitic diseases has experienced a major boost in recent years with the incorporation of bioimaging techniques. Visceral leishmaniasis, the second more neglected disease in the world, has effective treatments but with several disadvantages that make the search for new therapeutic solutions an urgent task. Animal models of visceral leishmaniasis that resemble the human disease have the disadvantage of using hamsters, which are an outbred breeding animal too large to obtain acceptable images with current bioimaging methodologies. Mouse models of visceral leishmaniasis seem, however, to be more suitable for early (acute) stages of the disease, but not for chronic ones. In our work, we describe a chronic Balb/c mouse model in which the infection primarily colonizes the spleen and well recreates the late stages of human disease. Thanks to the bioluminescent image, we have been able to identify experimentally, for the first time, a new primary lymphoid organ of colonization, the thymus, which appears infected from the beginning until the late phases of the infection.

T cell regeneration after immunological injury

Following periods of haematopoietic cell stress, such as after chemotherapy, radiotherapy, infection and transplantation, patient outcomes are linked to the degree of immune reconstitution, specifically of T cells. Delayed or defective recovery of the T cell pool has significant clinical consequences, including prolonged immunosuppression, poor vaccine responses and increased risks of infections and malignancies. Thus, strategies that restore thymic function and enhance T cell reconstitution can provide considerable benefit to individuals whose immune system has been decimated in various settings. In this Review, we focus on the causes and consequences of impaired adaptive immunity and discuss therapeutic strategies that can recover immune function, with a particular emphasis on approaches that can promote a diverse repertoire of T cells through de novo T cell formation.

Control of Persistent Salmonella Infection Relies on Constant Thymic Output Despite Increased Peripheral Antigen-Specific T Cell Immunity

Recent thymic emigrants are the youngest subset of peripheral T cells and their involvement in combating persistent bacterial infections has not been explored. Here, we hypothesized that CD4⁺ recent thymic emigrants are essential immune mediators during persistent Salmonella infection. To test this, we thymectomized adult mice either prior to, or during, persistent Salmonella infection. We found that thymic output is crucial in the formation of protective immune responses during the early formation of a Salmonella infection but is dispensable once persistent Salmonella infection is established. Further, we show that thymectomized mice demonstrate increased infection-associated mortality and bacterial burdens. Unexpectedly, numbers of Salmonella-specific CD4⁺ T cells were significantly increased in thymectomized mice compared to sham control mice. Lastly, we found that T cells from thymectomized mice may be impaired in producing the effector cytokine IL-17 at early time points of infection, compared to thymically intact mice. Together, these results imply a unique role for thymic output in the formation of immune responses against a persistent, enteric pathogen.

Streptococcus suis Serotype 2 Infection Causes Host Immunomodulation through Induction of Thymic Atrophy

Streptococcus suis serotype 2 is an important bacterial pathogen of swine and is also an emerging zoonotic agent that may be harmful to human health. Although the virulence genes of S. suis have been extensively studied, the mechanisms by which they damage the central immune organs have rarely been studied. In the current work, we wanted to uncover more details about the impact and mechanisms of S. suis on specific populations of thymic and immune cells in infected mice. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) assays revealed that S. suis infection induced apoptosis in CD3⁺, CD14⁺, and epithelial cells from the thymus. S. suis infection resulted in a rapid depletion of mitochondrial permeability and release of cytochrome c (CytC) and apoptosis-inducing factor (AIF) through upregulation of Bax expression and downregulation of Bcl-xl and Bcl2 expression in thymocytes. Moreover, S. suis infection increased cleavage of caspase-3, caspase-8, and caspase-9. Thus, S. suis induced thymocyte apoptosis through a p53- and caspase-dependent pathway, which led to a decrease of CD3⁺ cells in the thymus, subsequently decreasing the numbers of CD4⁺ and CD8⁺ cells in the peripheral blood. Finally, expression dysregulation of proinflammatory cytokines in the serum, including interleukin 2 (IL-2), IL-6, IL-12 (p70), tumor necrosis factor (TNF), and IL-10, was observed in mice after S. suis type 2 infection. Taken together, these results suggest that S. suis infection can cause atrophy of the thymus and induce apoptosis of thymocytes in mice, thus likely suppressing host immunity.

Understanding Infection-Induced Thrombosis: Lessons Learned From Animal Models

Thrombosis is a common consequence of infection that is associated with poor patient outcome. Nevertheless, the mechanisms by which infection-associated thrombosis is induced, maintained and resolved are poorly understood, as is the contribution thrombosis makes to host control of infection and pathogen spread. The key difference between infection-associated thrombosis and thrombosis in other circumstances is a stronger inflammation-mediated component caused by the presence of the pathogen and its products. This inflammation triggers the activation of platelets, which may accompany damage to the endothelium, resulting in fibrin deposition and thrombus formation. This process is often referred to as thrombo-inflammation. Strikingly, despite its clinical importance and despite thrombi being induced to many different pathogens, it is still unclear whether the mechanisms underlying this process are conserved and how we can best understand this process. This review summarizes thrombosis in a variety of models, including single antigen models such as LPS, and infection models using viruses and bacteria. We provide a specific focus on Salmonella Typhimurium infection as a useful model to address all stages of thrombosis during infection. We highlight how this model has helped us identify how thrombosis can appear in different organs at different times and thrombi be detected for weeks after infection in one site, yet largely be resolved within 24 h in another. Furthermore, we discuss the observation that thrombi induced to Salmonella Typhimurium are largely devoid of bacteria. Finally, we discuss the value of different therapeutic approaches to target thrombosis, the potential importance of timing in their administration and the necessity to maintain normal hemostasis after treatment. Improvements in our understanding of these processes can be used to better target infection-mediated mechanisms of thrombosis.

Soluble antigens from the neurotropic pathogen Angiostrongylus cantonensis directly induce thymus atrophy in a mouse model

The nematode Angiostrongylus cantonensis (A.C.) is a neurotropic pathogen; stage-III larva invade the human (non-permissive host) central nervous system (CNS) to cause eosinophilic meningitis or meningoencephalitis accompanied by immunosuppression. In an A.C.-infectedmouse (another non-permissive host) model, CNS damage-associated T cell immune deficiency and severe inflammation were proposed to result from activation of the hypothalamic-pituitary-adrenal (HPA) axis. However, glucocorticoids are anti-inflammatory agents. Additionally, while defects in thymic stromal/epithelial cells (TECs) are the major reason for thymic atrophy, TECs do not express the glucocorticoid receptor. Therefore, activation of the HPA axis cannot fully explain the thymic atrophy and inflammation. Using an A.C.-infected mouse model, we found that A.C.-infected mice developed severe thymic atrophy with dramatic impairments in thymocytes and TECs, particularly cortical TECs, which harbor CD4⁺CD8⁺ double-positive thymocytes. The impairments resulted from soluble antigens (sAgs) from A.C. in the thymuses of infected mice, as intrathymic injection of these sAgs into live mice and the addition of these sAgs to thymic cell culture resulted in thymic atrophy and cellular apoptosis, respectively. Therefore, in addition to an indirect effect on thymocytes through the HPA axis, our study reveals a novel mechanism by which A.C. infection in non-permissive hosts directly induces defects in both thymocytes and TECs via soluble antigens.

Lipopolysaccharide induces acute bursal atrophy in broiler chicks by activating TLR4-MAPK-NF-κB/AP-1 signaling

We investigated the mechanisms that induce atrophy of the chicken bursa of Fabricius (BF) upon lipopolysaccharide (LPS) treatment in young chicks. LPS treatment resulted in ∼36% decrease in bursal weight within 36 h (P < 0.01). Histological analysis showed infiltration of eosinophilic heterophils and nucleated oval shaped RBCs in or near blood vessels of the BF from LPS-treated chicks. Scanning electron micrographs showed severe erosion and breaks in the mucosal membrane at 12 h and complete exuviation of bursal mucosal epithelial cells at 36 h. We observed decreased cell proliferation (low PCNA positivity) and increased apoptosis (high TUNEL and ssDNA positivity) in the BF 12-72 h after LPS treatment. RNA-seq analysis of the BF transcriptome showed 736 differentially expressed genes with most expression changes (637/736) 12 h after LPS treatment. KEGG pathway analysis identified TLR4-MAPK-NF-κB/AP-1 as the key signaling pathway affected in response to LPS stimulation. These findings indicate LPS activates the TLR4-MAPK-NF-κB/AP-1 signaling pathway that mediates acute atrophy of the chicken bursa of Fabricius by inducing inflammation and apoptosis.

Salmonella infection: Interplay between the bacteria and host immune system

Salmonella infection causes morbidity and mortality throughout the world with the host immune response varying depending on whether the infection is acute and limited, or systemic and chronic. Additionally, Salmonella bacteria have evolved multiple mechanisms to avoid or subvert immunity to its own benefit and often the anatomical location of infection plays a role in both the immune response and bacterial fate. Here, we provide an overview of the interplay between the immune system and Salmonella, while discussing how different host and bacterial factors influence the outcome of infection.

Acute Thymic Involution and Mechanisms for Recovery

Acute thymic involution (ATI) is usually regarded as a virulence trait. It is caused by several infectious agents (bacteria, viruses, parasites, fungi) and other factors, including stress, pregnancy, malnutrition and chemotherapy. However, the complex mechanisms that operate during ATI differ substantially from each other depending on the causative agent. For instance, a transient reduction in the size and weight of the thymus and depletion of populations of T cell subsets are hallmarks of ATI in many cases, whereas severe disruption of the anatomical structure of the organ is also associated with some factors, including fungal, parasitic and viral infections. However, growing evidence shows that ATI may be therapeutically halted or reversed. In this review, we highlight the current progress in this field with respect to numerous pathological factors and discuss the possible mechanisms. Moreover, these new observations also show that ATI can be mechanistically reversed.

Differential susceptibility and maturation of thymocyte subsets during Salmonella Typhimurium infection: insights on the roles of glucocorticoids and Interferon-gamma

The thymus is known to atrophy during infections; however, a systematic study of changes in thymocyte subpopulations has not been performed. This aspect was investigated, using multi-color flow cytometry, during oral infection of mice with Salmonella Typhimurium (S. Typhimurium). The major highlights are: First, a block in the developmental pathway of CD4-CD8- double negative (DN) thymocytes is observed. Second, CD4+CD8+ double positive (DP) thymocytes, mainly in the DP1 (CD5loCD3lo) and DP2 (CD5hiCD3int), but not DP3 (CD5intCD3hi), subsets are reduced. Third, single positive (SP) thymocytes are more resistant to depletion but their maturation is delayed, leading to accumulation of CD24hiCD3hi SP. Kinetic studies during infection demonstrated differences in sensitivity of thymic subpopulations: Immature single positive (ISP) > DP1, DP2 > DN3, DN4 > DN2 > CD4+ > CD8+. Upon infection, glucocorticoids (GC), inflammatory cytokines, e.g. Ifnγ, etc are induced, which enhance thymocyte death. Treatment with RU486, the GC receptor antagonist, increases the survival of most thymic subsets during infection. Studies with Ifnγ-/- mice demonstrated that endogenous Ifnγ produced during infection enhances the depletion of DN2-DN4 subsets, promotes the accumulation of DP3 and delays the maturation of SP thymocytes. The implications of these observations on host cellular responses during infections are discussed.

Systemic toxoplasma infection triggers a long-term defect in the generation and function of naive T lymphocytes

Kugler et al. show that systemic infection with Toxoplasma gondii triggers a long-term impairment in thymic function, which leads to an immunodeficient state reflected in decreased antimicrobial resistance.

Because antigen-stimulated naive T cells either die as effectors or enter the activated/memory pool, continuous egress of new T lymphocytes from thymus is essential for maintenance of peripheral immune homeostasis. Unexpectedly, we found that systemic infection with the protozoan Toxoplasma gondii triggers not only a transient increase in activated CD4⁺ Th1 cells but also a persistent decrease in the size of the naive CD4⁺ T lymphocyte pool. This immune defect is associated with decreased thymic output and parasite-induced destruction of the thymic epithelium, as well as disruption of the overall architecture of that primary lymphoid organ. Importantly, the resulting quantitative and qualitative deficiency in naive CD4⁺ T cells leads to an immunocompromised state that both promotes chronic toxoplasma infection and leads to decreased resistance to challenge with an unrelated pathogen. These findings reveal that systemic infectious agents, such as T. gondii, can induce long-term immune alterations associated with impaired thymic function. When accumulated during the lifetime of the host, such events, even when occurring at low magnitude, could be a contributing factor in immunological senescence.

Accelerated Loss of TCR Repertoire Diversity in Common Variable Immunodeficiency

Although common variable immunodeficiency (CVID) has long been considered as a group of primary Ab deficiencies, growing experimental data now suggest a global disruption of the entire adaptive immune response in a segment of patients. Oligoclonality of the TCR repertoire was previously demonstrated; however, the manner in which it relates to other B cell and T cell findings reported in CVID remains unclear. Using a combination approach of high-throughput TCRβ sequencing and multiparametric flow cytometry, we compared the TCR repertoire diversity between various subgroups of CVID patients according to their B cell immunophenotypes. Our data suggest that the reduction in repertoire diversity is predominantly restricted to those patients with severely reduced class-switched memory B cells and an elevated level of CD21^lo B cells (Freiburg 1a), and may be driven by a reduced number of naive T cells unmasking underlying memory clonality. Moreover, our data indicate that this loss in repertoire diversity progresses with advancing age far exceeding the expected physiological rate. Radiological evidence supports the loss in thymic volume, correlating with the decrease in repertoire diversity. Evidence now suggests that primary thymic failure along with other well-described B cell abnormalities play an important role in the pathophysiology in Freiburg group 1a patients. Clinically, our findings emphasize the integration of combined B and T cell testing to identify those patients at the greatest risk for infection. Future work should focus on investigating the link between thymic failure and the severe reduction in class-switched memory B cells, while gathering longitudinal laboratory data to examine the progressive nature of the disease.

Transcriptome analysis indicated that Salmonella lipopolysaccharide-induced thymocyte death and thymic atrophy were related to TLR4-FOS/JUN pathway in chicks

Background

Thymus is the crucial site for T cell development and once believed to be immune privileged. Recently, thymus has gained special attention as it is commonly targeted by infectious agents which may cause pathogenic tolerance and subsequent immunosuppression.

Results

We analyzed thymic responses to the challenge with Salmonella typhimurium (STm) or lipopolysaccharide (LPS) derived from STm in chicks. Newly hatched chicks were injected intraperitoneally with 5 × 10⁴ CFU/mL STm or 50 mg/kg LPS. After LPS treatment, maximum thymocyte death (3 ~ 5-fold change) compared to controls was found at 12 h, and maximum loss of thymic weight (35 %) and reduced thymic index (20 %) were found at 36 h. After STm infection, maximum thymocyte death and thymic atrophy occurred at 36 and 72 h, respectively. No significant changes of thymic structure, chT1+ and CD4+/CD8+ T cell ratio were observed in thymus or spleen tissues after LPS treatment. Furthermore, transcriptome analysis revealed important roles for the TLR4-FOS/JUN signaling pathway in thymic injury. Thus, the major process of thymic atrophy in this study first involved activation of transcriptional factors FOS/JUN upon LPS binding to TLR4 that caused release of inflammatory factors, thereby inducing inflammatory responses and DNA damage and ultimately cell cycle arrest and thymic injury.

Conclusions

STm and Salmonella LPS could induce acute chick thymic injury. LPS treatment acted faster than STm. TLR4-FOS/JUN pathway may play an important role in LPS induced chick thymic injury.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2674-6) contains supplementary material, which is available to authorized users.

Identification of MiR-205 As a MicroRNA That Is Highly Expressed in Medullary Thymic Epithelial Cells

Thymic epithelial cells (TECs) support T cell development in the thymus. Cortical thymic epithelial cells (cTECs) facilitate positive selection of developing thymocytes whereas medullary thymic epithelial cells (mTECs) facilitate the deletion of self-reactive thymocytes in order to prevent autoimmunity. The mTEC compartment is highly dynamic with continuous maturation and turnover, but the genetic regulation of these processes remains poorly understood. MicroRNAs (miRNAs) are important regulators of TEC genetic programs since miRNA-deficient TECs are severely defective. However, the individual miRNAs important for TEC maintenance and function and their mechanisms of action remain unknown. Here, we demonstrate that miR-205 is highly and preferentially expressed in mTECs during both thymic ontogeny and in the postnatal thymus. This distinct expression is suggestive of functional importance for TEC biology. Genetic ablation of miR-205 in TECs, however, neither revealed a role for miR-205 in TEC function during homeostatic conditions nor during recovery from thymic stress conditions. Thus, despite its distinct expression, miR-205 on its own is largely dispensable for mTEC biology.

Soluble flagellin coimmunization attenuates Th1 priming to Salmonella and clearance by modulating dendritic cell activation and cytokine production

Soluble flagellin (sFliC) from Salmonella Typhimurium (STm) can induce a Th2 response to itself and coadministered antigens through ligation of TLR5. These properties suggest that sFliC could potentially modulate responses to Th1 antigens like live STm if both antigens are given concurrently. After coimmunization of mice with sFliC and STm there was a reduction in Th1 T cells (T-bet(+) IFN-γ(+) CD4 T cells) compared to STm alone and there was impaired clearance of STm. In contrast, there was no significant defect in the early extrafollicular B-cell response to STm. These effects are dependent upon TLR5 and flagellin expression by STm. The mechanism for these effects is not related to IL-4 induced to sFliC but rather to the effects of sFliC coimmunization on DCs. After coimmunization with STm and sFliC, splenic DCs had a lower expression of costimulatory molecules and profoundly altered kinetics of IL-12 and TNFα expression. Ex vivo experiments using in vivo conditioned DCs confirmed the effects of sFliC were due to altered DC function during a critical window in the coordinated interplay between DCs and naïve T cells. This has marked implications for understanding how limits in Th1 priming can be achieved during infection-induced, Th1-mediated inflammation.

Dominant Suppression of Inflammation via Targeted Mutation of the mRNA Destabilizing Protein Tristetraprolin

In myeloid cells, the mRNA-destabilizing protein tristetraprolin (TTP) is induced and extensively phosphorylated in response to LPS. To investigate the role of two specific phosphorylations, at serines 52 and 178, we created a mouse strain in which those residues were replaced by nonphosphorylatable alanine residues. The mutant form of TTP was constitutively degraded by the proteasome and therefore expressed at low levels, yet it functioned as a potent mRNA destabilizing factor and inhibitor of the expression of many inflammatory mediators. Mice expressing only the mutant form of TTP were healthy and fertile, and their systemic inflammatory responses to LPS were strongly attenuated. Adaptive immune responses and protection against infection by Salmonella typhimurium were spared. A single allele encoding the mutant form of TTP was sufficient for enhanced mRNA degradation and underexpression of inflammatory mediators. Therefore, the equilibrium between unphosphorylated and phosphorylated TTP is a critical determinant of the inflammatory response, and manipulation of this equilibrium may be a means of treating inflammatory pathologies.

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.

Immunity to intestinal pathogens: lessons learned from Salmonella

Salmonella are a common source of food- or water-borne infection and cause a wide range of clinical disease in human and animal hosts. Salmonella are relatively easy to culture and manipulate in a laboratory setting, and the infection of laboratory animals induces robust innate and adaptive immune responses. Thus, immunologists have frequently turned to Salmonella infection models to expand understanding of host immunity to intestinal pathogens. In this review, I summarize current knowledge of innate and adaptive immunity to Salmonella and highlight features of this response that have emerged from recent studies. These include the heterogeneity of the antigen-specific T-cell response to intestinal infection, the prominence of microbial mechanisms to impede T- and B-cell responses, and the contribution of non-cognate pathways for elicitation of T-cell effector functions. Together, these different issues challenge an overly simplistic view of host-pathogen interaction during mucosal infection, but also allow deeper insight into the real-world dynamic of protective immunity to intestinal pathogens.

Salmonella modulates B cell biology to evade CD8(+) T cell-mediated immune responses

Although B cells and antibodies are the central effectors of humoral immunity, B cells can also produce and secrete cytokines and present antigen to helper T cells. The uptake of antigen is mainly mediated by endocytosis; thus, antigens are often presented by MHC-II molecules. However, it is unclear if B cells can present these same antigens via MHC-I molecules. Recently, Salmonella bacteria were found to infect B cells, allowing possible antigen cross-processing that could generate bacterial peptides for antigen presentation via MHC-I molecules. Here, we will discuss available knowledge regarding Salmonella antigen presentation by infected B cell MHC-I molecules and subsequent inhibitory effects on CD8(+) T cells for bacterial evasion of cell-mediated immunity.

Cholinergic epithelial cell with chemosensory traits in murine thymic medulla

Specialized epithelial cells with a tuft of apical microvilli (“brush cells”) sense luminal content and initiate protective reflexes in response to potentially harmful substances. They utilize the canonical taste transduction cascade to detect “bitter” substances such as bacterial quorum-sensing molecules. In the respiratory tract, most of these cells are cholinergic and are approached by cholinoceptive sensory nerve fibers. Utilizing two different reporter mouse strains for the expression of choline acetyltransferase (ChAT), we observed intense labeling of a subset of thymic medullary cells. ChAT expression was confirmed by in situ hybridization. These cells showed expression of villin, a brush cell marker protein, and ultrastructurally exhibited lateral microvilli. They did not express neuroendocrine (chromogranin A, PGP9.5) or thymocyte (CD3) markers but rather thymic epithelial (CK8, CK18) markers and were immunoreactive for components of the taste transduction cascade such as Gα-gustducin, transient receptor potential melastatin-like subtype 5 channel (TRPM5), and phospholipase C_β2. Reverse transcription and polymerase chain reaction confirmed the expression of Gα-gustducin, TRPM5, and phospholipase C_β2. Thymic “cholinergic chemosensory cells” were often in direct contact with medullary epithelial cells expressing the nicotinic acetylcholine receptor subunit α3. These cells have recently been identified as terminally differentiated epithelial cells (Hassall’s corpuscle-like structures in mice). Contacts with nerve fibers (identified by PGP9.5 and CGRP antibodies), however, were not observed. Our data identify, in the thymus, a previously unrecognized presumptive chemosensitive cell that probably utilizes acetylcholine for paracrine signaling. This cell might participate in intrathymic infection-sensing mechanisms.

Resolving Salmonella infection reveals dynamic and persisting changes in murine bone marrow progenitor cell phenotype and function

The generation of immune cells from BM precursors is a carefully regulated process. This is essential to limit the potential for oncogenesis and autoimmunity yet protect against infection. How infection modulates this is unclear. Salmonella can colonize systemic sites including the BM and spleen. This resolving infection has multiple IFN-γ-mediated acute and chronic effects on BM progenitors, and during the first week of infection IFN-γ is produced by myeloid, NK, NKT, CD4(+) T cells, and some lineage-negative cells. After infection, the phenotype of BM progenitors rapidly but reversibly alters, with a peak ∼ 30-fold increase in Sca-1(hi) progenitors and a corresponding loss of Sca-1(lo/int) subsets. Most strikingly, the capacity of donor Sca-1(hi) cells to reconstitute an irradiated host is reduced; the longer donor mice are exposed to infection, and Sca-1(hi) c-kit(int) cells have an increased potential to generate B1a-like cells. Thus, Salmonella can have a prolonged influence on BM progenitor functionality not directly related to bacterial persistence. These results reflect changes observed in leucopoiesis during aging and suggest that BM functionality can be modulated by life-long, periodic exposure to infection. Better understanding of this process could offer novel therapeutic opportunities to modulate BM functionality and promote healthy aging.

c-Jun NH2 -terminal kinase is a critical node in the death of CD4+ CD8+ thymocytes during Salmonella enterica serovar Typhimurium infection

Thymic atrophy, due to the depletion of CD4(+) CD8(+) thymocytes, is observed during infections with numerous pathogens. Several mechanisms, such as glucocorticoids and inflammatory cytokines, are known to be involved in this process; however, the roles of intracellular signaling molecules have not been investigated. In this study, the functional role of c-Jun NH2 -terminal kinase (JNK) during infection-induced thymic atrophy was addressed. The levels of phosphorylated JNK in immature CD4(+) CD8(+) thymocytes from C57BL/6 (Nramp-deficient) and 129/SvJ (Nramp-sufficient) mice were increased upon oral infection of mice with Salmonella enterica serovar Typhimurium (S. typhimurium). Furthermore, inhibition of JNK signaling, but not ERK or p38 MAPK, prevented the in vitro death of infected thymocytes. Importantly, the in vivo inhibition of JNK signaling with SP600125 protected C57BL/6 CD4(+) CD8(+) thymocytes from depletion via multiple mechanisms as follows: lower intracellular ROS, inflammatory cytokines, Bax and caspase 3 activity, increase in Bcl-xL amounts, and prevention of the loss in mitochondrial membrane potential. Notably, thymic architecture was preserved in infected mice treated with SP600125. Overall, this study identifies a novel role for JNK as a crucial regulator of the death of CD4(+) CD8(+) thymocytes during S. typhimurium infection.

Lineage tracing and cell ablation identify a post-Aire-expressing thymic epithelial cell population

Thymic epithelial cells in the medulla (mTECs) play a critical role in enforcing central tolerance through expression and presentation of tissue-specific antigens (TSAs) and deletion of autoreactive thymocytes. TSA expression requires autoimmune regulator (Aire), a transcriptional activator present in a subset of mTECs characterized by high CD80 and major histocompatibility complex II expression and a lack of potential for differentiation or proliferation. Here, using an Aire-DTR transgenic line, we show that short-term ablation specifically targets Aire(+) mTECs, which quickly undergo RANK-dependent recovery. Repeated ablation also affects Aire(-) mTECs, and using an inducible Aire-Cre fate-mapping system, we find that this results from the loss of a subset of mTECs that showed prior expression of Aire, maintains intermediate TSA expression, and preferentially migrates toward the center of the medulla. These results clearly identify a distinct stage of mTEC development and underscore the diversity of mTECs that play a key role in maintaining tolerance.

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.

CD248 expression on mesenchymal stromal cells is required for post-natal and infection-dependent thymus remodelling and regeneration

The role of mesenchymal stromal cells (MSCs) in regulating immune responses in the thymus is currently unclear. Here we report the existence and role of a MSC population in the thymus that expresses the pericyte and MSC marker CD248 (endosialin). We show using a CD248-deficient mouse model, that CD248 expression on these cells is required for full post-natal thymus development and regeneration post-Salmonella infection. In CD248^−/− mice the thymus is hypocellular and regeneration is poorer, with significant loss of all thymocyte populations. This identifies the requirement of CD248 to maintain optimal thymic cellularity post-partum and infection.