Accelerated thymic atrophy as a result of elevated homeostatic expression of the genes encoded by the TNF/lymphotoxin cytokine locus

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.

Missing link between tissue specific expressing pattern of ERβ and the clinical manifestations in LGBLEL

Purpose

Lacrimal gland benign lymphoepithelial lesion (LGBLEL) is an IgG4-related disease of unknown etiology with a risk for malignant transformation. Estrogen is considered to be related to LGBLEL onset.

Methods

Seventy-eight LGBLEL and 13 control clinical samples were collected and studied to determine the relationship between estrogen and its receptors and LGBLEL development.

Results

The serological analysis revealed no significant differences in the levels of three estrogens be-tween the LGBLEL and control groups. However, immunohistochemical analyses indicated that the expression levels of ERβ and its downstream receptor RERG were relatively lower in LGBLEL samples than in control samples, with higher expression in the lacrimal gland and lower expression in the lymphocyte infiltration region. However, low expression of ERα was detected. The transcriptome sequence analysis revealed upregulated genes associated with LGBLEL enriched in lymphocyte proliferation and activation function; downregulated genes were enriched in epithelial and vascular proliferation functions. The key genes and gene networks were further analyzed. Interactions between B cells and epithelial cells were analyzed due to the identified involvement of leukocyte subsets and epithelial cells. B cell proliferation was found to potentially contribute to lacrimal gland apoptosis.

Conclusion

Therefore, the tissue-heterogeneous expression pattern of ERβ is potentially related to the clinical manifestations and progression of LGBLEL, although further investigations are required to confirm this finding.

Current Perspectives on the Role of TNF in Hematopoiesis Using Mice With Humanization of TNF/LT System

TNF is a multifunctional cytokine with its key functions attributed to inflammation, secondary lymphoid tissue organogenesis and immune regulation. However, it is also a physiological regulator of hematopoiesis and is involved in development and homeostatic maintenance of various organs and tissues. Somewhat unexpectedly, the most important practical application of TNF biology in medicine is anti-TNF therapy in several autoimmune diseases. With increased number of patients undergoing treatment with TNF inhibitors and concerns regarding possible adverse effects of systemic cytokine blockade, the interest in using humanized mouse models to study the efficacy and safety of TNF-targeting biologics in vivo is justified. This Perspective discusses the main functions of TNF and its two receptors, TNFR1 and TNFR2, in steady state, as well as in emergency hematopoiesis. It also provides a comparative overview of existing mouse lines with humanization of TNF/TNFR system. These genetically engineered mice allow us to study TNF signaling cascades in the hematopoietic compartment in the context of various experimental disease models and for evaluating the effects of various human TNF inhibitors on hematopoiesis and other physiological processes.

Thymic Hyperplasia with Lymphoepithelial Sialadenitis (LESA)-Like Features: Strong Association with Lymphomas and Non-Myasthenic Autoimmune Diseases

Thymic hyperplasia (TH) with lymphoepithelial sialadenitis (LESA)-like features (LESA-like TH) has been described as a tumor-like, benign proliferation of thymic epithelial cells and lymphoid follicles. We aimed to determine the frequency of lymphoma and autoimmunity in LESA-like TH and performed retrospective analysis of cases with LESA-like TH and/or thymic MALT-lymphoma. Among 36 patients (21 males) with LESA-like TH (age 52 years, 32-80; lesion diameter 7.0 cm, 1-14.5; median, range), five (14%) showed associated lymphomas, including four (11%) thymic MALT lymphomas and one (3%) diffuse large B-cell lymphoma. One additional case showed a clonal B-cell-receptor rearrangement without evidence of lymphoma. Twelve (33%) patients (7 women) suffered from partially overlapping autoimmune diseases: systemic lupus erythematosus (n = 4, 11%), rheumatoid arthritis (n = 3, 8%), myasthenia gravis (n = 2, 6%), asthma (n = 2, 6%), scleroderma, Sjögren syndrome, pure red cell aplasia, Grave's disease and anti-IgLON5 syndrome (each n = 1, 3%). Among 11 primary thymic MALT lymphomas, remnants of LESA-like TH were found in two cases (18%). In summary, LESA-like TH shows a striking association with autoimmunity and predisposes to lymphomas. Thus, a hematologic and rheumatologic workup should become standard in patients diagnosed with LESA-like TH. Radiologists and clinicians should be aware of LESA-like TH as a differential diagnosis for mediastinal mass lesions in patients with autoimmune diseases.

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.

Comparative analysis of thymic subpopulations during different modes of atrophy identifies the reactive oxygen species scavenger, N-acetyl cysteine, to increase the survival of thymocytes during infection-induced and lipopolysaccharide-induced thymic atrophy

The development of immunocompetent T cells entails a complex pathway of differentiation in the thymus. Thymic atrophy occurs with ageing and during conditions such as malnutrition, infections and cancer chemotherapy. The comparative changes in thymic subsets under different modes of thymic atrophy and the mechanisms involved are not well characterized. These aspects were investigated, using mice infected with Salmonella Typhimurium, injection with lipopolysaccharide (LPS), an inflammatory but non-infectious stimulus, etoposide (Eto), a drug used to treat some cancers, and dexamethasone (Dex), a steroid used in some inflammatory diseases. The effects on the major subpopulations of thymocytes based on multicolour flow cytometry studies were, first, the CD4^-  CD8^- double-negative (DN) cells, mainly DN2-4, were reduced with infection, LPS and Eto treatment, but not with Dex. Second, the CD8⁺  CD3^lo immature single-positive cells (ISPs) were highly sensitive to infection, LPS and Eto, but not Dex. Third, treatment with LPS, Eto and Dex reduced all three subpopulations of CD4⁺  CD8⁺ double-positive (DP) thymocytes, i.e. DP1, DP2 and DP3, but the DP3 subset was relatively more resistant during infection. Fourth, both CD4⁺ and CD8⁺ single-positive (SP) thymocytes were lowered by Eto and Dex, but not during infection. Notably, LPS lowered CD4⁺ SP subsets, whereas the CD8⁺ SP subsets were relatively more resistant. Interestingly, the reactive oxygen species quencher, N-acetyl cysteine, greatly improved the survival of thymocytes, especially DNs, ISPs and DPs, during infection and LPS treatment. The implications of these observations for the development of potential thymopoietic drugs are discussed.

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.

Modular transcriptional repertoire and MicroRNA target analyses characterize genomic dysregulation in the thymus of Down syndrome infants

Trisomy 21-driven transcriptional alterations in human thymus were characterized through gene coexpression network (GCN) and miRNA-target analyses. We used whole thymic tissue--obtained at heart surgery from Down syndrome (DS) and karyotipically normal subjects (CT)--and a network-based approach for GCN analysis that allows the identification of modular transcriptional repertoires (communities) and the interactions between all the system's constituents through community detection. Changes in the degree of connections observed for hierarchically important hubs/genes in CT and DS networks corresponded to community changes. Distinct communities of highly interconnected genes were topologically identified in these networks. The role of miRNAs in modulating the expression of highly connected genes in CT and DS was revealed through miRNA-target analysis. Trisomy 21 gene dysregulation in thymus may be depicted as the breakdown and altered reorganization of transcriptional modules. Leading networks acting in normal or disease states were identified. CT networks would depict the "canonical" way of thymus functioning. Conversely, DS networks represent a "non-canonical" way, i.e., thymic tissue adaptation under trisomy 21 genomic dysregulation. This adaptation is probably driven by epigenetic mechanisms acting at chromatin level and through the miRNA control of transcriptional programs involving the networks' high-hierarchy genes.

TNF superfamily members play distinct roles in shaping the thymic stromal microenvironment

The differentiation and proper function of thymic epithelial cells (TECs) depend on various tumor necrosis factor superfamily (TNFSF) signals that are needed to maintain the thymic stromal microenvironment. Nevertheless, the direct transcriptional effects of these signals on TECs remain unclear. To address this issue, we stimulated murine embryonic thymus tissue with selected TNFSF ligands and performed a gene expression profiling study. We show that Aire expression is a direct and specific effect of RANKL stimulation, whereas LTβ and TNFα are major inducers of chemokines in the thymic stroma and we propose differential NF-κB binding as one possible cause of these gene expression patterns. Our work provides further insight into the complex molecular pathways that shape the thymic microenvironment and maintain central tolerance.

Context-dependent roles for lymphotoxin-β receptor signaling in cancer development

The LTα1β2 and LIGHT TNF superfamily cytokines exert pleiotropic physiological functions through the activation of their cognate lymphotoxin-β receptor (LTβR). Interestingly, since the discovery of these proteins, accumulating evidence has pinpointed a role for LTβR signaling in carcinogenesis. Early studies have shown a potential anti-tumoral role in a subset of solid cancers either by triggering apoptosis in malignant cells or by eliciting an anti-tumor immune response. However, more recent studies provided robust evidence that LTβR signaling is also involved in diverse cell-intrinsic and microenvironment-dependent pro-oncogenic mechanisms, affecting several solid and hematological malignancies. Consequently, the usefulness of LTβR signaling axis blockade has been investigated as a potential therapeutic approach for cancer. Considering the seemingly opposite roles of LTβR signaling in diverse cancer types and their key implications for therapy, we here extensively review the different mechanisms by which LTβR activation affects carcinogenesis, focusing on the diverse contexts and different models assessed.

Lethal and nonlethal murine malarial infections differentially affect apoptosis, proliferation, and CD8 expression on thymic T cells

Although thymic atrophy and apoptosis of the double-positive (DP) T cells have been reported in murine malaria, comparative studies investigating the effect of lethal and nonlethal Plasmodium infections on the thymus are lacking. We assessed the effects of P. yoelii lethal (17XL) and nonlethal (17XNL) infections on thymic T cells. Both strains affected the thymus. 17XL infection induced DP T-cell apoptosis and a selective decrease in surface CD8 expression on developing thymocytes. By contrast, more severe but reversible effects were observed during 17XNL infection. DP T cells underwent apoptosis, and proliferation of both DN and DP cells was affected around peak parasitemia. A transient increase in surface CD8 expression on thymic T cells was also observed. Adult thymic organ culture revealed that soluble serum factors, but not IFN-γ or TNF-α, contributed to the observed effects. Thus, lethal and nonlethal malarial infections led to multiple disparate effects on thymus. These parasite-induced thymic changes are expected to impact the naïve T-cell repertoire and the subsequent control of the immune response against the parasite. Further investigations are required to elucidate the mechanism responsible for these disparate effects, especially the reversible involution of the thymus in case of nonlethal infection.

NIK promotes tissue destruction independently of the alternative NF-κB pathway through TNFR1/RIP1-induced apoptosis

NF-κB-inducing kinase (NIK) is well-known for its role in promoting p100/NF-κB2 processing into p52, a process defined as the alternative, or non-canonical, NF-κB pathway. Here we reveal an unexpected new role of NIK in TNFR1-mediated RIP1-dependent apoptosis, a consequence of TNFR1 activation observed in c-IAP1/2-depleted conditions. We show that NIK stabilization, obtained by activation of the non-death TNFRs Fn14 or LTβR, is required for TNFα-mediated apoptosis. These apoptotic stimuli trigger the depletion of c-IAP1/2, the phosphorylation of RIP1 and the RIP1 kinase-dependent assembly of the RIP1/FADD/caspase-8 complex. In the absence of NIK, the phosphorylation of RIP1 and the formation of RIP1/FADD/caspase-8 complex are compromised while c-IAP1/2 depletion is unaffected. In vitro kinase assays revealed that recombinant RIP1 is a bona fide substrate of NIK. In vivo, we demonstrated the requirement of NIK pro-death function, but not the processing of its substrate p100 into p52, in a mouse model of TNFR1/LTβR-induced thymus involution. In addition, we also highlight a role for NIK in hepatocyte apoptosis in a mouse model of virus-induced TNFR1/RIP1-dependent liver damage. We conclude that NIK not only contributes to lymphoid organogenesis, inflammation and cell survival but also to TNFR1/RIP1-dependent cell death independently of the alternative NF-κB pathway.

The thymic microenvironment differentially regulates development and trafficking of invariant NKT cell sublineages

The regulatory role of the thymic microenvironment during trafficking and differentiation of the invariant NKT (iNKT) cell lineage remains poorly understood. In this study, we show that fractalkine receptor expression marks emigrating subpopulations of the NKT1, NKT2, and NKT17 sublineages in the thymus and peripheral organs of naive mice. Moreover, NKT1 sublineage cells can be subdivided into two subsets, namely NKT1(a) and NKT1(b), which exhibit distinct developmental and tissue-specific distribution profiles. More specifically, development and trafficking of the NKT1(a) subset are selectively dependent upon lymphotoxin (LT)α1β2-LTβ receptor-dependent differentiation of thymic stroma, whereas the NKT1(b), NKT2, and NKT17 sublineages are not. Furthermore, we identify a potential cellular source for LTα1β2 during thymic organogenesis, marked by expression of IL-7Rα, which promotes differentiation of the NKT1(a) subset in a noncell-autonomous manner. Collectively, we propose a mechanism by which thymic differentiation and retention of the NKT1 sublineage are developmentally coupled to LTα1β2-LTβ receptor-dependent thymic organogenesis.

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.

Reduced regulatory T cell diversity in NOD mice is linked to early events in the thymus

The thymic natural regulatory T cell (Treg) compartment of NOD mice is unusual in having reduced TCR diversity despite normal cellularity. In this study, we show that this phenotype is attributable to perturbations in early and late stages of thymocyte development and is controlled, at least in part, by the NOD Idd9 region on chromosome 4. Progression from double negative 1 to double negative 2 stage thymocytes in NOD mice is inefficient; however, this defect is compensated by increased proliferation of natural Tregs (nTregs) within the single positive CD4 thymocyte compartment, accounting for recovery of cellularity accompanied by loss of TCR diversity. This region also underlies the known attenuation of ERK-MAPK signaling, which may preferentially disadvantage nTreg selection. Interestingly, the same genetic region also regulates the rate of thymic involution that is accelerated in NOD mice. These findings highlight further complexity in the control of nTreg repertoire diversity.

Severe thymic atrophy in a mouse model of skin inflammation accounts for impaired TNFR1 signaling

Transgenic mice expressing the caspase-cleaved form of the tyrosine kinase Lyn (LynΔN) develop a TNFα-dependent skin disease that accurately recapitulates human psoriasis. Participation of lymphocytes in this disease was confirmed by backcrossing LynΔN mice on a Rag-1 deficient background. The present study was therefore conducted to analyze whether modification of lymphocyte homeostasis does occur and participate in the phenotype of LynΔN mice. We show here that LynΔN mice consistently exhibit thymic atrophy that correlates with both a net decrease in the CD4+/CD8+ Double Positive (DP) and an increase in Single Positive (SP) thymocyte sub-populations, but also display an increase of splenic mature B cell. Interestingly, a normal immune phenotype was rescued in a TNFR1 deficient background. Finally, none of these immune alterations was detected in newborn mice before the onset of inflammation. Therefore, we conclude that chronic inflammation can induce thymic atrophy and perturb spleen homeostasis in LynΔN mice through the increased production of TNFα, LTß and TNFR1 signaling.

Deficiency of lymphotoxin-α does not exacerbate high-fat diet-induced obesity but does enhance inflammation in mice

Lymphotoxin-α (LTα) is secreted by lymphocytes and acts through tumor necrosis factor-α receptors and the LTβ receptor. Our goals were to determine whether LT has a role in obesity and investigate whether LT contributes to the link between obesity and adipose tissue lymphocyte accumulation. LT deficient (LT(-/-)) and wild-type (WT) mice were fed standard pelleted rodent chow or a high-fat/high-sucrose diet (HFHS) for 13 wk. Body weight, body composition, and food intake were measured. Glucose tolerance was assessed. Systemic and adipose tissue inflammatory statuses were evaluated by quantifying plasma adipokine levels and tissue macrophage and T cell-specific gene expression in abdominal fat. LT(-/-) mice were smaller (20%) and leaner (25%) than WT controls after 13 wk of HFHS diet feeding. LT(-/-) mice showed improved glucose tolerance, suggesting that, in WT mice, LT may impair glucose metabolism. Surprisingly, adipose tissue from rodent chow- and HFHS-fed LT(-/-) mice exhibited increased T lymphocyte and macrophage infiltration compared with WT mice. Despite the fact that LT(-/-) mice exhibited an enhanced inflammatory status at the systemic and tissue level even when fed rodent chow, they were protected from enhanced diet-induced obesity and insulin resistance. Thus, LT contributes to body weight and adiposity and is required to modulate the accumulation of immune cells in adipose tissue.

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.

Characterization of the expression of cytokeratins 5, 8, and 14 in mouse thymic epithelial cells during thymus regeneration following acute thymic involution

The thymus is a central lymphoid organ for T cell development. Thymic epithelial cells (TECs) constitute a major component of the thymic stroma, which provides a specialized microenvironment for survival, proliferation, and differentiation of immature T cells. In this study, subsets of TECs were examined immunohistochemically to investigate their cytokeratin (CK) expression patterns during thymus regeneration following thymic involution induced by cyclophosphamide treatment. The results demonstrated that both normal and regenerating mouse thymuses showed a similar CK expression pattern. The major medullary TECs (mTEC) subset, which is stellate in appearance, exhibited CK5 and CK14 staining, and the minor mTEC subset, which is globular in appearance, exhibited CK8 staining, whereas the vast majority of cortical TECs (cTECs) expressed CK8 during thymus regeneration. Remarkably, the levels of CK5 and CK14 expression were enhanced in mTECs, and CK8 expression was upregulated in cTECs during mouse thymus regeneration after cyclophosphamide-induced acute thymic involution. Of special interest, a relatively high number of CK5⁺CK8⁺ TEC progenitors occurred in the thymic cortex during thymus regeneration. Taken together, these findings shed more light on the role of CK5, CK8, and CK14 in the physiology of TECs during mouse thymus regeneration, and on the characterization of TEC progenitors for restoration of the epithelial network and for concomitant regeneration of the adult thymus.

Acute endotoxin-induced thymic atrophy is characterized by intrathymic inflammatory and wound healing responses

Background

Productive thymopoiesis is essential for a robust and healthy immune system. Thymus unfortunately is acutely sensitive to stress resulting in involution and decreased T cell production. Thymic involution is a complication of many clinical settings, including infection, malnutrition, starvation, and irradiation or immunosuppressive therapies. Systemic rises in glucocorticoids and inflammatory cytokines are known to contribute to thymic atrophy. Little is known, however, about intrathymic mechanisms that may actively contribute to thymus atrophy or initiate thymic recovery following stress events.

Methodology/Principal Findings

Phenotypic, histologic and transcriptome/pathway analysis of murine thymic tissue during the early stages of endotoxemia-induced thymic involution was performed to identify putative mechanisms that drive thymic involution during stress. Thymus atrophy in this murine model was confirmed by down-regulation of genes involved in T cell development, cell activation, and cell cycle progression, correlating with observed phenotypic and histologic thymus involution. Significant gene changes support the hypothesis that multiple key intrathymic pathways are differentially activated during stress-induced thymic involution. These included direct activation of thymus tissue by LPS through TLR signaling, local expression of inflammatory cytokines, inhibition of T cell signaling, and induction of wound healing/tissue remodeling.

Conclusions/Significance

Taken together, these observations demonstrated that in addition to the classic systemic response, a direct intrathymic response to endotoxin challenge concurrently contributes to thymic involution during endotoxemia. These findings are a substantial advancement over current understanding of thymus response to stress and may lead to the development of novel therapeutic approaches to ameliorate immune deficiency associated with stress events.

ELPylated anti-human TNF therapeutic single-domain antibodies for prevention of lethal septic shock

Tumour necrosis factor (TNF) is a major pro-inflammatory cytokine involved in multiple inflammatory diseases. The detrimental activity of TNF can be blocked by various antagonists, and commercial therapeutics based upon this principle have been approved for treatment of diseases including rheumatoid arthritis, Crohn's disease and psoriasis. In a search for new, improved anti-inflammatory therapeutics we have designed a single-domain monoclonal antibody (V(H) H), which recognizes TNF. The antibody component (TNF-V(H) H) is based upon an anti-human TNF Camelidae heavy-chain monoclonal antibody, which was linked to an elastin-like polypeptide (ELP). We demonstrate that ELP fusion to the TNF-V(H) H enhances accumulation of the fusion protein during biomanufacturing in transgenic tobacco plants. With this study, we show for the first time that this plant-derived anti-human TNF-V(H) H antibody was biologically active in vivo. Therefore, therapeutic application of TNF-V(H) H-ELP fusion protein was tested in humanized TNF mice and was shown to be effective in preventing death caused by septic shock. The in vivo persistence of the ELPylated antibody was ∼24 fold longer than that of non-ELPylated TNF-V(H) H.

Maturation-dependent licensing of naive T cells for rapid TNF production

The peripheral naïve T cell pool is comprised of a heterogeneous population of cells at various stages of development, which is a process that begins in the thymus and is completed after a post-thymic maturation phase in the periphery. One hallmark of naïve T cells in secondary lymphoid organs is their unique ability to produce TNF rapidly after activation and prior to acquiring other effector functions. To determine how maturation influences the licensing of naïve T cells to produce TNF, we compared cytokine profiles of CD4⁺ and CD8⁺ single positive (SP) thymocytes, recent thymic emigrants (RTEs) and mature-naïve (MN) T cells during TCR activation. SP thymocytes exhibited a poor ability to produce TNF when compared to splenic T cells despite expressing similar TCR levels and possessing comparable activation kinetics (upregulation of CD25 and CD69). Provision of optimal antigen presenting cells from the spleen did not fully enable SP thymocytes to produce TNF, suggesting an intrinsic defect in their ability to produce TNF efficiently. Using a thymocyte adoptive transfer model, we demonstrate that the ability of T cells to produce TNF increases progressively with time in the periphery as a function of their maturation state. RTEs that were identified in NG-BAC transgenic mice by the expression of GFP showed a significantly enhanced ability to express TNF relative to SP thymocytes but not to the extent of fully MN T cells. Together, these findings suggest that TNF expression by naïve T cells is regulated via a gradual licensing process that requires functional maturation in peripheral lymphoid organs.