Epithelial framework reorganization during human thymus involution

Morphometric Analysis of the Thymic Epithelial Cell (TEC) Network Using Integrated and Orthogonal Digital Pathology Approaches

The thymus, a central primary lymphoid organ of the immune system, plays a key role in T cell development. Surprisingly, the thymus is quite neglected with regards to standardized pathology approaches and practices for assessing structure and function. Most studies use multispectral flow cytometry to define the dynamic composition of the thymus at the cell population level, but they are limited by lack of contextual insight. This knowledge gap hinders our understanding of various thymic conditions and pathologies, particularly how they affect thymic architecture, and subsequently, immune competence. Here, we introduce a digital pathology pipeline to address these challenges. Our approach can be coupled to analytical algorithms and utilizes rationalized morphometric assessments of thymic tissue, ranging from tissue-wide down to microanatomical and ultrastructural levels. This pipeline enables the quantitative assessment of putative changes and adaptations of thymic structure to stimuli, offering valuable insights into the pathophysiology of thymic disorders. This versatile pipeline can be applied to a wide range of conditions that may directly or indirectly affect thymic structure, ranging from various cytotoxic stimuli inducing acute thymic involution to autoimmune diseases, such as myasthenia gravis. Here, we demonstrate applicability of the method in a mouse model of age-dependent thymic involution, both by confirming established knowledge, and by providing novel insights on intrathymic remodeling in the aged thymus. Our orthogonal pipeline, with its high versatility and depth of analysis, promises to be a valuable and practical toolset for both basic and translational immunology laboratories investigating thymic function and disease.

A Proposed Link Between Acute Thymic Involution and Late Adverse Effects of Chemotherapy

Epidemiologic data suggest that cancer survivors tend to develop a protuberant number of adverse late effects, including second primary malignancies (SPM), as a result of cytotoxic chemotherapy. Besides the genotoxic potential of these drugs that directly inflict mutational burden on genomic DNA, the precise mechanisms contributing to SPM development are poorly understood. Cancer is nowadays perceived as a complex process that goes beyond the concept of genetic disease and includes tumor cell interactions with complex stromal and immune cell microenvironments. The cancer immunoediting theory offers an explanation for the development of nascent neoplastic cells. Briefly, the theory suggests that newly emerging tumor cells are mostly eliminated by an effective tissue immunosurveillance, but certain tumor variants may occasionally escape innate and adaptive mechanisms of immunological destruction, entering an equilibrium phase, where immunologic tumor cell death "equals" new tumor cell birth. Subsequent microenvironmental pressures and accumulation of helpful mutations in certain variants may lead to escape from the equilibrium phase, and eventually cause an overt neoplasm. Cancer immunoediting functions as a dedicated sentinel under the auspice of a highly competent immune system. This perspective offers the fresh insight that chemotherapy-induced thymic involution, which is characterized by the extensive obliteration of the sensitive thymic epithelial cell (TEC) compartment, can cause long-term defects in thymopoiesis and in establishment of diverse T cell receptor repertoires and peripheral T cell pools of cancer survivors. Such delayed recovery of T cell adaptive immunity may result in prolonged hijacking of the cancer immunoediting mechanisms, and lead to development of persistent and mortal infections, inflammatory disorders, organ-specific autoimmunity lesions, and SPMs. Acknowledging that chemotherapy-induced thymic involution is a potential risk factor for the emergence of SPM demarcates new avenues for the rationalized development of pharmacologic interventions to promote thymic regeneration in patients receiving cytoreductive chemotherapies.

Transgenic Exosomes for Thymus Regeneration

During senescence, Wnt4 expression is down-regulated (unlike their Frizzled receptors), while PPARgamma expression increases in the thymus. Together, these changes allow for thymic degeneration to occur, observed as adipose involution. However, when restored, Wnt4 can efficiently counteract PPARgamma and prevent thymic senescence from developing. The Wnt-pathway activator miR27b has also been reported to inhibit PPARgamma. Our goal was to evaluate the Wnt4 and miR27b levels of Wnt4-transgenic thymic epithelial cell (TEC)-derived exosomes, show their regenerative potential against age-related thymic degeneration, and visualize their binding and distribution both in vitro and in vivo. First, transgenic exosomes were harvested from Wnt4 over-expressing TECs and analyzed by transmission electron microscopy. This unveiled exosomes ranging from 50 to 100 nm in size. Exosomal Wnt4 protein content was assayed by ELISA, while miR27b levels were measured by TaqMan qPCR, both showing elevated levels in transgenic exosomes relative to controls. Of note, kit-purified TEI (total exosome isolate) outperformed UC (ultracentrifugation)-purified exosomes in these parameters. In addition, a significant portion of exosomal Wnt4 proved to be displayed on exosomal surfaces. For functional studies, steroid (Dexamethasone or DX)-induced TECs were used as cellular aging models in which DX-triggered cellular aging was efficiently prevented by transgenic exosomes. Finally, DiI lipid-stained exosomes were applied on the mouse thymus sections and also iv-injected into mice, for in vitro binding and in vivo tracking, respectively. We have observed distinct staining patterns using DiI lipid-stained transgenic exosomes on sections of young and aging murine thymus samples. Moreover, in vivo injected DiI lipid-stained transgenic exosomes showed detectable homing to the thymus. Of note, Wnt4-transgenic exosome homing outperformed control (Wnt5a-transgenic) exosome homing. In summary, our findings indicate that exosomal Wnt4 and miR27b can efficiently counteract thymic adipose involution. Although extrapolation of mouse results to the human setting needs caution, our results appoint transgenic TEC exosomes as promising tools of immune rejuvenation and contribute to the characterization of the immune-modulatory effects of extracellular vesicles in the context of regenerative medicine.

Transient mitochondrial DNA double strand breaks in mice cause accelerated aging phenotypes in a ROS-dependent but p53/p21-independent manner

We observed that the transient induction of mtDNA double strand breaks (DSBs) in cultured cells led to activation of cell cycle arrest proteins (p21/p53 pathway) and decreased cell growth, mediated through reactive oxygen species (ROS). To investigate this process in vivo we developed a mouse model where we could transiently induce mtDNA DSBs ubiquitously. This transient mtDNA damage in mice caused an accelerated aging phenotype, preferentially affecting proliferating tissues. One of the earliest phenotypes was accelerated thymus shrinkage by apoptosis and differentiation into adipose tissue, mimicking age-related thymic involution. This phenotype was accompanied by increased ROS and activation of cell cycle arrest proteins. Treatment with antioxidants improved the phenotype but the knocking out of p21 or p53 did not. Our results demonstrate that transient mtDNA DSBs can accelerate aging of certain tissues by increasing ROS. Surprisingly, this mtDNA DSB-associated senescence phenotype does not require p21/p53, even if this pathway is activated in the process.

Nitroxidergic modulation of behavioural, cardiovascular and immune responses, and brain NADPH diaphorase activity upon morphine tolerance/dependence in rats

Opioid and non-opioid effects of acute and chronic morphine administration on behaviour, cardiovascular responses, cell proliferation and apoptosis and nitric-oxide synthase (NOS) activity were studied in rats. A novel score-point scale was introduced to quantify the signs of opioid withdrawal syndrome. NOS inhibitor L-NAME (N^G-nitro-L-arginine methyl ester) was applied to reveal the role of NOS/NO pathway in the modulation of morphine-induced in vivo and in vitro responses. The obtained data showed that chronic co-administration of L-NAME drastically attenuated naloxone-precipitated withdrawal syndrome and prevented the development of morphine tolerance to cardiovascular action of morphine. The apoptotic process was very much restricted by L-NAME supplementation of chronic morphine treatment, which resulted in few apoptotic cells, less low molecular weight genomic DNA and preservation of high molecular weight non-fragmented genomic DNA. The study provides new data for nitroxidergic modulation of opioid tolerance and dependence.

Wnt4 and LAP2alpha as pacemakers of thymic epithelial senescence

Age-associated thymic involution has considerable physiological impact by inhibiting de novo T-cell selection. This impaired T-cell production leads to weakened immune responses. Yet the molecular mechanisms of thymic stromal adipose involution are not clear. Age-related alterations also occur in the murine thymus providing an excellent model system. In the present work structural and molecular changes of the murine thymic stroma were investigated during aging. We show that thymic epithelial senescence correlates with significant destruction of epithelial network followed by adipose involution. We also show in purified thymic epithelial cells the age-related down-regulation of Wnt4 (and subsequently FoxN1), and the prominent increase in LAP2α expression. These senescence-related changes of gene expression are strikingly similar to those observed during mesenchymal to pre-adipocyte differentiation of fibroblast cells suggesting similar molecular background in epithelial cells. For molecular level proof-of-principle stable LAP2α and Wnt4-over-expressing thymic epithelial cell lines were established. LAP2α over-expression provoked a surge of PPARγ expression, a transcription factor expressed in pre-adipocytes. In contrast, additional Wnt4 decreased the mRNA level of ADRP, a target gene of PPARγ. Murine embryonic thymic lobes have also been transfected with LAP2α- or Wnt4-encoding lentiviral vectors. As expected LAP2α over-expression increased, while additional Wnt4 secretion suppressed PPARγ expression. Based on these pioneer experiments we propose that decreased Wnt activity and increased LAP2α expression provide the molecular basis during thymic senescence. We suggest that these molecular changes trigger thymic epithelial senescence accompanied by adipose involution. This process may either occur directly where epithelium can trans-differentiate into pre-adipocytes; or indirectly where first epithelial to mesenchymal transition (EMT) occurs followed by subsequent pre-adipocyte differentiation. The latter version fits better with literature data and is supported by the observed histological and molecular level changes.

Magnetic resonance imaging of radiation-induced thymic atrophy as a model for pathologic changes in acute feline immunodeficiency virus infection

The development of a protocol to reproducibly induce thymic atrophy, as occurs in feline immunodeficiency virus (FIV) infection and other immunosuppressive diseases, and to consistently estimate thymic volume, provides a valuable tool in the search of innovative and novel therapeutic strategies. Magnetic resonance imaging (MRI) using the short tau inversion recovery (STIR) technique, with fat suppression properties, was determined to provide an optimized means of locating, defining, and quantitatively estimating thymus volume in young cats. Thymic atrophy was induced in four, 8-10-week-old kittens with a single, directed 500 cGy dose of 6 MV X-rays from a clinical linear accelerator, and sequential MR images of the cranial mediastinum were collected at 2, 7, 14, and 21 days post irradiation (PI). Irradiation induced a severe reduction in thymic volume, which was decreased, on average, to 47% that of normal, by 7 days PI. Histopathology confirmed marked, diffuse thymic atrophy, characterized by reduced thymic volume, decreased overall cellularity, increased apoptosis, histiocytosis, and reduced distinction of the corticomedullary junction, comparable to that seen in acute FIV infection. Beginning on day 7 PI, thymic volumes rebounded slightly and continued to increase over the following 14 days, regaining 3-35% of original volume. These findings demonstrate the feasibility and advantages of using this non-invasive, in vivo imaging technique to measure and evaluate changes in thymic volume in physiologic and experimental situations. All experimental protocols in this study were approved by the Institutional Animal Care and Use Committee (IACUC) at Auburn University.

Aged human thymus hassall's corpuscles are immunoreactive for IGF-I and IGF-I receptor

Although Hassall's corpuscles have been proposed to act in both maturation of developing thymocytes and removal of apoptotic cells, their function remains an enigma. The involvement of insulin-like growth factor I (IGF-I) in the local autocrine and paracrine control of T-cell development in human thymus is still unclear. In this study, we investigated the structure and distribution of IGF-I and IGF-I receptor (IGF-IR)-immunopositive Hassall's corpuscles in aged human thymus using bright-field immunohistochemistry and immunoelectron microscopy. We report new immunocytochemical data for the presence of IGF-I/IGF-IR double-immunopositive Hassall's corpuscles in structurally preserved regions of age-involuted thymus and discuss the involvement of these unique thymic components in the local regulation of T-cell development and thymus plasticity during aging by IGF-I/IGF-IR-mediated cell signaling pathway.

Thymopoiesis in elderly human is associated with systemic inflammatory status

Immunosenescence studies of age-related immune system damage focused on clinical lymphopenic situations or androgenic blockade have revealed new insights about adult human immune reconstitution. However, as far as we know, the extent of lymphopoiesis in the thymus of elderly humans remains unclear. To this effect, we have analyzed 65 adult human thymuses (from 36 to 81 years; median age 68.6 years) obtained from patients who underwent cardiac surgery. Our results show a correlation between CD4(+)CD8(+) double-positive (DP) cells and both the age (inverse) and percentage (direct) of peripheral naive T cells, indicating that the thymus is still able to affect the peripheral lymphocyte pool even in the elderly. We also found significant correlation between the degree of thymopoiesis and the inflammation markers, as shown by the inverse correlations between DP and the percentage of neutrophils and IL-6 levels and the percentage of peripheral lymphocytes. Furthermore, in a multivariate linear regression the percentage of DP and IL-7 levels, but not age, were independently associated with the percentage of neutrophils. In conclusion, the thymus maintains, even in the elderly, an active thymopoiesis that rejuvenates the peripheral naive T-cell pool. Moreover, age-related thymopoietic decay is associated with the peripheral inflammation markers.

Characteristics of age-related changes in rat thymus: morphometric analysis and epithelial cell network in various thymic compartments

Structural alterations in thymuses of female rats during the first 2 years of life were evaluated by morphometric analysis and, then, correlated with organization of epithelial cells in various thymic compartments, examined for their cytokeratin immunoreactivity. With an advancing age, the thymuses demonstrated morphological modifications related to maturation and senescence, the dynamics of which varied between particular thymic compartments, and involved subpopulations of thymic epithelial cells. In the entire period of life the most dynamic changes were found in the cortex while the medulla was demonstrated to be a rather "stable" region. Morphometric studies revealed a negative correlation between the volume of thymic cortex and medulla and age of rats and a linear, positive relationship between the volume of connective tissue compartment and age. Changes in organization of epithelial network in the medulla preceded those observed in the cortex. Decreased proliferative activity of subset of medullary cells, which probably represented a self-renewable population, was accompanied by alterations in the immunocytochemically characterized (cytokeratines) differentiation process. At the same period of life, hypertrophy and hyperplasia of superficial epithelial cells seems to functionally replace medullary cells. This process begins around 3rd month of life and expands on all thymic compartments. The first changes in the cortex appeared around 8th month and were connected with reduced cytokeratin immunoreactivity. The involution observed in older animals was preceded by age-related alterations in epithelial network pattern which, in the course of stable morphometric parameters (between 5th and 12th month), showed character of a structural and functional adaptation.

Nerve growth factor immunoreactivity of mast cells in acute involuted human thymus

The acute involution of the thymus is induced by either exogenous or endogenous factors, including some infections (infection type involution). The present study was focused on both detection and immunocytochemical analysis of NGF immunopositive mast cells in child thymus with acute infection-induced involution. Autopsy thymus specimens from children with infection diseases (Sepsis, Encephalomyelitis, Varicella) were examined at light and electron microscopic level and compared to normal infantile thymuses. We observed a redistribution of NGF immunopositive mast cells in infection-affected child thymus, which lobular architecture was collapsed. A positive correlation between the degree of the involutive changes, increased distribution and enhanced NGF immunoreactivity of mast cells was defined. The possible involvement of NGF immunopositive mast cells in the process of acute thymus involution is discussed.

ABH Histo-Blood Group Antigens in Human Thymus Involution

BACKGROUND

Human histo-blood group antigens (HBGA) are genetically determined glycoproteins supposed to participate in cell differentiation, adhesion, cancer metastasis and angiogenesis. In tissues, HBGA are mostly expressed in epithelial cells (EC). The EC comprising the thymocyte microenvironment play an important role in the ontogeny of the thymus. The aim of the present work is to investigate ABH HBGA in senile thymus and to characterize their expression pattern related to the process of aging.

METHODS

Routine histology and immunohistochemical techniques were applied on thymus glands from senile and young individuals.

RESULTS

Involuted thymus exhibited large areas of adipose tissue containing scattered EC, all positive for HBGA. Stromal EC revealed different morphology and intrathymic localization but uniform cytoplasmic staining for ABH antigens. Endothelial cells of blood vessels and red blood cells were intensely stained for HBGA. Only single scattered lymphocytes possessed HBGA. In contrast with senile thymus, most lymphocyte populations in the gland of young individuals, as well as the Hassall's corpuscules, expressed HBGA.

CONCLUSIONS

The epithelial framework reorganization during age-related thymus involution involves modulation in ABH antigen expression in EC. These molecules are required by thymic EC to maintain the reduced but important crosstalk with lymphocytes during involution. The diminished reactivity for ABH antigens in the lymphocytes of aged thymus might reflect the impaired communication between these two cell types. We present novel evidence for permanent presence and modulation of ABH antigen reactivity in senile thymus, supporting the view that these molecules might be developmentally regulated.