The thymus road to a T cell: migration, selection, and atrophy.

The thymus plays a pivotal role in generating a highly-diverse repertoire of T lymphocytes while preventing autoimmunity. Thymus seeding progenitors (TSPs) are a heterogeneous group of multipotent progenitors that migrate to the thymus via CCR7 and CCR9 receptors. While NOTCH guides thymus progenitors toward T cell fate, the absence or disruption of NOTCH signaling renders the thymus microenvironment permissive to other cell fates. Following T cell commitment, developing T cells undergo multiple selection checkpoints by engaging with the extracellular matrix, and interacting with thymic epithelial cells (TECs) and other immune subsets across the different compartments of the thymus. The different selection checkpoints assess the T cell receptor (TCR) performance, with failure resulting in either repurposing (agonist selection), or cell death. Additionally, environmental cues such as inflammation and endocrine signaling induce acute thymus atrophy, contributing to the demise of most developing T cells during thymic selection. We discuss the occurrence of acute thymus atrophy in response to systemic inflammation. The thymus demonstrates high plasticity, shaping inflammation by abrogating T cell development and undergoing profound structural changes, and facilitating regeneration and restoration of T cell development once inflammation is resolved. Despite the challenges, thymic selection ensures a highly diverse T cell repertoire capable of discerning between self and non-self antigens, ultimately egressing to secondary lymphoid organs where they complete their maturation and exert their functions.

Thymus in Cardiometabolic Impairments and Atherosclerosis: Not a Silent Player?

The thymus represents a primary organ of the immune system, harboring the generation and maturation of T lymphocytes. Starting from childhood, the thymus undergoes involution, being replaced with adipose tissue, and by an advanced age nearly all the thymus parenchyma is represented by adipocytes. This decline of thymic function is associated with compromised maturation and selection of T lymphocytes, which may directly impact the development of inflammation and induce various autoinflammatory disorders, including atherosclerosis. For a long time, thymus health in adults has been ignored. The process of adipogenesis in thymus and impact of thymic fat on cardiometabolism remains a mysterious process, with many issues being still unresolved. Meanwhile, thymus functional activity has a potential to be regulated, since islets of thymopoeisis remain in adults even at an advanced age. The present review describes the intricate process of thymic adipose involution, focusing on the issues of the thymus' role in the development of atherosclerosis and metabolic health, tightly interconnected with the state of vessels. We also review the recent information on the key molecular pathways and biologically active substances that may be targeted to manipulate both thymic function and atherosclerosis.

TL1A and IL-18 synergy promotes GM-CSF-dependent thymic granulopoiesis in mice.

Acute systemic inflammation critically alters the function of the immune system, often promoting myelopoiesis at the expense of lymphopoiesis. In the thymus, systemic inflammation results in acute thymic atrophy and, consequently, impaired T-lymphopoiesis. The mechanism by which systemic inflammation impacts the thymus beyond suppressing T-cell development is still unclear. Here, we describe how the synergism between TL1A and IL-18 suppresses T-lymphopoiesis to promote thymic myelopoiesis. The protein levels of these two cytokines were elevated in the thymus during viral-induced thymus atrophy infection with murine cytomegalovirus (MCMV) or pneumonia virus of mice (PVM). In vivo administration of TL1A and IL-18 induced acute thymic atrophy, while thymic neutrophils expanded. Fate mapping with Ms4a3-Cre mice demonstrated that thymic neutrophils emerge from thymic granulocyte-monocyte progenitors (GMPs), while Rag1-Cre fate mapping revealed a common developmental path with lymphocytes. These effects could be modeled ex vivo using neonatal thymic organ cultures (NTOCs), where TL1A and IL-18 synergistically enhanced neutrophil production and egress. NOTCH blockade by the LY411575 inhibitor increased the number of neutrophils in the culture, indicating that NOTCH restricted steady-state thymic granulopoiesis. To promote myelopoiesis, TL1A, and IL-18 synergistically increased GM-CSF levels in the NTOC, which was mainly produced by thymic ILC1s. In support, TL1A- and IL-18-induced granulopoiesis was completely prevented in NTOCs derived from Csf2rb-/- mice and by GM-CSFR antibody blockade, revealing that GM-CSF is the essential factor driving thymic granulopoiesis. Taken together, our findings reveal that TL1A and IL-18 synergism induce acute thymus atrophy while  promoting extramedullary thymic granulopoiesis in a NOTCH and GM-CSF-controlled manner.

Network meta-analysis of the molecular mechanisms and signaling pathways underlying alcohol-induced thymic atrophy.

BACKGROUND: Thymic atrophy is characterized by loss of thymocytes, destruction of thymic architecture, and a subsequent decrease in naïve T cells with compromised immunity. Thymic atrophy occurs during aging. Environmental factors including alcohol misuse also induce thymic atrophy. Despite the link between alcohol misuse and thymic atrophy, the underlying mechanism is understudied. We aimed to identify molecules and signaling pathways that underly alcohol-induced thymic atrophy during aging. METHODS: F344 rats were given 3-day binge-ethanol (4.8 g/kg/day; 52% w/v; i.g.) and the thymus was collected and weighed. Molecular mechanisms underlying ethanol-induced thymic atrophy were investigated by network meta-analysis using the QIAGEN Ingenuity Pathway Analysis (IPA). The molecules associated with ethanol were identified from the QIAGEN Knowledge Base (QKB) and those associated with thymic atrophy were identified from QKB and Mouse Genome Informatics (MGI). Aging-mediated Differential Expression Genes (DEGs) from mouse thymocytes were obtained from the Gene Expression Omnibus (GEO) database (GSE132136). The relationship between the molecules and associated signaling pathways were studied using IPA. RESULTS: Binge-ethanol decreased thymic weight in F344 rats. Our meta-analysis using IPA identified molecules commonly shared by ethanol and thymic atrophy through which simulation with ethanol increased thymic atrophy. We then obtained aging-mediated DEGs from the atrophied thymocytes. We found that ethanol contributed to thymic atrophy through modulation of the aging-mediated DEGs. Our network meta-analysis suggests that ethanol may augment thymic atrophy through increased expression of cytokines (e.g., IL-6, IL-17A and IL-33) along with their regulators (e.g., STAT1 and STAT3). CONCLUSIONS: Exposure to alcohol may augment thymic atrophy by altering the activity of key inflammatory mediators, such as STAT family members and inflammatory cytokines. These findings provide insights into the signaling pathways and upstream regulators that underly alcohol-induced thymic atrophy during aging, suggesting that alcohol consumption could prepone thymic atrophy.

Studies on the immune status of calves with chronic inflammation and thymus atrophy.

The thymus is a primary lymphoid organ where the primary T cell repertoire is generated. Thymus atrophy is induced by various conditions, including infectious diseases, glucocorticoid treatment, and poor breeding management. Cattle with thymus atrophy tend to exhibit weak calf syndrome, a condition in which approximately half of neonates die shortly after birth. Calves with thymus atrophy that survive the first month typically contract chronic inflammatory diseases. In this study, we analyzed the populations of the peripheral blood mononuclear cells and thymocytes in calves with thymus atrophy. In addition, we evaluated polarization of master gene and cytokine mRNA expression in peripheral blood CD4+ cells in the calves. The population of CD4+CD8+ cells in thymus of the calves with thymus atrophy was lower than that of control calves. IL10 mRNA expression in peripheral blood CD4+ cells of calves with thymus atrophy was significantly lower than that of control calves. TBX21 mRNA expression in peripheral CD4+ cells of thymus atrophy calves was tended to be higher than that of the control group. In addition, FOXP3 mRNA expression in peripheral CD4+ cells of the thymus atrophy calves was tended to be lower than that of the control calves. Thymus atrophy calves exhibited chronic inflammatory disease leading, in severe situations, to conditions such as pneumonia with caseous necrosis. These severe inflammatory responses likely are due to decreases in IL10 mRNA expression, impairing control of macrophages, one of the main cell fractions of natural immunity.

Thymic changes due to leishmaniasis in dogs: An immunohistochemical study.

BACKGROUND: The thymus is necessary for the differentiation of T cells, a process that is regulated by the type of antigens found in thymocytes, the environment of surrounding cells and the thymus architecture. There is evidence that infectious diseases may result in morphological changes in this organ, such as premature atrophy and decreased thymocyte proliferation, that can affect the immune response. OBJECTIVES: We characterised the morphology and tissue distribution of haematopoietic and stromal cells in the thymuses of dogs naturally infected with Leishmania infantum, with the aim to determine the changes that may contribute to the pathophysiology of the disease. METHODS: Thymus samples were collected from 15 animals (aged 6 months to 5 years) ELISA-positive for leishmaniasis and from 10 dogs from non-endemic regions for leishmaniasis whose death was not related to infectious causes. The samples were submitted to histological processing and staining with Haematoxylin-Eosin to assess thymic morphometry and histopathological changes. Masson's trichrome staining was used to quantify the connective tissue present (collagen). The immunohistochemical method was used to determine the cellular constitution of the thymus, using antibodies that aimed at marking T lymphocytes (anti-CD3), B lymphocytes (anti-CD79a), macrophages (anti- MAC387), mesenchymal cells (anti-vimentin), epithelial cells (anti-cytokeratin), cells in mitosis (anti-Ki67) and cells in apoptosis (anti-caspase-3). RESULTS: The histopathological evaluation of infected dogs showed more signs consistent with thymus atrophy, including decreased parenchyma, infiltration of adipose and connective tissue near the capsule and between the lobules, lymphoid rarefaction mainly in the cortical region and loss of the cortical-medullary demarcation. In addition, we observed a decrease in the amounts of CD3 + T lymphocytes, macrophages (MAC387) and Ki67-positive cells and an increase in the number of cells positive for cytokeratin and CD79a (B lymphocytes). Finally, the parasite was detected in 46% of infected thymuses and may contribute for the observed changes. CONCLUSIONS: Apparently, leishmaniasis, like other infectious diseases, causes atrophy of the thymus and depletion of thymocytes with a relative increase in thymus epithelial cells. These morphological changes in the normal organisation of the thymus by mechanisms not yet well known may result in the abnormal release of T cells, with consequent damage to the host's immune response.

Epigenetic modifications in thymic epithelial cells: an evolutionary perspective for thymus atrophy.

BACKGROUND: The thymic microenvironment is mainly comprised of thymic epithelial cells, the cytokines, exosomes, surface molecules, and hormones from the cells, and plays a vital role in the development, differentiation, maturation and homeostasis of T lymphocytes. However, the thymus begins to degenerate as early as the second year of life and continues through aging in human beings, leading to a decreased output of naïve T cells, the limited TCR diversity and an expansion of monoclonal memory T cells in the periphery organs. These alternations will reduce the adaptive immune response to tumors and emerging infectious diseases, such as COVID-19, also it is easier to suffer from autoimmune diseases in older people. In the context of global aging, it is important to investigate and clarify the causes and mechanisms of thymus involution. MAIN BODY: Epigenetics include histone modification, DNA methylation, non-coding RNA effects, and chromatin remodeling. In this review, we discuss how senescent thymic epithelial cells determine and control age-related thymic atrophy, how this process is altered by epigenetic modification. How the thymus adipose influences the dysfunctions of the thymic epithelial cells, and the prospects of targeting thymic epithelial cells for the treatment of thymus atrophy. CONCLUSION: Epigenetic modifications are emerging as key regulators in governing the development and senescence of thymic epithelial cells. It is beneficial to re-establish effective thymopoiesis, identify the potential therapeutic strategy and rejuvenate the immune function in the elderly.

Influenza A virus-induced thymus atrophy differentially affects dynamics of conventional and regulatory T-cell development in mice.

Foxp3+ Treg cells, which are crucial for maintenance of self-tolerance, mainly develop within the thymus, where they arise from CD25+ Foxp3- or CD25- Foxp3+ Treg cell precursors. Although it is known that infections can cause transient thymic involution, the impact of infection-induced thymus atrophy on thymic Treg (tTreg) cell development is unknown. Here, we infected mice with influenza A virus (IAV) and studied thymocyte population dynamics post infection. IAV infection caused a massive, but transient thymic involution, dominated by a loss of CD4+ CD8+ double-positive (DP) thymocytes, which was accompanied by a significant increase in the frequency of CD25+ Foxp3+ tTreg cells. Differential apoptosis susceptibility could be experimentally excluded as a reason for the relative tTreg cell increase, and mathematical modeling suggested that enhanced tTreg cell generation cannot explain the increased frequency of tTreg cells. Yet, an increased death of DP thymocytes and augmented exit of single-positive (SP) thymocytes was suggested to be causative. Interestingly, IAV-induced thymus atrophy resulted in a significantly reduced T-cell receptor (TCR) repertoire diversity of newly produced tTreg cells. Taken together, IAV-induced thymus atrophy is substantially altering the dynamics of major thymocyte populations, finally resulting in a relative increase of tTreg cells with an altered TCR repertoire.

Priming with FLO8-deficient Candida albicans induces Th1-biased protective immunity against lethal polymicrobial sepsis.

The morphological switch between yeast and hyphae of Candida albicans is essential for its interaction with the host defense system. However, the lack of understanding of host-pathogen interactions during C. albicans infection greatly hampers the development of effective immunotherapies. Here, we found that priming with the C. albicans FLO8-deficient (flo8) mutant, locked in yeast form, protected mice from subsequent lethal C. albicans infection. Deficiency of Dectin-2, a fungus-derived α-mannan recognition receptor, completely blocked flo8 mutant-induced protection. Mechanistically, the flo8 mutant-induced Dectin-2/CARD9-mediated IL-10 production in DCs and macrophages to block thymus atrophy by inhibiting the C. albicans-induced apoptosis of thymic T cells, which facilitated the continuous output of naive T cells from the thymus to the spleen. Continuous recruitment of naive T cells to the spleen enhanced Th1-biased antifungal immune responses. Consequently, depletion of CD4+ T cells or blockade of IL-10 receptor function using specific antibodies in mice completely blocked the protective effects of flo8 mutant priming against C. albicans infection. Moreover, mannans exposed on the surface of the flo8 mutant were responsible for eliciting protective immunity by inhibiting the C. albicans-induced apoptosis of thymic T cells to sustain the number of naive T cells in the spleen. Importantly, priming with the flo8 mutant extensively protected mice from polymicrobial infection caused by cecal ligation and puncture (CLP) by enhancing Th1-biased immune responses. Together, our findings imply that targeting FLO8 in C. albicans elicits protective immune responses against polymicrobial infections and that mannans extracted from the flo8 mutant are potential immunotherapeutic candidate(s) for controlling infectious diseases.

Study on Improvement Effects of Ganoderma lucidum Polysaccharide Crude Extract on Estradiol-induced Thymus Atrophy in Mice / 中国药房

OBJECTIVE： To study the imp rovement effects of Ganoderma lucidum polysaccharides crude extract on estradiol-induced thymus atrophy in mice. METHODS ：Totally 60 female ICR mice were randomly divided into normal control group（normal saline ），model group （normal saline ），G. lucidum polysaccharides crude extract high-dose and low-dose groups （400，100 mg/kg，by crude drug ），with 15 mice in each group. Except for normal control group ，other groups were given estradiol intraperitoneally （0.1 mg/mice，6 times）every other day to establish thymic atrophy model. The next day after modeling finished，they were given relevant medicine intragastrically ，once a day ，for consecutive 14 d. Twenty-four hours after last medication，organ（thymus，spleen）index，MDA content and GST activity in plasma were determined. HE staining was adopted to observe the pathological changes of thymus and spleen tissue in mice. The thymus cell apoptosis was examined by TUNEL assay ， and the T cell subsets in peripheral blood were detected by flow cytometry. RESULTS ：Compared with normal control group ，the thymus index ，proportion of CD 3+CD4+T cell in peripheral blood and CD 4+/CD8+ ratio were decreased significantly in model group （P＜0.01）；spleen index ，MDA content in plasma and thymocyte apoptosis level as well as the proportion of CD 3+CD8+T cell in peripheral blood were all increased significantly （P＜0.05 or P＜0.01）. Thymic cortex and medullary boundary of mice was blurred；the intercellular space was enlarged ；some cells were damaged and apoptotic in cortex ；no pathological changes were found in the spleen. Compared with model group ，thymus index and GST activity in plasma as well as proportion of CD 3+CD4+T cell in peripheral blood and CD 4+/CD8+ ratio were all increased significantly in G. lucidum polysaccharides crude extract high-dose group（P＜0.05 or P＜0.01）；while MDA content in plasma ，the apoptosis level of thymocytes were all decreased significantly （P＜0.01 or P＜0.05）；and the pathological changes of thymus were improved significantly. MDA content in plasma was decreased significantly in G. lucidum polysaccharides crude extract low-dose group （P＜0.01），and other indexes/pathological changes were not obvious. CONCLUSIONS ：High dose （400 mg/kg）of G. lucidum polysaccharides crude extract can improve the thymus atrophy induced by estradiol in mice.

Detection of IL-1ß, IL-6 and TNF-α in Sprague-Dawely rats' atrophic thymus induced by lipopolysaccharide.

OBJECTIVE: This study aimed to investigate developmental changes of the thymus and intra- thymic IL-1ß, IL-6 and TNF-α expression in weaned Sprague-Dawley rats induced by lipopolysac- charide. METHODS: Forty healthy weaned rats aged 26 days and weighing 83±4 g were randomly and equally divided into two groups. The lipopolysaccharide group was treated daily with a single injection of lipopolysaccharide for 10 consecutive days, and the saline group was treated with an equal volume of sterilized saline. On the 1st, 4th, 7th and 10th day, histological changes and distribu- tion of IL-1ß-, IL-6- and TNF-α-positive cells were detected in the thymus by hematoxylin-eosin and immunohistochemistry staining, respectively. Subsequently, the expression levels of IL-1ß, IL-6 and TNF-α were evaluated in the thymus by the ELISA method. RESULTS: Thymus weight and index were significantly smaller in lipopolysaccharide-treated rats than in saline-treated rats (p⟨0.05), but no substantial changes were found in the thymus microstructure after lipopolysaccharide induction. Moreover, a large number of IL-1ß-, IL-6- and TNF-α-positive cells were observed with brownish-yellow color and mainly distributed in the thy- mus parenchyma, both integrated optical density and average optical density increased signifi- cantly in lipopolysaccharide-treated rats than those in saline-treated rats. Compared with the saline group, most of the thymic homogenates had higher levels of IL-1ß, IL-6 and TNF-α in the lipopolysaccharide group on different days. CONCLUSION: These findings indicate that the thymus atrophied after lipopolysaccharide induction in weaned Sprague-Dawley rats, and excessive production of intrathymic IL-1ß, IL-6 and TNF-α was probably involved in the atrophic process.

Immunohistochemistry by anti-cleaved-Lamin A: an improved approach to tackle the misuse of glucocorticoids in cattle.

The illegal use of glucocorticoids (GCs) as growth-promoters (GPs) is prohibited in farm animals in the European Union because the strong pharmacological activity of most synthetic GCs produces residues that are dangerous for human consumption. Among the alternative methods proposed to increase the efficacy of official controls, histology was the technique of choice in Italy on account of its high performance level. The aim of this study was to evaluate the reliability of immunohistochemistry (IHC) using anti-cleaved-Lamin A antibody to enhance the performance of the histological test applied to GC-related microscopic changes in the thymus. Veal calves (VC) and beef cattle (BC) were raised and both underwent different growth-promoting protocols or were left untreated. The morphology of the thymus parenchyma was evaluated for cortical atrophy with concurrent adipose tissue infiltration, and a score of 1 to 3 was attributed. A semi-quantitative IHC analysis was also performed by counting the number of positive thymocytes in 5 randomly selected high-power fields (HPFs). The distribution of the thymus atrophy scores was significantly different among the subgroups in both BC and VC. The IHC values were higher in untreated than in treated animals, for both BC and VC. The association between thymus atrophy score and IHC positivity showed higher median values in control than in treated animals (independently of the treatment protocol), for both BC and VC. Our data shows that IHC against anti-cleaved-Lamin A antibody is a reliable marker to detect illegal GC treatments, administered either alone or in association with other growth promoters, in both BC and VC. Combining IHC with the thymus atrophy score improves the accuracy of the histological method in correctly identifying treated animals and could represent a valuable, reproducible method to be applied to large-scale screening programmes.

Role of Hormonal Circuitry Upon T Cell Development in Chagas Disease: Possible Implications on T Cell Dysfunctions.

T cell response plays an essential role in the host resistance to infection by the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. This infection is often associated with multiple manifestations of T cell dysfunction, both during the acute and the chronic phases of disease. Additionally, the normal development of T cells is affected. As seen in animal models of Chagas disease, there is a strong thymic atrophy due to massive death of CD4+CD8+ double-positive cells by apoptosis and an abnormal escape of immature and potentially autoreactive thymocytes from the organ. Furthermore, an increase in the release of corticosterone triggered by T. cruzi-driven systemic inflammation is strongly associated with the alterations seen in the thymus of infected animals. Moreover, changes in the levels of other hormones, including growth hormone, prolactin, and testosterone are also able to contribute to the disruption of thymic homeostasis secondary to T. cruzi infection. In this review, we discuss the role of hormonal circuits involved in the normal T cell development and trafficking, as well as their role on the thymic alterations likely related to the peripheral T cell disturbances largely reported in both chagasic patients and animal models of Chagas disease.

Dysfunction of the thymus in mice with hypertension.

The aim of this study was to evaluate thymus function in mice with hypertension. A total of 60 C57BL/6J mice were randomized into control, sham surgery and two-kidney, one-clip groups (n=20 in each). At 4 or 8 weeks after surgery, mice were sacrificed, and blood, spleens, kidneys and thymuses were harvested. The results of reverse transcription-quantitative polymerase chain reaction analysis revealed that the mRNA levels of Forkhead box protein N1 (Foxn1) and autoimmune regulator (AIRE) in the thymus tissue of mice from the HTN group were significantly lower than those from the control group at 4 and 8 weeks (P<0.05). Foxn1 and AIRE expression was also reduced in the sham surgery group at 4 weeks after surgery, but had recovered 4 weeks later. Similar results were observed for the expression of signal-joint T cell receptor excision circles and the percentages of T cell subsets. The present study indicates that impaired thymus function is associated with hypertension in mice, which suggests that thymus function may be a novel target for the treatment of patients with hypertension.

Characterizing the thymic lesions in piglets infected with attenuated strains of highly pathogenic porcine reproductive and respiratory syndrome virus.

Piglets infected with the highly pathogenic PRRSV (HP-PRRSV) HuN4 strain develop severe thymus atrophy. However, the attenuated strain HuN4-F112 does not lead to lesions in organs. Here, we have characterized the thymic lesions in piglets infected with attenuated strains of HP-PRRSV HuN4 isolated at different passages in the attenuation process to produce HuN4-F112 from the parent HuN4 strain (HuN4-F5, HuN4-F15, HuN4-F23, HuN4-F30, and HuN4-F112). The thymic effects of infection were evaluated in terms of the thymus/body weight ratio, pathological changes, and thymocytes apoptosis. The ability of HP-PRRSV to induce thymus atrophy was reduced following attenuation after 23 passages; the HuN4-F23, but not HuN4-F30, caused thymus atrophy. The ability of the virus to induce thymocyte apoptosis decreased as it became attenuated. In addition, the viral load in the thymus was reduced as the virus was attenuated. The HuN4-F23 and HuN4-F30 strains might provide insight into the molecular mechanisms of HP-PRRSV pathogenesis. Taken together, our results indicate that the ability of HP-PRRSV to induce thymic atrophy is related to its pathogenicity.

Differential Expression of microRNAs in Thymic Epithelial Cells from Trypanosoma cruzi Acutely Infected Mice: Putative Role in Thymic Atrophy.

A common feature seen in acute infections is a severe atrophy of the thymus. This occurs in the murine model of acute Chagas disease. Moreover, in thymuses from Trypanosoma cruzi acutely infected mice, thymocytes exhibit an increase in the density of fibronectin and laminin integrin-type receptors, with an increase in migratory response ex vivo. Thymic epithelial cells (TEC) play a major role in the intrathymic T cell differentiation. To date, the consequences of molecular changes promoted by parasite infection upon thymus have not been elucidated. Considering the importance of microRNA for gene expression regulation, 85 microRNAs (mRNAs) were analyzed in TEC from T. cruzi acutely infected mice. The infection significantly modulated 29 miRNAs and modulation of 9 was also dependent whether TEC sorted out from the thymus exhibited cortical or medullary phenotype. In silico analysis revealed that these miRNAs may control target mRNAs known to be responsible for chemotaxis, cell adhesion, and cell death. Considering that we sorted TEC in the initial phase of thymocyte loss, it is conceivable that changes in TEC miRNA expression profile are functionally related to thymic atrophy, providing new clues to better understanding the mechanisms of the thymic involution seen in experimental Chagas disease.

Characterization of thymus atrophy in calves with subclinical BVD challenged with BHV-1.

Since the thymus is a target organ for the bovine viral diarrhea virus (BVDV), our experiment aimed to understand its relationship with the immunosuppressive effect by studying the consequences of a previous infection with BVDV on the thymus of calves challenged with bovine herpesvirus 1.1 (BHV-1). For this purpose, 12 animals were inoculated intranasally with non-cytopathic BVDV-1; 12 days later, 10 of them were coinfected intranasally with BHV-1. These animals were euthanized in batches of two at 0, 1, 2, 4, 7 or 14 dpi with BHV-1. Another 10 calves were inoculated solely with BHV-1 and euthanized in batches of two at 1, 2, 4, 7 or 14 dpi with BHV-1; two uninoculated calves were used as negative controls. Thymus samples from these animals were processed for viral detection and histopathological, immunohistochemical, and ultrastructural studies focused on BVDV/BHV-1 antigens, cortex:medulla ratio, apoptosis (TUNEL and caspase-3), collagen deposition, and factor VIII endothelial detection. Our study revealed the immunohistochemical presence of BVDV antigen in all animals in the BVDV-infected group, unlike BHV-1 detection, which was observed in animals in both infection groups only by molecular techniques. BVDV-preinfected animals showed severe atrophic changes associated with reduced cortex:medulla ratio, higher presence of cortical apoptosis, and increased collagen deposition and vascularization. However, calves solely infected with BHV-1 did not show atrophic changes. These findings could affect not only the numbers of circulating and local mature T cells but also the T cell-mediated immunity, which seems to be impaired during infections with this virus, thus favoring pathogenic effects during secondary infections.