Downregulation of interleukin-7 and hepatocyte growth factor in the thymic microenvironment is associated with thymus involution in tumor-bearing mice

Syringic acid improves cyclophosphamide-induced immunosuppression in a mouse model

Syringic acid (SA), a naturally occurring phenolic substance present in many edible plants and fruits, has been shown to have potential in immunoenhancement applications. In this study, we investigated the immunomodulatory effects of SA in mitigating cyclophosphamide (CYP)-induced immunosuppression in BALB/c mice using doxycycline as a positive control. SA administration prevented immune organ atrophy and morphological changes in the thymus, spleen, and bone marrow induced by CYP treatment in mice while also showing a dose-dependent enhancement of thymus and spleen indices compared to mice treated with CYP alone. Furthermore, SA improved thymocyte and splenocyte proliferation and exhibited significant antioxidant activity by reducing the elevated levels of malondialdehyde induced by CYP treatment. SA treatment effectively restored white blood cell (WBC) and lymphocyte counts to normal levels in CYP-treated animals, and the protective effects of CYP on immunological tissues were confirmed through histopathological examination. Moreover, SA treatment upregulated the expression of IL-6, IL-7, IL-15, and FoxN1. Finally, molecular docking studies revealed that binding energy values predicted minor inhibition potential toward IL-6, IL-7, FoxN1, IL-15, STAT3, STAT5, and JAK3. Overall, our findings suggest that SA treatment has the potential to reduce CYP-induced immunosuppression and may have applications as an immunologic adjuvant or functional food additive in chemotherapy.

Role of thymus in health and disease

The thymus is a primary lymphoid organ, essential for the development of T-cells that will protect from invading pathogens, immune disorders, and cancer. The thymus decreases in size and cellularity with age referred to as thymus involution or atrophy. This involution causes decreased T-cell development and decreased naive T-cell emigration to the periphery, increased proportion of memory T cells, and a restricted, altered T-cell receptor (TCR) repertoire. The changes in composition and function of the circulating T cell pool as a result of thymic involution led to increased susceptibility to infectious diseases including the recent COVID and a higher risk for autoimmune disorders and cancers. Thymic involution consisting of both structural and functional loss of the thymus has a deleterious effect on T cell development, T cell selection, and tolerance. The mechanisms which act on the structural (cortex and medulla) matrix of the thymus, the gradual accumulation of genetic mutations, and altered gene expressions may lead to immunosenescence as a result of thymus involution. Understanding the molecular mechanisms behind thymic involution is critical for identifying diagnostic biomarkers and targets for treatment help to develop strategies to mitigate thymic involution-associated complications. This review is focused on the consequences of thymic involution in infections, immune disorders, and diseases, identifying potential checkpoints and potential approaches to sustain or restore the function of the thymus particularly in elderly and immune-compromised individuals.

Interleukin (IL)-7 Signaling in the Tumor Microenvironment

Interleukin (IL)-7 plays an important immunoregulatory role in different types of cells. Therefore, it attracts researcher's attention, but despite the fact, many aspects of its modulatory action, as well as other functionalities, are still poorly understood. The review summarizes current knowledge on the interleukin-7 and its signaling cascade in context of cancer development. Moreover, it provides a cancer-type focused description of the involvement of IL-7 in solid tumors, as well as hematological malignancies.The interleukin has been discovered as a growth factor crucial for the early lymphocyte development and supporting the growth of malignant cells in certain leukemias and lymphomas. Therefore, its targeting has been explored as a treatment modality in hematological malignancies, while the unique ability to expand lymphocyte populations selectively and without hyperinflammation has been used in experimental immunotherapies in patients with lymphopenia. Ever since the early research demonstrated a reduced growth of solid tumors in the presence of IL-7, the interleukin application in boosting up the anticancer immunity has been investigated. However, a growing body of evidence indicative of IL-7 upregulation in carcinomas, facilitating tumor growth and metastasis and aiding drug-resistance, is accumulating. It therefore becomes increasingly apparent that the response to the IL-7 stimulus strongly depends on cell type, their developmental stage, and microenvironmental context. The interleukin exerts its regulatory action mainly through phosphorylation events in JAK/STAT and PI3K/Akt pathways, while the significance of MAPK pathway seems to be limited to solid tumors. Given the unwavering interest in IL-7 application in immunotherapy, a better understanding of interleukin role, source in tumor microenvironment, and signaling pathways, as well as the identification of cells that are likely to respond should be a research priority.

Computed tomographic evaluation of the thymus-does obesity affect thymic fatty involution in a healthy young adult population?

OBJECTIVE

To determine a relationship between increased body mass index (BMI) and fatty involution of the thymus in subjects aged between 20 and 30 years.

METHODS

CT images of 94 patients aged between 20 and 30 years were reviewed. Quantitative thymic mean attenuation was recorded and qualitative thymic attenuation was assigned to 1 of 4 possible grades. BMI and subcutaneous fat thickness were documented. Correlations between thymic attenuation, and BMI and subcutaneous fat thickness were assessed using linear regression models. Differences in thymic attenuation in overweight vs normal weight patients were assessed using t-test and Pearson Χ² analysis.

RESULTS

Low mean thymic attenuation values were associated with higher patient BMI (p = 0.024). Normal weight patients had a mean quantitative thymic attenuation of 15.5 Hounsfield unit and overweight patients had a mean quantitative thymic attenuation of -16.4 Hounsfield unit (p = 0.0218). There was a significant association between increasing subcutaneous fat thickness and reduced mean quantitative thymic attenuation (p < 0.0001). There was also a significant difference in subcutaneous fat thickness when comparing qualitatively assessed thymic Grade 0 with grades 2 and 3 (p = 0.027 and 0.001 respectively); and Grade 1 with Grade 3 (p = 0.001).

CONCLUSION

In patients between 20 and 30 years old, the degree of thymic fatty infiltration is related to BMI. Advances in knowledge: Multidetector CT can assess fatty involution of the thymus gland. This retrospective study demonstrates a relationship between BMI and thymus gland fatty involution. Subjects with increased subcutaneous fat have decreased mean thymus gland attenuation.

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.

Role of Vascular Endothelial Growth Factor (VEGF) in Thymus of Mice under Normal Conditions and with Tumor Growth

In our study, we for the first time investigated a role for VEGF as a factor regulating transendothelial migration of murine thymocytes in vitro. Effects of VEGF were examined in a model of thymocyte migration across a monolayer of EA.hy 926 endothelial cells. We showed that VEGF enhanced transendothelial migration of murine thymocytes and their adhesion to endothelial cells in a dose-dependent manner. VEGF did not influence thymocytes, but rather acted on endothelial cells by upregulating surface expression of adhesion molecule ICAM-1 and downregulating activity of 5'-nucleotidase. Effects from VEGF were comparable with those from TNF-α. Because it is known that administration of VEGF to intact animals results in thymic atrophy, it was assumed that it might play a role in developing thymic involution during tumor growth. Enhanced egress of thymocytes to the periphery was considered as a plausible mechanism underlying effects of VEGF. However, we revealed no difference in parameters of in vitro transendothelial migration for thymocytes from animals bearing a transplantable hepatoma 22a compared to control animals. VEGF mRNA expression in lysates of thymic stroma was found to be upregulated in mice with grafted tumors, whereas at the protein level the amount of VEGF did not differ. While examining expression of VEGF receptors on thymocytes by flow cytometry, both VEGFR-1 and VEGFR-2 were not detected, whereas the percentage of Nrp-1-positive thymocytes in animals with hepatoma 22a was as high as in the control group. Thus, we were unable to confirm a hypothesis regarding participation of VEGF in developing thymic involution during progression of experimental hepatoma. However, a set of novel data concerning a role for VEGF in stimulating transendothelial migration of thymocytes in vitro was obtained, and it may be of significance for understanding mechanisms underlying thymus functioning as well as a role of this cytokine in preparing endothelial cells for egress of thymocytes to the periphery.

Adipocyte and leptin accumulation in tumor-induced thymic involution

Cell-mediated immunity is an important defense mechanism against pathogens and developing tumor cells. The thymus is the main lymphoid organ involved in the formation of the cell-mediated immune response by the maturation and differentiation of lymphocytes that travel from the bone marrow, through the lymphatic ducts, to become T lymphocytes. Thymic involution has been associated with aging; however, other factors such as obesity, viral infection and tumor development have been shown to increase the rate of shrinkage of this organ. The heavy infiltration of adipocyte fat cells has been reported in the involuted thymuses of aged mice. In the present study, the possible accumulation of such cells in the thymus during tumorigenesis was examined by immunohistochemistry. A significant number of adipocytes around and infiltrating the thymuses of tumor-bearing mice was observed. Leptin is a pro-inflammatory adipocytokine that enhances thymopoiesis and modulates T cell immune responses. The levels of leptin and adiponectin, another adipocytokine that has anti-inflammatory properties, were examined by western blot analysis. While no changes were observed in the amounts of adiponectin present in the thymuses of the normal and tumor-bearing mice, significantly higher levels of leptin were detected in the thymocytes of the tumor-bearing mice. This correlated with an increase in the expression of certain cytokines, such as interleukin (IL)-2, interferon (IFN)-γ and granulocyte-macrophage colony-stimulating factor (GM-CSF). The co-culture of thymocytes isolated from normal mice with ex vivo isolated adipocytes from tumor-bearing mice yielded similar results. Our findings suggest that the infiltration and accumulation of adipocytes in the thymuses of tumor-bearing mice play an important role in their altered morphology and functions.

c-Met signalling is required for efficient postnatal thymic regeneration and repair

We have reported that in vivo administration of the hybrid cytokine rIL-7/HGFβ or rIL-7/HGFα, which contains interleukin-7 (IL-7) and the β- or α-chain of hepatocyte growth factor (HGF), significantly enhances thymopoiesis in mice after bone marrow transplantation. We have shown that the HGF receptor, c-Met, is involved in the effect of the hybrid cytokines. To address the role of c-Met signalling in thymocyte development and recovery, we generated conditional knockout (cKO) mice in which c-Met was specifically deleted in T cells by crossing c-Met(ft/ft) mice with CD4-Cre transgenic mice. We show here that although the number of total thymocytes and thymocyte subsets in young c-Met cKO mice is comparable to age-matched control (Ctrl) mice, the cKO mice were more susceptible to sub-lethal irradiation and dexamethasone treatment. This was demonstrated by low recovery in thymic cellularity in c-Met cKO mice after insult. Furthermore, the number of total thymocytes and thymocyte subsets was markedly reduced in 6- to 12-month-old cKO mice compared with age-matched Ctrl mice, and the thymic architecture of 12-month-old cKO mice was similar to that of 20-month-old wild-type mice. In addition, c-Met deficiency reduced cell survival and the expression of Bcl-xL in double-positive thymocytes, and decreased cell proliferation and the expression of cyclin E and cyclin-dependent kinase 5 in single-positive thymocytes. Our data indicate that c-Met signalling plays an important role in thymic regeneration after thymic insult. In addition, T-cell-specific inactivation of c-Met accelerates age-related thymic involution.

Insights into thymic involution in tumor-bearing mice

The thymus is a central lymphoid organ critical for the development and maintenance of an effective peripheral T-cell repertoire. Most important, it provides a specialized environment for the selection of rearranged clones that will function appropriately in the adaptive immune response. Thymic involution has been observed in several model systems; including graft-versus-host disease, aging, viral infection, and tumor development, however, the precise mechanisms involved in this phenomenon remain poorly defined. Here, we review some of our results related to the studies of the cell-mediated immunity in a mammary tumor model; more specifically, those related to the tumor-induced impaired T-cell development and thymic involution. Collectively, the understanding of the mechanisms and pathways associated with the tumor-induced thymic involution is essential for the development of innovative and safe therapies to fight against the immune suppression caused by the tumor development.