Migration patterns of thymus and bursa lymphocytes in normal chickens and Obese strain chickens with spontaneous thyroiditis

Obesity-induced thymic involution and cancer risk

Declining thymic functions associated either with old age (i.e., age-related thymic involution), or with acute involution as a result of stress, infectious disease, or cytoreductive therapies (e.g., chemotherapy/radiotherapy), have been associated with cancer development. A key mechanism underlying such increased cancer risk is the thymus-dependent debilitation of adaptive immunity, which is responsible for orchestrating immunoediting mechanisms and tumor immune surveillance. In the past few years, a blooming set of evidence has intriguingly linked obesity with cancer development and progression. The majority of such studies has focused on obesity-driven chronic inflammation, steroid/sex hormone and adipokine production, and hyperinsulinemia, as principal factors affecting the tumor microenvironment and driving the development of primary malignancy. However, experimental observations about the negative impact of obesity on T cell development and maturation have existed for more than half a century. Here, we critically discuss the molecular and cellular mechanisms of obesity-driven thymic involution as a previously underrepresented intermediary pathology leading to cancer development and progression. This knowledge could be especially relevant in the context of childhood obesity, because impaired thymic function in young individuals leads to immune system abnormalities, and predisposes to various pediatric cancers. A thorough understanding behind the molecular and cellular circuitries governing obesity-induced thymic involution could therefore help towards the rationalized development of targeted thymic regeneration strategies for obese individuals at high risk of cancer development.

Involution of the rat thymus in experimentally induced hypothyroidism

The thymus, as part of the immune-neuroendocrine axis, is greatly influenced by factors from most endocrine glands, especially the thyroid. Antithyroid drugs (carbimazole and methimazole) were used to induce hypothyroidism in rats. Histological and ultrastructural examination of the thymus showed progressive thymic involution after 4 weeks of drug treatment to the end of observations (7 weeks). The involution was characterised by increased thymocyte apoptosis and thymocyte phagocytosis by macrophages. This resulted in thymocyte depopulation, increases in numbers of interdigitating cells, alterations to mainly subcapsular and medullary epithelial cells, an apparent increase of mast cells and collagen in the capsule and septa, and increased numbers of B cells and plasma cells. Lymphoid cells immuno-reactive with MRC OX12 (which detects B cells) were observed within blood vessel walls, suggesting that they may have been moving in and out of the thymus. The administration of drugs causing hypothyroidism, therefore, also caused marked involution of the thymus.