Thymocytes created in the bone marrow migrate to the thymus where they mature into T cells of the adaptive immune system. The thymus, unfortunately, loses active tissue with age, and this progressively reduces the pace at which new T cells are created. Lacking replacements, the adaptive immune system becomes increasingly made of up senescent, exhausted, and malfunctioning cells. This is an important component of immune aging.
Researchers here note that the presence of excess fat tissue correlates with greater atrophy of the thymus. Given that chronic inflammation is hypothesized to be important in driving this thymic involution, this result is not all that surprising. Excess visceral fat is metabolically active and generates inflammatory signaling through a range of mechanisms. Thus better lifestyle choices improve the odds of having more active thymic tissue in later life, and a less aged immune system.
Fatty degeneration of thymus (or thymus involution) has long been considered a normal ageing process. However, there is emerging evidence that thymic involution is linked to T cell aging, chronic inflammation, and increased morbidity. Other factors, aside from chronological age, have been proposed to affect the involution rate. In the present study, we investigated the imaging characteristics of thymus on computed tomography (CT) in a Swedish middle-aged population. The major aims were to establish the prevalence of fatty degeneration of thymus and to determine its associations with demographic, lifestyle, and clinical factors, as well as inflammation, T cell differentiation, and thymic output.
In total, 1,048 randomly invited individuals (aged 50-64 years, 49% females) were included and thoroughly characterized. CT evaluation of thymus included measurements of attenuation, size and a 4-point scoring system, with scale 0-3 based on the ratio of fat and soft tissue. A majority, 615 (59%) showed complete fatty degeneration, 259 (25%) predominantly fatty attenuation, 105 (10%) half fatty and half soft-tissue attenuation, while 69 (6.6%) presented with a solid thymic gland with predominantly soft-tissue attenuation. Age, male sex, high BMI, abdominal obesity, and low dietary intake of fiber were independently associated with complete fatty degeneration of thymus. Also, fatty degeneration of thymus as well as low CT attenuation values were independently related to lower proportion of naïve CD8+ T cells, which in turn was related to lower thymic output, assessed by T-cell receptor excision circle (TREC) levels.