Different Properties of Involuted Thymus upon Nutritional Deficiency in Young and Aged Mice

Immune suppression in elderly individuals is one of the most important hygienic problems in aged societies. The primary immune organ thymus is histologically and functionally reduced by aging, which is known as thymic involution. The thymus is also involuted by nutritional deficiency, which frequently occurs in elderly individuals. However, there is no information on the thymic changes caused by nutritional deficiency with aging. Therefore, this study was conducted to examine the histological and molecular responses of the thymus to nutritional deficiency in young and aged mice. The thymic size was significantly smaller in 16- or 18-week-old aged mice than in 7-week-old young mice. Dietary restriction for 48 h reduced the thymic size in young mice, but not in aged mice. Immunostaining with anti-keratin 5 antibody revealed that the integrity of the corticomedullary boundary was maintained in the aged thymus, whereas dietary restriction induced its disorganization in both young and aged thymus. The numbers of immunoglobulin G (IgG)-positive cells were increased upon dietary restriction in aged, but not in young, thymus. Dietary restriction, but not aging, upregulated the mRNA levels of T-helper 2 (Th2)-related Il5, Il6, and Il10, whereas aging increased that of Th1-related interferon-γ (Ifng). The dietary restriction-induced upregulation of prostanoid-synthesizing enzymes was clearly observed in the young thymus but attenuated in the aged thymus. Thus, nutritional deficiency and aging cause an involuted thymus with different properties. Moreover, the thymus in aged mice does not show further reduction in size by nutritional deficiency but still responds differently compared with that in young mice.