The unilateral involution in the thymus of a 96-year-old male leads to the preservation of structural integrity in one thymic lobe, as assessed by the expression of medullar and cortical antigens and the presence of CD3+ cells.

The process of thymic involution begins soon after birth and continues through adult life. Although evolutionary conserved in all vertebrates, the thymic involution has no defined kinetics. Little is known about the pace of its regression in humans, except that there is a marked increase of thymic involution after puberty. This report describes the unusual structural findings in the thymus of a 96-year-old male. The morphological parameters of the organ were evaluated using H&E and immunohistochemistry (IHC) techniques. The macroscopic examination showed a typical organ's weight and size, except that the right thymic lobe presented a well-preserved organ and the left lobe was significantly adiposed. The H&E staining of the thymic sections from the left and right lobes confirmed advanced thymic adiposity in the left lobe and preserved thymic epithelial space containing hematoxylin-stained cells in the right lobe. The multiplex immunostaining of the right lobe sections with antibodies specific to cytokeratins -14 and -8, CD3, and CD4 revealed the presence of medullar and cortical epithelium and mix population of CD3+/CD4+ and CD3+/CD4- T cells. The T cells were associated with the medulla but not with the cortex of the thymus. The immunostaining with an antibody to FoxN1 showed that the protein was expressed in the thymic epithelium. Taken together, we provide evidence that the thymus of a 96-year-old man involuted different kinetics in each of the two thymic lobes. Furthermore, the presence of CD3+/CD4+ and CD3+/CD4-cells gives a hand to the hypothesis that a pool of T-cells may associate with this primary lymphatic organ for as long as there is the available thymic epithelium and be a source of lymphocytes aiding adaptive immune responses to old age.