Abstract
Normal human thymuses at various ages were immunohistologically examined in order to determine whether adult or aged thymus maintained the microenvironment for the T cell development and thymopoiesis was really ongoing. To analyze the thymic microenvironment, two monoclonal antibodies (MoAb) were employed. One is MoAb to IL-1 receptor (IL-1R) recognizing medullary and subcapsular cortical epithelial cells of normal infant human thymus. The other is UH-1 MoAb recognizing thymic epithelial cells within the cortex, which are negative with IL-1R-MoAb. Thymus of subjects over 20 years of age was split into many fragments and dispersed in the fatty tissue. However, the microenvironment of each fragment was composed of both IL-1R positive and UH-1 positive epithelial cells, and the UH-1 positive portion was populated with lymphocytes showing a follicle-like appearance. Lymphocytes in these follicle-like portions were mostly CD4+CD8+ double positive cells and contained many proliferating cells as well as apoptotic cells. Thus these follicle-like portions in adult and aged thymus were considered to be functioning as cortex as in infant thymus. Proliferative activity of thymocytes in the thymic cortex and the follicle-like portions definitely declined with advance of age, while incidence of apoptotic thymocytes increased with aging.
Highlights
The thymus is an important organ for the development of T cell dependent immune system
During immunohistological staining of mouse thymus by various monoclonal antibodies (MoAb), we incidentally found that IL-1 receptor (IL-1R) type I was strongly expressed in medullary thymic epithelial cells (Fig. 1)
Using the same MoAb, we examined the expression of IL-1R type I in human infant thymus and found that it was expressed in medullary epithelial cells and subcapsular cortical ones, but not in epithelial cells within the cortex (Fig. 2a,b)
Summary
The thymus is an important organ for the development of T cell dependent immune system. MacKall et al (1995) examined regeneration of CD4þ T cells in cancer patients after the intensive chemotherapy and found that regeneration of CD4þ T cells in the peripheral blood was very retarded in the young adult patients as compared with infant patients. They suggested that the thymus at young adult human was already definitely decreased in terms of the capacity to provide new T cells for peripheral lymphoid tissues
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