Intrathymic deficient expansion of T cell precursors in Down syndrome

To correlate the intrinsic cellular immunodeficiency, which is a major cause of increased susceptibility to polytopic infections in Down syndrome (DS) patients, with the histologic abnormalities observed in the thymus of these patients, we have studied thymus fragments and thymocyte cell suspensions from 15 non-institutionalized DS subjects. Comparing to the control age-matched samples, a reduced thymic cortex and a distinct depletion of CD1-positive (+) cells was observed by immuno-histologic examination. The phenotypic analysis of unselected thymocytes showed a significant reduction of CD3+, CD1+, CD4+, and CD8+ cells. When the total thymocyte population was separated into 10 fractions, using a continuous Percoll density gradient, a difference in cell distribution was observed. DS thymuses are almost devoid of high-density thymocytes (fractions 6-9) while more than 75% of the cells were recovered in the lightest 3 fractions (Frs). In addition, these thymuses were characterized by a marked depletion of CD1+ cells and by a conspicuous reduction of CD3+ cells normally present in the high-density Frs. On the other hand, the lightest 3 Frs of DS subjects were enriched in low-density CD1+ cells. Although enriched in these cells, normally characterized by a high mitotic activity, Fr1 DS thymocytes showed a reduced spontaneous proliferative capacity. When the expression of T cell receptor alpha- and beta-subunits was studied, the percentages of cells stained with anti-alpha and anti-beta antisera were found to be reduced in DS unfractionated thymocytes. The reduced number of high-density CD1+ thymocytes associated with a reduced spontaneous proliferative activity of low-density CD1+ thymocytes suggests that in DS thymuses there is a deficient expansion of immature T cells, resulting in a reduction of the various thymocyte subpopulations, including the thymocyte pool which differentiates into functionally mature T cells expressing the alpha-beta T cell receptor.