Delayed thymocyte maturation in the trisomy 16 mouse fetus.

Mouse fetuses with trisomy 16, an animal model for human trisomy 21 (Down syndrome), have severe defects in several hematopoietic stem cell populations and a marked reduction in thymocyte number. To determine whether there are other defects in the development of the trisomic thymus, the ontogeny of the cell surface antigenic determinants, Thy-1, Ly-1, CD3, CD4, CD8, and TCR v beta, was investigated. The trisomy 16 thymocytes were able to express all of determinants either during fetal life (days 14 to 19 of gestation) or in cultures of intact thymus lobes. However, in all instances (except for Thy-1, which already had a high proportion of expressing thymocytes by day 14), there was a delay in the time at which the determinants were first expressed, as manifested by reduced numbers of positively staining cells. Furthermore, there was also a delay in the rate at which the positively staining cells attained maximal Ag densities. Overall, there was an approximate 2 day lag in development of the fetal trisomic thymocytes. This lag permitted the identification of a large population of CD4-8+ cells prior to the appearance of CD4+8+ thymocytes. These findings are consistent with the identification of CD4-8+ as an intermediate stage between CD4-8- and CD4+8+ in fetal thymocyte ontogeny.