Prevalence of CD8/CD4 ratio in the fetal thymic parenchyme in Down's syndrome.

Aim: The maturation of most T-lymphocyte precursors takes place within the meshwork of thymic epithelial cells. Different steps of this process can be defined by immunologic phenotyping. The prothymocytes are positive for the terminal deoxynucleotidyl transferase (TdT) and give rise to cortical thymocytes, which express CD1, CD2, CD3, CD5, and both CD4 and CD8. These CD4 and CD8 double-positive cortical thymocytes differentiate into two lineages: CD4+ or CD8+ lymphocytes of the thymic medulla, by the tenth week of gestation. Our study points towards the determination of the CD8 cytotoxic/suppressor capacity of the fetal thymus in Down's syndrome.

Experimental Design: A quantitative comparison of T-lymphocytes (CD3, CD4, and CD8) in the thymic parenchyme in embryos after voluntary abortion during 2nd trimester of gestation and embryos with Down's syndrome, respectively, was performed.

Results: Our results showed: 1) A statistically significant depletion in the total number of T-cells (CD3 positive) in the cases of embryos with Down's syndrome over those after voluntary abortion, during the second trimester of gestation (p<0.0001, t-test). 2) A significant difference in the CD8/CD4 ratio in the cases of embryos with Down's syndrome, during the second trimester of gestation which was numerically stronger with the progress of fetal development (20th week: p<0.025; 24th week: p<0.01, chi-square).

Conclusions: The occurrence of increased CD8/CD4 ratio in the cases with Down's syndrome, in the second trimester of gestation, underlines the cytotoxic/suppressor property of the thymus in the affected fetuses.