Quantitative, Phenotypical, and Functional Characterization of Cellular Immunity in Children and Adolescents With Down Syndrome.

Background: Infections and autoimmune disorders are more frequent in Down syndrome, suggesting abnormality of adaptive immunity. Although the role of B cells and antibodies is well characterized, knowledge regarding T cells is limited.
Methods: Lymphocyte subpopulations of 40 children and adolescents with Down syndrome and 51 controls were quantified, and phenotype and functionality of antigen-specific effector T cells were analyzed with flow cytometry after polyclonal and pathogen-specific stimulation (with varicella-zoster virus [VZV] and cytomegalovirus [CMV]). Results were correlated with immunoglobulin (Ig) G responses.
Results: Apart from general alterations in the percentage of lymphocytes, regulatory T cells, and T-helper 1 and 17 cells, all major T-cell subpopulations showed higher expression of the inhibitory receptor PD-1. Polyclonally stimulated effector CD4+ T-cell frequencies were significantly higher in subjects with Down syndrome, whereas their inhibitory receptor expression (programmed cell death 1 [PD-1] and cytotoxic T-lymphocyte antigen 4 [CTLA-4]) was similar to that of controls and cytokine expression profiles were only marginally altered. Pathogen-specific immunity showed age-appropriate levels of endemic infection, with correlation of CMV-specific cellular and humoral immunity in all subjects. Among VZV IgG-positive individuals, a higher percentage of VZV-specific T-cell-positive subjects was seen in those with Down syndrome. Conclusions: Despite alterations in lymphocyte subpopulations, individuals with Down syndrome can mount effector T-cell responses with similar phenotype and functionality as controls but may require higher effector T-cell frequencies to ensure pathogen control.