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Chromosome 22q11.2 deletion syndrome is a common disorder characterized by thymic hypoplasia, conotruncal cardiac defect and hypoparathyroidism. Patients have a risk of infections and autoimmunity associated with T lymphocytopenia. To assess the immunological constitution of patients, the numerical changes and cytokine profile of circulating T cells were analysed by flow cytometry and real-time polymerase chain reaction (PCR). CD3+, CD4+, T cell receptor (TCR)αβ+ or CD8αα+ cell counts were lower, and CD56+ cell counts were higher in patients than in controls during the period from birth to adulthood. The ageing decline of CD3+ or CD4+ cell counts was slower in patients than in controls. The proportion of CD8αα+ cells increased in controls, and the slope index was larger than in patients. On the other hand, both the number and proportion of Vα24+ cells increased in patients, and the slope indexes tended to be larger than in controls. The positive correlation of the number of T cells with CD8αα+ cells was observed only in patients, and that with Vα24+ cells was seen only in controls. No gene expression levels of interferon (IFN)-γ, interleukin (IL)-10, transforming growth factor (TGF)-β, cytotoxic T lymphocyte antigen 4 (CTLA4) or forkhead box p3 (Foxp3) in T cells differed between patients and controls. There was no significant association between the lymphocyte subsets or gene expression levels and clinical phenotype including the types of cardiac disease, hypocalcaemia and frequency of infection. These results indicated that T-lymphocytopenia in 22q11.2 deletion patients became less severe with age under the altered composition of minor subsets. The balanced cytokine profile in the limited T cell pool may represent a T cell homeostasis in thymic deficiency syndrome. [ABSTRACT FROM AUTHOR]