The Decrease of Regulatory T Cells through the Low Levels of FOXP3 Expression Mediated by NFAT1 in Infancy with Autoimmune Neutropenia.

CD4⁺ /CD25⁺ regulatory T cells have been recognized to regulate the maintenance of self-tolerance, and several human autoimmune diseases in adulthood. To date, little is known about the standard value of regulatory T cells in infancy and childhood and about its involvement in pediatric autoimmune disease. Autoimmune neutropenia (AIN) in infancy is a common disorder characterized by chronic neutropenia, the detection of antineutrophil antibodies in sera, and spontaneous resolution of neutropenia within several months to a few years. In this study we examined the frequencies of regulatory T cells in various normal age groups and AIN patients to elucidate the involvement of regulatory T cells in AIN known as a representative autoimmune disease in infancy and early childhood. Blood samples from healthy neonates (n = 22), infants (1 month through 3 years in age, n = 21), adults (n = 12) and AIN patients (1 month through 3 years in age, n = 22) were obtained after informed consent. The frequencies of regulatory T cells were assessed by the expression of CD4, CD25, and FOXP3 using flow cytometry. The mRNA expressions of FOXP3 and its transcription factor, NFAT in CD4⁺ /CD25⁺ regulatory T cells were determined by quantitative real-time PCR. The frequency of CD4⁺ /CD25^high regulatory T cells known as pure regulatory T cells fraction was highest in neonates and decreased by aging. Notably, the number of CD4⁺ /CD25^high regulatory T cells in AIN patients significantly decreased, compared with that of age-matched healthy infants (P < 0.01) and the ratio of intracytosolic FOXP3 expression of each regulatory T cell fraction in AIN patients was lower than that of age-matched healthy infants (P < 0.05). Next, we purified CD4⁺ /CD25⁺ T cells by using FACS-Aria (more than 95% of purity) and the mRNA expressions in CD4⁺ /CD25⁺ T cells were quantified. The expressions of FOXP3 and NFAT1 mRNA in AIN patients were significantly lower than those of healthy infants (P < 0.05). The low level of FOXP3 mRNA in patients well correlated with the expression level of NFAT1 mRNA, implying that the expression of FOXP3 was mediated by NFAT1. Furthermore, the decreased frequency of CD4⁺ /CD25^high regulatory T cells and FOXP3-positive cells in AIN patients were restored to normal levels when the neutrophil count was spontaneously recovered. These results strongly suggest that regulatory T cells play an important role in the immunopathophysiology of AIN in childhood.