Significant augmentation of regulatory T cell numbers occurs during the early neonatal period

Summary Regulatory T cells (T regs ) control immune responses by suppressing various inflammatory cells. T regs in newborn babies may play an important role in preventing excessive immune responses during their environmental change. We examined the number and phenotype of T regs during the neonatal period in 49 newborn babies. T regs were characterized by flow cytometry using cord blood (CB) and peripheral blood (PB) from the early (7–8 days after birth) and late (2–4 weeks after birth) neonatal periods. CD4 + forkhead box protein 3 (FoxP3 + ) T cells were classified into resting T regs (CD45RA + FoxP3 low ), activated T regs (CD45RA – FoxP3 high ) and newly activated T cells (CD45RA – FoxP3 low ). Compared with CB and PB during the late neonatal period, the percentage of T regs and all T reg subpopulations in the CD4 + lymphocyte population were increased significantly during the early neonatal period. Furthermore, the proportion and absolute number of activated T regs were increased markedly compared with other T reg subpopulations, such as resting T regs and newly activated T cells (non‐T regs ), in the early neonatal period. Increased T regs concomitantly expressed the suppressive molecule cytotoxic T lymphocyte antigen‐4 (CTLA‐4). The up‐regulated expression of chemokine receptor 4 (CCR4) and down‐regulated expression of CCR7 were also observed in expanded T regs . When cord blood cells were cultured in vitro with CD3 monoclonal antibodies (mAb) for 5 days, CD4 + CD45RA – FoxP3 high cells were increased significantly during the culture. Thus, the presence of increased activated T regs in early neonates may play an important role in immunological regulation by suppressing excessive T cell activation caused by the immediate exposure to ubiquitous antigens after birth.