Analysis of the human neonatal thymus: Evidence for a transient thymic involution

The neonatal period is marked by the impairment of the major components of both innate and adaptive immunity. We report a severe depletion of cortical CD4(+)CD8(+) double-positive thymocytes in the human neonatal thymus, This drastic reduction in immature double-positive cells, largely provoked by a n increased rate of cell death, could be observed as early as 1 day after b irth, delaying the recovery of the normal proportion of this thymocyte subs et until the end of the first month of postnatal life. Serum cortisol level s were not increased in newborn donors, indicating that the neonatal thymic involution is a physiological rather than a stress-associated pathological event occurring in the perinatal period. Newborn thymuses also showed incr eased proportions of both primitive CD34(+)CD1(-) precursor cells and matur e TCR alpha beta(high)CD69(-)CD1(-) CD45RO(+)/RA(dull) and CD45RO(dull)/RA( +) cells, which presumably correspond to recirculating T lymphocytes into t he thymus, A notable reinforcement of the subcapsular epithelial cell layer as well as an increase in the intralobular extracellular matrix network ac companied modifications in the thymocyte population. Additionally neonatal thymic dendritic cells were found to be more effective than dendritic cells isolated from children's thymuses at stimulating proliferative responses i n allogeneic T cells. All these findings can account for several alteration s affecting the peripheral pool of T lymphocytes in the perinatal period.