Effect of gestational age on lymphocyte phenotypes in hospitalized preterm infants

Background: Preterm infants exhibit an increased susceptibility to infections. To assess the contribution of adaptive immunity to this vulnerability, it is crucial to study its postnatal development. Objectives: We sought to define profiles of adaptive immune-cell subsets in large cohorts of preterm infants, investigating the influence of gestational age (GA) and perinatal factors on their development. Methods: Two German tertiary care neonatal intensive care unit cohorts (cohort 1: n = 499; cohort 2: n = 78) of hospitalized preterm infants (GA, 22.9-36.4 weeks) underwent flow cytometric phenotyping of peripheral blood lymphocyte subsets within the first 49 days of life. MetadeconfoundR package was used to evaluate (confounding) effects of clinical conditions on lymphocyte profiles. Results: GA at birth was a primary determinant of profiles of lymphocyte subsets. The most premature infants displayed persistently lower CD4⁺ T_H-cell frequencies, an early transient increase in B cells, and a later expansion of natural killer cells (T_H-low B-high natural killer–high phenotype). Detailed analysis revealed a less naive but more effector and regulatory CD4⁺ T-cell phenotype in preterm infants with lower GA at birth. Amniotic infection syndrome further accentuated this “premature” immune profile, which was also more prevalent in infants with typical complications of prematurity. In contrast, female sex was associated with higher CD4⁺ T_H-cell frequencies. Conclusions: This study provides a comprehensive characterization of adaptive immune development in hospitalized preterm infants during the first weeks of life, demonstrating a strong dependence on GA and modulation by perinatal factors. The identified distinct developmental profiles offer a valuable reference framework for interpreting immune phenotyping data and highlight potential associations between immunologic immaturity and clinical outcomes in this vulnerable population.