Abstract 1601 Poster Session II, Sunday, 5/2 (poster 105)

T-cell immune system includes two maturational stages designated as naive T-cells that express CD45RA and memory T-cells that express CD45RO. Naive T-cells are transformed to memory T-cells in response to antigenic stimulation. Memory T-cells are responsible for the enhanced cell mediated immune response on exposure to previously encountered antigens. Previously we have shown that there is a marked predominance of naive T-cells in newborn infants and that full term newborns are capable of increasing expression of memory T-cells in response to early onset sepsis. Premature infants have a variety of deficiencies in cell mediated host defense mechanisms and have an increased prevalence of late onset sepsis. We hypothesized that immaturity of cell-mediated immunity in the preterm infant will be manifested as a failure to increase expression of memory T-cells in response to late-onset sepsis. The expression of cell surface markers was examined in peripheral blood lymphocytes using flow cytometry. The expression of naive and memory T-cells was compared in three groups of premature infants. Twenty healthy premature infants (GA 29±1 wk) without chorioamnionitis in the first day of life (group 1), 18 healthy premature infants (GA 30±1 wk) without chorioamnionitis, or clinical or culture positive sepsis at 19±2 days of age (group 2) and 20 premature infants (GA 27±1 wk) with late onset culture positive sepsis at 22±2 days of age (group 3). The percent expression of naive T-cells in group 3 was similar to the expression of these cells in group 1 and 2 (61±3% vs 63±2%; p=ns). Also, there was no difference in the absolute number of these naive cells among the groups (3119±482 vs 3152±344 vs 4070±439 cells/µl; p=ns). There was an increase in the postnatal expression of memory T-cells in septic and non septic premature infants. However, the percent expression of memory T-cells was significantly higher in group 3 compared to group 1 and 2 (18±2% vs 8±1% vs 10±2%; p<0.001). Also, the absolute number of the memory T-cells was significantly higher in group 3 than group 1 and 2 (917±140 vs 297±40 vs 549±67 cells/µl;p<0.001).

We conclude that the marked predominance of naive T-cells in premature infants is consistent with the T-cell immune system immaturity. However, premature infants are capable of increasing expression of memory T-cells postnatally. This increase was more marked in premature infants with late onset sepsis. We speculate that the postnatal increase in memory T-cells expression may be related to non-specific as well as infectious antigenic stimulation.