Human newborn infants are believed to be more susceptible to sepsis due to immature host defense mechanisms. T-cells are essential for antigenic recognition. We previously have shown that there is a marked predominance of naive T-cells in newborns that is consistent with T-cell immune system immaturity. NK cells mediate cellular cytotoxicity independent of previous antigenic exposure. The role of NK cells in cytotoxicity against viral infection has been extensively studied. The role of these cells as a compensatory defense mechanism against neonatal sepsis is unclear. The purpose of the present study is to examine the expression of NK cells in newborn infants with and without early onset sepsis. We used flow cytometry to determine expression of cell surface markers on cord blood lymphocytes from 20 healthy newborn infants (GA 37±1 wks), peripheral blood lymphocytes from 15 newborn infants (GA 36±1 wks) with clinical or culture positive sepsis and 24 healthy adults. Comparisons among these 3 groups were made using ANOVA. Data presented are means and SE. The expression of NK cells was significantly higher in healthy newborn infants than adults (1160±144 vs 238±25 cells/μl, p<0.05). In septic newborn infants, NK cell expression was significantly lower at the time of diagnosis at 1.5±1 days of life than in healthy newborn infants (389±59 vs 1160±144 cells/μl, p<0.05).
We conclude that NK cell expression is higher in the healthy newborn infant than in the adult. We speculate that this may be compensatory for their immature T-cell immune system. Further work is necessary to determine whether neonatal NK cells are as competent as those of the adult and whether the reduction in NK cells in the septic newborn is the cause or result of infection.
