F-actin Content of Neonate and Adult Neutrophils

We utilized flow cytometric analysis of NBDphallacidin-stained cells to measure F-actin content, expressed as fluorescent channel numbers, and we compared the microfilamentous cytoskeletal organization in neutrophils from healthy neonates (36 to 38 weeks gestational age) and adults. Basal F-actin content in neonate cord blood neutrophils is higher than that of adults. The elevation is intrinsic to the cell and not related to parturition because basal F-actin content of neonatal cells obtained by venipuncture (days 1 to 8 of age) is also elevated (38 +/- 10, N = 15) when compared to adults (21 +/- 7.0, N = 27). The rate of N-formyl-methionyl-leucyl-phenylalanine (fMLP)-induced actin polymerization is similar in adult and neonatal neutrophils and is maximal by 30 to 45 seconds at 25 degrees C. Adult, neonatal cord, and neonatal venipuncture neutrophils increase F-actin content to a similar extent following 0.5 mumol/L fMLP activation (52 +/- 18, N = 27; 58.7 +/- 18, N = 18; 51.5 +/- 7.0, N = 15, respectively). However, the relative increase in F-actin content following fMLP activation is much greater in adult (2.37-fold) than neonatal neutrophils (1.28-fold). This difference is due to the elevated basal F-actin content of neonatal cells. Comparison of distribution of F-actin content among basal, neonatal neutrophils reveals two subpopulations of neutrophils with respect to F-actin content--approximately 25% with F-actin content similar to that of adult neutrophils and 75% with F-actin content greater than that of adult cells. Following fMLP activation, the subpopulations disappear. The results suggest that abnormalities in microfilamentous cytoskeletal organization of neonatal cells may, in part, be responsible for decreased chemotactic response of neonatal neutrophils.