A mixed population of immature and mature leucocytes in umbilical cord blood results in a reduced expression and function of CR3 (CD11b/CD18)

Neonatal neutrophils express less membrane and cytoplasmic CR3 (iC3b-recept or, Mac-1, alpha(M)beta(2)-integrin) than do adult neutrophils, and it has been suggested that this renders neonatal neutrophils deficient in diapedes is and bactericidal activity. The reason(s) for this deficiency are unknown . In this study, CR3 expression and the CR3-dependent respiratory burst act ivity of individual neonatal neutrophils are quantified in comparison with adult leucocytes using flow cytometry. Monocytes and neutrophils are define d as CD14(high)CD15(low) and CD14(low)CD15(high), respectively. Although ne onatal neutrophils bore less CR3 on average than did adult neutrophils, neo natal neutrophils were more heterogeneous and many neonatal neutrophils exp ressed adult levels of CR3. Because of higher neutrophil concentrations in card versus adult blood, the calculated number of neutrophils in cord blood expressing high amounts of CR3 was equivalent to that of adult blood. Simi lar findings were made with monocytes. The size of the CR3-dependent respir atory burst stimulated by particulate beta-glucan correlated directly with the expression of CR3 by individual neutrophils. With neonatal and adult ne utrophils having comparable CR3 densities, the respiratory burst activities were equivalent, Wright-Giemsa differential staining of the subset of neon atal neutrophils with low CR3 levels isolated by fluorescence-activated cel l sorting showed a higher proportion of immature cells than the sorted popu lation expressing high CR3 levels. Therefore, higher proportions of immatur e cells in cord blood probably explain previous reports of deficient CR3 ex pression and function. The typical neutrophilia of cord blood may compensat e for this apparent deficiency by providing adult concentrations of mature neutrophils.