Intermediate CD14 ++ CD16 + blood monocytes are elevated in preterm neonates

Monocytes are a major part of the innate immune system. Three populations of blood monocyte subsets in humans have been defined according to their differential expression of CD14 and CD16: classical CD14 ++ CD16 - , non-classical CD14 + CD16 ++ and intermediate CD14 ++ CD16 + monocytes. In adults these subsets are well described, but very little is known on the development and activity of these cells in very-preterm neonates. Blood samples from preterm neonates born at gestational age from 24 to 32 weeks were taken from day 0 up to 11 weeks after birth and compared with term neonates born by cesarean section and adult controls. At 10 days or less after birth intermediate CD14 ++ CD16 + monocytes were strongly elevated in preterm neonates (49.1±9.2% of all monocytes at 24-26 weeks gestational age), they were still high at 20.7±6.3 % in preterm neonates born at 30-32 weeks gestational age while in term neonates they accounted for 6.9±3.3% compared to 3.3±0.98% in adult control donors. CD62L expression was 4-fold higher in intermediate monocytes from preterm neonates compared to cells from term neonates and adults, but HLA-DR was strongly decreased in preterm intermediate monocytes. While both intermediate monocytes and non-classical monocytes were strong producers of Lipopolysaccharide induced-TNF in preterm and term neonates the level of TNF production was 5-fold lower compared to what is seen in adult monocyte subsets. The strong expansion and aberrant phenotype of intermediate monocytes in preterm neonates may be explained by the immaturity of the innate immune system in the preterms and this may contribute to the increased susceptibility to infection.