Natural killer cell activity in very low birth weight infants.

The exact role of natural killer (NK) cells in host defense is unclear, but they may be important as an early response to certain infections. We evaluated NK cell phenotype and activity in premature very low birth weight infants (VLBWI) (n = 52) with an average gestational age of 29.3 wk (24-35 wk) and an average birth weight of 1124 g (537-1480 g). All patients initially were evaluated within 7 d of birth. Samples also were obtained at 2, 4, and 6 wk in some infants. The proportion of mononuclear cells expressing the phenotypic marker of NK cells (NKH-1; CD56) was significantly lower in VLBWI than in adults (2.5 +/- 1.4 versus, 12.5 +/- 7.8%, p less than 0.0001) or term infants (2.5 +/- 1.4 versus 9.5 +/- 7.1%, p less than 0.0001). VLBWI also had significantly diminished NK activity expressed as the percentage of specific lysis compared with adults (4.7 +/- 4.4 versus 32.3 +/- 14.5%, p less than 0.0001) or term infants (4.7 +/- 4.4 versus 15.5 +/- 10.8%, p less than 0.0001). Both the number of cells expressing the NK phenotype and the NK lytic activity in VLBWI increased in the 6 wk after birth. NK activity in VLBWI was enhanced by IL-2 and in most cases by interferon-gamma.