Changes in plasma FcRIII demonstrate increasing receptor production during late pregnancy and after preterm birth.

We have previously described reduced expression of the Fc gamma receptor type III on the cell membrane (M-FcRIII) of neutrophils from very preterm neonates. To investigate the mechanism underlying this reduced receptor expression, we have measured neutrophil-derived soluble FcRIII (S-FcRIII) in the plasma of fetuses and neonates from 19 wk gestation. S-FcRIII in fetal plasma and in preterm neonates born before 32 wk gestation was consistently low [mean 13.6 +/- 1.2% (mean adult S-FcRIII = 100%, range 30-240%)]. In utero, S-FcRIII starts to rise from 33 wk and increases more than 4-fold to reach adult levels by term. S-FcRIII measured sequentially in preterm infants born as early as 24 wk of gestation showed a rapid postnatal increase to reach adult levels within 3 wk of birth. The changes in S-FcRIII paralleled changes in M-FcRIII expression on the cell surface. These observations point to a reduced rate of FcRIII production by fetal neutrophils as opposed to an increase in receptor release. The parallel increase in S-FcRIII and M-FcRIII suggests that there may be a programmed activation of FcRIII synthesis within individual cells late in the 3rd trimester of fetal development. In addition, FcRIII production may be switched on early by preterm birth.