Quantitation and characterization of plasma HLA in neonates of different gestational ages.

The functional significance of plasma HLA class I antigens is unclear. They are thought to have an immunomodulatory role and be tolerogenic in transplant settings including the materno-fetal semi-allograft. There is, however, no available data on the concentrations of soluble HLA class I antigens in fetuses or newborns. We therefore determined plasma HLA class I antigen levels in 93 neonates born at different gestational ages and compared them to those in 66 healthy adults. The mean plasma HLA concentration in cord blood obtained from these neonates (0.30 +/- 0.15 microgram/ml, mean +/- SD) was significantly lower (p < 0.0001) than in the adults (0.77 +/- 0.44 microgram/ml). No correlation between the plasma HLA levels and the gestational ages of the neonates was detected. Characterizing the plasma HLA class I antigens by immunoprecipitation and immunoblotting, four different molecular weight forms, 44, 39, 36 and 34 kDa, were recognized. Their distribution in neonates was not different from that in adults. Since the circulating leukocytes are a probable source of plasma HLA class I antigens, we measured the surface HLA expression on leukocytes in 4 neonates and 4 adults by immunofluorescent flow cytometry. The fluorescence intensities on neonatal granulocytes and lymphocytes were 50% of those on corresponding adult cells. This finding suggests that the reduced HLA expression by neonatal leukocytes may be partially responsible for the lower concentration of HLA class I antigens in neonatal plasma.