COMPLEMENT FACTOR-IX DEFICIENCY IN SERUM OF HUMAN NEONATES

The serum concentration of complement factor C9 (C9) was 260 +/- 47-mu-g/ml (+/-SE) in 14 mothers and <42-mu-g/ml in each of their 14 neonates. During incubation for 60 min, 11 of 14 maternal sera and 3 of 14 neonatal sera reduced the survival of Escherichia coli O7w:K1:NM to <20% of the original inoculum (P < .03). Eleven neonatal sera did not kill the bacteria. Supplemental C9 (60-mu-g/ml) enhanced the bactericidal capacity of 10 neonatal sera. I-125-labeled C9 was deposited onto E. coli by neonatal sera, but less efficiently than by pooled adult sera. Supplemental IgG enhanced I-125-labeled C9 deposition and potentiated the bactericidal activity of exogenous C9. Therefore, neonatal sera contained diminished concentrations of C9 and killed E. coli inefficiently. In neonatal sera, supplemental C9 was deposited onto E. coli and enhanced bactericidal activity. These effects of C9 were potentiated by supplemental IgG.