Assessment of dried blood spot and umbilical cord blood samples for the quantification of maternally transferred IgG against the group B streptococcal alpha-like surface proteins in neonates at Kolding Hospital, Denmark: a proof-of-concept, cross-sectional study.

Abstract

Maternally transferred IgG against group B Streptococcus (GBS) reduces the risk of GBS disease in infants. Previous studies have used umbilical cord serum as a proxy for neonatal serum to quantify GBS-specific IgG in neonates. The maternal GBS-AlpN vaccine targets four N-terminals of the alpha-like surface proteins (AlpNs; RibN, AlphaCN, Alp1N, and Alp2/3N), which are present in 92-99% of invasive GBS isolates. Early vaccine licensure could rely on the serological threshold of risk reduction (SToRR) based on neonatal AlpN-IgG concentrations. Dried blood spot samples (DBSSs), routinely collected 48-72 h after birth, could serve this purpose. We conducted a proof-of-concept, cross-sectional study, aiming to ascertain whether AlpN-IgG concentrations in DBSSs and umbilical cord serum reflect concentrations in neonatal serum. Pregnant women were recruited at 35-37 weeks of gestation during a routine midwife consultation at the Department of Obstetrics, Kolding Hospital, Denmark, between June 10, 2023, and Jan 31, 2024. The maternal exclusion criteria included blood transfusion within 30 days before delivery and known immunodeficiency. The neonatal exclusion criteria included blood transfusion before DBSS collection, severe congenital malformation, life-threatening conditions, birthweight of 2500 g or less, and insufficient neonatal serum volume. Umbilical cord blood samples were collected immediately after birth. Paired neonatal heel blood samples and DBSSs were collected 48-72 h after birth. Using ELISA, we measured the levels of AlpN-specific IgG in cord serum, neonatal serum, and DBSS eluates using two 3-mm punches. Agreement between sample types was assessed using the concordance correlation coefficient (CCC). Agreement was considered very high for a CCC of 0·90 or higher; strong for a CCC of 0·80-0·89; moderate for a CCC of 0·65-0·79; and poor for a CCC lower than 0·65. Deming regression was used to evaluate if AlpN-specific conversion factors between the sample types were required. A conversion factor was applied if the 95% CI for the intercept did not include zero or if the 95% CI for the slope did not include one. A total of 27 neonates were included with paired neonatal serum samples and DBSSs; among these, 23 had paired cord serum samples. The agreement between DBSSs and neonatal serum was very high for AlphaCN-IgG (CCC 0·94, 95% CI 0·88-0·97) and Alp2/3N-IgG (0·94, 0·87-0·97); strong for Alp1N-IgG (0·80, 0·60-0·90); and moderate for RibN-IgG (0·79, 0·63-0·88). DBSSs were equivalent to neonatal serum for AlphaCN-IgG, Alp1N-IgG, and Alp2/3N-IgG. RibN-IgG required DBSS-to-neonatal serum conversion factors. Cord serum showed very high agreement with neonatal serum for IgG against all four AlpNs: RibN-IgG (0·94, 0·88-0·97), AlphaCN-IgG (0·96, 0·97-1·00), Alp1N-IgG (0·96, 0·92-0·98), and Alp2/3N-IgG (0·90, 0·78-0·96). Cord serum was equivalent to neonatal serum for AlphaCN-IgG and Alp2/3N-IgG. Cord-to-neonatal serum conversion factors were applied for RibN-IgG and Alp1N-IgG. DBSS analysis serves as a reliable method for quantifying neonatal AlpN-specific IgG. Our findings support the use of DBSSs to establish an AlpN-specific SToRR that could facilitate licensure of the maternal AlpN-GBS vaccine. AlpN-specific IgG concentrations in cord serum closely reflect those in neonatal serum, supporting the use of the former as a surrogate marker. Both DBSSs and cord serum samples represent practical alternatives to neonatal serum, especially in settings in which sample collection is challenging. Additional studies are warranted to confirm the applicability of the established conversion factors across diverse populations and settings. MinervaX.