IgA response in preterm neonates shows little evidence of antigen-driven selection

Abstract

After birth, contact to environmental Ags induces the production of IgA, which represents a first line of defense for the neonate. We sought to characterize the maturation of the repertoire of IgA H chain transcripts in circulating blood B cells during human ontogeny. We found that IgA H chain transcripts were present in cord blood as early as 27 wk of gestation and that the restrictions of the primary Ab repertoire (IgM) persisted in the IgA repertoire. Thus, B cells harboring more "mature" V(H) regions were not preferred for class switch to IgA. Preterm and term neonates expressed a unique IgA repertoire, which was characterized by short CDR-H3 regions, preference of the J(H) proximal D(H)7-27 gene segment, and very few somatic mutations. During the first postnatal months, these restrictions were slowly released. Preterm birth did not measurably accelerate the maturation of the IgA repertoire. At a postconceptional age of 60 wk, somatic mutation frequency of IgA H chain transcripts reached 25% of the adult values but still showed little evidence of Ag-driven selection. These results indicate that similar to IgG, the IgA repertoire expands in a controlled manner after birth. Thus, the IgA repertoire of the newborn has distinctive characteristics that differ from the adult IgA repertoire. These observations might explain the lower affinity and specificity of neonatal IgA Abs, which could contribute to a higher susceptibility to infections and altered responses to vaccinations, but might also prevent the development of autoimmune and allergic diseases.

FIGURE 1. N(D)N length of IgA transcripts

Each data point represents the mean of one blood sample (see Table I). N(D)N length in preterm infants increased with postconceptional age (r=0.538, p<0.0001). Near term, the N(D)N length in preterm and term neonates reached adult levels. The polynominal non-linear best fit curve is shown for IgA sequences. Adult data are shown as mean, quartiles, and tukey whiskers.

FIGURE 2. Usage of D_H gene families in IgA transcripts

The single member of the D_H7 gene family, D_H7-27, was overrepresented in IgA transcripts from preterm neonates, and its frequency decreased during ontogeny.

FIGURE 3. Average somatic mutation frequency of IgA transcripts

Each data point represents the mean of one blood sample (see Table I). Somatic mutation frequency increased with postconceptional age (preterm, r=0.678, p<0.0001 / term, r=0.687, p<0.0001). At a postconceptional age of ~60 weeks the somatic mutation frequency was similar in preterm and term neonates. The polynominal non-linear best fit curve is shown for IgA sequences. Adult data are shown as mean, quartiles, and tukey whiskers.

FIGURE 4. Correlation between the somatic mutation frequencies of IgA and IgG transcripts from the same blood samples

IgG transcripts had been obtained in a previous study (12) from the same 15 blood samples that were used to analyze IgA transcripts (r=0.5234, p< 0.05, excluding adult samples).

FIGURE 5. Antigen selection of IgA transcripts

Inference of Ag selection in IgA transcripts from preterm neonates (A), term neonates (B) and adults (C). Shown is the ratio of replacement mutations in CDR-H1 and CDR-H2 (R_CDR) to the total number of mutations in the V region (M_V) plotted against M_V. The shaded area represents the 95% confidence limits for the probability of random mutations. A data point falling outside these confidence limits represents a sequence that has a high proportion of replacement mutations in the CDR. The probability that these mutations occurred randomly is p < 0.05. 2 % of preterm and term neonate IgA transcripts exhibited statistical signs of antigen selection in comparison to 34 % of adult IgA transcripts (p<0.05, two-tailed Chi² Test, respectively). Numbers of sequences are written above the dots; size of dots increases with the number of sequences with the same parameters.