Increased junctional diversity in fetal B cells results in a loss of protective anti-phosphorylcholine antibodies in adult mice

Fetal Igs are less diverse than adult Igs, largely because of the lack of N addition in the absence of Tdt. To test whether the absence of Tdt is esse ntial, we generated Tg mice that express Tdt and add N regions in fetal B c ells. When challenged as adults with PC-containing Streptococcus pneumoniae , these mice fail to make the hallmark T15 anti-PC Ab encoded by canonical rearrangements of Ig H and L chain genes. The anti-PC Abs from these mice a re altered by premature N addition and do not protect against death from vi rulent pneumococcal infection. These results show that maintenance of lower Ig diversity in early life is essential for the acquisition of a complete functional adult repertoire.