We have analyzed the developmental patterns of two groups of B cell precursors in nonimmunized BALB/c mice with respect to their relative proportions, absolute frequencies, V gene usage, fine specificity, and avidity for antigen. One group of B cells (group I) secretes antibodies specific for PC and PC-containing bacteria, whereas the other group (group II) produces antibodies recognizing only nonenvironmental PC-protein conjugates. A marked shift in the proportions of group I and group II occurs during ontogeny: while the group I B cells dominate (>85%) the adult antibody repertoire, the group II B cells have equal representation in neonatal mice from Days 1 to 7, and remain as a significant portion until 2 weeks of age. Examination of the absolute frequencies of group I and group II B cells revealed that the frequency of group II B cells remained relatively stable throughout ontogeny, whereas group I B cells expanded rapidly after 7 days of age to predominate in the adult. Genetic analysis indicated that early group I antibodies were encoded by VH and VL genes different from adult group I antibodies which are mostly encoded by a single VH (S107) and VL (Vk22) gene combination (the T15 idiotype). On the other hand, early group II antibodies used VH genes comparable to their adult counterparts. The majority of early group I antibodies have lower avidity for PC than adult T15+ antibodies, whereas the avidity of neonatal group II antibodies varies considerably and is comparable with that of the adult group II antibodies. Our results suggest that the ontogeny of phosphocholine-specific B cells may be regulated according to their fine specificity rather than to their avidity or V gene usage.
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