The early-life determinant LIN28B constrains immunoglobulin light chain secondary rearrangements independent of BCR specificity

The early-life B cell repertoire is disproportionately enriched for self-reactive specificities in mice and humans, raising the question of how this ontogenic permissiveness is achieved. The predominant B cell central tolerance mechanism edits away self-reactivity by secondary rearrangements of the immunoglobulin light chain (IgL) following strong B cell receptor (BCR) engagement during the immature B cell stage. Here, we demonstrate a layer of developmental regulation, imposed by the early-life restricted RNA-binding protein LIN28B, that suppresses the incidence and capacity for IgL secondary rearrangements during ontogeny. Genetic dissection demonstrated that the underlying mechanisms operate independent of BCR specificity or pre-BCR requirement, dissociating the receptor editing fate from strict BCR instruction. We identified an adult-specific receptor editing-biased pre-B cell state marked by CD25 expression and metabolic quiescence. LIN28B subverted this state, shifting the balance from secondary rearrangements to positive selection and bone marrow egress. Together, our results demonstrate that the central tolerance threshold is an ontogenically tuned parameter, providing insights into the self-reactivity bias that characterizes the early-life B cell repertoire.