Regulation of the IGK Locus and B Cell Development

Abstract

Allelic exclusion coupled to lineage and stage specific regulation ensure the Igκ locus undergoes RAG mediated V-J recombination only at the small pre-BII stage of lymphocyte development. These mechanisms also ensure the final antigen receptor is monospecific and allow self-specific receptors to be recognised and altered at later stages of development. Allelic exclusion is controlled by mechanism involving either epigenetic based allele selection early in development or probabilistic activation of a single allele at the small pre-BII stage. Current models favour the probabilistic model based on an elegant GFP reporter system. We present a reappraisal of this models based on an absence of the originally detected probabilistic activation. We find the absence to be explained in part by an aberrant splice event generated by developmentally regulated Igκ germline promoter usage. The activity of the promoter was investigated using in-vitro models of critical events during the B cell development but their role remains elusive. Intense investigation for the last 20 years has determined that chromatic regulation underpins the stage and lineage specification of rearrangement. The protein factors responsible for this regulation remain unknown. Using transgene reporters of cis-acting sequences and in-vitro model systems of B cell development, we find an involvement for members of the zinc finger family of Ikaros transcription factors, IRF4 and LEF1. We determined a bipartite role for Ikaros whereby its activity could both suppress rearrangement prior to pre-BCR signalling and promote rearrangement thereafter, possibly through an IRF4 based mechanism. Finally we present evidence for a role for Ikaros protein in later stages of B cell development. We find modulation of Ikaros activity directly influences the ability of cells to differentiate to the plasma cell fate. We used a transgenic model to allow genome-wide mapping of the transcriptional events regulated by Ikaros and find multiple small modulations of factors and pathways.