Ig gene rearrangement steps are initiated in early human precursor B cell subsets and correlate with specific transcription factor expression.

The role of specific transcription factors in the initiation and regulation of Ig gene rearrangements has been studied extensively in mouse models, but data on normal human precursor B cell differentiation are limited. We purified five human precursor B cell subsets, and assessed and quantified their IGH, IGK, and IGL gene rearrangement patterns and gene expression profiles. Pro-B cells already massively initiate D(H)-J(H) rearrangements, which are completed with V(H)-DJ(H) rearrangements in pre-B-I cells. Large cycling pre-B-II cells are selected for in-frame IGH gene rearrangements. The first IGK/IGL gene rearrangements were initiated in pre-B-I cells, but their frequency increased enormously in small pre-B-II cells, and in-frame selection was found in immature B cells. Transcripts of the RAG1 and RAG2 genes and earlier defined transcription factors, such as E2A, early B cell factor, E2-2, PAX5, and IRF4, were specifically up-regulated at stages undergoing Ig gene rearrangements. Based on the combined Ig gene rearrangement status and gene expression profiles of consecutive precursor B cell subsets, we identified 16 candidate genes involved in initiation and/or regulation of Ig gene rearrangements. These analyses provide new insights into early human precursor B cell differentiation steps and represent an excellent template for studies on oncogenic transformation in precursor B acute lymphoblastic leukemia and B cell differentiation blocks in primary Ab deficiencies.