'Ups and downs' of the immunoglobulin heavy chain locus

The immunoglobulin heavy chain (IgH) gene locus spans several megabases. We show that IgH activation during B cell differentiation occurs in discrete, independently-regulated domains. Initially, a 120 kb domain of germline DNA is hyperacetylated that extends from D_FL16.1, the 5′-most D_H gene segment, to the intergenic region between Cμ and Cδ. Germline V_H genes were not hyperacetylated at this stage, which accounts for D_H to J_H recombination occurring first during B cell development. Subsequent activation of the V _H locus happens in at least three differentially regulated domains: an IL-7 regulated domain consisting of the 5′ J558 family, an intermediate domain and the 3′ V_H genes, which are hyperacetylated in response to DJ_H recombination. These observations lead to mechanisms for two well documented phenomena in B cell ontogeny—the sequential rearrangement of D_H followed by V _H gene segments, and the preferential recombination of D_H-proximal V_H genes in pro-B cells. We suggest that stepwise activation may be a general mechanism by which large segments of the genome are readied for expression.; Production of immunoglobulin heavy chain (IgH) protein feeds back to terminate further V_H gene recombination, a phenomenon also referred to as allelic exclusion. We provide evidence to support the proposition that allelic exclusion is the consequence of terminating signals that activate V_H genes for recombination. For the largest V_HJ558 family of genes this occurs by attenuating IL-7/IL-7R signals in pre-B cells. Loss of these signals reverts the V_H locus to a chromatin state that is associated with hypo-acetylated histones and less accessible to nucleases. Furthermore, hyperacetylation and accessibility of unrearranged V_H genes can be restored in allelically excluded splenic B cells by activating this pathway. Thus, transient signals mediate V_H gene activation and inactivation during development.; Finally, phenotypic analysis, as well as assays for gene activation and immunoglobulin gene rearrangements, confirm the essential role of IL-7 signaling for the development of committed B cell progenitors in the bone marrow. Further, we show that the vast majority of B cells that are present in IL-7−/− and IL-7Rα−/− bone marrow are recirculating peripheral B cells, most likely B-1 B cells of fetal or neonatal origin.