Organization and Expression of Mouse Vh Genes
A fundamental question in immunogenetics involves the mechanism by which the antigen receptor genes (immunoglobulin and T cell receptor) become targeted for rearrangement in a tissue and stage appropriate manner during lymphocyte development. Work from numerous investigators has shown that locus “accessibility” is the result of a dynamic process that includes alterations in both nuclear positioning and chromatin structure. Recent studies in our lab have focused on the regulation of variable region gene accessibility within the mouse immunoglobulin heavy chain locus (Igh). We are particularly interested in how usage of the many different Vh gene segments is controlled during rearrangement.
Figure 1. The drawing illustrates the order of the different Vh gene families of the IgH locus.
Accessibility of D-distal Vh genes is uniquely dependent on IL-7 signaling. We have recently explored the biochemical basis of this pathway using a cell line model developed in our lab. In this system, IL-7 induces accessible chromatin, as measured by germline transcription and histone H4 acetylation, at the D-distal VhA1 gene but not at a D-proximal Vh gene (V11). We used dominant negative and constitutively active Stat5 retroviruses to show that active Stat5 is both necessary and sufficient for the induction of VhA1 germline transcripts. Chromatin immunoprecipitation assays demonstrated the recruitment of Stat5 to the VhA1 gene. These results provide a molecular mechanism by which individual Vh genes of the Igh locus can be independently regulated during B cell development.
Figure 2. IL-7 treatment induces enhanced DNaseI sensitivity in the Vh locus.
We are working to understand how HS1 and other hypersensitive sites located within the Igh locus function to regulate VDJ recombination. Our hypothesis predicts that transcription factors known to play a central role in controlling B cell development interact with such sites and influence the rearrangement process. To date we have confirmed the presence of two such factors, PU.1 and Pax5, at HS1, as diagrammed below.
Figure 3. The cartoon illustrates our working model of the mechanism controlling Igh gene expression.