Wesley Dunnick, Ph.D.
Professor,
Department of Microbiology and Immunology

wesadunn@umich.edu

Mechanism and Regulation of Immunoglobulin Switch Recombination

    Early in the immune response, the dominant antibody is IgM with a μ heavy chain; later it becomes IgG, IgE, or IgA, with γ , ε , or α heavy chains, respectively. Immunoglobulin heavy chains are encoded by two genes--one for the variable region and a second for the constant region; the switch in heavy chain gene expression is accomplished by movement of a single variable region gene amongst two constant region genes. This recombination event is unprecedented in biology, in that it is neither a site-specific, nor is it based on homologous recombination. To understand how the DNA recombination works, we are studying the sequence requirements of this recombination event. The B cell regulates switch recombination in that only one, or sometimes two, of the six possible heavy chain genes participate in the DNA rearrangement event. My laboratory also seeks to understand the molecular basis of this gene-specificity. 

      Since switch recombination occurs only in B cells undergoing antigen driven differentiation, we study this event on recombinant DNA molecules inserted into normal B cells by preparation of transgenic mice. Our latest version is a 230 kb transgene that includes an assembled variable region, the entire heavy chain constant region locus, and a set of regulatory elements 3' of the constant regions. This transgene undergoes switch recombination like that of the endogenous locus. Hence, we can study switch recombination of the wild type genes, and we can also study switch recombination of recombinant genes mutated by a relatively simple technology. For example, we have shown that deletion of the 3' end of the heavy chain locus, which includes a complex set of regulatory elements, results in loss of switch recombination to the ε and all four Υ heavy chain genes. Other laboratories have shown that proteins (cytokines) produced by other lymphoid cells signal the B cell to switch to a specific gene. This cytokine signal also results in gene-specific transcription. For example, interleukin-4 induces first transcription, and then switch rearrangement, of the mouse γ1 gene. Our 230 kb transgene also expresses these germline transcripts, and we have already found that mutation of sites that bind cytokine-regulated factors can result in changes in the amount of germline transcription. 

Selected Publications:

Collins, J. T., Shi, J. Burrell, B. E., Bishop, D. K, and Dunnick, W. A. Induced expression of murine gamma2a by CD40 ligation independently of interferon-gamma. J. Immunol. 177:5414-5419, 2006.

Dunnick, W. A., Shi, J., Graves , K. A., and Collins, J. T. The 3' end of the heavy chain constant region locus enhances germline transcription and switch recombination of the four γ genes. J. Exp. Med. 201:1459-1466, 2005.

Dunnick, W. A., Shi, J., Graves , K. A., and Collins, J. T. Germline transcription and switch recombination of a transgene containing the entire heavy chain constant region locus: Effect of a mutation in a STAT6 binding site in the γ1 promoter. J. Immunol. 173: 5531-5539, 2004.

Berton , M. T., Linehan, L. A., Wick, K. L. R., and Dunnick, W. A. NF-κB elements associated with the Stat6 site in the germline γ1 immunoglobulin promoter are not necessary for the transcriptional response to CD40 ligand. Int. Immunol. 16: 1741-1749, 2004.