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. We utilize 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 a 28-kb deletion, including four regulatory elements 3' of the constant region genes, results in loss of switch recombination to the ε, α, and all four γ 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. We have shown that promoter regions for this gene-specific transcription control the cytokine-regulated transcription and switch rearrangement.
Dunnick, W. A., Collins, J. T., Shi, J., Westfield, G., Fontaine, C., Hakimpour, P., and Papavasiliou, F. N. Switch recombination and somatic hypermutation are controlled by the heavy chain 3' enhancer region. J. Expt. Med. 206: 2613-2623, 2009.
Dunnick, W. A., Shi, J., Holden, V., Fontaine, C. and Collins, J. T. The role of germline promoters and I exons in cytokine-induced gene-specific class switch recombination. J. Immunol. 186: 350-358, 2011. Highlighted "In This Issue".
Zhu, L., Chang, C.-H., and Dunnick, W. A. Excessive amounts of mu heavy chain block B cell development. Int. Immunol. 23: 545-551, 2011.
Dunnick, W. A., Shi, J., Zerbato, J. M., Fontaine, C. A., and Collins, J. T. Enhancement of class switch recombination by the cumulative activity of four separate elements. J. Immunol. 187:4733-4743, 2011.