The immunoglobulin class switch: Structural characterization of recombinant switch regions.

The immunoglobulin class switch is mediated by a DNA rearrangement involving specific genomic segments referred to as switch regions. Switch regions are composed of t and emly repeated sequences spanning from 2-10kb upstream of each heavy chain constant region. Switch recombination occurs during the differentiation of the antibody secreting plasma cell to facilitate expression of alternative heavy chain isotypes, and so alternative effector functions within a given cell lineage. The switch recombination machinery appears to function in an isotype-specific fashion. Analysis of molecularly cloned recombinant switch regions was undertaken in order to establish a broader underst and ing of the DNA sequence requirements of the switch process. The structural features of recombinant switch regions also provide insight into the mechanisms of the switch recombination machinery. The recombinations are generally confined to the switch region t and em arrays, with the exception of S$\\mu$, wherein the requirement for t and em organization of sequences participating in the recombination event is not as strict. Comparison of nucleotide sequences at the sites of recombination within S$\\gamma$3 and S$\\gamma$1 indicates that recombination takes place at short sequences (5-7bp) related to GAGCT/GGGGT or YAGGTTG, which are present in all switch regions. Thus, the isotype specificity of the recombination event is not conferred by the DNA sequences at the site of recombination. Analysis of one of the molecular clones, $\\gamma$M35, reveals evidence for a successive switch event in which recombination between S$\\mu$ and S$\\gamma$3 was followed by recombination 57bp downstream with S$\\gamma$1. Switch recombinations between homologous switch region sequences (e.g. S$\\gamma$3 and S$\\gamma$1 in $\\gamma$M35) do not rely upon extensive alignment or basepairing of homologous sequences. Inversion of switch region sequences occurred during the recombination event represented by a second molecular clone, $\\gamma$M42, indicating that the switch recombinase machinery does not function in a strictly orientation-dependent fashion. Finally, the presence of duplicated sequences in a recombinant switch region suggests that DNA synthesis plays a role in the switch recombination process. A mechanism to account for the observed structural features of recombinant switch regions is discussed.PhDBiologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162113/1/8907126.pd