7. DNA recombination causes class switching
24.6 Allelic exclusion is triggered by productive rearrangement |
| Key terms defined in this section |
| Allelic exclusion describes the expression in any particular lymphocyte of only one allele coding for the expressed immunoglobulin. |
Each B cell expresses a single type of light chain and a single type of heavy chain, because only a single productive rearrangement of each type occurs in a given lymphocyte, to produce one light and one heavy chain gene. Because each event involves the genes of only one of the homologous chromosomes, the alleles on the other chromosome are not expressed in the same cell. This phenomenon is called allelic exclusion.
The occurrence of allelic exclusion complicates the analysis of somatic recombination. A probe reacting with a region that has rearranged on one homologue will also detect the allelic sequences on the other homologue. We are therefore compelled to analyze the different fates of the two chromosomes together.
The usual pattern displayed by a rearranged active gene can be interpreted in terms of a deletion of the material between the recombining V and C loci.
Two types of gene organization are seen in active cells:
Probes to the active gene may reveal both the rearranged and germline patterns of organization. We assume then that joining has occurred on one chromosome, while the other chromosome has remained unaltered.
Two different rearranged patterns may be found, indicating that the chromosomes have suffered independent rearrangements. In some of these instances, material between the recombining V and C gene segments is entirely absent from the cell line. This is most easily explained by the occurrence of independent deletions (resulting from recombination) on each chromosome.
When two chromosomes both lack the germline pattern, usually only one of them has passed through a productive rearrangement to generate a functional gene. The other has suffered a nonproductive rearrangement; this may take several forms, but in each case the gene sequence cannot be expressed as an immunoglobulin chain. (It may be incomplete, for example because D-J joining has occurred but V-D joining has not followed; or it may be aberrant, with the process completed, but failing to generate a gene that codes for a functional protein.)
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Figure 24.16 A successful rearrangement to produce an active light or heavy chain suppresses further rearrangements of the same type, and results in allelic exclusion. |
The coexistence of productive and nonproductive rearrangements suggests the existence of a feedback loop to control the recombination process. A model is outlined in Figure 24.16. Suppose that each cell starts with two loci in the unrearranged germline configuration Ig0. Either of these loci may be rearranged to generate a productive gene Ig+ or a nonproductive gene Ig V .
If the rearrangement is productive, the synthesis of an active chain provides a trigger to prevent rearrangement of the other allele. The active cell has the configuration Ig0/Ig+.
If the rearrangement is nonproductive, it creates a cell with the configuration Ig0/Ig V. There is no impediment to rearrangement of the remaining germline allele. If this rearrangement is productive, the expressing cell has the configuration Ig+/Ig V . Again, the presence of an active chain suppresses the possibility of further rearrangements.
Two successive nonproductive rearrangements produce the cell Ig V/Ig V. In some cases an Ig V/Ig V cell can try yet again. Sometimes the observed patterns of DNA can only have been generated by successive rearrangements.
The crux of the model is that the cell keeps trying to recombine V and C gene segments until a productive rearrangement is achieved. Allelic exclusion is caused by the suppression of further rearrangement as soon as an active chain is produced. The use of this mechanism in vivo is demonstrated by the creation of transgenic mice whose germline has a rearranged immunoglobulin gene. Expression of the transgene in B cells suppresses the rearrangement of endogenous genes (for review see Storb, 1987).
Allelic exclusion is independent for the heavy- and light-chain loci. Heavy chain genes usually rearrange first. Allelic exclusion for light chains must apply equally to both families (cells may have either active κ or λ light chains). It is likely that the cell rearranges its κ genes first, and tries to rearrange λ only if both κ attempts are unsuccessful.
There is an interesting paradox in this series of events. The same consensus sequences and the same V(D)J recombinase are involved in the recombination reactions at H, κ, and λ loci. Yet the three loci rearrange in a set order. What ensures that heavy rearrangement precedes light rearrangement, and that κ precedes λ? The loci may become accessible to the enzyme at different times, possibly as the result of transcription. Transcription occurs even before rearrangement, although of course the products have no coding function. The transcriptional event may change the structure of chromatin, making the consensus sequences for recombination available to the enzyme.
| Reviews | |
| Storb, U. (1987). Transgenic mice with immunoglobulin genes. Ann. Rev. Immunol.. 5, 151-174. | |
