IMMUNOLOGY:
Expansion of the Allelic Exclusion Principle?

Cells normally have two copies of autosomal genes, one inherited from the mother and one from the father. For most genes, both copies (or alleles) are used by the cell, but for certain genes, cells randomly select only one allele to encode RNA and protein for that gene. For various cells of the immune system and for olfactory neurons, this so-called monoallelic expression is one of the mechanisms ensuring that a single kind of receptor is displayed on the surface of each cell. For example, each B cell must produce a single antibody from among the extremely large number of possible antibodies that could be made. A well characterized, complex DNA rearrangement mechanism leads to the expression of this single, specific antibody; a key aspect of this mechanism is that it results in the monoallelic expression of each expressed immunoglobulin gene. This phenomenon is called allelic exclusion, a particular type of monoallelic expression where not only is the other allele not expressed, but other genes from the same family are turned off as well (1). Similarly, T cells and certain cells expressing natural killer (NK) cell receptor genes (2) use allelic exclusion to maintain each cell's specificity. Individual neurons of the olfactory system express only one of a family of olfactory receptors using mechanisms that also result in allelic exclusion (3). Now, a report on page 2118 of this issue by Holländer et al. (4) describes monoallelic expression of the mouse gene encoding interleukin-2 (IL-2), an important immunomodulatory cytokine expressed and secreted by most activated CD4⁺ T cells (5). In contrast to the diverse protein which are monoallelically expressed, IL-2 is not a receptor and its expression does not confer a cell-specific phenotype. How does monoallelic transcription fit into the picture of the regulation of the IL-2 gene and the biological function of IL-2?

The IL-2 gene is coordinately regulated by T cell receptor signaling and other signals from accessory receptors. Because its transcriptional regulation depends on multiple signals, the IL-2 gene has been extensively studied as a model for how distinct signal transduction pathways can be integrated into a specific transcriptional response (5). The extremely tight regulation of the IL-2 gene has been explained by the cooperative binding of transcription factors regulated by a variety of signaling pathways. All of the necessary factors must be present for any of them to bind, leading to an all-or-nothing effect on transcription (6). There is also evidence that changes in the chromatin structure of the IL-2 gene are important for its regulation (7).

The most compelling evidence for monoallelic expression of the IL-2 gene comes from an allele-discriminating reverse transcriptase-polymerase chain reaction (RT PCR) analysis of the IL-2 mRNA in single, activated CD4⁺ T cells. In these experiments, the two alleles are distinguished by a polymorphism. Holländer et al. observe that individual cells contain either maternal transcripts or paternal transcripts, and do not observe any cells expressing both alleles. It will be interesting to see whether future experiments demonstrate that monoallelic expression is an absolute phenomenon in CD4⁺ T cells. Alternatively, certain T cells may express both alleles. This would be similar to the case of the LY49 NK cell receptor gene, which was first observed to be strictly monoallelic but subsequently proved to be expressed from both alleles in some cells (8).

Also consistent with monoallelic expression of the IL-2 gene is the observation that the gene replicates asynchronously. Asynchronous replication is associated with various monoallelically expressed genes including the olfactory receptor genes, imprinted genes, and X-inactivated genes (3, 9). In imprinted genes and the inactive X chromosome, a correlation can be made between which allele is replicated earlier and which allele is transcribed. In the olfactory receptor genes, which allele is transcribed and which allele is replicated first are both random with respect to parental legacy. However, it has not been possible to determine whether there is a correlation between early (or late) replication and transcription because the olfactory receptor genes are only transcribed in postmitotic cells. Whether the replication asynchrony of the IL-2 gene is random with respect to parental legacy and correlated with transcription is not yet known.

The timing of replication for the lymphocyte antigen receptor genes has not been extensively analyzed. The mouse immunoglobulin heavy chain (IgH) constant region gene is synchronously replicating (9). This could reflect the predominance of the well-characterized negative feedback mechanism in regulating allelic exclusion in immunoglobulin genes. Alternatively, other areas of the IgH locus may reveal asynchronous replication. For genes where asynchronous replication is observed, the asynchrony is observed in embryonic cells and in adult cells where the genes are not expressed. This indicates that the asynchrony of replication reflects a distinct marking of the two alleles in all cell types, irrespective of tissue-specific gene expression. If the asynchrony of replication of the IL-2 gene is also present in cells other than T cells, this would indicate that before T cell development, one IL-2 allele is rendered unavailable for future activation.

All other known examples of random monoallelic expression of autosomal genes (immunoglobulins, T cell receptors, olfactory receptors, and LY49 NK cell receptors) involve genes encoding diverse receptors in systems in which receptor expression is restricted so that cells have distinct specificities. In all of these cases, monoallelic expression is a fundamental aspect of the transcriptional restriction of receptor expression. Why would the cytokine IL-2, which is expressed in most activated CD4⁺ T cells be expressed monoallelically? Interestingly, both IL-2 and the IL-2 receptor are expressed during thymocyte development around the time of establishment of allelic exclusion in T cell receptor genes. For mature T cells, the well-characterized integration of signal transduction pathways by the transcription factors could account for the observed regulation without having to invoke monoallelic expression (6). Thus, monoallelic expression may reflect a new aspect of the regulation of IL-2 gene expression, perhaps one involving an interplay between nuclear architecture and chromatin structure (10).

References

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