Mechanisms of generating T cell receptor diversity

T cells are able to respond to foreign antigens by means of a diverse repertoire of antigen-specific receptors (T cell receptors or TCRs). There are two classes of TCRs ($\alpha\beta$ and $\gamma\delta$) that are found on mutually exclusive populations of T cells. Diversity in each receptor subunit ($\alpha,\ \beta,\ \gamma, or\ \delta$) is generated by several mechanisms with the key process being gene rearrangement. Sequence analysis of TCR subunits has revealed a region that is variable in amino acid composition and is equivalent to the third complementary determining region (CDR3) found in immunoglobulins (Igs). This region is encoded at the position where gene segments join and it has been shown to be important in defining specificity of an Ig or TCR for antigen. The mechanisms of generating diversity at this junction include the deletion of nucleotides from the ends of the genes and/or addition of nucleotides in this region. It has not been determined if both $\alpha\beta$ and $\gamma\delta$ T cells use the same recombinational machinery in these processes.To compare the molecular mechanisms involved in the generation of diversity in $\alpha\beta$ and $\gamma\delta$ TCRs, I have cloned a single member family variable (V) gene that is used by both T cell types. Analysis of transcripts from $\alpha$ and $\delta$ chains that use this gene showed a similar pattern of nucleotide deletion at the junction for both $\alpha$ and $\delta$ transcripts. In addition, there was no discernible difference in the extent of nucleotide additions at the junction. These results suggest that the mechanisms involved in the generation of junctional diversity for $\alpha$ and $\delta$ chains are similar and independent of whether the V gene would be rearranged to encode either a TCR $\alpha$ or $\delta$ subunit.It has been proposed that the expression of transcripts from unrearranged V genes may be an important prerequisite for gene rearrangement. Consistent with this hypothesis, transcripts from unrearranged TCR V region genes were detected in the thymus. Sequence analysis of a family of five V$\alpha$ genes revealed that two of the members had a unique motif at their carboxy termini. It is known that proteins with this motif undergo a post-translational modification called isoprenylation. These results raised the possibility that transcripts from unrearranged V genes could encode proteins with novel T cell-specific function. To define the factors that are important in the regulation of V gene transcription, several different approaches were taken. For example, lymphoid-derived cell lines were examined for expression of transcripts from unrearranged V genes. Results indicated that transcription from V regions of the TCR subunits $\alpha,\ \beta,\ gamma,$ and $\delta$, are differentially regulated.Together, the studies presented here show that during differentiation in the thymus, T cells express a complex molecular "machinery" to first determine which genes will be rearranged and second to generate extensive diversity in the rearranged genes.U of I OnlyETDs are only available to UIUC Users without author permissio