Allelic Exclusion of TCR α-Chains upon Severe Restriction of Vα Repertoire

Abstract

Development of thymocytes through the positive selection checkpoint requires the rearrangement and expression of a suitable T cell receptor (TCR) α-chain that can pair with the already-expressed β-chain to make a TCR that is selectable. That is, it must have sufficient affinity for self MHC-peptide to induce the signals required for differentiation, but not too strong so as to induce cell death. Because both alleles of the α-chain continue to rearrange until a positively-selectable heterodimer is formed, thymocytes and T cells can in principle express dual α-chains. However, cell-surface expression of two TCRs is comparatively rare in mature T cells because of post-transcriptional regulatory mechanisms termed “phenotypic allelic exclusion”. We produced mice transgenic for a rearranged β-chain and for two unrearranged α-chains on a genetic background where endogenous α-chains could not be rearranged. Both Vα3.2 and Vα2 containing α-chains were efficiently positively selected, to the extent that a population of dual α-chain-bearing cells was not distinguishable from single α-chain-expressors. Surprisingly, Vα3.2-expressing cells were much more frequent than the Vα2 transgene-expressing cells, even though this Vα3.2-Vβ5 combination can reconstitute a known selectable TCR. In accord with previous work on the Vα3 repertoire, T cells bearing Vα3.2 expressed from the rearranged minilocus were predominantly selected into the CD8+ T cell subpopulation. Because of the dominance of Vα3.2 expression over Vα2 expressed from the miniloci, the peripheral T cell population was predominantly CD8+ cells.

Expansion of an Unusual Virtual Memory CD8+ Subpopulation Bearing Vα3.2 TCR in Themis-Deficient Mice

“…We observed that the proportion of Vα3.2 + CD8 + T cells and Vα3.2 + Vβ5.1/5.2 + cells was triple or more in the periphery of Themis –/– mice than Themis +/+ mice. We have previously found in rearranging TCR minigene mice that co-expression of Vα3.2 and Vβ5.2 is a favored combination ( 70 ). This increase in the number of CD8 + T cells carrying Vα3.2 TCR occurred in the periphery, as the proportion of these cells in the thymus were similar between Themis –/– and Themis +/+ mice.…”

Genesis of the αβ T-cell receptor

“…In principle, allelic exclusion ensures that only one receptor may be expressed on the surface of the cell, but this process is leaky: 7% of T-cells have two productive β -chains [ 19 , 20 ], and %1 express both of them on the surface [ 21 – 23 ]. Allelic exclusion in the α chain is less well quantified as it relies on different mechanisms [ 24 , 25 ], with estimates ranging from 7% [ 8 ] to 30% [ 22 ] of cells with two functionally expressed α chains.…”

Genesis of theαβT-cell receptor

“…In principle, allelic exclusion ensures that only one receptor may be expressed on the surface of the cell, but this process is leaky: 7% of T-cells have two productive β-chains [19,20], and %1 express both of them on the surface [21][22][23]. Allelic exclusion in the α chain is less well quantified as it relies on different mechanisms [24,25], with estimates ranging from 7% [8] to 30% [22] of cells with two functionally expressed α chains.…”