CD4−CD8− thymocytes that express the T cell receptor may have previously expressed CD8

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Abstract
Amongst CD4−CD8− (double negative) thymocytes there is a sizeable population (variable from strain to strain) of cells expressing surface T cell receptor (TCR). These TCR+ double negatives are predominantly non-cycling, have very little precursor activity, and, unlike the TCR− CD4−CD8− thymocytes, appear not to be part of the mainstream of thymocyte development. A unique feature of this population is the biased Vβ-gene region usage. In CBA mice, 60–70% of TCR+ CD4−CD8− cells express receptors that utilize Vβ8 gene products, compared with peripheral T cells from the same strain which are only 20–30% Vβ8+. This suggests that the high Vβ8 usage may be the result of some selective process. A growing body of experimental data suggests that TCR specificity selection occurs at the CD4+CD8+ stage of thymocyte development. In order to gain some insight into the previous history of the TCR+ double negatives, in particular whether or not they have previously expressed CD8 and therefore been eligible for selection, we have determined the methylation state of the CD8 gene and compared it to other thymocyte populations. We show that the TCR+ CD4−CD8− thymocytes are demethyiated at some sites in the CD8 gene, consistent with previous CD8 expression. However, the demethylation pattern is distinct from that seen on typical peripheral T cells or on mature thymocytes, suggesting that the TCR+ CD4−CD8− thymocytes are not derived from mature thymocytes or peripheral T cells which have returned to the thymus and downregulated CD8 expression. We also show that a proportion of the thymic double negatives which express TCRγδ are demethylated at some sites in the CD8 gene.