T cell receptor gene expression in sheep: differential usage of TcR1 in the periphery and thymus

We have isolated cDNA clones for each chain of the sheep T cell receptor (TcR) and used these to analyze TcR gene expression in thymocyte and peripheral T cell subsets. Outer cortical thymocytes expressed a low level of mature message for all TcR chains suggesting that intrathymic precursors for the alpha/beta and gamma/delta lineages occur in this population. Inner cortical and medullary thymocytes expressed high levels of mature alpha/beta transcripts, low levels of mature delta transcripts but no detectable gamma message. Mature alpha/beta transcripts were detected in peripheral CD4+ lymphocytes and these as well as CD8+ cells expressed a surface heterodimer of 85 kDa which resolved into 40- and 50-kDa subunits after reduction. Peripheral CD4-CD8-lymphocytes, which in sheep are marked by the T19 antigen and may account for up to 60% of T cells in blood, expressed a surface heterodimer of 75 kDa. The T19+ cells had high levels of mature delta and truncated beta transcripts in their cytoplasm but did not express the C gamma gene detected in DN thymocytes, although they seem to share V gamma and/or J gamma elements. Both forms of the sheep TcR are associated with CD3 molecules on the cell surface. These results show that (a) in contrast to the situation in rodents and humans, a large proportion of peripheral sheep lymphocytes use TcR gamma/delta; (b) the proportion of T cells in the periphery which use TcR gamma/delta is greater than in thymus; and (c) CD4-CD8- cells in thymus and periphery (T19+) use the same C delta gene but appear to use different C gamma genes, suggesting that in sheep there may be more than one lineage of lymphocytes expressing TcR gamma/delta.