Different expression of the recombination activity gene RAG‐1 in various populations of thymocytes, peripheral T cells and gut thymus‐independent intraepithelial lymphocytes suggests two pathways of T cell receptor rearrangement

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Abstract
The presence of transcripts of the recombination activating gene RAG-1 was studied by in situ hybridization on selected populations of murine thymocytes, peripheral lymphocytes and gut intraepithelial lymphocytes (IEL), obtained by cell sorting. RAG-1 mRNA was found in a majority of “double-positive” (DP) thymocytes, but was absent in “single-positive” thymocytes and peripheral T lymphocytes. The only other T lineages in which about 10%–20% of the cells contained RAG-1 mRNA, and in smaller amounts, were “double-negative” (DN), T cell receptor (TcR) γδ- cortical thymocytes and gut CD3− IEL. These observations suggest that (a) the high expression of RAG-1 transcripts in DP thymocytes is related to the process of expansion-selection of these cells, probably accompanied by repeated TcR rearrangements, and that (b) in contrast. CD3− IEL from the gut (which are thymus independent) as well as some DN thymocytes undergo limited TcR rearrangement giving rise locally to TcR+ T cells without prior extensive process of local expansion-selection. A small percentage of peripheral B cells also contained RAG-1 mRNA, raising the possibility that this protein may also be involved in immunoglobulin class switching.