Expression of recombination-activating gene in mature peripheral T cells in Peyer's patch

Recombination-activating gene (RAG) 1 and 2 are essential for the gene rearrangement of antigen receptors of both T and B cells. To investigate RAG gene expression in peripheral lymphoid organs other than the thymus and bone marrow, we established mice in which a green fluorescent protein (GFP) gene is knocked-in the RAG2 gene locus (RAG2-GFP mice). In the thymus and bone marrow of heterozygous RAG2-GFP mice, as expected, GFP expression was detected in the appropriate stages of developing T and B cells. Interestingly, only a fraction of Thy-1.2⁺ cells in the Peyer's patch were found to be GFP⁺ amongst the peripheral lymphoid organs. The GFP⁺ cells expressed high levels of surface TCRβ and CD3, suggesting mature T cells with rearranged TCRαβ. However, they showed activated/memory phenotypes, i.e. CD45RB^low, CD69^high, CD44^high and CD62L^low, and belonged to a CD4⁺CD8⁺ population expressing c-kit, IL-7R and pTα characteristic of immature developing lymphocytes. Moreover, RAG⁺ Peyer's patch T cells seem to be of thymic origin as judged by their expression of CD8αβ. These results show that there exists a fraction of mature T cells expressing RAG genes in the Peyer's patch, implying a potential for a secondary rearrangement of TCR in extrathymic tissues.