Restricted expression of recombination activating gene (RAG-1) in mouse lymphoid tissues.
In an attempt to determine the distribution of recombinase activity in the mouse thymus, spleen and lymph nodes, we used the in situ hybridization method to examine the expression of the recombination activating genes RAG-1 and RAG-2. Expression of RAG-1 was found in most cortical thymocytes but not in the majority of medullary thymocytes. Although hybridization signals of RAG-2 were not as intense as those of RAG-1, the localization of RAG-2 transcripts was similar to that of RAG-1. In the spleen, expression of RAG-1 was found only in limited cells near the sinus, and the majority of the cells within the follicle were negative for RAG-1 transcript. In nude mice, RAG-1-expressing cells were detected in the same regions, which suggests that in situ hybridization signals of RAG-1 in the spleen are due to the cells of B cell origin. In the lymph nodes, expression of RAG-1 was found only in the medullary region. Expression of RAG-2 transcript in the spleen and the lymph nodes, if any, was too faint to determine the specific localization. These results suggest that most of the cortical thymocytes and some cells in the spleen are capable of rearranging T cell receptor genes and immunoglobulin genes, respectively, but the possibility of some other explanation could not be ruled out in RAG-1 expressing cells of the spleen and the lymph nodes.