Extrathymic TCR Gene Rearrangement in Human Small Intestine: Identification of New Splice Forms of Recombination Activating Gene-1 mRNA with Selective Tissue Expression

Abstract

Two new 5′-untranslated region (5′UTR) exons were identified in the human gene for the lymphocyte-specific endonuclease recombination activating gene-1 (RAG1) required for the somatic recombination yielding functional Ag receptors. These 5′UTR exons were used in three different splice forms by jejunal lymphocytes of the T cell lineage. RAG1 mRNA containing the previously described 5′UTR exon was not expressed in these cells. Conversely, one of the new 5′UTR exons was not expressed in thymus. The new RAG1 mRNA splice forms were all expressed in immature T cells (CD2+CD7+CD3−). This cell population also expressed high levels of mRNA for the pre-T α-chain. In situ hybridization demonstrated jejunal cells expressing the new splice forms of RAG1 mRNA, both intraepithelially and in lamina propria. Pre-T α-chain mRNA-expressing cells were detected at the same sites. These results strongly suggest ongoing TCR gene rearrangement in human small intestinal mucosa, yielding T cells specially adapted for this environment. This seems to be achieved by two parallel processes, extrathymic T cell development and peripheral Ag-driven TCR editing.

Dextran Sulfate Sodium‐induced Colitis Generates a Transient Thymic Involution – Impact on Thymocyte Subsets

“…In addition, other reports have shown that the intestinal mucosa can act as sites for extrathymic maturation of T lymphocytes in humans [33,34] as well as in immunocompromized mice, e.g. TAP-deficient mice lacking MHC class I [34] and TcRab-deficient mice which under certain circumstances develop colitis [3,35].…”

Intestinal αβ T Cells Differentiate and Rearrange Antigen Receptor Genes In Situ in the Human Infant

“…In mice, intestinal RAG expression has been recognized for a decade (1,4), although it is usually ascribed to CD8␣␣ cells. RAG expression has been reported in human intestine (12,34,35), specifically by immature CD3 Ϫ CD2 ϩ TCR Ϫ IEL and CD3 ϩ TCR Ϫ IEL (34) in the jejunum, and by immature CD3 Ϫ CD2 ϩ CD7 ϩ IEL and LPL in the small intestines of adults and children (35). It was recently reported that T cells in the human small intestine and bone marrow express two specific splice variants of the RAG-1 gene (1A/2 and 1A/1B/2) not found in thymocytes (35).…”

“…RAG expression has been reported in human intestine (12,34,35), specifically by immature CD3 Ϫ CD2 ϩ TCR Ϫ IEL and CD3 ϩ TCR Ϫ IEL (34) in the jejunum, and by immature CD3 Ϫ CD2 ϩ CD7 ϩ IEL and LPL in the small intestines of adults and children (35). It was recently reported that T cells in the human small intestine and bone marrow express two specific splice variants of the RAG-1 gene (1A/2 and 1A/1B/2) not found in thymocytes (35). It was proposed that T cells using the RAG-1 1A/2 and 1A/1B/2 variants migrate directly from the bone marrow to the small intestine, where they use the 1A/2-and 1A/1B/2-specific RAG-1 variants during TCR gene recombination in situ (35).…”

“…It was recently reported that T cells in the human small intestine and bone marrow express two specific splice variants of the RAG-1 gene (1A/2 and 1A/1B/2) not found in thymocytes (35). It was proposed that T cells using the RAG-1 1A/2 and 1A/1B/2 variants migrate directly from the bone marrow to the small intestine, where they use the 1A/2-and 1A/1B/2-specific RAG-1 variants during TCR gene recombination in situ (35). As our RAG-1 primers are located in exon 2, we would detect all isoforms of RAG-1, and so our finding that 15 of 19 intestinal samples express RAG-1 could correspond to the expression of a gut-specific RAG isoform and certainly merits investigation.…”

Lymphocyte reconstitution after allogeneic bone marrow transplantation in a previously thymectomized patient—no evidence of extrathymic T-cell maturation

“…Indeed, intraepithelial lymphocytes located in the small intestinal mucosa were found to express recombination activating genes RAG 1, RAG 2 and pre-T a-chain, which are all required for T-cell receptor (TCR) rearrangement. [2][3][4] Although these findings strongly suggest the presence of extrathymic T-cell maturation (ETCM), no direct evidence of this phenomenon has been provided so far.…”