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Gut 2004;53:480-486. doi: 10.1136/gut.2003.022293
Background: Id2, an inhibitor of basic helix-loop-helix transcription factors, regulates cell differentiation. Id[2.sup.-/-] mice exhibit a variety of phenotypes in the immune system.
Aims: In this study we investigated whether Id2 plays a role in intestinal intraepithelial lymphocytes (IELs), which constitute the main defence against pathogens in the intestinal tract.
Methods: Flow cytometry and bone marrow transplantation were used to analyse and characterise subsets of IELs of Id[2.sup.-/-] mice. Gene expression was analysed by real-time polymerase chain reaction. Intestinal barrier function was evaluated by treating mice with 5-fluorouracil (5-FU).
Results: Among the four members of the Id gene family, Id2 was selectively expressed in all T cell subsets in the small intestinal IELs. Id[2.sup.-/-] mice showed alteration in the proportions of T cell subsets and a substantial reduction in the number of IELs, especially those of the CD[4.sup.+] and CD8[alpha][[beta].sup.+] T cell subsets, indicating a more pronounced effect on thymus derived IELs. Expression of [[alpha].sub.E] integrin was reduced in CD[4.sup.+] and CD8[alpha][[beta].sup.+] T cell subsets in IELs of Id[2.sup.-/-] mice. IELs isolated from C57BL/6 mice reconstituted with Id[2.sup.-/-] bone marrow cells showed a similar phenotype to that of Id[2.sup.-/-] mice, indicating that the defects are intrinsic to bone marrow derived cells. Expression of genes encoding intestinal epithelial cell derived cytokines was reduced in Id[2.sup.-/-] mice. The 5-FU treatment revealed impaired intestinal barrier function of Id[2.sup.-/-] mice.
Conclusions: The Id2 gene is essential for constituting the intestinal mucosal barrier, particularly with respect to IELs. Id2 null mutant mice may provide a good experimental model for studying the ontogeny of IELs and intestinal inflammation and infection.
The intestinal mucosal barrier, which consists of intestinal epithelial cells (IECs) and intestinal intraepithelial lymphocytes (IELs), is the first line of the host immune defence in the gut. IELs constitute a major lymphocyte population residing in close proximity to the intestinal lumen, (1-3) and the size of the IEL population is equivalent to, or larger than, the population of peripheral lymphocytes in the spleen. (1) These cells have been considered to comprise several ontogenetically and phenotypically distinct T cell subpopulations that can be identified by their expression of T cell receptor (TCR), CD4 and CD8: TCR[alpha][[beta].sup.+]CD[4.sup.+] T cells, CD8[alpha][[beta].sup.+] T cells expressing TCR[alpha][beta], and CD8[alpha][[alpha].sup.+] T cells expressing TCR[alpha][beta] or TCR[gamma][delta], together with minor subpopulations of TCR[alpha][[beta].sup.+]CD[4.sup.+]CD8[alpha][[alpha].sup.+] and TCR[gamma][[delta].sup.+]CD[4.sup.-]CD[8.sup.-] T cells. (2) Among these T cells, CD8[alpha][[alpha].sup.+] T cells are unique to the IELs as they are not detected in conventional peripheral lymphoid organs such as the spleen, lymph nodes, and Peyer's patches. (2-4) In addition, CD8[alpha][[alpha].sup.+] IELs do not recirculate in the periphery whereas TCR[alpha][[beta].sup.+]CD[4.sup.+] and TCR[alpha][[beta].sup.+]CD8[alpha][[beta].sup.+] T cells may represent circulating CD4 and CD8[alpha][beta] T cells. (5) Thus IELs are developmentally categorised into two groups: thymically derived TCR[alpha][[beta].sup.+]CD[4.sup.+] and TCR[alpha][[beta].sup.+]CD8[alpha][[beta].sup.+] T cells, and extrathymically derived CD8[alpha][[alpha].sup.+] T cells, which may differ with respect to their use of different cytokine signalling pathways to regulate their differentiation. (3-5)
IELs have a number of important immunological functions, such as cytotoxic activity, secretion of cytokines, including interleukin (IL)-2, IL-3, IL-5, tumour necrosis factor [alpha], transforming growth factor [beta] (TGF-[beta]), and interferon [gamma] (IFN-[gamma]), and modulation of epithelial cell death and regeneration. (6) Previous studies have demonstrated an essential role for IELs against infections caused by certain microorganisms and parasites. CD8[alpha][[beta].sup.+] T cells are the major population active against Toxoplasma gondii infection (7) and markedly contribute to elimination of virus infected intestinal mucosa cells. (8) Other T cell subsets are also known to contribute to the defence system in the intestinal tract. (9) On the other hand, IECs can produce various cytokines and play an important role in trafficking and maintenance of IELs. (10-12) For example, [[alpha].sub.E][[beta].sub.7] is one of the adhesion molecules that mediate homing of lymphocytes to the gut, and upregulation of its expression has been shown to be completely dependent on epithelial derived TGF-[beta]. (11) Studies with cytokine receptor deficient mice have also pointed to the importance of IEC derived IL-2, Il-7, and IL-15 for IEL …