Age‐related changes of T cell subsets in intestinal intraepithelial lymphocytes of mice

Age‐related changes in T cell subsets were examined in intestinal intraepithelial lymphocytes (i‐IEL), which contain unique T cells differentiating extrathymically. In 2‐month‐old mice bred under conventional condition, i‐IEL consisted of a large number of CD4⁻CD8α/α⁺ cells bearing either T cell receptor (TcR)α/β or TcRγ/δ and only a few CD4⁺CD8α⁻ cells. In aged mice (6 months old and 24 months old), unique CD4⁺CD8α/α⁺ i‐IEL bearing TcRα/β increased in number and conversely the proportion of TcRγ/δ⁺ i‐IEL was decreased. Such an increase in number of CD4⁺CD8α/α⁺ cells was detected in i‐IEL from aged (14‐months old) nude mice, but not in aged (14 months old) germ‐free mice, suggesting that a significant fraction of TcRα/β T cells such as CD4⁺CD8α⁺ i‐IEL can develop along an extrathymic pathway under the influence of intestinal microflora with age.