CD4⁺ CD8⁺ murine intestinal intraepithelial Iymphocytes

We have studied a population of CD4⁺ intestinal intraepithellal Iymphocytes using two-color flow cytometrlc analyses, and in highly purified fluorescent-activated cell-sorted preparations. Although CD4⁺ T cells present within the epithelial immune compartment comprised only ∼10-20% of the total intestinal epithelial lymphoid cells, an unusually high proportion of those CD4⁺ lymphocytes expressed a CD4⁺CD8⁺ phenotype which is rarely encountered in peripheral T cells. By comparison, CD4⁺ lymphocytes from spleen or lymph nodes existed exclusively as single-positive T cells. Analyses of CD4 and CD8 expression on lymphocytes from Peyer's patches, the lamina propria, and IEL indicated that CD4⁺CD8⁺ lymphocytes were unique to the IEL. Using CD4⁺CD8⁺ preparations obtained by fluorescent-activated cell sorting, CD4⁺CD8⁺ epithelial T cells were found also to express CD3 and Thy-1 surface markers. This heretofore undescribed extrathymic population of double-positive T cells constitutes a unique peripheral T cell subset which may be involved in intestinal T cell maturation and development, or could represent a highly specialized effector population.