Severe combined immunodeficient (SCID) mice injected with co-isogenic CD4+/CD45RBhigh lymph node T cells from normal donors develop a wasting disease that is caused by hyperplasia of the intestinal epithelium. SCID mice injected with purified lymph node CD4+ T cells or CD4+/CD45RBlow T cells do not develop the disease. The IEL compartment from SCID mice injected with highly purified CD4+/CD45RBhigh T cells or CD4+ T cells contained significant numbers of T cells that expressed both CD4 and CD8

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... lpha, but not CD8 beta. The CDr+/CD8 alpha + T cells were unique to the IEL compartment of the small intestine and were not observed in significant numbers in the lamina propria, mesenteric lymph node, nor IEL compartment of the large intestine. By using Ly-5 mismatched donors and recipients, we determined that the CD4+/CD8 alpha + T cells were derived from the donor T cells. The expression of CD8 alpha was stable in vitro, and CD8 alpha mRNA was detected in sorted CD4+/CD8 alpha + T cells by reverse transcriptase-PCR (RT-PCR). Recombinase-activating gene (RAG)-1 and -2 mRNA was not detected in the intra-epithelial lymphocyte CD4+/CD8 alpha + T cell population. Thus, it appears that under conditions unique to the epithelial layer of the small intestine, mature post-thymic CD4+ T cells can be induced to express CD8 alpha.