A similar expression pattern of adhesion molecules between intermediate TCR cells in the liver and intraepithelial lymphocytes in the intestine.

Two major populations of extrathymically differentiated T cells exist in the liver and intestine. Such T cells in the liver have TCR of intermediate intensity (i.e., intermediate TCR cells) and constitutively express IL-2 receptor beta-chain (IL-2R beta), whereas those in the intestine, especially intraepithelial lymphocytes, have TCR of bright intensity, consisting of a mixture of IL-2R beta+ and IL-2R beta-. All mature thymocytes and thymus-derived T cells seen in the peripheral immune organs are TCR-bright+ IL-2R beta- under resting conditions. When the expression pattern of adhesion molecules, including CD44, L-selectin, LFA-1 and ICAM-1, was compared among these T-cell populations, they displayed quite unique patterns of expression. All extrathymic T cells in the liver, intestine, and even other organs were CD44+ L-selectin-LFA-1++ICAM-1+, whereas thymocytes and thymus-derived T cells were CD44-L-selectin+LFA-1+ICAM-1-. This inverted expression of adhesion molecules between extrathymic T cells and thymus-derived T cells might be associated with their unique tissue-localization.