Intraepithelial Lymphocytes in Human Gut Have Lytic Potential and a Cytokine Profile That Suggest T Helper 1 and Cytotoxic Functions

The functional properties of intraepithelial lymphocytes (IEL) in normal human jejunum, ileum, and colon were investigated. Cytokine mRNA expression in IEL and enterocytes was determined by reverse transcriptase-PCR and IFN-γ⁺ IEL by immunohistochemistry. Polyclonal activators were used to study proliferation and IFN-γ secretion of IEL, and an anti-CD3-mediated redirected cytotoxicity assay was used to determine the lytic potential of IEL. Freshly isolated IEL at all three gut levels expressed mRNA for IL-1β, IL-2, IL-8, IFN-γ, and TNF-α. Approximately 10% of IEL produced IFN-γ, suggesting that IEL are immunologically active in vivo, performing similar functions along the intestine. IEL could be stimulated further in vitro to express IL-10, TNF-β, and TGF-β1, while no Th2-type cytokines were induced, suggesting suppressive and cytolytic functions for IEL. All three jejunal IEL subpopulations (CD4^-CD8^-TCR-γδ⁺, CD4⁺TCR-αβ⁺, CD8⁺TCR-αβ⁺) expressed the same four cytokines, IL-2, IL-8, IFN-γ, and TNF-α, indicating that CD4⁺TCR-αβ⁺ IEL are Th1 cells and that TCR-γδ⁺ IEL and CD8⁺TCR-αβ⁺ IEL include cytotoxic effector cells. Indeed, freshly isolated jejunal IEL displayed cytolytic activity. IEL were induced to proliferation by anti-CD3/TCR complex mAbs and leukoagglutinin, but not by Con A. There was no correlation between the magnitude of the proliferative response and the amounts of secreted IFN-γ. Enterocytes expressed IL-1β and IL-8, and sometimes TNF-α. Although jejunal enterocytes express HLA-DR and hsp60, Ag presentation by these cells may induce anergy since their cytokine profile is different from that of classical APCs.