Mechanisms of Immune Suppression for CD8+T Cells by Human Corneal Endothelial Cells via Membrane-Bound TGFβ

Abstract

To determine whether CD8(+) T cells in vitro are suppressed or converted into regulators (Treg cells) by human corneal endothelial (HCE) cells. HCE cell lines were established from primary HCE cells, and allogeneic CD8(+) T cells were isolated from peripheral blood mononuclear cells of healthy donors. T-cell activation was assessed for proliferation by [(3)H]-thymidine incorporation. Expression of TGFbeta on HCE cells was evaluated by flow cytometry, RT-PCR, and immunohistochemistry. Anti-TGFbeta-neutralizing antibodies or TGFbeta siRNA was used to abolish the HCE-mediated inhibitory function. CD8(+) T cells exposed to HCE cells were used as Treg cells, to induce Treg cells in vitro by exposure to HCE cells. Expression of CD25 or Foxp3 on Treg cells was evaluated by flow cytometry. Cultured HCE cells produced the membrane-bound active TGFbeta isoform 2. HCE surface TGFbeta2 was necessary, to inhibit CD8(+) T-cell activation via direct cell contact. In addition, although HCE cells were found to constitutively express costimulatory molecules, such as programmed death-ligand 1 (PD-L1) and PD-L2, and secreted thrombosponsin-1, only membrane-bound TGFbeta2 was actually delivered to the CD8(+) T cells. HCE cells were also found to convert some CD8(+) T-cell populations into Treg cells via their membrane-bound TGFbeta. These HCE-induced Treg cells produced soluble forms of TGFbeta1, but not of TGFbeta2, and they also acquired a regulatory phenotype that expressed CD25(high) and Foxp3. Ocular resident tissue-exposed T cells can be suppressed or induced to become regulators within the human peripheral microenvironment.