Alloactivation of Naïve CD4+CD8−CD25+T Regulatory Cells: Expression of CD8α Identifies Potent Suppressor Cells That Can Promote Transplant Tolerance Induction

Therapy with alloantigen-specific CD4 + CD25 + T regulatory cells (Treg) for induction of transplant tolerance is desirable, as nave thymic Treg (tTreg) are not alloantigen-specific and are weak suppressor cells. Nave tTreg from DA rats cultured with fully allogeneic PVG stimulator cells in the presence of rIL-2 express IFN-gamma receptor (IFNGR) and IL-12 receptor beta2 (IL-12R2) and are more potent alloantigen-specific regulators that we call Ts1 cells. This study examined additional markers that could identify the activated alloantigen-specific Treg as a subpopulation within the CD4 + CD25 + Foxp3 + Treg. After culture of nave DA CD4 + CD8 -CD25 + T cells with rIL-2 and PVG alloantigen, or rIL-2 without alloantigen, CD8 was expressed on 10-20% and CD8 on <5% of these cells. These cells expressed ifngr and Il12rb2. CD8 + cells had increased Ifngr that characterizes Ts1 cells as well was Irf4, a transcription factor induced by TCR activation. Proliferation induced by re-culture with rIL-12 and alloantigen was greater with CD4 + CD8 + CD25 + Treg consistent with the CD8 + cells expressing IL-12R. In MLC, the CD8 + fraction suppressed responses against allogeneic stimulators more than the mixed Ts1 population, whereas the CD4 + CD8 -CD25 + T cells were less potent. In an adoptive transfer assay, rIL-2 and alloantigen activated Treg suppress rejection at a ratio of 1:10 with nave effector cells, whereas alloantigen and rIL-2 activated tTreg depleted of the CD8 + cells were much less effective. This study demonstrated that expression of CD8 by rIL-2 and alloantigen activation of CD4 + CD8 -CD25 + Foxp3 + T cells was a marker of activated and potent Treg that included alloantigen-specific Treg.