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 naïve thymic Treg (tTreg) are not alloantigen-specific and are weak suppressor cells. Naïve 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-12Rβ2) 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 naïve DA CD4(+)CD8(−)CD25(+)T cells with rIL-2 and PVG alloantigen, or rIL-2 without alloantigen, CD8α was expressed on 10–20% and CD8β on <5% of these cells. These cells expressed ifngr and Il12rb2. CD8α(+) 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(+)CD8α(+)CD25(+)Treg consistent with the CD8α(+) cells expressing IL-12R. In MLC, the CD8α(+) 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 naïve effector cells, whereas alloantigen and rIL-2 activated tTreg depleted of the CD8α(+) cells were much less effective. This study demonstrated that expression of CD8α 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.