CD8⁺ Treg - From Mouse To Man

T cell-mediated regulation of the immune response to self and foreign antigens is essential to maintain immune homeostasis and prevent autoimmune tissue destruction. The majority of studies addressing this issue, both in mice and humans, have focused on the contribution of CD4⁺CD25⁺FOXP3⁺ regulatory T cells. The contribution of regulatory CD8⁺ T cells has not been appreciated until recently. Recent studies have identified a small subset of IL-15 dependent CD8⁺ regulatory T cells (Treg) that is essential for maintenance of self- tolerance and prevention of autoimmune disease in mice. Expression of a triad of cell surface markers – CD44⁺CD122⁺Ly49⁺ – has been used to distinguish and purify CD8⁺Treg (Kim et al., Nature 2010; Kim et al., PNAS 2011).

Here we have defined the human homologue of CD8⁺ Treg. The Ly49 receptor, identified as a stable surface marker on CD8⁺ Treg, is a member of a multigenic/multiallelic receptor family recognizing classical MHC class I molecules. The functional homologue of murine Ly49 is the killer cell immunoglobulin-like receptor (KIR). Our analysis has revealed that, while expression of KIR subtype combinations appears to be stochastic and co-expression of these KIR receptors is random, the inhibitory KIR2DL2/3 and KIR3DL1 subtypes are dominantly expressed by human CD3⁺CD8⁺ T cells. Similar to murine CD8⁺ Treg, CD8⁺ T cells that express KIR2DL2/3 or KIR3DL1 also express CD44 and CD122. Moreover, consistent with murine CD8⁺ Treg, incubation with IL-15 results in activation and proliferation of KIR⁺CD8⁺ T cells that maintain a stable surface phenotype. Gene array analyses in mice has indicated that Helios, a highly conserved zinc finger transcription factor and member of the Ikaros family of transcription factors, is expressed by CD44⁺CD122⁺Ly49⁺ CD8⁺ Treg. Helios is involved in T cell development and expressed by ∼70% of FoxP3⁺CD4⁺ Treg but not by mature B cells, dendritic cells or myeloid cells. Our analyses identified a Helios⁺ subset in the CD8⁺ Treg population, which (compared to the Helios^- subset) embodies many of the functional characteristics of CD8⁺ Treg in mice. Co-expression of Helios is also apparent in some KIR⁺CD8⁺ T cells in human samples. In mice, we have shown that CD8⁺ Treg target CD4 T_FH cells and thus maintain self-tolerance. In vitro suppression assays revealed that KIR⁺CD8⁺ cells but not KIR^–CD8⁺ cells confer inhibitory activity on CD4⁺CXCR5⁺ T_FHtarget cells in humans.

Taken together, our findings implicate KIR⁺CD8⁺ cells as the human homologue of murine CD8⁺ Treg, including expression of transcription factor Helios, responsiveness to IL-15 and suppression of CD4⁺ T_FH cells. Understanding the genetic and biological features of this CD8⁺ T cell subset in humans opens the possibility of exploiting their regulatory activity for the development of immunotherapy in the context of autoimmune disease and cancer.