CTL function — not just an on/off switch

The role of transcription factors in the development of CD4⁺ T-cell lineages has been the subject of much research. T-bet has been identified as a master switch for controlling commitment to the T helper 1 (T_H1)-cell lineage and GATA3 is important for T_H2-cell development. However, the factor(s) involved in determining the development of effector CD8⁺ T-cell function have been less well understood. Now, Steve Reiner’s group show that the T-box transcription factor Eomesodermin (Eomes) complements the actions of T-bet in determining the development of cytolytic function in CD8⁺ T cells.

Mice that lack T-bet have a defect in the development of T_H1 cells, and CD4⁺ T cells and natural killer (NK) cells from these mice show defective production of interferon-γ (IFN-γ). Despite the absence of T-bet, IFN-γ production and cytolytic effector function in CD8⁺ T cells is unaffected, implying that these functions can develop independently of T-bet. However, to their surprise, the Reiner group found that introduction of a dominant-negative form of T-bet (DN T-bet) decreased the expression of IFN-γ by CD8⁺ T cells from T-bet-deficient animals. This seemed to indicate that, as T-bet itself is absent, a T-bet-related factor is important for IFN-γ production by CD8⁺ T cells.

To identify the T-bet-related factor in activated CD8⁺ T cells, the authors used degenerate oligonucleotides to a conserved region of the T-box domain to amplify complementary DNAs. Eighteen clones were obtained; eight of these encoded T-bet itself and ten encoded Eomes — a T-box factor that is important for initiating the fate of mesodermal cells in vertebrates. Using specific molecular probes, marked Eomes expression was shown to be restricted to activated CD8⁺ T cells. Similar to the effect of DN T-bet, DN Eomes inhibited IFN-γ production by wild-type and T-bet-deficient CD8⁺ T cells.

Overexpression of Eomes or T-bet by T_H2 cells or by T-bet-deficient CD4⁺ T cells was sufficient to induce IFN-γ production. The expression of Eomes coordinated with the expression of the lytic molecules perforin and granzyme B by activated NK cells and CD8⁺ T cells. Overexpression of Eomes or T-bet by developing T_H2 cells was sufficient to induce the expression of perforin and granzyme B.

Next, the authors investigated whether a causal relationship exists between Eomes expression and cytolytic function. Introduction of DN Eomes or DN T-bet (which targets both T-bet and Eomes) into CD8⁺ T cells from wild-type mice led to a greater defect in granzyme B induction than gene deletion of T-bet alone. Comparison of Eomes^+/+ and ^+/– mice showed that haploinsufficiency of Eomes was accompanied by a marked reduction of perforin messenger RNA in activated cells. Together, these results show that the development of cytolytic function of CD8⁺ T cells is not quite as simple as flicking a single master switch. Eomes seems to complement T-bet in controlling the production of IFN-γ and cytolytic molecules by these cells. Further work will be required to determine the precise mechanisms that control CD8⁺ effector T-cell development.