Regulatory T cells are potent inhibitors of autoimmune responses, but how they maintain self-reactive T cells in a naïve state is still unclear. Using a model of selective Treg reconstitution of immunodeficient mice, we have demonstrated that Tregs regulate DC expression of the costimulatory molecules CD80 and CD86 in the steady state. By this means, Tregs control the activation threshold of naïve CD4 T cells. When Tregs are rendered deficient in CTLA-4, their effects on DC costimulation and activation of naïve CD4 T cells are simultaneously blocked, indicating that Tregs suppress naïve T cell activation in vivo via a DC intermediary, rather than by direct T:T interaction.
How CTLA-4 is involved in this process is unclear. Two CD80/CD86-binding isoforms of CTLA-4 are conserved between mouse and man: a membrane bound form (mCTLA-4), and a soluble secreted form (sCTLA-4). Each of these isoforms has previously been implicated in Treg-dependent regulation of CD80/CD86 in vitro. However the relative contributions of these two splice variants have not been elucidated, particularly under in vivo conditions.
In our Treg-reconstitution model we observed that Treg-dependent normalisation of CD80/CD86 expression was associated with large increases in sCTLA-4. In contrast, mCTLA-4 was expressed at low levels in both Treg-reconstituted mice and WT controls. sCTLA-4 has previously been reported to be the default splice variant in unstimulated human CD4 T cells. Consistent with this, Treg cultured ex vivo in the absence of a TCR stimulus produced mainly sCTLA-4, whereas Tregs that received a TCR stimulus downregulated sCTLA-4 and upregulated production of mCTLA-4.
Together, these results suggest that steady-state regulation of DC costimulation, and thus the T cell activation threshold, is mediated via Treg-production of soluble CTLA-4. The precise mechanism by which this occurs is under investigation.