On the induction and function of Foxp3+ regulatory T cells

Sammanfattning: This thesis is based on three original papers with the overall aim to study the mechanisms of induction and function of Foxp3+CD4+CD25+ regulatory T cells. Regulatory T cells induce down-modulation of costimulatory molecules CD80 and CD86 on dendritic cells in vitro. In the first study we further show that the extent of down-modulation is functionally significant since regulatory T cell conditioned DCs induce poor T cell proliferation responses. We further show that down-modulation is induced rapidly and is dependent on CTLA-4, expressed by the regulatory T cells. Regulatory T cells have been reported to kill antigen-presenting cells. Yet we demonstrate here that down-modulation is not a result of selective killing of DCs expressing high level of costimulatory molecules. We propose that regulatory T cells down-modulate B7-molecules on DCs in a CTLA-4-dependent way, thereby enhancing suppression of T cell activity. Previous studies have demonstrated that anti-CD40L or anti-B7 require the presence of CD4+CD25+ regulatory T cells for induction of antigen specific hypo-responsiveness. Other tolerance strategies involving regulatory T cells have shown a dependency on IL-10. The objective of the second study was to investigate the role of Foxp3+regulatory T cells and IL-10 in the induction of transplant tolerance by treatment with CTLA4Ig, anti-CD40L and anti-LFA-1. We demonstrate here that neither T cell derived IL-10 nor the presence of regulatory T cells is essential for induction of graft acceptance in mice treated with costimulation blockade. However, CD4+ T cells depleted of regulatory cells convert into CD4+CD25+ T cells in the periphery of treated mice and histological analysis revealed accumulation of a high number of CD4+Foxp3+ T cells specifically in long term accepted grafts. Suggesting that CD4+Foxp3+ T cells may be involved in the long-term acceptance of allografts induced by costimulation blockade. Repeated immunization of mice with staphylococcal enterotoxin B (SEB) induces extensive deletion of target CD4+ T cells. Some target cells are however spared and become anergic. In the third study we report that the T cell anergy correlates with an increased proportion of Foxp3+ cells among target T cells, mainly caused by a reduced number of Foxp3- cells. The anergic CD4+ target T cells from rag-2 deficient TCR-transgenic mice was previously shown to contain regulatory T cells with distinct suppressor cell function. We show here that these anergic cells lack Foxp3-expression, but that cells from the rag-2 deficient animals can be induced to express this factor when appropriately stimulated in vivo or in vitro. These data indicate that the distinct suppressor cell function of the anergic CD4+ T cells from rag-2 deficient and rag-2 sufficient mice is due to differential Foxp3 expression by these cells. Further, the absence of Foxp3 expressing cells in naive rag-2 deficient TCR transgenic mice reveals the induction of functionally distinct regulatory cells by repeated SEB immunization.