TGF-beta, T-cell tolerance and anti-CD3 therapy

Abstract

Recent studies by Chatenoud and co-workers suggest that non-mitogenic F(ab′)₂ fragments of anti-CD3 antibodies, which cannot bind the Fc receptor, induce a prolonged period of tolerance and prevent diabetes in nonobese diabetic (NOD) mice. Tolerance is established by regulatory T cells through the production of transforming growth factor-β1 (TGF-β1), but the mechanism by which TGF-β1 confers this activity is unclear. Analysis of mice deficient in TGF-β1 suggests that TGF-β1 raises the threshold at which intracellular calcium activates T cells to a level that prevents an autoimmune response.

Figure 1

Regulatory T (T_reg) cells prevent diabetes in nonobese diabetic (NOD) mice. (a) CD4⁺CD25⁺ T cells develop in the thymus and prevent peripheral T-cell activation and autoimmune disease. T_reg cells inhibit proliferation and cytokine production by T helper (Th) cells on stimulation in vitro and in vivo. On stimulation, T_reg cells produce transforming growth factor-β1 (TGF-β1), which in turn inhibits the activation of Th cells. CD4⁺CD25^hi T_reg cells also inhibit Th-cell responses in a TGF-β1-independent manner, although their role in vivo is still unclear. (b) Deficiency in functional CD4⁺CD25⁺ T_reg cells causes diabetes in NOD mice. In these mice, Th cells are spontaneously activated and T_reg cells do not produce TGF-β1, thereby resulting in autoimmune type 1 diabetes. (c) Induction of functional T_reg cells prevents diabetes in NOD mice through TGF-β1-mediated inhibition of Th-cell activation. Treatment of NOD mice with anti-CD3 generates functional CD4⁺CD25⁺ T_reg cells from CD4⁺ CD25⁻ T cells. Blockade of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4)-mediated co-stimulation inhibits the production of TGF-β1 by T_reg cells and thus reverses the protection mediated by anti-CD3 treatment. Consequently, blockade by anti-TGF-β1 also reverses the protection mediated by T_reg cells.

Figure 2

Production of transforming growth factor-β1 (TGF-β1) is essential for the function of regulatory T (T_reg) cells. On activation of CD4⁺CD25⁻ T cells by anti-CD3, T helper (Th) cells produce pro-inflammatory cytokines such as interleukin (IL)-2, interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). The anti-inflammatory cytokines TGF-β1 and IL-10 (not shown) are also induced to a lesser extent. Treatment of CD4⁺CD25⁻ T cells with the non-mitogenic anti-CD3 F(ab′)₂ fragment induces expression of CD25, resulting in CD4⁺CD25⁺ T_reg cells. Activated T_reg cells produce mainly TGF-β1 and IL-10, but do not produce IL-2, IFN-γ or TNF-α. Co-stimulation through CD28 is essential for the development of T_reg cells and their survival, whereas co-stimulation through cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) is essential for their functional response. Expression of the transcription factor gene Foxp3 is essential for the development and function of T_reg cells, which inhibit Th-cell activation through the juxtacrine activity of TGF-β1. Blue cylinder, CD25; orange cylinder, CD28; pink cylinder, CD4; yellow cylinder, CTLA-4; green cylinder, T-cell receptor; red cylinder, CD3.