The adaptor protein, downstream of tyrosine kinases-1 (Dok-1), and the phosphatase SHIP are both tyrosine phosphorylated in response to T cell stimulation. However, a function for these molecules in T cell development has not been defined. To clarify the role of Dok-1 and SHIP in T cell development in vivo, we compared the T cell phenotype of wild-type, Dok-1 knockout (KO), SHIP KO, and Dok-1/SHIP double-knockout (DKO) mice. Dok-1/SHIP DKO mice were runted and had a shorter life span compared with either Dok-1 KO or SHIP KO mice. Thymocyte numbers from Dok-1/SHIP DKO mice were reduced by 90%. Surface expression of both CD25 and CD69 was elevated on freshly isolated splenic CD4+ T cells from SHIP KO and Dok-1/SHIP DKO, suggesting these cells were constitutively activated. However, these T cells did not proliferate or produce IL-2 after stimulation. Interestingly, the CD4+ T cells from SHIP KO and Dok-1/SHIP DKO mice produced higher levels of TGF-β, expressed Foxp3, and inhibited IL-2 production by CD3-stimulated CD4+CD25− T cells in vitro. These findings suggest Dok-1 and SHIP function in pathways that influence regulatory T cell development.

Tolerance to self-antigens is an ongoing process that begins centrally during T-cell maturation in the thymus and continues throughout the cell's life in the periphery by a network of regulated restraints. Remaining self-reactive T-cells that escape intrathymic deletion may be silenced within the peripheral immune system by specialized regulatory CD4+ cells. By analogy, regulatory CD4+ cells that control immunity to "acquired self" should arise in circumstances where the immune system acquires tolerance to foreign MHC, such as the tolerance that develops following the exposure to foreign MHC antigens during the neonatal period. We have used this classic model of neonatal tolerance to examine the role of regulatory CD4+ cells in acquired tolerance to disparate class I and class II MHC. Adoptive transfer of unfractionated but not CD4+-depleted spleen cells from neonatal tolerant mice into SCID recipients inhibited skin graft rejection by immunocompetent CD8+ T cells. Using 5-bromo-2'-deoxyuridine incorporation, standard cytotoxic T-lymphocyte assays, short-term interferon-gamma ELISPOT, and intracellular FACS analysis to study CD8+ T-cell effector function, we demonstrated that neonatal tolerant mice contain CD4+CD25+ cells that suppress the development of anti-donor CD8+ T-cell responses in vitro. We conclude that regulatory CD4+CD25+ cells initiate and/or maintain tolerance by preventing the development of CD8+ T-cell alloreactivity.

We previously reported that prolonged graft survival in neonatally tolerant mice was associated with enhanced Th2/Th1 cytokines. To determine whether Th2 CD4 cells function in tolerance, we examined whether we could prevent tolerance by blocking Th2 CD4 maturation, using anti-interleukin (IL)-4 monoclonal antibody treatment during neonatal antigen exposure. Anti-IL-4 treatment restored the ability BALB/c of mice to reject A/J skin grafts and blocked the induction of tolerance through multiple mechanisms. Anti-IL-4 treatment blocked the development of donor microchimerism and recovered the ability of mice to proliferate and to generate appropriate delayed-type hypersensitivity (DTH) and cytotoxic T lymphocyte (CTL) responses against A/J in a dose-dependent manner. Low-dose anti-IL-4 recovered DTH responses and interferon (IFN)-gamma production, but failed to completely prevent IL-4 production or to recover the CTL activity. No A/J-reactive IFN-gamma-producing CD8 cells were detected in these mice. In contrast, mice treated with higher doses of anti-IL-4 generated normal CTL responses against A/J, and contained A/J-reactive IFN-gamma-producing CD8 cells. The recovery of CTL responses and IFN-gamma-producing CD8 cells was associated with a more complete blocking of Th2 cytokine production. Therefore, the presence of IL-4 may play an important role in the induction of neonatal tolerance by shifting maturation of CD4 cells toward Th2 cells and away from Th1 cells, and also by preventing maturation of alloreactive CD8 CTL cells.

No abstract

In the past, tolerance mechanisms have focused on processes that involve elimination (deletion) or paralysis (anergy) of immune responses. It is now becoming clearer that peripheral tolerance to antigen depends on the generation of regulatory cells that function to maintain the tolerant state. The development of peripheral tolerance may require that the immune system utilize several strategies, including deletion, anergy, and immunoregulatory pathways, and these strategies may overlap. Recent investigations using animal models of transplantation tolerance have demonstrated that immunoregulatory CD4 mechanisms may play a central role in limiting organ-destructive immune responses. In this Overview, we discuss the rationale behind the need for invoking active regulatory mechanisms in peripheral immunologic tolerance and summarize the data that support or refute a CD4 regulatory mechanism.