The cytokine transforming growth factor-beta (TGF-beta) converts naïve T cells into regulatory T (Treg) cells that prevent autoimmunity. However, in the presence of interleukin-6 (IL-6), TGF-beta has also been found to promote the differentiation of naïve T lymphocytes into proinflammatory IL-17 cytokine-producing T helper 17 (T(H)17) cells, which promote autoimmunity and inflammation. This raises the question of how TGF-beta can generate such distinct outcomes. We identified the vitamin A metabolite retinoic acid as a key regulator of TGF-beta-dependent immune responses, capable of inhibiting the IL-6-driven induction of proinflammatory T(H)17 cells and promoting anti-inflammatory Treg cell differentiation. These findings indicate that a common metabolite can regulate the balance between pro- and anti-inflammatory immunity.

TCRαβ thymocytes differentiate to either CD8αβ cytotoxic T lymphocytes or CD4+ T helper cells. This functional dichotomy is controlled by key transcription factors, including the T helper master regulator, ThPOK, which suppresses the cytolytic program in MHC class II-restricted CD4+ thymocytes. ThPOK continues to repress CD8-lineage genes in mature CD4+ T cells, even as they differentiate to T helper effector subsets. Here we show that the T helper-fate was not fixed and that mature antigen-stimulated CD4+ T cells could terminate Thpok expression and reactivate CD8-lineage genes. This unexpected plasticity resulted in the post-thymic termination of the T helper-program and the functional differentiation of distinct MHC class II-restricted CD4+ cytotoxic T lymphocytes.

Abstract4-1BB and 4-1BBL can control adaptive immunity, but we demonstrated that their interaction also suppressed myelopoiesis. 4-1BBL was found to be expressed on hematopoietic stem cells, and differentiating common myeloid (CMPs) and granulocyte-macrophage progenitors (GMPs), and 4-1BB was inducible on activated myeloid progenitors. Steady state numbers of GMPs, myeloidlineage cells, and mature dendritic cells, were elevated in 4-1BB-and 4-1BBL-deficient mice, indicative of a negative functional role, and this was confirmed in bone marrow chimeras and in vitro where the absence of 4-1BB/4-1BBL interactions led to enhanced differentiation into dendritic cell lineages. The regulatory activity was mediated through 4-1BBL, with binding by 4-1BB inhibiting differentiation of myeloid progenitors. Thus, 4-1BB and 4-1BBL have a novel function in limiting myelopoiesis and dendritic cell development.

Members of the TNFR family are thought to deliver costimulatory signals to T cells and modulate their function and survival. In this study, we compare the role of two closely related TNFR family molecules, OX40 and 4-1BB, in generating effector CD8 T cells to Ag delivered by adenovirus. OX40 and 4-1BB were both induced on responding naive CD8 T cells, but 4-1BB exhibited faster and more sustained kinetics than OX40. OX40-deficient CD8 T cells initially expanded normally; however, their accumulation and survival at late times in the primary response was significantly impaired. In contrast, 4-1BB-deficient CD8 T cells displayed hyperresponsiveness, expanding more than wild-type cells. The 4-1BB-deficient CD8 T cells also showed enhanced maturation attributes, whereas OX40-deficient CD8 T cells had multiple defects in the expression of effector cell surface markers, the synthesis of cytokines, and in cytotoxic activity. These results suggest that, in contrast to current ideas, OX40 and 4-1BB can have a clear functional dichotomy in modulating effector CD8 T cell responses. OX40 can positively regulate effector function and late accumulation/survival, whereas 4-1BB can initially operate in a negative manner to limit primary CD8 responses.

CpG DNA has been recognized as a powerful stimulant of dendritic cells (DCs). In this study, we demonstrate that CpG DNA inhibits spontaneous apoptosis of DCs. CpG DNA up-regulated cellular inhibitor of apoptosis proteins (cIAPs) as well as Bcl-2 and Bcl-xL, but down-regulated active caspase-3. Although CpG DNA activated p38 mitogen-activated protein kinase, extracellular signal-related kinase, and phosphatidylinositide-3′-OH kinase (PI3K), only the blocking of PI3K inhibited the CpG DNA-induced DC survival. Moreover, while the expression of Bcl-2 and Bcl-xL depends on both PI3K and p38 mitogen-activated protein kinase, the up-regulation of cIAPs and the down-regulation of active caspase-3 by CpG DNA require PI3K activation, suggesting PI3K-dependent up-regulation of cIAPs in the antiapoptotic activity of CpG DNA in DCs. This study indicates that CpG DNA provides a survival signal to DCs, which might be one of mechanisms by which bacterial DNA stimulates and maintains the innate immune responses.