Positive and negative regulation of pathogen induced dendritic cell function by G-protein coupled receptors

The induction of IL-12 from dendritic cells (DC) is a major initiating step in host resistance to intracellular pathogens. We have studied the regulation of this response using an in vivo model in which IL-12 production by splenic CD8alpha+ DC is followed after injection of a soluble extract (STAg) of the protozoan parasite Toxoplasma gondii. Our findings indicate that the potent IL-12 response observed is highly dependent on both the chemokine receptor CCR5 and G(i)-protein coupled signaling. In addition, we have examined the basis of the unresponsiveness of DC to secondary STAg injection which occurs following primary exposure to this parasite stimulus. Our results demonstrate that this refractory state correlates with the down-regulation of CCR5 expression on DC which, in turn, appears to depend on the induction of endogenous lipoxin A(4) (LXA(4)), a product of arachidonic acid metabolism. Since LXA(4) is known to also signal through a G-protein coupled receptor pathway, these findings taken together support a major role for G-protein signaling in the regulation of microbial-induced DC function.

Lipoxin-mediated inhibition of IL-12 production by DCs: a mechanism for regulation of microbial immunity

Lipoxins are eicosanoid mediators that show potent inhibitory effects on the acute inflammatory process. We show here that the induction of lipoxin A(4) (LXA(4)) accompanied the in vivo suppression of interleukin 12 (IL-12) responsiveness of murine splenic dendritic cells (DCs) after microbial stimulation with an extract of Toxoplasma gondii. This paralysis of DC function could not be triggered in mice that were deficient in a key lipoxygenase involved in LXA(4) biosynthesis. In addition, DCs pre-treated with LXA(4) became refractory to microbial stimulation for IL-12 production in vitro and mice injected with a stable LXA(4) analog showed reduced splenic DC mobilization and IL-12 responses in vivo. Together, these findings indicate that the induction of lipoxins in response to microbial stimulation can provide a potent mechanism for regulating DC function during the innate response to pathogens.

T-bet is rapidly induced by interferon-gamma in lymphoid and myeloid cells

Differentiation of naive CD4(+) T cells into IFN-gamma-producing T helper 1 (T(H)1) cells is pivotal for protective immune responses against intracellular pathogens. T-bet, a recently discovered member of the T-box transcription factor family, has been reported to play a critical role in this process, promoting IFN-gamma production. Although terminal T(H)1 differentiation occurs over days, we now show that challenge of mice with a prototypical T(H)1-inducing stimulus, Toxoplasma gondii soluble extract, rapidly induced IFN-gamma and T-bet; T-bet induction was substantially lower in IFN-gamma-deficient mice. Naive T cells expressed little T-bet, but this transcription factor was induced markedly by the combination of IFN-gamma and cognate antigen. Human myeloid antigen-presenting cells showed T-bet induction after IFN-gamma stimulation alone, and this induction was antagonized by IL-4 and granulocyte/macrophage colony-stimulating factor. Although T-bet was induced rapidly and directly by IFN-gamma, it was not induced by IFN-alpha, lipopolysaccharide, or IL-1, indicating that this action of IFN-gamma was specific. Moreover, T-bet induction was dependent on Stat1 but not Stat4. These data argue for a model in which IFN-gamma gene regulation involves an autocrine loop, whereby the cytokine regulates a transcription factor that promotes its own production. These findings substantially alter the current view of T-bet in IFN-gamma regulation and promotion of cell-mediated immune responses.

CD40 triggering of heterodimeric IL-12 p70 production by dendritic cells in vivo requires a microbial priming signal

CD40 ligation triggers IL-12 production by dendritic cells (DC) in vitro. Here, we demonstrate that CD40 cross-linking alone is not sufficient to induce IL-12 production by DC in vivo. Indeed, resting DC make neither the IL-12 p35 nor IL-12 p40 subunits and express only low levels of CD40. Nevertheless, after DC activation by microbial stimuli that primarily upregulate IL-12 p40 and augment CD40 expression, CD40 ligation induces a significant increase in IL-12 p35 and IL-12 p70 heterodimer production. Similarly, IL-12 p70 is produced during T cell activation in the presence but not in the absence of microbial stimuli. Thus, production of bioactive IL-12 by DC can be amplified by T cell-derived signals but must be initiated by innate signals.

CCR5 provides a signal for microbial induced production of IL-12 by CD8 alpha+ dendritic cells

The activation of dendritic cells (DC) to produce interleukin 12 (IL-12) is thought to be a key step in the initiation of cell-mediated immunity to intracellular pathogens. Here we show that ligation of the C-C chemokine receptor (CCR) 5 can provide a major signal for the induction of IL-12 synthesis by the CD8 alpha+ subset of DC and that this pathway is important in establishing interferon gamma-dependent resistance to the protozoan parasite Toxoplasma gondii. These findings support the concept that the early induction of chemokines by invading pathogens is a critical step not only for the recruitment of DC but also for the determination of their subsequent immunologic function.

Analysis of fractalkine receptor CX(3)CR1 function by targeted deletion and green fluorescent protein reporter gene insertion

The seven-transmembrane receptor CX(3)CR1 is a specific receptor for the novel CX(3)C chemokine fractalkine (FKN) (neurotactin). In vitro data suggest that membrane anchoring of FKN, and the existence of a shed, soluble FKN isoform allow for both adhesive and chemoattractive properties. Expression on activated endothelium and neurons defines FKN as a potential target for therapeutic intervention in inflammatory conditions, particularly central nervous system diseases. To investigate the physiological function of CX(3)CR1-FKN interactions, we generated a mouse strain in which the CX(3)CR1 gene was replaced by a green fluorescent protein (GFP) reporter gene. In addition to the creation of a mutant CX(3)CR1 locus, this approach enabled us to assign murine CX(3)CR1 expression to monocytes, subsets of NK and dendritic cells, and the brain microglia. Analysis of CX(3)CR1-deficient mice indicates that CX(3)CR1 is the only murine FKN receptor. Yet, defying anticipated FKN functions, absence of CX(3)CR1 interferes neither with monocyte extravasation in a peritonitis model nor with DC migration and differentiation in response to microbial antigens or contact sensitizers. Furthermore, a prominent response of CX(3)CR1-deficient microglia to peripheral nerve injury indicates unimpaired neuronal-glial cross talk in the absence of CX(3)CR1.

Paralysis of dendritic cell IL-12 production by microbial products prevents infection-induced immunopathology

Interleukin-12 plays a major role in immunity to intracellular pathogens by governing the development of IFNgamma-dependent host resistance. Nevertheless, unregulated IL-12 synthesis can lead to immunopathology, an outcome prevented by the concurrent expression of interleukin-10. Dendritic cells (DC) are an important source of the initial IL-12 stimulated by microbial agents. Here, we show that, following systemic triggering, DC can no longer be restimulated to produce IL-12 in vivo while continuing to respond in vitro. When infected with Toxoplasma gondii during this refractory state, mice mount impaired acute IFNgamma responses and, in the case of IL-10-deficient animals, are protected from cytokine-induced mortality. These findings demonstrate a previously unrecognized form of immunologic paralysis involving DC that can protect from infection-induced immunopathology.