IL-33 administration is associated with facilitation of Th type-2 (Th2) responses and cardioprotective properties in rodent models. However, in heart transplantation, the mechanism by which IL-33, signaling through ST2L, the membrane-bound form of ST2, promotes transplant survival is unclear. We report that IL-33 administration, while facilitating Th2 responses, also increases immunoregulatory myeloid cells and CD4+ Foxp3+ regulatory T cells (Treg) in mice. IL-33 expands functional myeloid-derived suppressor cells (MDSC), -CD11b+ cells that exhibit intermediate (int) levels of Gr-1 and potent T cell suppressive function. Furthermore, IL-33 administration causes a St2-dependent expansion of suppressive CD4+ Foxp3+ Treg, including a ST2L+ population. IL-33 monotherapy following fully allogeneic mouse heart transplantation resulted in significant graft prolongation, associated with increased Th2-type responses and decreased systemic CD8+ IFN-γ+ cells. Also, despite reducing overall CD3+ cell infiltration of the graft, IL-33 administration markedly increased intragraft Foxp3+ cells. Whereas control graft recipients displayed increases in systemic CD11b+ Gr-1hi cells, IL-33-treated recipients exhibited increased CD11b+ Gr-1int cells. Enhanced ST2 expression was observed in the myocardium and endothelium of rejecting allografts, however the therapeutic effect of IL-33 required recipient St2 expression and was dependent on Treg. These findings reveal a new immunoregulatory property of IL-33. Specifically, in addition to supporting Th2 responses, IL-33 facilitates regulatory cells, particularly functional CD4+ Foxp3+ Treg that underlie IL-33-mediated cardiac allograft survival.

Plasmacytoid (p)DC are type-I IFN-producing cells known for their capacity to promote anti-viral innate and adaptive immune responses. Despite their potent anti-viral function, when compared to conventional DC (cDC), pDC exhibit poor immunostimulatory ability and their interaction with T cells often favors the generation of Treg. pDC are activated primarily in response to single-stranded (ss) RNA and ss DNA through TLR7 and TLR9, respectively, but also through TLR-independent mechanisms. Non-lymphoid tissue pDC, such as those residing in the airways, gut, and liver, play a significant role in regulating mucosal immunity and are critical for the development of tolerance to inhaled or ingested Ags. Herein we discuss properties that define tolerogenic pDC and how their unique characteristics translate into an ability to regulate immunity and promote the development of tolerance. We cover the importance of pDC during intrathymic Treg development and the maintenance of peripheral tolerance, as well as their regulatory role in transplantation, autoimmunity, and cancer. We highlight recent findings regarding danger- and pathogen-associated molecular pattern (DAMP and PAMP, respectively) signaling in the regulation of pDC function, and how the ability of pDC to promote tolerance translates into the potential clinical applications of these cells as therapeutic targets to regulate immune reactivity.

The nucleotide-binding oligomerization domain (NOD)2/CARD15 protein, which senses muramyl dipeptide (MDP), a product of bacterial peptidoglycan, appears to play an important role in regulating intestinal immunity. Although the liver is exposed to gut-derived MDP, the influence of NOD2 ligation on hepatic APC, in particular dendritic cells (DC), is unknown. Freshly isolated mouse liver and spleen plasmacytoid (p)DC expressed higher levels of NOD2 message than conventional myeloid (m)DC. Following MDP stimulation in vivo, liver pDC, but not mDC, up-regulated expression of IFN regulatory factor 4 (IRF-4), a negative regulator of TLR signaling, and induced less allogeneic T cell proliferation and IFN-γ production. The adoptive transfer of liver pDC from MDP-treated mice failed to prime allogeneic T cells in vivo. By contrast, splenic DC IRF-4 levels and T cell stimulatory activity remained unchanged. Liver pDC from MDP-stimulated mice also displayed greater IκBα, cell surface B7-H1, and B7-H1 relative to CD86 than control liver pDC. No similar effects were observed for liver mDC or spleen DC. Absence of B7-H1 on liver pDC reversed the inhibitory effect of MDP. After ex vivo stimulation with LPS or CpG, liver pDC but not mDC from MDP-treated animals secreted less IL-12p70, IL-6, and TNF-α and induced weaker allogeneic T cell proliferation than those from controls. Moreover, CpG-stimulated liver pDC from MDP-treated mice secreted less IFN-α than their splenic counterparts, and systemic levels of IFN-α were reduced in MDP-treated animals after CpG administration. These findings suggest that differential effects of NOD2 ligation on liver pDC may play a role in regulating hepatic innate and adaptive immunity.

Plasmacytoid dendritic cells (pDC) constitute the body’s principal source of type I interferon (IFN) and are comparatively abundant in the liver. Among various cytokines implicated in liver ischemia and reperfusion (I/R) injury, type I IFNs have been described recently as playing an essential role in its pathogenesis. Moreover, type I IFNs have been shown to up-regulate hepatocyte expression of IFN regulatory factor 1 (IRF-1), a key transcription factor that regulates apoptosis and induces liver damage after I/R. Our aim was to ascertain the capacity of IFN-α released by liver pDC to induce liver damage through hepatic IRF-1 up-regulation after I/R injury. Our findings show that liver pDC mature and produce IFN-α in response to liver I/R. Liver pDC isolated after I/R induced elevated levels of IRF-1 production by hepatocytes compared with liver pDC isolated from sham-operated mice. Notably, hepatic IRF-1 expression was reduced significantly by neutralizing IFN-α. In vivo, IFN-α neutralization protected the liver from I/R injury by reducing hepatocyte apoptosis. This was associated with impaired expression of IRF-1 and pro-apoptotic molecules such as Fas ligand, its receptor (Fas) and death receptor 5 which are regulated by IRF-1. Furthermore, pDC-depleted mice failed to up-regulate hepatic IFN-α and displayed less liver injury associated with reduced levels of hepatic IL-6, tumor necrosis factor-α and hepatocyte apoptosis after I/R compared with controls. Conclusion: these data support the hypothesis that IFN-α derived from liver pDC plays a key role in the pathogenesis of liver I/R injury by enhancing apoptosis as a consequence of induction of hepatocyte IRF-1 expression.

Freshly-isolated hepatic dendritic cells (DC) are comparatively immature, relatively resistant to maturation, and can down-modulate effector T cell responses. Molecular mechanisms that underlie these properties are ill-defined. DNAX-activating protein of 12 kDa (DAP12) is an ITAM-bearing transmembrane adaptor protein, that integrates signals through several receptors, including triggering receptor expressed on myeloid cells (TREM)-1, -2; and CD200R. Notably, DC propagated from DAP12-deficient mice exhibit enhanced maturation in response to TLR ligation. Given the constitutive exposure of liver DC to endotoxin draining from the gut, we hypothesized that DAP12 might regulate liver DC maturation. We show that while DAP12 is expressed by both freshly-isolated liver and spleen myeloid DC, LPS-stimulated liver DC maintain DAP12 mRNA expression at higher levels than splenic DC. Moreover, inhibition of DAP12 expression by liver DC using small interfering (si)RNA, promotes their phenotypic and functional maturation, resulting in enhanced TNFα, IL-6 and IL-12p70 production, reduced secretion of IL-10 and enhanced CD4+ and CD8+ T cell proliferation. Furthermore, DAP12 silencing correlates with decreased STAT3 phosphorylation in mature liver DC, and with diminished expression of the IL-1R-associated kinase (IRAK)-M, a negative regulator of TLR signaling. These findings highlight, for the first time, a regulatory role for DAP12 in hepatic DC maturation, and suggest a mechanism whereby this function may be induced/maintained.