MHC Class II-restricted antigen presentation by plasmacytoid dendritic cells drives proatherogenic T cell immunity.

BACKGROUND: Plasmacytoid dendritic cells (pDCs) bridge innate and adaptive immune responses and are important regulators of immuno-inflammatory diseases. However, their role in atherosclerosis remains elusive. METHODS AND RESULTS: Here, we used genetic approaches to investigate the role of pDCs in atherosclerosis. Selective pDC deficiency in vivo was achieved using CD11c-Cre × Tcf4(-/flox) bone marrow transplanted into Ldlr(-/-) mice. Compared with control Ldlr(-/-) chimeric mice, CD11c-Cre × Tcf4(-/flox) mice had reduced atherosclerosis levels. To begin to understand the mechanisms by which pDCs regulate atherosclerosis, we studied chimeric Ldlr(-/-) mice with selective MHCII deficiency on pDCs. Significantly, these mice also developed reduced atherosclerosis compared with controls without reductions in pDC numbers or changes in conventional DCs. MHCII-deficient pDCs showed defective stimulation of apolipoprotein B100-specific CD4(+) T cells in response to native low-density lipoprotein, whereas production of interferon-α was not affected. Finally, the atheroprotective effect of selective MHCII deficiency in pDCs was associated with significant reductions of proatherogenic T cell-derived interferon-γ and lesional T cell infiltration, and was abrogated in CD4(+) T cell-depleted animals. CONCLUSIONS: This study supports a proatherogenic role for pDCs in murine atherosclerosis and identifies a critical role for MHCII-restricted antigen presentation by pDCs in driving proatherogenic T cell immunity.

T-bet is essential for Th1-mediated, but not Th17-mediated, CNS autoimmune disease.

T cells that produce both IL-17 and IFN-γ, and co-express ROR-γt and T-bet, are often found at sites of autoimmune inflammation. However, it is unknown whether this co-expression of T-bet with ROR-γt is a prerequisite for immunopathology. We show here that T-bet is not required for the development of Th17-driven experimental autoimmune encephalomyelitis (EAE). The disease was not impaired in T-bet(-/-) mice and was associated with low IFN-γ production and elevated IL-17 production among central nervous system (CNS) infiltrating CD4(+) T cells. T-bet(-/-) Th17 cells generated in the presence of IL-6/TGF-ß/IL-1 and IL-23 produced GM-CSF and high levels of IL-17 and induced disease upon transfer to naïve mice. Unlike their WT counterparts, these T-bet(-/-) Th17 cells did not exhibit an IL-17→IFN-γ switch upon reencounter with antigen in the CNS, indicating that this functional change is not critical to disease development. In contrast, T-bet was absolutely required for the pathogenicity of myelin-responsive Th1 cells. T-bet-deficient Th1 cells failed to accumulate in the CNS upon transfer, despite being able to produce GM-CSF. Therefore, T-bet is essential for establishing Th1-mediated inflammation but is not required to drive IL-23-induced GM-CSF production, or Th17-mediated autoimmune inflammation.

Th17 plasticity in human autoimmune arthritis is driven by the inflammatory environment.

In several murine models of autoimmune arthritis, Th17 cells are the dominant initiators of inflammation. In human arthritis the majority of IL-17-secreting cells within the joint express a cytokine phenotype intermediate between Th17 and Th1. Here we show that Th17/1 cells from the joints of children with inflammatory arthritis express high levels of both Th17 and Th1 lineage-specific transcription factors, RORC2 and T-bet. Modeling the generation of Th17/1 in vitro, we show that Th17 cells "convert" to Th17/1 under conditions that mimic the disease site, namely low TGFbeta and high IL-12 levels, whereas Th1 cells cannot convert to Th17. Th17/1 cells from the inflamed joint share T-cell receptor (TCR) clonality with Th17 cells, suggesting a shared clonal origin between Th17 and Th17/1 cells in arthritis. Using CD161, a lectin-like receptor that is a marker of human Th17, we show synovial Th17 and Th17/1 cells, and unexpectedly, a large proportion of Th1 cells express CD161. We provide evidence to support a Th17 origin for Th1 cells expressing CD161. In vitro, Th17 cells that convert to a Th1 phenotype maintain CD161 expression. In the joint CD161+ Th1 cells share features with Th17 cells, with shared TCR clonality, expression of RORC2 and CCR6 and response to IL-23, although they are IL-17 negative. We propose that the Th17 phenotype may be unstable and that Th17 cells may convert to Th17/1 and Th1 cells in human arthritis. Therefore therapies targeting the induction of Th17 cells could also attenuate Th17/1 and Th1 effector populations within the inflamed joint.

TLR-4 ligation of dendritic cells is sufficient to drive pathogenic T cell function in experimental autoimmune encephalomyelitis.

BACKGROUND: Experimental autoimmune encephalomyelitis (EAE) depends on the initial activation of CD4(+) T cells responsive to myelin autoantigens. The key antigen presenting cell (APC) population that drives the activation of naïve T cells most efficiently is the dendritic cell (DC). As such, we should be able to trigger EAE by transfer of DC that can present the relevant autoantigen(s). Despite some sporadic reports, however, models of DC-driven EAE have not been widely adopted. We sought to test the feasibility of this approach and whether activation of the DC by toll-like receptor (TLR)-4 ligation was a sufficient stimulus to drive EAE. FINDINGS: Host mice were seeded with myelin basic protein (MBP)-reactive CD4+ T cells and then were injected with DC that could present the relevant MBP peptide which had been exposed to lipopolysaccharide as a TLR-4 agonist. We found that this approach induced robust clinical signs of EAE. CONCLUSIONS: DC are sufficient as APC to effectively drive the differentiation of naïve myelin-responsive T cells into autoaggressive effector T cells. TLR-4-stimulation can activate the DC sufficiently to deliver the signals required to drive the pathogenic function of the T cell. These models will allow the dissection of the molecular requirements of the initial DC-T cell interaction in the lymphoid organs that ultimately leads to autoimmune pathology in the central nervous system.

Perivascular mast cells regulate vein graft neointimal formation and remodeling.

Objective. Emerging evidence suggests an important role for mast cells in vein graft failure. This study addressed the hypothesis that perivascular mast cells regulate in situ vascular inflammatory and proliferative responses and subsequent vein graft neointimal lesion formation, using an optimized local mast cell reconstitution method. Methods and Results. Neointimal hyperplasia was induced by insertion of a vein graft into the right carotid artery in wild type and mast cell deficient Kit(W-sh/W-sh) mice. In some experiments, mast cells were reconstituted systemically (tail vein injection of bone marrow-derived mast cells) or locally (directly into the right neck area) prior to vein grafting. Vein graft neointimal lesion formation was significantly (P < 0.05) reduced in Kit(W-sh/W-sh) mice. Mast cell deficiency reduced the number of proliferating cells, and inhibited L-selectin, CCL2, M-CSF and MIP-3α expression in the vein grafts. Local but not systemic mast cell reconstitution restored a perivascular mast cell population that subsequently promoted neointimal formation in mast cell deficient mice. Conclusion. Our data demonstrate that perivascular mast cells play a key role in promoting neointima formation by inducing local acute inflammatory and proliferative responses. These results suggest that ex vivo intraoperative targeting of mast cells may have therapeutic potential for the prevention of pathological vein graft remodeling.

Induction of passive EAE using myelin-reactive CD4+ T cells.

Experimental autoimmune encephalomyelitis (EAE) is an autoimmune disease of the central nervous system (CNS) often used as a model for the early inflammatory stages of multiple sclerosis and also as a model of organ-specific autoimmune disease.This protocol describes the induction of passive EAE in mice, either using T cells isolated from mice primed with myelin antigens, or through the use of naïve TCR transgenic T cells activated in vitro in the presence of myelin-derived antigens.