Marianna Ioannou

Crucial Role of Granulocytic Myeloid-Derived Suppressor Cells in the Regulation of Central Nervous System Autoimmune Disease

There is a need in autoimmune diseases to uncover the mechanisms involved in the natural resolution of inflammation. In this article, we demonstrate that granulocytic myeloid-derived suppressor cells (G-MDSCs) abundantly accumulate within the peripheral lymphoid compartments and target organs of mice with experimental autoimmune encephalomyelitis prior to disease remission. In vivo transfer of G-MDSCs ameliorated experimental autoimmune encephalomyelitis, significantly decreased demyelination, and delayed disease onset through inhibition of encephalitogenic Th1 and Th17 immune responses. Exposure of G-MDSCs to the autoimmune milieu led to up-regulation of the programmed death 1 ligand that was required for the G-MDSC–mediated suppressive function both in vitro and in vivo. Importantly, myeloid-derived suppressor cells were enriched in the periphery of subjects with active multiple sclerosis and suppressed the activation and proliferation of autologous CD4+ T cells ex vivo. Collectively, this study revealed a pivotal role for myeloid-derived suppressor cells in the regulation of multiple sclerosis, which could be exploited for therapeutic purposes.

The negative costimulatory molecule PD-1 modulates the balance between immunity and tolerance via miR-21

Disruption of the programmed death‐1 (PD‐1) pathway leads to breakdown of peripheral tolerance and initiation of autoimmunity. The molecular pathways that mediate this effect remain largely unknown. We report here that PD‐1 knockout (PD‐1−/−) mice develop more severe and sustained Ag‐induced arthritis (AIA) than WT animals, which is associated with increased T‐cell proliferation and elevated levels of IFN‐γ and IL‐17 secretion. MicroRNA analysis of Ag‐specific CD4+ T cells revealed a significant upregulation of microRNA 21 (miR‐21) in PD‐1−/− T cells compared with WT controls. In addition, PD‐1 inhibition, via siRNA, upregulated miR‐21 expression and enhanced STAT5 binding in the miR‐21 promoter area. Computational analysis confirmed that miR‐21 targets directly the expression of programmed cell death 4 (PDCD4) and overexpression of miR‐21 in cells harboring the 3′UTR of PDCD4 resulted in reduced transcription and PDCD4 protein expression. Importantly, in vitro delivery of antisense‐miR‐21 suppressed the Ag‐specific proliferation and cytokine secretion by PD‐1−/− T cells, whereas adoptive transfer of Ag‐specific T cells, overexpressing miR‐21, induced severe AIA. Collectively, our data demonstrate that breakdown of tolerance in PD‐1−/− mice activates a signaling cascade mediated by STAT5, miR‐21, and PDCD4 and establish their role in maintaining the balance between immune activation and tolerance.

In Vivo Ablation of Plasmacytoid Dendritic Cells Inhibits Autoimmunity through Expansion of Myeloid-Derived Suppressor Cells

Autoimmunity ensues upon breakdown of tolerance mechanism and priming of self-reactive T cells. Plasmacytoid dendritic cells (pDCs) constitute a unique cell subset that participates in the activation of autoreactive T cells but also has been shown to be critically involved in the induction of self-tolerance. However, their functional importance during the priming phase of an organ-specific autoimmune response remains unclear. In this study, we demonstrate that absence of pDCs during myelin antigenic challenge resulted in amelioration of experimental autoimmune encephalomyelitis and reduced disease severity. This was accompanied by significantly decreased frequency of myelin-specific T cells in the draining lymph nodes and inhibition of Th1 and Th17 immune responses. Unexpectedly, in vivo ablation of pDCs increased myelopoiesis in the bone marrow and specifically induced the generation of CD11bhiGr1+ myeloid-derived suppressor cells (MDSCs). Furthermore, we demonstrate that pDC depletion enhanced the mobilization of MDSCs in the spleen, and that sorted MDSCs could potently suppress CD4+ T cell responses in vitro. Importantly, pDC-depleted mice showed increased levels of MCP-1 in the draining lymph nodes, and in vivo administration of MCP-1 increased the frequency and absolute numbers of MDSCs in the periphery of treated mice. Together, our results reveal that absence of pDCs during the priming of an autoimmune response leads to increased mobilization of MDSCs in the periphery in an MCP-1–dependent manner and subsequent amelioration of autoimmunity.

Cell-based modulation of autoimmune responses in multiple sclerosis and experimental autoimmmune encephalomyelitis: therapeutic implications.

Multiple sclerosis (MS) is a prototypic autoimmune inflammatory disorder of the central nervous system (CNS). MS pathogenesis is a complex phenomenon that is influenced by genetic and environmental factors that lead to the dysregulation of immune homeostasis and tolerance. It has been shown that pathogenic T lymphocyte subsets, such as T helper 1 (Th1) and Th17 cells, play a crucial role in the autoimmune cascade influencing disease initiation, progression and subsequent tissue damage during MS. On the other hand, several mechanisms have been described in both patients and animal models of MS with the potential to modulate myelin-specific autoimmune responses and to facilitate amelioration of disease pathology. To this end, regulatory T cells (Tregs) are considered to be a powerful cell subset not only in the maintenance of homeostasis but also in the re-establishment of tolerance. Along these lines, other cell subsets such as dendritic cells (DCs), myeloid-derived suppressor cells (MDSCs), γδ T cells and natural killer (NK) cells have been shown to regulate the autoimmune response in the CNS under certain circumstances. This review will attempt to summarize the relevant knowledge of the regulatory mechanisms exerted by immune cells in MS that could hold the promise for the design of novel therapeutic strategies.

Elimination of Granulocytic Myeloid-Derived Suppressor Cells in Lupus-Prone Mice Linked to Reactive Oxygen Species-Dependent Extracellular Trap Formation.

Emerging evidence supports a crucial role of myeloid‐derived suppressor cells (MDSCs) in the regulation of autoimmune diseases. However, their role in systemic lupus erythematosus (SLE) remains unknown. This study sought to address the role of MDSCs in the pathogenesis of SLE.

Elimination of granulocytic myeloid‐derived suppressor cells in lupus‐prone mice due to ROS‐dependent extracellular trap formation

Emerging evidence supports a crucial role of myeloid‐derived suppressor cells (MDSCs) in the regulation of autoimmune diseases. However, their role in systemic lupus erythematosus (SLE) remains unknown. This study sought to address the role of MDSCs in the pathogenesis of SLE.

De Novo–Induced Self-Antigen–Specific Foxp3+ Regulatory T Cells Impair the Accumulation of Inflammatory Dendritic Cells in Draining Lymph Nodes

Foxp3+ regulatory T cell (Treg)-based immunotherapy holds promise for autoimmune diseases. However, this effort has been hampered by major caveats, including the low frequency of autoantigen-specific Foxp3+ Tregs and lack of understanding of their molecular and cellular targets, in an unmanipulated wild-type (WT) immune repertoire. In this study, we demonstrate that infusion of myelin in WT mice results in the de novo induction of myelin-specific Foxp3+ Tregs in WT mice and amelioration of experimental autoimmune encephalomyelitis. Myelin-specific Foxp3+ Tregs exerted their effect both by diminishing Ag-bearing inflammatory dendritic cell (iDC) recruitment to lymph nodes and by impairing their function. Transcriptome analysis of ex vivo–isolated Treg-exposed iDCs showed significant enrichment of transcripts involved in functional properties of iDCs, including chemotaxis-related genes. To this end, CCR7 expression by iDCs was significantly downregulated in tolerant mice and this was tightly regulated by the presence of IL-10. Collectively, our data demonstrate a novel model for deciphering the Ag-specific Foxp3+ Treg-mediated mechanisms of tolerance and delineate iDCs as a Foxp3+ Treg cellular target in unmanipulated mice.

Tpl2 kinase regulates FcγR signaling and immune thrombocytopenia in mice

The MAPK3 Tpl2 controls innate and adaptive immunity by regulating TLR, TNF‐α, and GPCR signaling in a variety of cell types. Its ablation gives rise to an anti‐inflammatory phenotype characterized by resistance to LPS‐induced endotoxin shock, DSS‐induced colitis, and TNF‐α‐induced IBD. Here, we address the role of Tpl2 in autoimmunity. Our data show that the ablation and the pharmacological inhibition of Tpl2 protect mice from antiplatelet antibody‐induced thrombocytopenia, a model of ITP. Thrombocytopenia in this model and in ITP is caused by phagocytosis of platelets opsonized with antiplatelet antibodies and depends on FcγR activation in splenic and hepatic myeloid cells. Further studies explained how Tpl2 inhibition protects from antibody‐induced thrombocytopenia, by showing that Tpl2 is activated by FcγR signals in macrophages and that its activation by these signals is required for ERK activation, cytoplasmic Ca2+ influx, the induction of cytokine and coreceptor gene expression, and phagocytosis.

Defects within the Myeloid Derived Supressor Cells (MDSCS) Compartment may Facilitate Aberrant Immune Responses in Systemic Lupus Erythematosus (SLE)

Background and objectives There is a currently unmet need to define the mechanisms involved in the homeostatic control of the immune response in autoimmune diseases. MDSCs (characterised in mice as CD11b+Gr1+ and in humans as CD14-HLA-DRlowCD15+CD33+) represent a heterogeneous population of myeloid precursors of macrophages, dendritic cells and granulocytes with a distinct regulatory role in suppressing T-cell responses. We sought to delineate the role of these cells both in murine and human lupus. Materials and methods We used the lupus prone (NZB x NZW) F1 female mice [3 months old, pre-SLE (n=6) and 6-8 months old, SLE mice (n=6)]. C57BL/6 (B6) female mice [3 mo (n=2) or 6 mo (n=6)] were used as controls. B6 mice immunised with myelin peptide in adjuvant (Experimental Autoimmune Encephalomyelitis) were used as a disease control. Cells were isolated from the bone marrow and spleen of the indicated groups and were stained with fluorescent-conjugated antibodies. Cells were stained for the aforementioned markers and for 7AAD. Analysis was performed in humans as well, using samples from SLE patients (n=18), healthy controls (n=20) and multiple sclerosis (MS) patients (n=31) as disease control. The phenotype and enumeration of cell populations were performed by flow cytometry. Results Compared to healthy B6 mice, splenic MDSCs of pre-SLE F1 animals were decreased both in frequency and absolute numbers (*p=0.012), indicating a defect in the MDSC compartment in F1 lupus mice. MDSCs were significantly expanded in the spleen of F1 diseased animals compared to healthy F1 controls (*p=0.017) albeit at significantly lower levels compared to mice with EAE (***p<0.0001). In humans, subjects with active lupus (n=8) exhibited higher numbers of MDSCs in the peripheral blood compared to inactive patients (n=10) and healthy controls, but at lower levels compared to patients with active MS (n=14). Ongoing experiments using in vivo transfer of murine MDSCs address their potential to halt renal disease progression and the cells/molecules involved. Conclusions Together these data suggest defective MDSC accumulation and/or expansion in the periphery in lupus which may contribute to the immune deregulation observed in this disease. Identification of the cell subsets and the molecules involved may provide additional therapeutic targets for the restoration of immune tolerance in lupus.

Regulation of autoimmune inflammation by myeloid-derived suppressor cells

Background and objectives Restoration of immune homeostasis and self-tolerance represent the ultimate goal of autoimmune diseases. Although a variety of therapeutic targets are employed, a large number of patients with autoimmune syndromes fail to either respond to current therapy or to achieve long-lasting remission after its cessation. Myeloid-derived suppressor cells (MDSCs) encompass a newly described population of cells that potently suppress immune responses however their role in autoimmune inflammatory diseases is poorly understood. Materials and methods Using flow cytometry, the authors assessed the presence of CD11b Gr1+ MDSCs in the spleen and draining lymph nodes of mice with trinitrobenzene sulfonate (TNBS)-induced colitis and control-treated mice. Furthermore, the authors monitored the expansion of the monocytic (CD11b Ly6C+ Ly6G−) and granulocytic (CD11b Ly6G+ Ly6C−) subsets of MDSCs in the peripheral lymphoid compartments. Results The authors observed an increased accumulation of CD11b Gr1+ MDSCs in the spleen of TNBS-treated mice, compared to control group. Among the MDSCs, the granulocytic Ly6G+ subset was significantly enriched in TNBS-treated mice both in spleen and lymph nodes. Extensive phenotypic characterisation of sorted Ly6G+ MDSCs revealed an increased expression of the inhibitory molecule PD-L1, but not PD-L2, indicating that MDSCs might mediate their function via the PD-1/PD-L1 pathway. Conclusions This data demonstrate a preferential expansion of granulocytic Ly6G+ MDSCs during the effector phase of the inflammatory response. Ongoing work would address the potential role of this cell subset in inhibiting established disease and elucidate the mechanisms used by MDSCs for the suppression of autoimmune inflammatory diseases.