Multi-faceted inhibition of dendritic cell function by CD4+Foxp3+ regulatory T cells.

CTLA-4 is required for CD4+Foxp3+ regulatory T (Treg) cell function, but its mode of action remains incompletely defined. Herein we generated Ctla-4ex2fl/flFoxp3-Cre mice with Treg cells exclusively expressing a naturally occurring, ligand-independent isoform of CTLA-4 (liCTLA-4) that cannot interact with the costimulatory molecules CD80 and CD86. The mice did not exhibit any signs of effector T cell activation early in life, however, at 6 months of age they exhibited excessive T cell activation and inflammation in lungs. In contrast, mice with Treg cells completely lacking CTLA-4 developed lymphoproliferative disease characterized by multi-organ inflammation early in life. In vitro, Treg cells exclusively expressing liCTLA-4 inhibited CD80 and CD86 expression on dendritic cells (DC). Conversely, Treg cells required the extra-cellular part of CTLA-4 to up-regulate expression of the co-inhibitory molecule PD-L2 on DCs. Transcriptomic analysis of suppressed DCs revealed that Treg cells induced a specific immunosuppressive program in DCs.

Induction of autoimmune disease by deletion of CTLA-4 in mice in adulthood.

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is essential for immunological (self-) tolerance, but due to the early fatality of CTLA-4 KO mice, its specific function in central and peripheral tolerance and in different systemic diseases remains to be determined. Here, we further examined the role of CTLA-4 by abrogating CTLA-4 expression in adult mice and compared the resulting autoimmunity that follows with that produced by congenital CTLA-4 deficiency. We found that conditional deletion of CTLA-4 in adult mice resulted in spontaneous lymphoproliferation, hypergammaglobulinemia, and histologically evident pneumonitis, gastritis, insulitis, and sialadenitis, accompanied by organ-specific autoantibodies. However, in contrast to congenital deficiency, this was not fatal. CTLA-4 deletion induced preferential expansion of CD4(+)Foxp3(+) Treg cells. However, T cells from CTLA-4-deficient inducible KO mice were able to adoptively transfer the diseases into T cell-deficient mice. Notably, cell transfer of thymocytes de novo produced myocarditis, otherwise not observed in donor mice depleted in adulthood. Moreover, CTLA-4 deletion in adult mice had opposing impacts on induced autoimmune models. Thus, although CTLA-4-deficient mice had more severe collagen-induced arthritis (CIA), they were protected against peptide-induced experimental autoimmune encephalomyelitis (EAE); however, onset of protein-induced EAE was only delayed. Collectively, this indicates that CTLA-4 deficiency affects both central and peripheral tolerance and Treg cell-mediated suppression.

Regulatory T cells control epitope spreading in autoimmune arthritis independent of cytotoxic T-lymphocyte antigen-4.

CD4+  Foxp3+ regulatory T (Treg) cells can control both cellular and humoral immune responses; however, when and how Treg cells play a predominant role in regulating autoimmune disease remains elusive. To deplete Treg cells in vivo at given time-points, we used a mouse strain, susceptible to glucose-6-phosphate isomerase peptide-induced arthritis (GIA), in which the deletion of Treg cells can be controlled by diphtheria toxin treatment. By depleting Treg cells in the GIA mouse model, we found that a temporary lack of Treg cells at both priming and onset exaggerated disease development. Ablation of Treg cells led to the expansion of antigen-specific CD4+ T cells including granulocyte-macrophage colony-stimulating factor, interferon-γ and interleukin-17-producing T cells, and promoted both T-cell and B-cell epitope spreading, which perpetuated arthritis. Interestingly, specific depletion of cytotoxic T-lymphocyte antigen-4 (CTLA-4) on Treg cells only, was sufficient to protect mice from GIA, due to the expansion of CTLA-4- Treg cells expressing alternative suppressive molecules. Collectively, our findings suggest that Treg cells, independently of CTLA-4, act as the key driving force in controlling autoimmune arthritis development.

CTLA-4 expressed by FOXP3+ regulatory T cells prevents inflammatory tissue attack and not T-cell priming in arthritis.

Cytotoxic T-lymphocyte antigen 4 (CTLA-4) -mediated regulation of already tolerized autoreactive T cells is critical for understanding autoimmune responses. Although defects in CTLA-4 contribute to abnormal FOXP3+ regulatory T (Treg) cell function in rheumatoid arthritis, its role in autoreactive T cells remains elusive. We studied immunity towards the dominant collagen type II (CII) T-cell epitope in collagen-induced arthritis both in the heterologous setting and in the autologous setting where CII is mutated at position E266D in mouse cartilage. CTLA-4 regulated all stages of arthritis, including the chronic phase, and affected the priming of autologous but not heterologous CII-reactive T cells. CTLA-4 expression by both conventional T (Tconv) cells and Treg cells was required but while Tconv cell expression was needed to control the priming of naive autoreactive T cells, CTLA-4 on Treg cells prevented the inflammatory tissue attack. This identifies a cell-type-specific time window when CTLA-4-mediated tolerance is most powerful, which has important implications for clinical therapy with immune modulatory drugs.

Expression of type I interferon by splenic macrophages suppresses adaptive immunity during sepsis.

Early during Gram-negative sepsis, excessive release of pro-inflammatory cytokines can cause septic shock that is often followed by a state of immune paralysis characterized by the failure to mount adaptive immunity towards secondary microbial infections. Especially, the early mechanisms responsible for such immune hypo-responsiveness are unclear. Here, we show that TLR4 is the key immune sensing receptor to initiate paralysis of T-cell immunity after bacterial sepsis. Downstream of TLR4, signalling through TRIF but not MyD88 impaired the development of specific T-cell immunity against secondary infections. We identified type I interferon (IFN) released from splenic macrophages as the critical factor causing T-cell immune paralysis. Early during sepsis, type I IFN acted selectively on dendritic cells (DCs) by impairing antigen presentation and secretion of pro-inflammatory cytokines. Our results reveal a novel immune regulatory role for type I IFN in the initiation of septic immune paralysis, which is distinct from its well-known immune stimulatory effects. Moreover, we identify potential molecular targets for therapeutic intervention to overcome impairment of T-cell immunity after sepsis.

Germ-free mice deficient of reactive oxygen species have increased arthritis susceptibility.

The NADPH oxidase 2 (NOX2) complex is responsible for the production of ROS in phagocytic cells. Genetic defects in NOX2 lead to opportunistic infections and inflammatory manifestations such as granulomas in humans, also known as chronic granulomatous disease (CGD). This condition is mirrored in mice with defective ROS production and interestingly both species are predisposed to autoimmune diseases. An unresolved question is whether the hyper-inflammation and tendency to develop autoimmunity are secondary to the increased infections, or whether these are parallel phenomena. We generated germ-free ROS deficient Ncf1 mutant mice that when reared in specific pathogen-free condition, are highly susceptible to collagen-induced arthritis compared with wild-type mice. Strikingly, arthritis incidence and severity was almost identical in germ-free and specific pathogen-free ROS-deficient mice. In addition, partial reduction of the microbial flora by antibiotics treatment did not alter the disease course. Taken together, this shows that ROS has a clear immune regulatory function that is decoupled from its function in host defence.

Endophilin A2 deficiency protects rodents from autoimmune arthritis by modulating T cell activation.

The introduction of the CTLA-4 recombinant fusion protein has demonstrated therapeutic effects by selectively modulating T-cell activation in rheumatoid arthritis. Here we show, using a forward genetic approach, that a mutation in the SH3gl1 gene encoding the endocytic protein Endophilin A2 is associated with the development of arthritis in rodents. Defective expression of SH3gl1 affects T cell effector functions and alters the activation threshold of autoreactive T cells, thereby leading to complete protection from chronic autoimmune inflammatory disease in both mice and rats. We further show that SH3GL1 regulates human T cell signaling and T cell receptor internalization, and its expression is upregulated in rheumatoid arthritis patients. Collectively our data identify SH3GL1 as a key regulator of T cell activation, and as a potential target for treatment of autoimmune diseases.

Eosinophils Suppress the Migration of T Cells Into the Brain of Plasmodium berghei-Infected Ifnar1-/- Mice and Protect Them From Experimental Cerebral Malaria.

Cerebral malaria is a potentially lethal disease, which is caused by excessive inflammatory responses to Plasmodium parasites. Here we use a newly developed transgenic Plasmodium berghei ANKA (PbAAma1OVA) parasite that can be used to study parasite-specific T cell responses. Our present study demonstrates that Ifnar1-/- mice, which lack type I interferon receptor-dependent signaling, are protected from experimental cerebral malaria (ECM) when infected with this novel parasite. Although CD8+ T cell responses generated in the spleen are essential for the development of ECM, we measured comparable parasite-specific cytotoxic T cell responses in ECM-protected Ifnar1-/- mice and wild type mice suffering from ECM. Importantly, CD8+ T cells were increased in the spleens of ECM-protected Ifnar1-/- mice and the blood-brain-barrier remained intact. This was associated with elevated splenic levels of CCL5, a T cell and eosinophil chemotactic chemokine, which was mainly produced by eosinophils, and an increase in eosinophil numbers. Depletion of eosinophils enhanced CD8+ T cell infiltration into the brain and increased ECM induction in PbAAma1OVA-infected Ifnar1-/- mice. However, eosinophil-depletion did not reduce the CD8+ T cell population in the spleen or reduce splenic CCL5 concentrations. Our study demonstrates that eosinophils impact CD8+ T cell migration and proliferation during PbAAma1OVA-infection in Ifnar1-/- mice and thereby are contributing to the protection from ECM.

Specific depletion of Ly6C(hi) inflammatory monocytes prevents immunopathology in experimental cerebral malaria.

Plasmodium berghei ANKA (PbA) infection of C57BL/6 mice leads to experimental cerebral malaria (ECM) that is commonly associated with serious T cell mediated damage. In other parasitic infection models, inflammatory monocytes have been shown to regulate Th1 responses but their role in ECM remains poorly defined, whereas neutrophils are reported to contribute to ECM immune pathology. Making use of the recent development of specific monoclonal antibodies (mAb), we depleted in vivo Ly6C(hi) inflammatory monocytes (by anti-CCR2), Ly6G+ neutrophils (by anti-Ly6G) or both cell types (by anti-Gr1) during infection with Ovalbumin-transgenic PbA parasites (PbTg). Notably, the application of anti-Gr1 or anti-CCR2 but not anti-Ly6G antibodies into PbTg-infected mice prevented ECM development. In addition, depletion of Ly6C(hi) inflammatory monocytes but not neutrophils led to decreased IFNγ levels and IFNγ+CD8+ T effector cells in the brain. Importantly, anti-CCR2 mAb injection did not prevent the generation of PbTg-specific T cell responses in the periphery, whereas anti-Gr1 mAb injection strongly diminished T cell frequencies and CTL responses. In conclusion, the specific depletion of Ly6C(hi) inflammatory monocytes attenuated brain inflammation and immune cell recruitment to the CNS, which prevented ECM following Plasmodium infection, pointing out a substantial role of Ly6C+ monocytes in ECM inflammatory processes.