Pathogenic T cells persist after reversal of autoimmune disease by immunosuppression with regulatory T cells.

Autoimmune disease can be prevented with immunosuppressive agents; however, the effectiveness of these treatments in advanced stage of disease and the fate of pathogenic T cells following such treatments are not clear. In this study we demonstrate that a single dose of in vitro-induced Treg cells (iTreg cells) resulted in the functional repair and restitution of stomach tissue that had been severely damaged in advanced autoimmune gastritis. iTreg cells caused depletion or inactivation of autoreactive naïve T cells that were antigen inexperienced, however, autoreactive effector/memory T cells persisted in treated mice, resulting in residual cellular infiltrates within the repaired stomach tissue. The persisting autoreactive T cells were able to rapidly cause autoimmune disease if iTreg cells were removed. Similar data were obtained from mice treated continuously with corticosteroid, in that there was substantial restitution of the gastric mucosa; however, effector T cells persisted and rapidly caused pathology following drug removal. Therefore, iTreg cells or corticosteroid can suppress pathogenic autoreactive cells in advanced autoimmune disease, reversing tissue damage and improving tissue function. However, the persistence of pathogenic T cells represents a disease risk.

Both IFN-γ and IL-17 are required for the development of severe autoimmune gastritis.

IL-17, produced by a distinct lineage of CD4(+) helper T (Th) cells termed Th17 cells, induces the production of pro-inflammatory cytokines from resident cells and it has been demonstrated that over-expression of IL-17 plays a crucial role in the onset of several auto-immune diseases. Here we examined the role of IL-17 in the pathogenesis of autoimmune gastritis, a disease that was previously believed to be mediated by IFN-γ. Significantly higher levels of IL-17 and IFN-γ were found in the stomachs and stomach-draining lymph nodes of mice with severe autoimmune gastritis. Unlike IL-17, which was produced solely by CD4(+) T cells in gastritic mice, the majority of IFN-γ-producing cells were CD8(+) T cells. However, CD8(+) T cells alone were not able to induce autoimmune gastritis. T cells that were deficient in IL-17 or IFN-γ production were able to induce autoimmune gastritis but to a much lower extent compared with the disease induced by wild-type T cells. These data demonstrate that production of neither IL-17 nor IFN-γ by effector T cells is essential for the initiation of autoimmune gastritis, but suggest that both are required for the disease to progress to the late pathogenic stage that includes significant tissue disruption.

Arl5b is a Golgi-localised small G protein involved in the regulation of retrograde transport.

Regulation of membrane transport is controlled by small G proteins, which include members of the Rab and Arf families. Whereas the role of the classic Arf family members are well characterized, many of the Arf-like proteins (Arls) remain poorly defined. Here we show that Arl5a and Arl5b are localised to the trans-Golgi in mammalian cells, and furthermore have identified a role for Arl5b in the regulation of retrograde membrane transport from endosomes to the trans-Golgi network (TGN). The constitutively active Arl5b (Q70L)-GFP mutant was localised efficiently to the Golgi in HeLa cells whereas the dominant-negative Arl5b (T30N)-GFP mutant was dispersed throughout the cytoplasm and resulted in perturbation of the Golgi apparatus. Stable HeLa cells expressing GFP-tagged Arl5b (Q70L) showed an increased rate of endosome-to-Golgi transport of the membrane cargo TGN38 compared with control HeLa cells. Depletion of Arl5b by RNAi resulted in an alteration in the intracellular distribution of mannose-6-phosphate receptor, and significantly reduced the endosome-to-TGN transport of the membrane cargo TGN38 and of Shiga toxin, but had no affect on the anterograde transport of the cargo E-cadherin. Collectively these results suggest that Arl5b is a TGN-localised small G protein that plays a key role in regulating transport along the endosome-TGN pathway.

A convenient model of severe, high incidence autoimmune gastritis caused by polyclonal effector T cells and without perturbation of regulatory T cells.

Autoimmune gastritis results from the breakdown of T cell tolerance to the gastric H(+)/K(+) ATPase. The gastric H(+)/K(+) ATPase is responsible for the acidification of gastric juice and consists of an α subunit (H/Kα) and a ß subunit (H/Kß). Here we show that CD4(+) T cells from H/Kα-deficient mice (H/Kα(-/-)) are highly pathogenic and autoimmune gastritis can be induced in sublethally irradiated wildtype mice by adoptive transfer of unfractionated CD4(+) T cells from H/Kα(-/-) mice. All recipient mice consistently developed the most severe form of autoimmune gastritis 8 weeks after the transfer, featuring hypertrophy of the gastric mucosa, complete depletion of the parietal and zymogenic cells, and presence of autoantibodies to H(+)/K(+) ATPase in the serum. Furthermore, we demonstrated that the disease significantly affected stomach weight and stomach pH of recipient mice. Depletion of parietal cells in this disease model required the presence of both H/Kα and H/Kß since transfer of H/Kα(-/-) CD4(+) T cells did not result in depletion of parietal cells in H/Kα(-/-) or H/Kß(-/-) recipient mice. The consistency of disease severity, the use of polyclonal T cells and a specific T cell response to the gastric autoantigen make this an ideal disease model for the study of many aspects of organ-specific autoimmunity including prevention and treatment of the disease.

Immune Control of Legionella Infection: An in vivo Perspective.

Legionella pneumophila is an intracellular pathogen that replicates within alveolar macrophages. Through its ability to activate multiple host innate immune components, L. pneumophila has emerged as a useful tool to dissect inflammatory signaling pathways in macrophages. However the resolution of L. pneumophila infection in the lung requires multiple cell types and abundant cross talk between immune cells. Few studies have examined the coordination of events that lead to effective immune control of the pathogen. Here we discuss L. pneumophila interactions with macrophages and dendritic cell subsets and highlight the paucity of knowledge around how these interactions recruit and activate other immune effector cells in the lung.

Cutting edge: pulmonary Legionella pneumophila is controlled by plasmacytoid dendritic cells but not type I IFN.

Plasmacytoid dendritic cells (pDCs) are well known as the major cell type that secretes type I IFN in response to viral infections. Their role in combating other classes of infectious organisms, including bacteria, and their mechanisms of action are poorly understood. We have found that pDCs play a significant role in the acute response to the intracellular bacterial pathogen Legionella pneumophila. pDCs were rapidly recruited to the lungs of L. pneumophila-infected mice, and depletion of pDCs resulted in increased bacterial load. The ability of pDCs to combat infection did not require type I IFN. This study points to an unappreciated role for pDCs in combating bacterial infections and indicates a novel mechanism of action for this cell type.

Molecular pathogenesis of infections caused by Legionella pneumophila.

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

Extrathymic mechanisms of T cell tolerance: lessons from autoimmune gastritis.

While the thymus plays a key role in the prevention of many autoimmune phenomena it is clear that robust mechanisms external to the thymus are also vital in controlling self-reactive T cells. Here we review the current concepts in the field of extrathymic tolerance and use recent studies of autoimmune gastritis to illustrate how T cells directed to a prominent, clinically relevant autoantigen, namely the gastric proton pump, can be silenced with little or no thymic involvement. Autoimmune gastritis represents one of the most thoroughly characterised autoimmune systems and the knowledge and tools available to study this disease will continue to allow a thorough assessment of the genetic, cellular and molecular events that underlie tolerance and autoimmunity.

Role of regulatory T cells in gastrointestinal inflammatory disease.

Regulatory T cells curb unwanted immune responses and regulate responses to microflora and it is now clear that regulatory T cells play an important role in a number of chronic inflammatory diseases of the gut. First, regulatory T cells are crucial in controlling immune responses to gastric autoantigens and thus preventing autoimmune gastritis and pernicious anemia. Second, regulatory T cells may modulate the response to Helicobacter pylori, thus affecting the ability of the immune system to clear the pathogen and mediate damage to the gastric mucosa. Finally, regulatory T cells play an important role in preventing damaging inflammatory responses to commensal organisms in the lower gut, thus guarding against inflammatory bowel diseases. In the present review, we examine the actions of regulatory T cells in the gut and conclude that further understanding of regulatory T cell biology may lead to new therapeutic approaches to chronic gastrointestinal disease.

Two genetic loci independently confer susceptibility to autoimmune gastritis.

Autoimmune gastritis is a CD4+ T cell-mediated disease induced in genetically susceptible mice by thymectomy on the third day after birth. Previous linkage analysis indicated that Gasa1 and Gasa2, the major susceptibility loci for gastritis, are located on mouse chromosome 4. Here we verified these linkage data by showing that BALB.B6 congenic mice, in which the distal approximately 40 Mb of chromosome 4 was replaced by C57BL/6 DNA, were resistant to autoimmune gastritis. Analysis of further BALB.B6 congenic strains demonstrated that Gasa1 and Gasa2 can act independently to cause full expression of susceptibility to autoimmune disease. Gasa1 and Gasa2 are located between D4Mit352-D4Mit204 and D4Mit343-telomere, respectively. Numerical differences in Foxp3+ regulatory T cells were apparent between the BALB/c and congenic strains, but it is unlikely that this phenotype accounted for differences in autoimmune susceptibility. The positions of Gasa1 and Gasa2 correspond closely to the positions of Idd11 and Idd9, two autoimmune diabetes susceptibility loci in nonobese diabetic (NOD), mice and this prompted us to examine autoimmune gastritis in NOD mice. After neonatal thymectomy, NOD mice developed autoimmune gastritis, albeit at a slightly lower incidence and severity of disease than in BALB/c mice. Diabetes-resistant congenic NOD.B6 mice, harbouring a B6-derived interval encompassing the Gasa1/2-Idd9/11 loci, demonstrated a slight reduction in the incidence of autoimmune gastritis. This reduction was not significant compared with the reduction observed in BALB.B6 congenic mice, suggesting a difference in the genetic aetiology of autoimmune gastritis in NOD and BALB mice.