Inhibition of Antigen-Specific and Nonspecific Stimulation of Bovine T and B Cells by Lymphostatin from Attaching and Effacing Escherichia coli

Lymphostatin, a virulence factor of attaching and effacing Escherichia coli, inhibits proliferation and cytokine responses of human T cells in a manner associated with cell cycle arrest but not apoptosis or necrosis

Lymphostatin is a virulence factor of enteropathogenic E. coli (EPEC) and non-O157 serogroup enterohaemorrhagic E. coli. Previous studies using whole-cell lysates of EPEC showed that lymphostatin inhibits the mitogen-activated proliferation of bulk human peripheral blood mononuclear cells (PBMCs) and the production of cytokines IL-2, IL-4, IL-5, and IFN-γ. Here, we used highly purified lymphostatin and PBMC-derived T cells to show that lymphostatin inhibits anti-CD3/anti-CD28-activated proliferation of human CD4⁺ and CD8⁺ T cells and blocks the synthesis of IL-2, IL-4, IL-10 and IFN-γ without affecting cell viability and in a manner dependent on an N-terminal DTD glycosyltransferase motif. Such inhibition was not observed with T cells activated by phorbol 12-myristate 13-acetate and ionomycin, implying that lymphostatin targets T cell receptor signaling. Analysis of the expression of CD69 indicated that lymphostatin suppresses T cell activation at an early stage and no impacts on apoptosis or necrosis were observed. Flow cytometric analysis of the DNA content of lymphostatin-treated CD4⁺ and CD8⁺ T cells showed a concentration- and DTD-dependent accumulation of the cells in the G0/G1 phase of the cell cycle, and corresponding reduction of the percentage of cells in S phase. Consistent with this, we found a marked reduction in the abundance of cyclins D3, E and A and loss of phosphorylated Rb over time in activated T cells from 8 donors treated with lymphostatin. Moreover, the cyclin-dependent kinase (cdk) inhibitor p27^kip1, which inhibits progression of the cell cycle at G1 by acting on cyclin E-cdk2 or cyclin D-cdk4 complexes, was found to be accumulated in lymphostatin-treated T cells. Analysis of the abundance of phosphorylated kinases involved in signal transduction found that 30 of 39 were reduced in abundance following lymphostatin treatment of T cells from 5 donors, albeit not significantly so. Our data provide novel insights into the mode of action of lymphostatin on human T lymphocytes.

Thymic T-Cell Production Is Associated With Changes in the Gut Microbiota in Young Chicks

Increasing studies show that gut microbiota play a central role in immunity, although the impact of the microbiota on mediation of thymic T cells throughout life is not well understood. Chickens have been shown to be a valuable model for studying basic immunology. Here, we show that changes in the gut microbiota are associated with the development of thymic T cells in young chickens. Our results showed that T-cell numbers in newborn chicks sharply increased from day 0 and peaked at day 49. Interestingly, the α-diversity score pattern of change in gut microbiota also increased after day 0 and continued to increase until day 49. We found that early antibiotic treatment resulted in a dramatic reduction in gut alpha diversity: principal component analysis (PCA) showed that antibiotic treatment resulted in a different cluster from the controls on days 9 and 49. In the antibiotic-treated chickens, we identified eight significantly different (p < 0.05) microbes at the phylum level and 14 significantly different (p < 0.05) microbes at the genus level, compared with the controls. Importantly, we found that antibiotic treatment led to a decreased percentage and number of T cells in the thymus when measured at days 9 and 49, as evaluated by flow cytometry. Collectively, our data suggest that intestinal microbiota may be involved in the regulation of T cells in birds, presenting the possibility that interventions that actively modify the gut microbiota in early life may accelerate the maturation of humoral immunity, with resulting anti-inflammatory effects against different pathogens.

Activity of Lymphostatin, A Lymphocyte Inhibitory Virulence Factor of Pathogenic Escherichia coli, is Dependent on a Cysteine Protease Motif

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LifA shares a cysteine protease motif with bacterial toxins and secreted effectors.

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C1480A substituted LifA has reduced inhibitory activity against T cells.

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LifA is cleaved in T cells and this requires C1480 and endosome acidification.

Lymphostatin (LifA) is a 366 kDa protein expressed by attaching & effacing Escherichia coli. It plays an important role in intestinal colonisation and inhibits the mitogen- and antigen-stimulated proliferation of lymphocytes and the synthesis of proinflammatory cytokines. LifA exhibits N-terminal homology with the glycosyltransferase domain of large clostridial toxins (LCTs). A DTD motif within this region is required for lymphostatin activity and binding of the sugar donor uridine diphosphate N-acetylglucosamine. As with LCTs, LifA also contains a cysteine protease motif (C1480, H1581, D1596) that is widely conserved within the YopT-like superfamily of cysteine proteases. By analogy with LCTs, we hypothesised that the CHD motif may be required for intracellular processing of the protein to release the catalytic N-terminal domain after uptake and low pH-stimulated membrane insertion of LifA within endosomes. Here, we created and validated a C1480A substitution mutant in LifA from enteropathogenic E. coli strain E2348/69. The purified protein was structurally near-identical to the wild-type protein. In bovine T lymphocytes treated with wild-type LifA, a putative cleavage product of approximately 140 kDa was detected. Appearance of the putative cleavage product was inhibited in a concentration-dependent manner by bafilomycin A1 and chloroquine, which inhibit endosome acidification. The cleavage product was not observed in cells treated with the C1480A mutant of LifA. Lymphocyte inhibitory activity of the purified C1480A protein was significantly impaired. The data indicate that an intact cysteine protease motif is required for cleavage of lymphostatin and its activity against T cells.

Interaction of Bovine Lymphocytes with Products of Shiga Toxin-Producing Escherichia coli

Shiga toxin-producing Escherichia coli (STEC) produce a number of virulence factors that interfere with lymphocyte functions, including mitogen- and antigen-activated proliferation and pro-inflammatory cytokine synthesis. Here we describe how to isolate lymphocyte subsets from bovine peripheral blood as well as methods that we have used to study the effects of STEC products on lymphocyte proliferation and cytokine production. We also describe an assay that allows for the detection of association of a given protein with lymphocytes.

The Role of Escherichia coli Shiga Toxins in STEC Colonization of Cattle

Many cattle are persistently colonized with Shiga toxin-producing Escherichia coli (STEC) and represent a major source of human infections with human-pathogenic STEC strains (syn. enterohemorrhagic E. coli (EHEC)). Intervention strategies most effectively protecting humans best aim at the limitation of bovine STEC shedding. Mechanisms enabling STEC to persist in cattle are only partialy understood. Cattle were long believed to resist the detrimental effects of Shiga toxins (Stxs), potent cytotoxins acting as principal virulence factors in the pathogenesis of human EHEC-associated diseases. However, work by different groups, summarized in this review, has provided substantial evidence that different types of target cells for Stxs exist in cattle. Peripheral and intestinal lymphocytes express the Stx receptor globotriaosylceramide (Gb₃syn. CD77) in vitro and in vivo in an activation-dependent fashion with Stx-binding isoforms expressed predominantly at early stages of the activation process. Subpopulations of colonic epithelial cells and macrophage-like cells, residing in the bovine mucosa in proximity to STEC colonies, are also targeted by Stxs. STEC-inoculated calves are depressed in mounting appropriate cellular immune responses which can be overcome by vaccination of the animals against Stxs early in life before encountering STEC. Considering Stx target cells and the resulting effects of Stxs in cattle, which significantly differ from effects implicated in human disease, may open promising opportunities to improve existing yet insufficient measures to limit STEC carriage and shedding by the principal reservoir host.

Direct Manipulation of T Lymphocytes by Proteins of Gastrointestinal Bacterial Pathogens

Gastrointestinal bacterial infection represents a significant threat to human health, as well as a burden on food animal production and welfare. Although there is advanced knowledge about the molecular mechanisms underlying pathogenesis, including the development of immune responses to these pathogens, gaps in knowledge persist. It is well established that gastrointestinal bacterial pathogens produce a myriad of proteins that affect the development and effectiveness of innate immune responses. However, relatively few proteins that directly affect lymphocytes responsible for humoral or cell-mediated immunity and memory have been identified. Here, we review factors produced by gastrointestinal bacterial pathogens that have direct T cell interactions and what is known about their functions and mechanisms of action. T cell-interacting bacterial proteins that have been identified to date mainly target three major T cell responses: activation and expansion, chemotaxis, or apoptosis. Further, the requirement for more focused studies to identify and understand additional mechanisms used by bacteria to directly affect the T cell immune response and how these may contribute to pathogenesis is highlighted. Increased knowledge in this area will help to drive development of better interventions in prevention and treatment of gastrointestinal bacterial infection.