Immunosuppression induced by Salmonella involves inhibition of tyrosine phosphorylation in murine T lymphocytes

Oral Wild-Type Salmonella Typhi Challenge Induces Activation of Circulating Monocytes and Dendritic Cells in Individuals Who Develop Typhoid Disease

A new human oral challenge model with wild-type Salmonella Typhi (S. Typhi) was recently developed. In this model, ingestion of 104 CFU of Salmonella resulted in 65% of subjects developing typhoid fever (referred here as typhoid diagnosis -TD-) 5–10 days post-challenge. TD criteria included meeting clinical (oral temperature ≥38°C for ≥12h) and/or microbiological (S. Typhi bacteremia) endpoints. One of the first lines of defense against pathogens are the cells of the innate immune system (e.g., monocytes, dendritic cells -DCs-). Various changes in circulating monocytes and DCs have been described in the murine S. Typhimurium model; however, whether similar changes are present in humans remains to be explored. To address these questions, a subset of volunteers (5 TD and 3 who did not develop typhoid despite oral challenge -NoTD-) were evaluated for changes in circulating monocytes and DCs. Expression of CD38 and CD40 were upregulated in monocytes and DCs in TD volunteers during the disease days (TD-0h to TD-96h). Moreover, integrin α4β7, a gut homing molecule, was upregulated on monocytes but not DCs. CD21 upregulation was only identified in DCs. These changes were not observed among NoTD volunteers despite the same oral challenge. Moreover, monocytes and DCs from NoTD volunteers showed increased binding to S. Typhi one day after challenge. These monocytes showed phosphorylation of p38MAPK, NFkB and Erk1/2 upon stimulation with S. Typhi-LPS-QDot micelles. In contrast, monocytes from TD volunteers showed only a moderate increase in S. Typhi binding 48h and 96h post-TD, and only Erk1/2 phosphorylation. This is the first study to describe different activation and migration profiles, as well as differential signaling patterns, in monocytes and DCs which relate directly to the clinical outcome following oral challenge with wild type S. Typhi.

Typhoid fever continues to be a public health problem and novel more effective vaccines are needed. One of the limitations in the development of new vaccines is an incomplete understanding of the host-pathogen interactions. To gain new insights into these interactions a new human oral challenge model with wild-type Salmonella Typhi (S. Typhi) was recently developed. In this model, 65% of the challenged subjects developed typhoid fever (referred here as typhoid diagnosis-TD-). Monocytes and dendritic cells (DCs) are part of the innate immune system and one of the first lines of defense against pathogens. The changes induced in these cells by S. Typhi infection were studied in a subset of volunteers (5 TD and 3 who did not develop TD despite the same oral challenge-NoTD-). Monocytes and DCs showed upregulation of different activation molecules between TD and NoTD volunteers. Furthermore, monocytes from NoTD volunteers showed enhanced S. Typhi binding and activation of signaling pathways associated with the pattern recognition receptor (PRR) TLR4, one day after challenge. In contrast, monocytes from TD volunteers had a moderate increase in S. Typhi binding and different signaling profiles. Therefore, multiple differences in monocytes and DCs from TD and NoTD volunteers following wild type S. Typhi challenge were identified.

Inhibition of mitogen-induced murine lymphocyte proliferation by Helicobacter pylori cell-free extract

BACKGROUND AND AIMS

Previous studies have shown that lysates of Helicobacter pylori inhibit mitogen-induced proliferation of human peripheral blood mononuclear cells. The objective of the present study was to determine whether H. pylori cell-free extract (HPCE) has similar effects on murine lymphoid cells and could, therefore, be used to further delineate the mechanisms of alteration of lymphocyte function by H. pylori.

METHODS

The HPCE was prepared from a H. pylori reference strain and from five clinical strains with varying status of cagA and vacA. Mouse splenic and mesenteric lymph node cells were cultured in microwell plates in the presence or absence of varying concentrations of HPCE (0.625-12.5 microg/mL). T cell mitogens were added into the culture 2 h later and the cells were cultured at 37 degrees C in 5% CO2 for a further 72 h. Cell proliferation was determined by a non-radioactive rapid dye assay and the percentage inhibition caused by HPCE was calculated.

RESULTS

Pre-exposure to HPCE significantly inhibited concanavalin A-induced proliferation of murine spleen and mesenteric lymph node cells (up to 100% inhibition; P < or = 0.01). The HPCE also inhibited lymphocyte proliferation stimulated by mitogens phorbol-myristate-acetate and ionomycin and by the anti-CD3epsilon monoclonal antibody (P < or = 0.05). The inhibition was dose-dependent, but independent of the presence of virulence genes cagA or vacA. Treatment of HPCE at 80 degrees C for 30 min, but not at 55 degrees C for 60 min, completely abolished its inhibitory action. The HPCE, pretreated with pronase E, proteinase K, trypsin, acid or alkali also completely lost its inhibitory effect (P < or = 0.01), while in contrast, treatment with carboxypeptidase and leucine aminopeptidase had no effect.

CONCLUSION

Helicobacter pylori produces heat-labile proteins or peptides that suppress T cell mitogen-induced proliferation of murine lymphoid cells in a similar manner to that observed with human peripheral blood mononuclear cells. The mouse cell culture system can, therefore, be used as a model to further study the mechanisms of action and antigen specificity of these immunomodulatory factors.

The polysaccharide portion of lipopolysaccharide regulates antigen-specific T-cell activation via effects on macrophage-mediated antigen processing

The lipopolysaccharide (LPS) structure of Salmonella typhimurium has been correlated with the virulence of wild-type strain LT2. Mutants of LT2 with truncated polysaccharide portions of LPS are less virulent than strains with a complete LPS structure. Polyclonal T cells and monoclonal T-cell hybridomas were more reactive to heat-killed rough mutants than to heat-killed smooth strains, as measured by interleukin-2 (IL-2) production. Using a large panel of strains with truncated LPS molecules, we found that T-cell reactivity decreased with certain lengths of polysaccharide. The decreased response was not due to differential phagocytic uptake, IL-12 production, or major histocompatibility complex class II surface expression by macrophages. Also, LT2 did not mediate any global suppression since addition of LT2 did not diminish the response of T cells specific for antigens unrelated to Salmonella. In an experiment in which processing times were varied, we found that antigens from rough strains were processed and presented more quickly than those associated with smooth strains. At longer processing times, epitopes from LT2 were presented well. We hypothesize that the slower antigen processing and presentation of wild-type Salmonella may be caused by masking of surface antigens by the longer polysaccharide portion of smooth LPS. This blocking of effective antigen presentation may contribute to the virulence of Salmonella.

DNA sequencing of the gene encoding Salmonella typhimurium-derived T-cell inhibitor (STI) and characterization of the gene product, cloned STI

In a previous study, we found a novel protein which inhibited T-cell responsiveness to interleukin-2 (IL-2) in Salmonella typhimurium and called it S. typhimurium-derived T-cell inhibitor (STI). In this study, we analyzed the DNA sequence of the gene encoding STI. The STI gene was cloned into a plasmid vector, pUC118, and expressed in Escherichia coli JM109. Like native STI, the cloned STI inhibited IL-2-dependent CTLL-2 cell growth. Furthermore, this growth inhibition involved down-regulation of IL-2 receptor expression. These results indicate that the cloned STI expressed in E. coli was identical to native STI. Sequencing revealed that the STI gene contained an open reading frame of 2298 base pairs encoding a precursor form of 765 amino acid residues (molecular mass 83605) that is processed into a mature form of 745 amino acid residues with molecular mass 81 548. Homology analysis revealed that its amino acid sequence was highly homologous with that of the beta-glucosidase of E. coli K-12. We designated the gene encoding STI sti.

Salmonella infection-induced non-responsiveness of murine splenic T-lymphocytes to interleukin-2 (IL-2) involves inhibition of IL-2 receptor gamma chain expression

In a previous study we demonstrated that Salmonella typhimurium-induced immunosuppression involved T-cell non-responsiveness to interleukin-2 (IL-2). In this study we observed that Salmonella-induced T-cell non-responsiveness to IL-2 was not reversed completely by treatment with N(G)-monomethyl-L-arginine, which is known to inhibit nitric oxide (NO) secretion by macrophages in culture. Furthermore, when purified splenic T-lymphocytes from Salmonella-infected mice were activated with an anti-CD3 antibody, the responsiveness of these T-cells to IL-2 was suppressed significantly. Results of flow cytometric analysis using an anti-IL-2 receptor gamma chain (IL-2Rgamma) antibody showed that IL-2Rgamma expression in mitogen-activated T-cells was down-regulated by Salmonella infection. These results suggest that Salmonella infection-induced T-cell non-responsiveness to IL-2 involves a defective function of T-cells themselves and appears to be regulated by inhibition of IL-2Rgamma expression in T-cells.

A purified protein from Salmonella typhimurium inhibits high-affinity interleukin-2 receptor expression on CTLL-2 cells

Previously, we have demonstrated that the immunosuppression induced by the purified substance Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) involves T-cell non-responsiveness to interleukin-2 (IL-2). In the present study, it was found that STI inhibited the growth of CTLL-2 cells, which are an IL-2-dependent cytotoxic T-cell line. Analysis of IL-2 receptor (IL-2R) function showed that STI inhibited high-affinity receptor expression and internalization by CTLL-2 cells. Furthermore, FACS analysis demonstrated that STI inhibited both beta chain and gamma chain expression of IL-2R on the cells. These results suggest that the suppression of T-cell proliferation induced by STI results from a defect in IL-2R function.

Purified protein from Salmonella typhimurium inhibits the interleukin-2 response of murine splenic T-lymphocytes activated with anti-CD3 antibody

Previously, we demonstrated that the immunosuppression induced by a purified preparation of Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) can be observed in terms of suppression of the proliferation of murine spleen cells stimulated with a mitogenic lectin. In the present study, I observed that STI inhibited the interleukin-2 (IL-2) response of purified murine splenic T lymphocytes stimulated with anti-CD3 antibody. The flow cytometric analysis of IL-2 receptor (IL-2R) expression on T cells showed that STI specifically suppressed the expression of IL-2R beta and IL-2R gamma. Furthermore, when the IL-2-dependent T-cell line CTLL-2 was incubated with STI, the growth of CTLL-2 cells was significantly inhibited. These results suggest that the target cells for STI are T cells themselves, and that the suppression of T-cell proliferation induced by STI might involve a defect in the IL-2 receptor (IL-2R) function of T cells.

A purified protein from Salmonella typhimurium inhibits proliferation of murine splenic anti-CD3 antibody-activated T-lymphocytes

In previous study, we observed that the purified substance Salmonella typhimurium-derived inhibitor of T-cell proliferation (STI) had an immunosuppressive effect, demonstrated as the suppression of mitogenic lectin-induced proliferation of murine spleen cells. In the present study, we confirmed the immunosuppressive effect of STI, which suppressed the proliferation of murine splenic T-lymphocytes activated with the anti-CD3 antibody (Ab) and phorbol 12-myristate-13 acetate (PMA) and this phenomenon was accompanied by augmentation of interferon-gamma (IFN-gamma) secretion and inhibition of interleukin-2 (IL-2) secretion. Furthermore, the augmentation of IFN-gamma secretion caused IL-2 receptor alpha chain (IL-2R alpha) over expression on T-cells. However, the addition of an anti-IFN-gamma Ab and recombinant IL-2 (rIL-2) did not reverse the suppressed T-cell proliferation, although the level of IL-2R alpha expression on T-cells recovered to around normal. Furthermore, Western blotting using an anti-phosphotyrosine Ab showed that IL-2R-mediated tyrosine phosphorylation of protein substrates in T-cells was inhibited by incubation with STI for 48 h and this inhibition was not reversed by adding the anti-IFN-gamma Ab and rIL-2. These results suggest that STI-induced suppression of T-cell proliferation involves a defect in IL-2R function and/or IL-2 signaling pathway in T-cells.

Changes associated with tyrosine phosphorylation during short-term hypoxia in retinal microvascular endothelial cells in vitro

The occlusion of capillary vessels results in low oxygen tension in adjacent tissues which triggers a signaling cascade that culminates in neovascularization. Using bovine retinal capillary endothelial cells (BRCEC), we investigated the effects of short-term hypoxia on DNA synthesis, phosphotyrosine induction, changes in the expression of basic fibroblast growth factor receptor (bFGFR), protein kinase C (PKC alpha), heat shock protein 70 (HSP70), and SH2-containing protein (SHC). The effect of protein tyrosine kinase (PTK) and phosphatase inhibitors on hypoxia-induced phosphotyrosine was also studied. Capillary endothelial cells cultured in standard normoxic (pO2 = 20%) conditions were quiesced in low serum containing medium and then exposed to low oxygen tension or hypoxia (pO2 = 3%) in humidified, 5% CO2, 37 degrees C, tissue culture chambers, on a time-course of up to 24 h. DNA synthesis was potentiated by hypoxia in a time-dependent manner. This response positively correlated with the cumulative induction of phosphotyrosine and the downregulation of bFGFR (M(r) approximately 85 kDa). Protein tyrosine kinase inhibitors, herbimycin-A, and methyl 2,5-dihydroxycinnamate, unlike genistein, markedly blocked hypoxia-induced phosphotyrosine. Prolonged exposure of cells to phosphatase inhibitor, sodium orthovanadate, also blocked hypoxia-induced phosphotyrosine. The expression of HSP70, PKC alpha, and SHC were not markedly altered by hypoxia. Taken together, these data suggest that short-term hypoxia activates endothelial cell proliferation in part via tyrosine phosphorylation of cellular proteins and changes in the expression of the FGF receptor. Thus, endothelial cell mitogenesis and neovascularization associated with low oxygen tension may be controlled by abrogating signaling pathways mediated by protein tyrosine kinase and phosphatases.

A cell-free Salmonella typhimurium extract induces inhibition of tyrosine phosphorylation in murine splenic T-lymphocytes

In a previous study, we observed that suppression of T-cell proliferation induced by Salmonella infection is associated with inhibition of tyrosine phosphorylation in T-cells, and that a cell-free Salmonella typhimurium LT2 extract (LT2 extract) also suppressed mitogen-induced T-cell proliferation. In the present study, therefore, we attempted to clarify whether the T-cell suppression induced by LT2 extract involved inhibition of tyrosine phosphorylation in T-cells. Western blotting using anti-phosphotyrosine antibodies showed that the mitogen-induced tyrosine phosphorylation of 120-, 106-, 94-, 76-, 68-, 57- and 36-kDa proteins in murine splenic T-cells was inhibited by treatment with LT2 extract. These results suggest that the suppression of T-cell proliferation induced by LT2 extract is also associated with inhibition of tyrosine phosphorylation in T-cells.

Immunosuppression induced by Salmonella infection is correlated with augmentation of interleukin-2 receptor alpha chain expression in murine splenic lymphocytes

In a previous study, we observed that the suppression of T-cell proliferation induced by Salmonella cell-free extract was associated with augmentation of IL-2 receptor (IL-2R) alpha chain expression. In this study, we also observed this kind of augmentation of IL-2R alpha in Salmonella-infected mice. Phytohaemagglutinin (PHA)-stimulated proliferation of murine spleen cells was significantly suppressed when the mice were infected with Salmonella typhimurium. However, expression of the alpha chain but not the beta chain of IL-2R in lymphocytes was augmented by the infection. Analysis of the IL-2R-positive cell-population showed that the augmentation of IL-2R alpha was not specific to certain cell subpopulations. Furthermore, the inhibition of PHA-stimulated murine spleen cell proliferation and the augmentation of IL-2R alpha expression induced by the infection in lymphocytes was completely reversed by treatment with anti-interferon-gamma monoclonal antibody (anti-IFN-gamma Ab). These results suggest that the suppression of T-cell proliferation induced by Salmonella infection was associated with augmentation of IL-2R alpha expression in an IFN-gamma production-dependent manner in the same way as the suppression of T-cell proliferation induced by Salmonella cell-free extract.

Inhibition of T-cell proliferation induced by a cell-free Salmonella typhimurium extract does not involve a nitric oxide-mediated mechanism

In a previous study, we observed that a cell-free Salmonella typhimurium extract induced suppression of mitogen-induced T-cell proliferation and that this suppression involved nonresponsiveness of T-cells to interleukin-2 (IL-2) and augmentation of IL-2 receptor (IL-2R) expression. In this study, we found that inhibition of phytohemagglutinin (PHA)-stimulated murine spleen cell proliferation induced by a cell-free S. typhimurium extract was reversed by treatment with an anti-interferon-gamma monoclonal antibody (anti-IFN-gamma Ab), but not by interleukin-4 or NG-monomethyl-L-arginine, which is known to inhibit nitric oxide (NO)-secretion from spleen cells in culture. However, IL-2R expression was augmented by treatment with the extract, although this was independent of an NO-mediated mechanism. Only anti-IFN-gamma Ab treatment reduced the augmented IL-2R expression to a normal level. These results suggest that the suppression of T-cell proliferation induced by the Salmonella cell-free extract is associated with augmentation of IL-2R expression in an NO production-independent manner.

A cell-free Salmonella typhimurium extract modulates interleukin-2 (IL-2) receptor expression but not IL-2-stimulated responses of murine splenic lymphocytes

In a previous study, we observed that a cell-free Salmonella typhimurium extract induced suppression of mitogen-induced T-cell proliferation and this suppression involved non-responsiveness of T-cells to interleukin-2 (IL-2). In this study, we found that a cell-free S. typhimurium extract modulated IL-2 receptor (IL-2R) expression on phytohemagglutinin (PHA)-stimulated murine spleen cells and this was a mechanism of T-cell non-responsiveness to IL-2, but did not affect IL-2 binding to IL-2R and the consequent responses. Western blotting using anti-phosphotyrosine antibodies showed that IL-2R-mediated tyrosine phosphorylation of protein substrates in PHA-activated murine splenic T-cells, which express a high-affinity IL-2R (alpha- and beta-chains), was not affected by treatment with the S. typhimurium cell-free extract. Furthermore, PHA-activated spleen T-cells responded to recombinant IL-2 and this was not inhibited by the extract. Surprisingly, IL-2R expression was augmented by treatment with the extract, although this was independent of IL-2 production. These results suggest that the suppression of T-cell proliferation induced by the Salmonella cell-free extract was associated with augmentation of IL-2R expression, rather than down-regulation of the IL-2 response. This may be a mechanism responsible for the Salmonella extract-evoked suppression of mitogen-induced T-cell proliferation.