Salmonella typhi Ty2 OmpC porin induces bactericidal activity on U937 monocytes

The YompC protein of Yersinia enterocolitica: molecular and physiological characterization

The structural gene coding for YompC has been identified in the genome of a pathogenic strain of Yersinia enterocolitica O:9, and was subsequently cloned and sequenced. Detailed alignment of the deduced amino acid sequence showed that YompC is a member of the OmpC porin family with the highest degree of homology to Klebsiella pneumoniae. The mutant lacking YompC porin was constructed by insertional inactivation of the yompC gene which resulted from the integration of suicide vector at the yompC locus. In intact cells of Y. enterocolitica, loss of the YompC protein reduced the outer membrane permeability for beta-lactam antibiotics and tetracycline and resulted in a 2-5-fold increase in resistance to these compounds, depending on their chemical properties. Mutation in the ompR regulatory gene resulted in the loss of both YompC and YompF porins, which led to a greater increase of resistance to antibiotics, as compared with the YompC mutant strain. Moreover, the binding assay with HEp-2 cells suggests that YompC may play a role in the adhesion properties of Y. enterocolitica strains.

A Salmonella typhi OmpC fusion protein expressing the CD154 Trp140-Ser149 amino acid strand binds CD40 and activates a lymphoma B-cell line

CD154 is a type II glycoprotein member of the tumour necrosis factor (TNF) ligand family, which is expressed mainly on the surface of activated T lymphocytes. The interaction with its receptor CD40, plays a central role in the control of several functions of the immune system. Structural models based on the homology of CD154 with TNF and lymphotoxin indicate that binding to CD40 involves three regions surrounding amino acids K143, R203 and Q220, and that strands W140-S149 and S198-A210 are critical for such interactions. Also, it has been reported that two recombinant CD154 fragments, including amino acid residues Y45-L261 or E108-L261 are biologically active, whereas other polypeptides, including S149-L261, are not. Therefore, we decided to construct a fusion protein inserting the W140-S149 amino acid strand (WAEKGYYTMS) in an external loop of the outer membrane protein C (OmpC) from Salmonella enterica serovar Typhi and assess its ability to bind CD40 and activate B cells. The sodium dodecyl sulphate-polyacrylamide gel electrophoresis demonstrated that the chimeric OmpC-gp39 protein conserved its ability to form trimers. Binding to CD40 was established by three variants of enzyme-linked immunosorbent assay, a direct binding assay by coating plates with a recombinant CD40-Fc protein and through two competition assays between OmpC-gp39 and recombinant CD154 or soluble CD40-Fc. Flow cytometry analysis demonstrated that OmpC-gp39 increased the expression levels of major histocompatibility complex II, CD23, and CD80, in Raji human B-cell lymphoma similarly to an antibody against CD40. These results further support that the CD154/CD40 interaction is similar to the TNF/TNF receptor. This is the first report of a bacterial fusion protein containing a small amino acid strand form a ligand that is able to activate its cognate receptor.

Lysis of salmonella typhi intracellularly infected U937 cells by human natural killer cells: effect of protein kinase inhibitors

We examined the effect of Salmonella typhi (wild-type Ty2 and mutant strain TYT1231)-infected U937 cells on natural killer cell (NKC) cytotoxicity of peripheral blood mononuclear cells (PBMCs) and highly purified NKC (HPNKCs; CD16(+)/CD56(+) > 95%; the rest corresponding to CD3(+) T cells). We also analyzed the possible role of various protein kinases involved in natural cytotoxicity on these processes. PBMC cytotoxicity against S typhi-infected U937 cells was significantly higher (paired Student t test; P < 0.05) than its lytic effect against noninfected cells (control) at the various effector-to-target cell ratios used (30:1 [24.4 +/- 9.7, 25.1 +/- 11.8, and 17.5 +/- 8.6]; 50:1 [26.6 +/- 9.7, 26.7 +/- 12.8, and 21.2 +/- 7.5] and 70:1 [32.4 +/- 14.4, 30.1 +/- 12.4, and 23.1 +/- 7.2], respectively). PBMC NKC activity seemed to be dependent on such ratios and was similar against both Salmonella strains studied. Approximately half of the individual samples tested (n = 12; 8 male and 4 female subjects of comparable age) showed at least a 20% specific lysis increase against their own control; essentially no changes or smaller increases in NKC activity were observed in all other samples. Similar results were obtained using HPNKCs as effector cells (5:1 ratio [38.9 +/- 12.3, 43.3 +/- 11.2, and 27.5 +/- 4.9] and 10:1 ratio [51.3 +/- 9.1, 46.1 +/- 9.8, and 37.7 +/- 15.5, respectively]). In general, specimens significantly lysed after incubation with PBMCs responded in a similar manner to a challenge with HPNKCs. PBMC and HPNKC cytotoxicity against S typhi wild-type-infected U937 cells was significantly decreased in a dose-dependent manner by the addition of genistein (50-200 micromol) or GFX (0.5-2.0 micromol) to the cytotoxicity assay mixture. NKC activity was almost completely inhibited at the highest genistein and GFX concentrations. In similar experiments, wortmannin (100-500 nmol) failed to inhibit PBMC cytotoxicity and significantly decreased HPNKC activity only at the highest concentration tested. These results show that in the process of NKC recognition and lysis of S typhi-infected U937 cells, there is not a requisite for full bacterial intracellular survival capacity and that S typhi-infected U937 cells are a significantly better target than noninfected U937 cells. NKC signaling pathways activated during the S typhi-infected U937 cell recognition and lysis process are mainly protein tyrosine kinase and protein kinase-C, and they can be blocked by the same protein kinase inhibitors known to inhibit natural cytotoxicity.

A chromosomal region surrounding the ompD porin gene marks a genetic difference between Salmonella typhi and the majority of Salmonella serovars

In this work it is shown that the majority of Salmonella serovars most frequently associated with the systemic infection of vertebrate hosts produce a major outer-membrane porin, OmpD. However, OmpD is absent from the outer-membrane protein profiles of Salmonella typhi strain Ty2 and 26 clinical isolates of S. typhi examined by SDS-PAGE. To determine whether the ompD gene is present in S. typhi, primers internal to the ompD coding sequence were used to amplify the gene by PCR. With the exception of S. typhi strains, the ompD gene was amplified from the genomes of all Salmonella serovars tested. Consistently, a specific ompD probe did not hybridize with DNA isolated from the S. typhi strains. Taken together, these results demonstrate that S. typhi does not produce OmpD due to the absence of the ompD gene. Furthermore, it was investigated whether the deletion of ompD extended to smvA. This gene is adjacent to ompD in the Salmonella typhimurium chromosome and encodes a protein involved in the resistance to methyl viologen, a superoxide-generating agent. Although PCR failed to amplify the smvA gene from the S. typhi strain Ty2 genome, it was possible to amplify it from the chromosome of the clinical strains. On the other hand, hybridization analyses showed that the smvA gene is present in all the S. typhi strains tested. In contrast to the other Salmonella serovars, S. typhi strain Ty2 and the clinical isolates showed sensitivity to methyl viologen, suggesting that smvA gene is inactive in S. typhi. In conclusion, the ompD-smvA region is variable in structure among Salmonella serovars. It is hypothesized that the absence of ompD may suggest a role in host specificity.

Effect of Salmonella-infected human monocytes on natural killer cell cytotoxicity. In vitro studies

Various chemicals, including some bacteria-derived components, modulate natural killer cell (NKC) activity. We have analyzed the effect of wild-type Ty2 and of mutant strain TYT1231 Salmonella typhi-infected monocytes (U937 cells and human autologous monocytes) on NKC cytotoxicity of peripheral blood mononuclear cell (PBMC) and highly purified NKC (HPNKC; CD16+/56+ > 95%; the rest corresponding to CD3+ T-cells). PBMC's co-culture with either S. typhi strain infected U937 cells (medium or non-infected U937 cells as controls) resulted in the induction of lymphocyte activated killer (LAK) cell activity showing cytotoxicity against target human NKC-resistant lymphoblastoid Daudi cell line. Comparable experiments using autologous monocytes gave similar results. Co-culture of HPNKC preparations with either S. typhi strain infected U937 cells resulted in increased LAK cell activity against target Daudi cells in each and everyone of the five samples tested; paired Student's t-test p < 0.01 for both times (20 and 40 h) tested. Similar to the results observed in the experiments using PBMC, we did not find significant differences in the ability between medium and non-infected cells, or between wild-type S. typhi Ty2 and mutant strain TYT1231 infected U937 cells, to induce LAK activity in HPNKC preparations. PBMC co-incubation with either S. typhi strain infected U937 cells or autologous monocytes resulted in significant increases in IL-12, TNF-alpha, and IFN-gamma secretion. In similar experiments using HPNKC samples instead, infected U937 cells significantly increased IL-12 and IFN-gamma, but not TNF-alpha secretion. PBMC co-incubation with non-infected U937 cells, but not with non-infected monocytes, significantly increased supernatant IL-12 and TNF-alpha levels (no significant changes in IFN-gamma were recorded). Secreted cytokines remained essentially unchanged after co-incubating HPNKC preparation with non-infected U-937 cells. Incubation of PBMC or HPNKC preparations with either S. typhi strain infected U937 cells failed to produce significant changes in the expression of NKC lineage (CD16+/56+) or activation (CD28+, CD69+ and CD95+) markers. The ability of infected monocytes to induce LAK activity, release NKC cytokines and upmodulate NKC's CD95+ marker expression was essentially the same for both infecting Salmonella strains used. These results suggest a role for NKC in the physiological defensive response against intracellularly infected monocytes representing, perhaps one of the earliest antimicrobial mechanisms of the innate immune system.

The porin OmpC ofSalmonella typhimuriummediates adherence to macrophages

Macrophages recognize, adhere to, and phagocytose Salmonella typhimurium. The major outer membrane protein OmpC is a candidate ligand for macrophage recognition. To confirm this we used transposon mutagenesis to develop an ompC-deficient mutant in a known virulent strain of S. typhimurium; mutant and wild type were compared in macrophage adherence and association assays. Radiolabeled wild type S. typhimurium bound to macrophages at five-fold higher levels than did the ompC mutant. In association assays, macrophages in monolayers bound and internalized three-fold more wild type than mutant, while macrophages in suspension bound and internalized 40-fold more wild type than mutant. The ompC gene of our test strain of S. typhimurium contains several discrete differences compared with the ompC genes of Salmonella typhi and Escherichia coli. The deduced OmpC amino acid sequence of S. typhimurium shares 77 and 98% identity with OmpC amino acid sequences of E. coli and S. typhi, respectively. Evidence from this study supports a role for the OmpC protein in initial recognition by macrophages and distinguishes regions of this protein that potentially participate in host-cell recognition of bacteria by phagocytic cells.Key words: Salmonella, porin, macrophage, outer membrane protein, DNA sequencing.