Characterization of the binding of soluble CD14 to human endothelial cells and mechanism for CD14-dependent cell activation by LPS

The binding of soluble CD14 (sCD14) to human umbilical vein endothelial cells (HUVEC) was examined in order to understand the role of CD14 in potentiating LPS activity. Both purified sCD14 and [125I]-sCD14 potentiated LPS-stimulated ICAM-1 expression in HUVEC. This potentiation was blocked by anti-CD14 monoclonal antibodies (mAbs) 63D3, UCHM-1 and RM052. Saturation binding assay revealed that [125I]-sCD14 bound to HUVEC with a ligand-acceptor dissociation constant of 290 nM. The binding of [125 I]-sCD14 was inhibited by sCD14 with a sCD14-acceptor dissociation constant of 24 nM. The density of CD14 acceptors was estimated to be 4-8 x 105 sites per cell. The [125I]-sCD14 binding was inhibited by anti-human CD14 mAbs UCHM-1 and RM052 but not 63D3. The bound [125I]-sCD14 could be washed off by acid buffer, pH 3.0, and its localization on the cell membrane was confirmed by light microscopic autoradiography. Based on the previously published description of LPS-sCD14 interactions and our observation that anti-CD14 mAbs inhibiting sCD14-acceptor binding also blocked the potentiation of LPS activity by sCD14, we propose that bridging or crosslinking between the putative LPS-receptor complex with the sCD14-acceptor complex via LPS-sCD14 interactions is the mechanism of CD14-dependent activation of endothelial cells (EC) by LPS. Re-examination of the published data suggests that this mechanism is a universal one for EC, leukocytes and lymphocytes.

Multiple mechanisms of action for inhibitors of histidine protein kinases from bacterial two-component systems

Many pathogenic bacteria utilize two-component systems consisting of a histidine protein kinase (HPK) and a response regulator (RR) for signal transduction. During the search for novel inhibitors, several chemical series, including benzoxazines, benzimidazoles, bis-phenols, cyclohexenes, trityls, and salicylanilides, were identified that inhibited the purified HPK-RR pairs KinA-Spo0F and NRII-NRI, with 50% inhibitory concentrations (IC50s) ranging from 1.9 to >500 microM and MICs ranging from 0.5 to >16 microg/ml for gram-positive bacteria. However, additional observations suggested that mechanisms other than HPK inhibition might contribute to antibacterial activity. In the present work, representative compounds from the six different series of inhibitors were analyzed for their effects on membrane integrity and macromolecular synthesis. At 4x MIC, 17 of 24 compounds compromised the integrity of the bacterial cell membrane within 10 min, as measured by uptake of propidium iodide. In this set, compounds with lower IC50s tended to cause greater membrane disruption. Eleven of 12 compounds inhibited cellular incorporation of radiolabeled thymidine and uridine >97% in 5 min and amino acids >80% in 15 min. The HPK inhibitor that allowed >25% precursor incorporation had no measurable MIC (>16 microg/ml). Fifteen of 24 compounds also caused hemolysis of equine erythrocytes. Thus, the antibacterial HPK inhibitors caused a rapid decrease in cellular incorporation of RNA, DNA, and protein precursors, possibly as a result of the concomitant disruption of the cytoplasmic membrane. Bacterial killing by these HPK inhibitors may therefore be due to multiple mechanisms, independent of HPK inhibition.

Gram-positive sepsis. Mechanisms and differences from gram-negative sepsis

This article has reviewed the mechanisms by which gram-positive bacteria lead to septic shock, with regard to bacterial structure and toxicology and the host responses elicited both in animal models and in the clinical setting. Gram-positive organisms are better suited to invade host tissues and elicit, in general, a brisker phagocytic response than gram-negative organisms. The lack of endotoxin in the outer cell wall is compensated for by the presence of exposed peptidoglycan and a range of other toxic secreted products. It appears that cell wall components of gram-positive bacteria may signal via the same receptor as gram-negative endotoxin, although the type of signal and coreceptor may differ. Both animal and clinical data suggest that, unlike endotoxin-mediated shock, gram-positive infection produces a modest TNF response only and does not respond well to anti-TNF therapies. This leads one to conclude that the mechanisms leading to shock in gram-positive infection may be multifactorial and perhaps more difficult to treat. A thorough review of gram-positive mechanisms of sepsis is hampered by a lack of basic research in this field. Understanding of gram-negative bacterial structure and the regulation of virulence genes is at an advanced stage, yet the molecular tools to analyse virulence factors in the gram-positive genome have only recently become available. There is a paucity of good animal models of gram-positive infection and a lack of microbiologic data from some of the major trials in sepsis that might have given greater insight into the mechanisms leading to shock in various infections.

A lipoteichoic acid fraction of Enterococcus hirae activates cultured human monocytic cells via a CD14-independent pathway to promote cytokine production, and the activity is inhibited by serum components

To elucidate the cellular activation mechanisms of lipoteichoic acid (LTA) compared with those of lipopolysaccharide (LPS), a quantitatively major LTA fraction, QM-1M, was prepared from hot phenol-water extracts of Enterococcus hirae (ATCC 9790) by hydrophobic octyl-Sepharose chromatography and by ion-exchange membrane (QMA-Mem Sep 1010) chromatography as a 60% 1-propanol- and 1 M NaCl-eluted fraction. Unlike the reference Escherichia coli LPS, QM-1M did not demonstrate any ability to induce cytokines in a human whole blood culture system in this study, whereas QM-1M induced a few cytokines such as interleukin (IL)-8 and tumor necrosis factor-alpha in human monocytic THP-1 cell and human peripheral mononuclear cell (PBMC) cultures in the absence of serum. Fetal calf and human sera decreased the above cytokine induction by QM-1M in THP-1 and PBMC cultures, whereas sera increased activities of the reference LPS. IL-8 induction in the absence of serum in response to QM-1M was demonstrated to proceed through a CD14-independent pathway unlike the reference LPS.

Interactions of lymphocytes from patients with psoriatic arthritis or healthy controls and cultured endothelial cells

Psoriatic arthritis (PA) is an inflammatory rheumatic disease that can concomitantly occur in patients with psoriasis vulgaris. Psoriatic synovitis shows alterations of the synovial microvasculature. Inflammatory cells adhere to endothelial cells (EC) and migrate through the vascular wall of postcapillary venules located in the subintimal layer of the synovial membrane. The aim of our study was to investigate, first, the phenotype of lymphocytes (LC) of PA patients using flow cytometry (FC) with regard to activation antigens and adhesion molecules; second, the adhesion of LC of PA patients on cultivated resting or activated (with thrombin, LPS, IFN-gamma, or TNF-alpha) human umbilical vein endothelial cells (HUVEC) by counting the Feulgen-stained nuclei of both adherent LC and HUVEC using image analysis; and third, the synthesis of IL-6 and IL-8 in both LC and HUVEC 24 hr after cell contact. These cytokines were determined qualitatively by immunofluorescence and quantitatively at the single-cell level by FC as well as in the supernatants of the cultures using commercial cytokine ELISAs. Fourth, we investigated whether or not the LC adhesion on HUVEC as well as the cytokine production could be inhibited by monoclonal antibodies against LC- or EC-specific adhesion molecules. In contrast to controls PA patients showed an increased surface expression of CD11a, b, and c as well as of CD44 but a reduced surface expression of CD49d/CD29, and CD49e/CD29, and cell-bound fibronectin on CD3+ LC. The activation markers CD25 and HLA-DR were found to be slightly enhanced in PA. The cell adhesion was generally enhanced in PA patients vs controls. It could be reduced with monoclonal antibodies (MoAbs) against CD11a and CD18 on IFN-gamma- or TNF-alpha-activated HUVEC but was generally enhanced after treatment of HUVEC with MoAbs against CD54, CD62E, or CD106. Due to LC adhesion on HUVEC IL-6 and IL-8 were produced in significantly higher amounts in PA patients compared to controls. This effect occurred already in resting but was enhanced in activated HUVEC. While IL-6 is mainly produced by HUVEC but also in smaller quantities by LC, IL-8 is synthesized only by HUVEC and could be modified by preincubation with MoAbs against LC- or EC-specific adhesion molecules in parallel to the cell adhesion. The experiments show that the main adhesion pathway in LC homing of PA patients is the interaction of the LC adhesion molecule CD11a/CD18 with CD54 on EC followed by an enhanced synthesis of proinflammatory and chemotactic cytokines. These results favor the hypothesis that the pathological alterations of the microvasculature in PA patients are generated by altered homing processes.