Accelerated internalization and detoxification of endotoxin by anti-lipopolysaccharide antibody is an Fc receptor--mediated process.

BACKGROUND: Interaction between lipopolysaccharide (LPS), LPS-binding protein, and the CD14 receptor at the surface of LPS-responsive cells results in inflammatory cytokine release and internalization and detoxification of LPS. Monoclonal antibodies (mAbs) raised against the deep-core lipid A or the O-linked polysaccharide moieties of LPS accelerate internalization and detoxification of LPS without stimulating cytokine release. This study was conducted to test the hypothesis that the antibody-mediated internalization of LPS is an Fc receptor (FcR)--mediated process. METHODS: Fluoroisothiocyanate (FITC)-conjugated Escherichia coli O111:B4 LPS was incubated with RAW 264.7 cells and allowed to internalize for 2 hours in the presence and absence of anti-LPS, anti-CD14, and isotype control mAbs, and Fab fragments from the anti-CD14, anti--Fc receptor, and control mAbs. Tumor necrosis factor--alpha (TNF-alpha) release was measured by WEHI 164 cell bioassay. FITC-LPS uptake was measured by flow cytometry. Statistical analysis was by analysis of variance and Fisher exact test. RESULTS: Addition of anti-LPS antibodies resulted in a 30- to 40-fold acceleration of LPS internalization (P <.01) in agreement with previous studies. This increase was blunted by anti-CD14 and also by isotype control holo-antibody (P <.01), but not by Fab fragments from anti-CD14 or isotype control antibody. Both anti-FcR antibodies and Fab fragments blocked anti-LPS antibody--stimulated uptake of FITC-LPS. Both intact anti-CD14 holo-antibody and Fab fragments blocked TNF-alpha release (P <.01). CONCLUSIONS: Clearance and detoxification of LPS are thought to be essential to the host response to endotoxin. It has been shown that antibodies to LPS accelerate its internalization by monocytic cell lines without increasing the elaboration of cytokines. We found that specific blockade of CD14 by Fab fragments could block TNF-alpha release but not alter the accelerated internalization of LPS produced by anti-LPS antibodies. In contrast, a nonspecific blockade of internalization was produced by competing antibody, which suggests a mechanistic role for the FcR. Specific blockade of FcR by either holo-antibody or Fab fragments blocked accelerated internalization, which confirms a FcR mechanism. We conclude that the accelerated internalization of LPS produced by anti-LPS antibody is an Fc receptor--mediated process. These results have significance for the development of adjuvant immunotherapy for gram-negative bacterial sepsis.

Near-infrared and nuclear magnetic resonance spectroscopic assessment of tissue energetics in an isolated, perfused canine hind limb model of dysoxia.

This controlled laboratory study examined the efficacy of near-infrared spectroscopy (NIRS) and 31P-nuclear magnetic resonance (NMR) spectroscopy in measuring regional tissue oxygenation in a isolated, perfused hind limb model of tissue dysoxia. Isolated hind limb perfusion was carried out in 20 mongrel dogs and oxygen delivery was varied by manipulating either hemoglobin concentration, oxygen saturation, or flow. Hind limbs from anesthetized mongrel dogs (n = 20) were separated and isolated perfusion performed. NIRS probes for recording relative O2 saturation of tissue hemoglobin (HbO2) and cytochrome a,a3 and NMR probes for measuring 31P-high energy phosphates were placed over the limb. Measurements of physiologic parameters, blood gases, lactate, NIRS values for HbO2 and cytochrome a,a3 redox state, and 31P-phosphate levels were recorded at set intervals throughout the experiment. Measures of tissue oxygen consumption (VO2) correlated with tissue oxygenation as measured by HbO2 and cytochrome a,a3 redox state (NIRS), as well as by 31P-high energy phosphate levels (NMR) throughout the experiment. Delivery-dependent tissue oxygenation was detected at a higher DO2 by NIRS than by VO2 or NMR. Tissue oxygenation as measured by NIRS and NMR shows excellent correlation with oxygen delivery in an isolated, perfused model of shock. NIRS may allow early detection of tissue dysoxia using rapid non-invasive techniques.

Binding specificity of polymyxin B, BPI, LALF, and anti-deep core/lipid a monoclonal antibody to lipopolysaccharide partial structures.

The deep core/lipid A (DCLA) region of gram-negative bacterial lipopolysaccharide (LPS) is common to most gram-negative pathogens and contains anionic phosphoryl groups plus numerous acyl chains as part of the toxic lipid A moiety. Several disparate agents that antagonize the effects of LPS exhibit extensive physicochemical similarities (hydrophobicity, cationic charge) within their binding domains. It is presumed that binding to the DCLA region by each of these antagonists-cross-reactive anti-LPS monoclonal antibodies (mAbs), polymyxin B (PmB), plus bactericidal permeability-increasing protein (BPI) and Limulus anti-LPS factor (LALF)-may be related to these properties. Therefore, we hypothesized that in addition to secondary and tertiary protein conformation, electrostatic interactions involving the negatively charged phosphoryl groups, hydrophobic interactions involving the acyl chains of lipid A, or both might be important factors that promote LPS antagonism. Binding of PmB, BPI, LALF, or anti-DCLA mAb 1B6 to Salmonella minnesota monophosphoryl lipid A (MPLA), diphosphoryl lipid A (DPLA), and Salmonella minnesota Re (which possess a common structural moiety, but vary considerably in structure and charge) was examined. Highly phosphorylated DNA and bovine serum albumin served as unrelated structural controls. BPI bound MPLA, which is hydrophobic and minimally charged, while mAb 1B6 bound anionic DNA; neither PmB nor LALF were reactive with MPLA or DNA. We surmised that hydrophobic interactions play a role in BPI binding to LPS, and although electrostatic interactions appear to be important for binding of mAb 1B6 to DCLA, they may not contribute to as great an extent for PmB, BPI, or LALF. Thus our data support the contention that the contribution of these specific physicochemical factors varies among endotoxin antagonists.

Gram-negative bacterial sepsis and the sepsis syndrome.

Gram-negative sepsis syndrome is an increasingly common complication in medical and surgical patients. The molecular and cellular mechanisms underlying this dreaded complication are yielding to investigation. These studies have led to a multiplicity of targets for novel therapies. Despite highly promising results in many animal studies, clinical studies have been disappointing.

Near-infrared spectroscopy measurement of regional tissue oxyhemoglobin saturation during hemorrhagic shock.

Adequate resuscitation of patients from shock states depends on restoration of oxygen delivery (DO2) to tissues. Direct measurement of systemic DO2 during shock states requires invasive techniques such as pulmonary artery catheterization. These experiments were performed to examine the ability of near-infrared spectroscopy (NIRS), to measure regional tissue oxygenation in a large-animal model of hemorrhagic shock, and to compare these measures to global measures of oxygen delivery. Splenectomized female pigs (n = 11) were anesthetized, instrumented, and monitored. NIRS probes were placed on the leg, in the stomach via nasogastric tube, and on the liver during laparotomy. Hemorrhagic shock was induced by phlebotomy of 28% of blood volume. After 1 hour, resuscitation was with shed blood and crystalloid until cardiac output plateaued. Measurements of physiologic parameters, blood gases, lactate, intramucosal pH, and NIRS values for regional tissue hemoglobin oxygen saturation (StO2), and cytochrome a,a3 redox state were recorded at intervals throughout the experiment. Tissue oxygenation as measured by oxyhemoglobin saturation and cytochrome a,a3 redox (NIRS) correlated with measures of systemic DO2 throughout the experiment. The liver probe demonstrated blunted changes in tissue oxygenation suggesting relatively protected circulation. Intramucosal pH did not correlate well with DO2. Regional tissue oxygenation as measured by NIRS shows excellent correlation with global oxygen delivery. NIRS may allow estimation of systemic oxygen delivery using rapid non-invasive techniques.