Philip R. Craddock

Oxygen radicals mediate endothelial cell damage by complement-stimulated granulocytes. An in vitro model of immune vascular damage.

During hemodialysis, alternative pathway complement activation leads to pulmonary sequestration of granulocytes, with loss of pulmonary vascular endothelial integrity and, at times, protein-rich pulmonary edema. An in vitro model of this phenomenon was constructed utilizing 51Cr-labeled human umbilical vein endothelial cell cultures. In this system, granulocytes, when exposed to activated complement (C), induce endothelial damage; this injury is mediated primarily by oxygen radicals produced by the granulocytes. C5a appears to be the C component responsible for granulocyte-induced cytotoxicity; studies with cytochalasin B-treated granulocytes suggest that close approximation of the granulocytes and endothelial cells is necessary for maximal cell injury.

Hemodialysis leukopenia. Pulmonary vascular leukostasis resulting from complement activation by dialyzer cellophane membranes.

Acute leukopenia occurs in all patients during the first hour of hemodialysis with cellophanemembrane equipment. This transient cytopenia specifically involves granulocytes and monocytes, cells which share plasma membrane reactivity towards activated complement components. The present studies document that complement is activated during exposure of plasma to dialyzer cellophane, and that upon reinfusion of this plasma into the venous circulation, granulocyte and monocyte entrapment in the pulmonary vasculature is induced. During early dialysis, conversion of both C3 and factor B can be demonstrated in plasma as it leaves the dialyzer. Moreover, simple incubation of human plasma with dialyzer cellophane causes conversion of C3 and factor B, accompanied by depletion of total hemolytic complement and C3 but sparing of hemolytic C1. Reinfusion of autologous, cellophane-incubated plasma into rabbits produces selective granulocytopenia and monocytopenia identical to that seen in dialyzed patients. Lungs from such animals reveal striking pulmonary vessel engorgement with granulocytes. The activated complement component(s) responsible for leukostasis has an approximate molecular weight of 7,000-20,000 daltons. Since it is generated in C2-deficient plasma and is associated with factor B conversion, it is suggested that activation of complement by dialysis is predominantly through the altermative pathway.

Complement (C5-a)-induced granulocyte aggregation in vitro. A possible mechanism of complement-mediated leukostasis and leukopenia.

Activated plasma complement will induce biphasic aggregation of human granulocytes dectable by standard nephelometric techniques. The responsible active component was suggested to be C5a by molecular weight and heat-stability assays; moreover, aggragating activity was ablated by anti-C5 but not anti-C3 antibodies. C5a prepared by trypsinization of purified C5 reproduced the aggregating activity of whole activated plasma, whereas plasma from a C5-deficient donor did not support aggregation. Embolization of granulocyte aggregates might be a previously unsuspected cause of leukostasis and pulmonary damage in various clinical situations where intravascular complement activation occurs.

Complement-induced granulocyte aggregation: an unsuspected mechanism of disease.

The capacity of blood cells to aggregate, best exemplified by the response of platelets to vascular injury, is generally thought to be beneficial. However, if aggregation occurs inappropriately—tha...

Association of complement activation and elevated plasma-C5a with adult respiratory distress syndrome. Pathophysiological relevance and possible prognostic value.

Clinical and experimental observations suggest that aggregation of polymorphonuclear granulocytes (PMN) in response to activated complement (C) might contribute to the genesis of the adult respiratory distress syndrome (ARDS), aggregating PMN causing pulmonary dysfunction by becoming lodged in the lung as leucoemboli. PMN-aggregating activity can be detected in C-activated plasma and reflects C5a levels. In 61 patients at risk for ARDS a strong and highly significant correlation was found between the presence of PMN-aggregating activity in the plasma and the development of ARDS; this correlation was also significant when patients with sepsis were excluded from analysis. In patients followed prospectively detection of elevated C5a seemed to be a useful predictor of ARDS. Since corticosteroids have been shown to inhibit PMN aggregation both in vitro and in vivo, the evidence for a role for PMN aggregation in the genesis of ARDS supports the use of corticosteroids in this disorder.

Corticosteroids inhibit complement-induced granulocyte aggregation. A possible mechanism for their efficacy in shock states.

Granulocyte (PMN) aggregation and embolization may underlie complement (C)-mediated organ dysfunction in such syndromes as hemodialysis neutropenia and Purtschers ischem;c retinopathy. Because of clinical and pathologic parallels, we have further suggested a role for this phenomenon in the genesis of the adult respiratory distress syndrome (ARDS). Because corticosteroids are commonly used in immune diseases, and have particularly been claimed efficacious in shock and ARDS, we tested the capability of methylprednisolone (MP), hydrocortisone (HC), and dexamethasone (DEX) to inhibit PMN aggregation. Aggregation engendered in vitro by zymosan-activated plasma (ZAP) was inhibited by MP and HC at concentrations approximating plasma levels achieved with the large bolus (30 mg/kg i.v) therapy advocated in shock states; DEX was almost without effect. Using intravital fluorescence microscopy, we observed PMN aggregation and embolization in the mesenteric vessels of rats given intra-arterial infusions of ZAP; this was also prevented by pretreatment with 30 mg/kg MP. Steroid inhibition of aggregation seemed not to involve disruption of receptor function, because aggregation induced by alternative agents, n-formyl-Met-Leu-Phe and the ionophore A23187, was also inhibited by MP. Moreover, corticosteroid inhibition of PMN prostaglandin synthesis is also an unlikely explanation for our results, since aspirin and ibuprofen failed to block aggregation and arachidonic acid neither effected aggregation itself nor ameliorated the steroid effect. Our studies provide a plausible rationale for the empiric observation that high-dose corticosteroids may benefit patients with syndromes associated with microvascular leukostasis.

Reversal of Hemodialysis Granulocytopenia and Pulmonary Leukostasis: A CLINICAL MANIFESTATION OF SELECTIVE DOWN-REGULATION OF GRANULOCYTE RESPONSES TO C5adesarg

The transient granulocytopenia of hemodialysis results indirectly from plasma complement activation by dialyzer cellophane membranes. The C5a(desarg) so produced can induce reversible granulocyte aggregation in vitro and in vivo, and we hypothesized that the pulmonary leukostasis responsible for the granulocytopenia results from embolization of aggregates formed under the influence of C5a(desarg) produced in the dialyzer. These studies were designed to measure C5a(desarg) generation during dialysis by granulocyte aggregometry and to determine the reason for the transience of the leukostasis. C5a(desarg) generation was equally evident throughout dialysis, persisting well after granulocytopenia had reversed, and dialyzer-induced complement activation was insufficient to produce significant depletion of plasma complement titers. That granulocyte deactivation might be responsible for the transience was suggested by the absence of the usual granulocytopenia in a patient with uniquely high levels of C5a(desarg) in his predialysis plasma. Granulocytes drawn from seven stable uremic patients after granulocytopenia had reversed exhibited a dose-related, selective and irreversible refractoriness to stimulation with C5a(desarg), but their responses to n-formyl-Met-Leu-Phe remained normal. Identical deactivation was produced in normal cells by short- or long-term exposure of C5a(desarg) in vitro. These studies suggest that C5a(desarg) is indeed generated by the dialyzer throughout hemodialysis and that the transience of the leukostasis and granulocytopenia is due to selective down-regulation of cellular responses to C5a(desarg)-a phenomenon that hitherto has been described only in vitro and that may be important in limiting the deleterious effects of adherent granulocytes on the endothelium in patients with intravascular complement activation.

Ibuprofen inhibits granulocyte responses to inflammatory mediators. A proposed mechanism for reduction of experimental myocardial infarct size

The use of nonsteroidal antiinflammatory agents to reduce myocardial infarct size has demonstrated a dichotomy between ibuprofen, which reduces myocardial infarct size, and aspirin, which does not. A feline model of coronary ischemia using ligation of the anterior descending artery demonstrated that intravenous ibuprofen (2.5–20 mg/kg) given immediately and 2 h after ligation significantly decreased (by about 40%) myocardial infarct size. In contrast, aspirin did not diminish infarct size at any achieved dose; in fact, at some doses it tended to increase infarct size. In vitro studies with purified granulocytes demonstrated a similar dichotomy between ibuprofen and aspirin. Ibuprofen inhibits granulocyte aggregation, superoxide production, lysosomal enzyme release, and granulocytemediated endothelial cytotoxicity, while aspirin is without effect on these modalities. We propose that ibuprofens beneficial effect in experimental myocardial ischemia is related to its ability to inhibit activated granulocytes and thus to diminish myocardial cell death in experimental myocardial infarction.

Granulocytes utilize different energy sources for movement and phagocytosis

Granulocytes depend on anaerobic glycolysis for the energy required for chemotaxis, phagocytosis, and microbial killing. Two potential sources of the needed glucose are available: exogenous glucose and intracellular glycogen. These studies demonstrate that chemotaxin-induced movement of granulocytes induces accelerated uptake of exogenous glucose while phagocytosis does not, presumably utilizing instead the relatively slow process of glycogenolysis. As measured by incorporation of extracellular radiolabeled hexoses [1-14C]glucose or [3H]deoxyglucose), the soluble chemotaxin-aggregants of granulocytes, nF-met-leu-phe, CSades arg, bacterial filtrate, or arachidonic acid all augment transmembrane hexose uptake. This insulin-like activity closely parallels the dose-related effects of these agents on induction of granulocyte aggregation and chemotaxis. Insulin, itself, affects glucose transport minimally and mainly at supraphysiologic concentrations. In contrast, phagocytic stimuli fail to enhance hexose uptake at all, despite stimulating catabolism of glucose, which in turn is probably generated by glycogenolysis. These data show that granulocytes, whose motile function occurs in glucose-rich milieu, alter in tandem their cellular glucose uptake with their movement response. For phagocytosis, which often occurs in hypoglycotic, purulent exudates, granulocytes depend on stored energy supplies—probably glycogen. This coordination may be crucial in supporting granulocyte antimicrobial activity during acute inflammation.

Deleterious effects of endotoxin on cultured endothelial cells: an in vitro model of vascular injury.

The effects of endotoxin-triggered granulocytes on the viability of endothelial cells in vitro was investigated. Endotoxin or its lipid A component caused granulocytes to adhere to and significantly damage cultured endothelial cells. Fresh serum is not necessary but does amplify both adherence and endothelial injury. Much of the endothelial injury was inhibited by free-radical scavengers or by blocking granulocyte adhesion to endothelial cells and appears to result from free radical production by the stimulated granulocyte. Studies in this model suggest a pathogenic role for the endotoxin-triggered granulocyte in the Shwartzman reaction and perhaps related clinical disorders.