Macrophage priming by interferon gamma: a selective process with potentially harmful effects.

The tissue-fixed macrophage is a key cellular element in the initiation and regulation of inflammation. Understanding the regulation of macrophage activation may provide valuable clues to the mechanisms involved in both beneficial and deleterious effects of inflammation. The lymphokine interferon-gamma (IFN-gamma) is capable of producing paradoxical immunoinflammatory effects. In the immunocompromised host it up-regulates a variety of immune functions and improves survival, but it is also capable of producing harmful effects by sensitizing immunocompetent animals to subclinical doses of endotoxin. These paradoxical effects suggest that the state of activation or priming of the host immune system is a key determinant of its response to endotoxemia. Because tumor necrosis factor (TNF) and procoagulant activity (PCA) elaboration by the tissue-fixed macrophage play a central role in the host response to endotoxin, we asked whether the paradoxical effects of IFN-gamma may be caused by priming of the macrophage for TNF and/or PCA production. In vitro, IFN-gamma produces a marked augmentation in TNF but does not alter PCA elaboration in response to endotoxin, demonstrating the selectivity of IFN-gamma priming of the macrophage. In vivo, IFN-gamma pretreatment followed by an established subclinical endotoxin exposure enhances toxicity while simultaneously increasing peak serum TNF levels. Exogenous priming by IFN-gamma alters the activation state of the macrophage and modifies the host response to endotoxin. Because this response is also dependent on the host's underlying immune state, IFN-gamma treatment in the immunocompetent host has the potential to produce deleterious effects by eliciting an exaggerated TNF response during endotoxemia.

Platelet-activating factor augments tumor necrosis factor and procoagulant activity.

Infusion of platelet activating factor (PAF) reproduces the host physiologic response to endotoxemia and sepsis. Tumor necrosis factor (TNF) and procoagulant activity (PCA) are two other potentially deleterious central inflammatory mediators produced in large quantities by tissue-fixed macrophages (M phi). The relationship, if any, between PAF and TNF or PCA production is unknown. Rabbit alveolar M phi were treated in vitro with PAF alone and prior to endotoxin (LPS). PAF alone had no effect on M phi PCA or TNF. PAF (10(-9)-10(-6) M) cotreatment enhanced M phi PCA and TNF levels in a dose response from two- to sixfold above that of LPS treatment alone. PAF (10(-6) M) pretreatment of M phi at T -4 to -6 hr produces an eight- to ninefold enhancement in both TNF and PCA levels. Thus, both coincubation and pretreatment or "priming" of the M phi with PAF prior to LPS stimulation greatly increase M phi production of PCA and TNF. The ability to augment the production of these two potent inflammatory mediators may explain in part the mechanism of action of PAF in vivo.

Endotoxin requirements for alveolar macrophage stimulation.

Acute pulmonary failure or ARDS in severely injured patients continues to be a significant problem. The most important clinical risk factor identified is sepsis syndrome. Sepsis syndrome is the clinical correlate of a malignant systemic inflammatory process and is directed in large part by the tissue-fixed macrophage (M phi), such as the alveolar M phi. The M phi is capable of producing most of the central inflammatory mediators responsible for the pathophysiology seen during sepsis and organ injury. Two major mediators are procoagulant activity (PCA), leading to diffuse microvascular thrombosis, and tumor necrosis factor (TNF), causing much of the physiologic derangement of sepsis. Endotoxins (LPS) derived from Gram-negative bacterial cell walls are the primary inflammatory stimulus for the tissue-fixed M phi production of inflammatory mediators. It is not completely known how LPS interacts with its various cellular targets, but it is hoped that knowledge of the molecular interactions involved in stimulation of the M phi by endotoxin will lead to therapies to modulate the response and prevent deleterious processes such as ARDS. In the present studies, LPS from E. coli 0111:B4 was shown in a dose response to stimulate large levels of both PCA and TNF in alveolar M phi. LPS from Bacteroides fragilis and Lipid X (the monosaccharide precursor of endotoxin) were unable to cause stimulation of the M phi in vitro. However, both moieties, B. fragilis LPS and Lipid X, were able to effectively and specifically compete with E. coli LPS and block M phi stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Is macrophage-induced microthrombosis during endotoxemia dependent on prostaglandin synthesis?

Steroids and nonsteroidal anti-inflammatory drugs (NSAID), in addition to inhibiting prostaglandin synthesis, have recently been shown to block production of procoagulant activity (PCA) by alveolar macrophages (A-M phi). This inhibition may ameliorate one deleterious effect of endotoxin (LPS), since LPS-induced PCA may contribute to the diffuse microvascular thrombosis (DIC) and subsequent organ failure seen in endotoxemia. However, the results of treatment of endotoxemia with steroids of NSAID are variable or short-lived. To help elucidate the basis for these results, the effect of these agents on production of PCA by another M phi population, the hepatic M phi (H-M phi PCA is functionally discrete from A-M phi PCA) was evaluated. Four agents, (hydrocortisone, indomethacin, ibuprofen and acetylsalicylic acid (ASA)) used to both block production of prostaglandins and ameliorate the deleterious effects of LPS in vivo were tested on the production of rabbit M phi PCA in vitro in homogeneous cultures. PCA was measured by a standardized one-step coagulation assay. Treatment with LPS (1 to 10 g/ml) produced maximal stimulation of PCA production with a peak at 8 hr with PCA levels 20- to 30+-fold higher than controls. The effect of pretreatment (T-10 min) of M phi cultures with the various inhibitors on PCA production at 8 hr post-LPS was evaluated, (N = 6 to 10 experiments).(ABSTRACT TRUNCATED AT 250 WORDS)

Microthrombosis during endotoxemia: potential role of hepatic versus alveolar macrophages.

Macrophages (M phi) produce multiple inflammatory mediators which may contribute to the pathophysiological processes seen during sepsis. Since diffuse microthrombosis has been implicated as a potential etiology for organ dysfunction and failure in sepsis, the present study examined the production of procoagulant activity (PCA) by M phi in response to endotoxin, characterized the activity, and evaluated methods to modify the response. Since hepatic and pulmonary dysfunction is a common complication of sepsis, rabbit M phi were isolated from both pulmonary (A-M phi) and hepatic (H-M phi) sites. Both M phi populations produced PCA in response to endotoxin in vitro. There is a rapid rise in activity with a peak at 20- to 30+ fold over background levels at 8 hr poststimulation. Although similar in their ability to enhance coagulation, the two M phi PCAs were shown to differ markedly in other biochemical and functional assays. The A-M phi PCA in contrast to the H-M phi PCA, was resistant to heat inactivation, serine protease inhibition, and warfarin pretreatment, while indomethacin (a prostaglandin synthesis inhibitor) blocked A-M phi PCA production but not the H-M phi response. Corticosteroids totally blocked PCA production by both M phi populations. Endotoxin, therefore, induces a rapid increase in M phi PCA, and the magnitude and rapidity of the response argue for a potentially significant pathophysiologic role, in vivo. Although derived from a common progenitor, A-M phi and H-M phi produce a functionally discreet PCA. This differential response may partially explain the contradictory results obtained in studies using various cellular metabolism inhibitors, e.g., indomethacin and steroids.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential for endotoxin-activated Kupffer's cells to induce microvascular thrombosis.

The potential contribution of Kupffer's cells, le, hepatic macrophages (HM phi s) to the diffuse microvascular thrombosis seen during septicemia was evaluated by measuring the ability of a homogeneous population of explanted HM psi s to express procoagulant activity (PCA). Addition of as little as 100 ng/mL of endotoxin stimulated a 30-fold increase over control values of PCA within eight hours. This PCA was membrane associated and functioned externally to the macrophage. Sensitivity to heat (56 degrees C) and diisopropyl fluorophosphate differentiated this PCA from typical tissue thromboplastin activity. The increase in PCA was blocked by pretreatment with warfarin sodium (a phytonadione blocker) and could be restored by addition of phytonadione. These studies showed that endotoxin induces in HM psi s a significant increase in PCA, functioning like coagulation factor VII. These results support a role for Kupffer's cells in the initiation of microvascular thrombosis in endotoxemia.