Meconium Is a Potent Activator of Complement in Human Serum and in Piglets

Abstract

Meconium aspiration syndrome (MAS) is a clinical condition in the newborn infant with a significant morbidity and mortality. The complex pathophysiology of MAS, leading to both pulmonary and systemic complications, is characterized by an incompletely understood inflammatory reaction. Treatment is symptomatic, mainly limited to airway cleaning and ventilatory support. In this study, we show for the first time that meconium is a potent activator of complement, a key mediator of inflammation. In vitro, meconium activated the alternative complement pathway in human umbilical cord serum as judged by a substantial increase in the alternative pathway convertase C3bBbP. The activation proceeded through C3 (C3bc) and the terminal C5-9 pathway (terminal SC5b-9 complement complex), whereas the classical and lectin pathways were not activated (C1rs-C1-inhibitor complexes and C4bc). The lipid fraction, containing, e.g. free fatty acids, and the water fraction, containing, e.g. bile acids, contributed equally to the complement activation. A blocking antibody to factor D (alternative pathway) completely inhibited the meconium-induced complement activation, whereas blocking antibodies to mannose-binding lectin (lectin pathway) and C2 (classical and lectin pathway) had no effect. In vivo, meconium induced systemic complement activation in a piglet model of MAS, paralleling the increase in lung dysfunction. In conclusion, meconium is a potent activator of the complement system both in vitro and in vivo. Complement may be important in the pathogenesis of MAS, and specific complement inhibition might be a possible treatment approach in MAS. The aspiration of meconium-stained amniotic fluid (MSAF) in the newborn can lead to meconium aspiration syndrome (MAS), which is a clinical syndrome with significant morbidity and mortality (1). MAS might be defined as respiratory distress in an infant who is born through MSAF and whose symptoms cannot be otherwise explained (2). The pathophysiology of lung injury in MAS is complex and includes both mechanical obstruction by meconium and pronounced inflammatory responses (3). It has been postulated that other conditions, including intrauterine infection and chronic and acute asphyxia, rather than inhaled meconium, may explain the primary pathology of MAS (4). The pathophysiology of MAS (hypoxemia, pulmonary inflammation, and disturbed pulmonary vasoregulation) bears similarities with acute respiratory distress syndrome (5), the latter frequently being a manifestation of the systemic inflammatory response syndrome (SIRS) in the presence or absence of bacterial infection (6, 7).Lung inflammation by itself can lead to systemic inflammation (8). Leakage of meconium content through the alveolar wall to the lung capillaries may bring substances from meconium in direct contact with blood, leading to systemic inflammation. Furthermore, lung rupture and meconium embolism in chronic intrauterine meconium aspiration has been described (9), which may be an additional mechanism for meconium exposure to blood...

Studies of meconium-induced lung injury: inflammatory cytokine expression and apoptosis

“…Jones et al 4 demonstrated increased production of inflammatory cytokines in preterm infants with MAS. Castelheim 10 showed compliment activation in human monocytes culture of MAS model.…”

“…17 Now we describe the pathways of induction of inflammatory response following cell influx. 4 Preterm and term newborns Production of TNFa, IL1b, and IL8 is regulated by IL-10; IL-10 not detected in preterm infants with or without MAS de Beaufort et al 8 Newborn infants Overexpression of IL-8 in MAS in vitro; increased migration of neotrophils; anti-IL-8 antibodies inhibit neutrophil migration Uotila and Kaapa 9 Human monocytes In culture increase of cycloxygenase-2 expression in vitro Castellheim et al 10 Human monocytes in culture Complement activation in MAS Animal models of MAS Tyler et al 3 Animal models Surfactant inactivation, chemical pneumonitis, airway obstruction, direct toxic damage of animal lung tissues Zagariya et al 6 Rabbit pups Intense inflammatory reactions involved in meconium-induced injury in vivo; expression of endothelin 1 in MAS Davey et al 11 Piglets Acute decrease in gas exchange and dynamic lung compliance; increase of total lung protein; inhibition of surfactant Holopainen et al 12 Piglets Inflammation, airway epithelium targeted, necrotic changes, phospholipase 2 activity increased. Mollnes et al 13 Animal models Complement activation in MAS Khan et al 14 Mice Increase of IL-13 expression in MAS in vivo; lymphocytic and eosinophilic inflammation Zagariya et al 15 Rabbit pups Phospholipase A2 activates apoptosis, influx of T-lymphocytes and macrophages in vivo; increase of TNFa, IL-1b, IL-6 and IL-8 in MAS in vivo Zagariya et al 16 Rabbit pups Cell death by apoptosis, influx of neutrophils and macrophages in vivo Tessler et al 17 Newborn infants Reactive oxygen species in vitro…”

“…10 Meconium itself may stimulate neutrophil chemotaxis and it supposedly induces the lung inflammatory cells to produce proinflammatory chemotactic cytokines. 4,8,10,18 Ex vivo studies using a piglet model show that moderate and high concentrations of aspirated meconium rapidly activate circulating neutrophils. 19 Experimental study in piglet model suggests that meconium-induced inflammation is localized in the meconium-contaminated areas of the lungs with no generalized inflammatory injury.…”

“…10,12,16 It is further apparent that intrapulmonary meconium stimulates the lung inflammatory cells to express inflammatory cytokines, such as TNF-a, IL-1b, IL-6 and IL-8, and may thereby propagate development of parenchyma cell injury in the exposed animal and human lungs. 8,10,16 A novel signaling pathway involved in MAS is the role of IL-13. This molecule has been shown to be markedly elevated in a murine model of MAS and could be a mediator of persistent airway dysfunction and remodeling in survivors of MAS.…”

The role of complement in meconium aspiration syndrome

“…36 Adding meconium to human umbilical serum induced a substantial increase in TCC formation, which was found to be mediated mainly through the alternative activation pathway. As judged by their relative amounts in whole meconium, the lipid and water fractions contributed equally to this activation.…”