Pulmonary and Systemic Endotoxin Tolerance in Preterm Fetal Sheep Exposed to Chorioamnionitis

Abstract

In a model of human chorioamnionitis, fetal sheep exposed to a single injection, but not repeated injections, of intra-amniotic endotoxin develop lung injury responses. We hypothesized that repeated exposure to intra-amniotic endotoxin induces endotoxin tolerance. Fetal sheep were given intra-amniotic injections of saline (control) or Escherichia coli LPS O55:B5 (10 mg) either 2 days (2-day group, single exposure), 7 days (7-day group, single exposure), or 2 plus 7 days (2- and 7-day repeat exposure) before preterm delivery at 124 days gestation (term = 150 days). Endotoxin responses were assessed in vivo in the lung and liver, and in vitro in monocytes from the blood and the lung. Compared with the single 2-day LPS exposure group, the (2 plus 7 days) repeat LPS-exposed lambs had: 1) decreased lung neutrophil and monocyte inducible NO synthase (NOSII) expression, and 2) decreased lung cytokine and liver serum amyloid A3 mRNA expression. In the lung, serum amyloid A3 mRNA expression decreased in the airway epithelial cells but not in the lung inflammatory cells. Unlike the single 7-day LPS exposure group, peripheral blood and lung monocytes from the repeat-LPS group did not increase IL-6 secretion or hydrogen peroxide production in response to in vitro LPS. Compared with controls, TLR4 expression did not change but IL-1R-associated kinase M expression increased in the monocytes from repeat LPS-exposed lambs. These results are consistent with the novel finding of endotoxin tolerance in preterm fetal lungs exposed to intra-amniotic LPS. The findings have implications for preterm infants exposed to chorioamnionitis for both responses to lung injury and postnatal nosocomial infections.

The impact of respiratory viruses on lung health after preterm birth

“…Exposure to perinatal infection and intrauterine inflammation have been associated with increased production of inflammatory cytokines and premature activation of the immune system which can skew immune responses toward a hyporesponsive (tolerant) phenotype and result in long-term immune programming effects [26,27]. Similarly, exposure to other non-infectious influences including oxidative stress and shear forces generated from supplemental oxygen and mechanical ventilation have also been associated with increased secretion of pro-inflammatory cytokines resulting in sustained systematic inflammation and which can further compromise immune responses [18].…”

Effects of intra-amniotic lipopolysaccharide exposure on the fetal lamb lung as gestation advances

“…We used the single exposure at 118 d and the two exposures at 118 and 123 d to parallel our previous reports that demonstrated fetal immune tolerance to LPS (13,16,18). The deliveries at 2, 10, and 17 d after the second exposure allowed evaluation of the persistence of responses during late fetal development.…”

“…Neutrophils, monocytes, and lymphocytes are increased in the alveolar lavages of preterm lambs after a single dose of lipopolysaccharide (LPS) (13). Additionally, intra-amniotic LPS exposure induces (i) maturation of monocytes to alveolar macrophages with increased inflammatory response potentials by inducing hematopoietic transcription factor PU.1 (14) and (ii) cross-tolerance to different Toll-like receptor (TLR) agonists (15).…”

“…However, our knowledge of the characteristics of these immune responses is limited to short-term exposure to proinflammatory agonists. The persistence of fetal immune modulations following a proinflammatory stimulus and how the duration of exposure to antenatal inflammation may affect the fetal response as gestation advances till near term is poorly understood (13,16). Whether such changes in the immune system persist after a proinflammatory stimulus may enhance understanding of whether preterm/term infants are more susceptible to infections, sepsis, and/or pneumonia later in life (17).…”

“…Additionally, intra-amniotic LPS exposure induces (i) maturation of monocytes to alveolar macrophages with increased inflammatory response potentials by inducing hematopoietic transcription factor PU.1 (14) and (ii) cross-tolerance to different Toll-like receptor (TLR) agonists (15). Thus, the naive fetal immune system is capable of complex modulations that can alter subsequent immune function (13). However, our knowledge of the characteristics of these immune responses is limited to short-term exposure to proinflammatory agonists.…”