Our results were derived from secondary analyses of data from a large cohort of extremely preterm infants who were enrolled in the NICHD Neonatal Research Network multicenter “Inflammatory Cytokines and Neurodevelopmental outcomes in extremely low birth weight infants” study. As was expected, the subgroup of infants who were administered O2
continually through 24 hours of life were more immature, had significantly lower mean birth weight and needed intubation, surfactant and antibiotic therapy more often than the group with brief (<6 hours) oxygen exposure. MMP 9 and β chemokine (MIP 1 α and β, MCP 1 and RANTES) concentrations in our population are higher than previously reported values for older children, using different techniques (22
). Infants with prolonged early O2
exposure had elevated MCP 1 and lower RANTES on day 3 than the group with brief O2
administration. MCP 1 on day 3 showed a significant positive and MMP 9 on day 1 and RANTES on day 3 showed modest negative correlations with the highest administered O2
. The striking finding of our study was that early prolonged O2
exposure was independently associated with higher plasma MCP 1 on day 3, even after adjusting for other clinical variables.
In our cohort, circulating MMP 9 was considerably higher than has been reported in healthy adults (mean (SEM) 315.3 (29.9) ng/ml) and older children (mean 400.4–553 ng/ml), although concentrations measured using different techniques (zymography, ELISA and luminex technology) can not be directly compared (23
). Median MMP 9 levels in fetal plasma, in the setting of preterm labor and preterm rupture of membranes are also lower than in our cohort (mean 89.3 and 102.5 ng/ml) (24
). Only one previous published study provided normative data on plasma activities of MMP 9, measured by zymography and found these to be highest in preterm infants 33–36 weeks gestation and to decline by 50% after day 1 (25
). Contrary to previous studies that show an elevation in bronchoalveolar lavage fluid or lung tissue MMP 9 in association with hyperoxia, we found only a modest negative correlation between MMP 9 on day 1 and highest FiO2
at 3 days (13
). There are some potential reasons for this. MMP 9 is a pro-enzyme that is activated in the tissues; therefore, blood spot MMP 9 may not accurately reflect tissue-specific concentrations or enzymatic activities. In addition, the biologic effect of the MMPs is determined by the balance between the enzymes and their tissue inhibitors, the levels of which we did not evaluate (26
β chemokine concentrations on days 1 and 3 of life in our study are broadly concordant with the limited previous published data, although somewhat higher (26
). Serum MCP 1, MIP 1α, and RANTES levels very similar to our data were reported in a study comparing these mediators among perinatally asphyxiated and perinatally infected term and preterm neonates and healthy controls, although the assay methodology was traditional ELISA (27
). Our results validate the observation of others that preterm infants mount a robust chemokine response in the first few days of life. Using a different assay methodology of recycling immuno-affinity chromatography, however, Dammann et al reported very low median levels of MIP 1α and RANTES among a small group of 15 extremely low gestation infants, still higher than in the term counterparts (28
Both early prolonged O2
administration and the highest FiO2
were associated with significantly lower RANTES on day 3. A significantly lower plasma RANTES has been previously reported in perinatally infected neonates and necrotizing enterocolitis and is shown to accurately predict the development of disseminated intravascular coagulation in severely infected infants (29
). In adults, circulating RANTES levels inversely correlate with APACHE scores (r= −0.7) and adverse outcomes (median 5.6 ng/ml in non-survivors vs. 16.4 in survivors, p< 0.05) (32
). In a study involving adult hemodialyzed patients, a significant negative correlation was observed between RANTES levels and copper-zinc superoxide dismutase levels, an established marker of oxidative stress (33
). Since, in our data, the group of infants with prolonged oxygen exposure was smaller, more immature and generally “sicker” and O2
administration was not independently predictive of plasma RANTES, we speculate that a lower RANTES may simply be associated with severity of illness, with which oxygen exposure is inextricably linked.
MCP 1 on day 3 was significantly higher among infants with early prolonged O2
exposure, correlated significantly with the highest FiO2 and on median regression, O2
was an independent predictor of plasma MCP1. These observations are in accord with the limited previous data in neonates. Maximal tracheal aspirate MCP 1 concentrations have been shown to be significantly higher in infants who were O2
dependent at 28 days and 36 weeks post-conceptional age and to correlate with the development of bronchopulmonary dysplasia and adverse outcomes (34
). Higher tracheal aspirate MCP 1 during the first week have been reported among infants with respiratory distress syndrome and pulmonary hemorrhage (35
). MCP 1 has been implicated in the pathogenesis of acute and chronic lung injury in animal studies and is a pathophysiologic mediator of excitotoxic brain injury in neonatal rats (35
). It is thought to be the predominant mediator of monocyte-macrophage activation (35
). The mechanism of MCP 1 increase remains unclear. Therefore, it is certainly plausible that O2
may be the trigger for an elevated MCP 1 and its associated morbidities.
Our study has some limitations. We used sustained early oxygen exposure from 6 to 24 hours of age as a surrogate measure of greater oxygen exposure. While we compared two groups of infants with distinct durations of O2
exposure, the actual FiO2
and the partial pressure of oxygen in blood is not available and may have varied among infants. In addition, circulating chemokine and metalloproteinase concentrations may not reflect lung tissue concentrations or tissue-specific oxidative injury. We did not have data on traditional assays of oxidative injury such as oxidized glutathione or urinary isoprostanes or on chorioamnionitis. Chorioamnionitis has been demonstrated to induce pulmonary and systemic inflammatory response, with elevated MMP 9 and cytokines in bronchoalveolar lavage fluid, lung tissue and cord blood (38
). In addition, it is associated with a higher risk of preterm birth and may modulate early oxygen requirements through its effect on respiratory distress syndrome (41
). We recognize that, because of the marked baseline differences between the two groups, some of the major results may be due to an unexplained variance in the groups. Nonetheless, the fairly consistent association between oxygen and higher MCP 1 revealed in our exploratory analyses are novel, potentially important data on which to base further studies.
In summary, oxygen administration was associated with higher plasma MCP 1. This intriguing finding suggests that further investigation is needed to improve our understanding of its role in oxidative injury and utility as a quantifiable marker of oxidative stress in preterm infants. We speculate that MCP 1 may even provide a broad mechanistic link between oxygen, inflammation, and the causally related morbidities.