Peripheral blood mononuclear leukocyte production of both cytokines increased after lipopolysaccharide stimulation compared with unstimulated levels. The median (interquartile range) increase in pg/mL was 992.8 (259.9–1,553.5) for TNF-α and 1,079.5 (303.9–2,419.6) for IL-10.
A total of 292 women had paired assays for IL-10 and 319 women had paired assays for TNF-α. Table 2 shows the median change with range and interquartile range from baseline to follow-up in concentrations (lipopolysaccharide stimulated minus unstimulated) for IL-10 and TNF-α for the three groups defined by gestational age at delivery. The median change in concentrations in IL-10 was different between the three groups (P=.01). The increase in IL-10 was less in women delivering at 35–36 weeks of gestation (48.9 pg/mL) compared with women delivering at term (159.3 pg/mL) and decreased from baseline to follow-up among women delivering before 35 weeks of gestation (median decrease of 65.2 pg/mL) (Table 2). The pairwise comparisons revealed the changes in concentrations were different between those delivering before 35 weeks of gestation compared with those delivering at term (P=.01). The change in median concentrations in TNF-α from baseline to follow-up also differed among the three groups defined by gestational age at delivery (P=.03), but the pattern was not consistent. Women delivering at 35–36 weeks of gestation had a drop in concentrations from baseline to follow-up (median decrease of 356.0 pg/mL). The median increase from baseline to follow-up among women delivering at term was 86.9 pg/mL; women delivering before 35 weeks of gestation also had an increase, 132.1 pg/mL (Table 2). The pairwise comparisons revealed the changes in concentrations were different between those delivering at 35–36 weeks of gestation compared with those delivering at term (P=.01).
Similar to the findings in the trial of 852 women, the rate of preterm birth in this ancillary study of 343 women varied by fish diet history. The rate of preterm birth at less than 37 weeks of gestation was 33.7% among those who ate at least one fish meal per week and 44.4% among those who ate less than one fish meal per week (P=.004, relative risk 0.76, 95% confidence interval [CI] 0.63–0.92). This association remained after controlling for treatment assignment, earliest gestational age of prior preterm delivery, number of prior preterm deliveries, smoking, race and ethnicity, body mass index, and clinical center (P=.03, odds ratio 0.68, 95% CI 0.48–0.96). We hypothesized that modulation of the inflammatory response may be a mechanism by which fish diet reduces preterm birth; however, there was no difference in the change in concentrations from baseline to follow-up for IL-10 or TNF-α between the two groups defined by dietary fish intake, less than one fish meal per week compared with at least one fish meal per week (Table 3). Our previous analysis of the association between fish diet and preterm birth showed a U-shaped pattern with probability of preterm birth decreasing with increasing fish intake but then increasing again. The protective effect of dietary fish was not observed in women eating four or more fish meals per week.29 Therefore, we also estimated the association between change in IL-10 and TNF-α concentrations among three fish diet groups: less than one meal per week, one to three meals per week, and four or more meals per week. There were no differences in these cytokine measurements among these three groups (data not shown) (IL-10, P=.58; TNF-α, P=.26). There were no differences in change in concentrations from baseline to follow-up for IL-10 or TNF-α between the omega-3 and placebo treatment groups (Table 3).
In this cohort of women with a prior spontaneous preterm delivery, we observed a decrease in LPS-stimulated peripheral blood mononuclear leukocyte production of IL-10 across the second trimester in women destined to deliver before 35 weeks of gestation. Women who subsequently delivered at term demonstrated an increase in IL-10 production and, although women delivering late preterm also had increased IL-10 production, the increase was less than among those delivering at term. It is believed that IL-10 plays a role in the maintenance of pregnancy and downregulation of IL-10 favors an inflammatory state.15,30,31 Interleukin-10 can block preterm labor induced by intrauterine infusion of lipopolysaccharide in rodents13 and IL-1β-induced preterm labor in primates.12 In a study of seven women either in the first trimester or at term before labor and seven age-matched nonpregnant control participants, peripheral blood mononuclear leukocyte production of IL-10 was higher than control participants in the first trimester but dropped to nonpregnant levels at term leading the investigators to hypothesize that withdrawal of anti-inflammatory agents, including anti-inflammatory cytokines, occurs to accelerate an inflammatory process necessary for term labor.15 Our data suggest this process may occur prematurely in women destined to deliver before term.
The study results did not support our hypothesis that the different effects of fish diet and omega-3 supplementation on preterm birth may be the result of differences in modulating the immune response. Our results are in agreement with those from a randomized clinical trial in pregnant women for primary allergy prevention conducted in Sweden. Among 145 women, lipopolysaccharide-induced TNF-α and IL-10 secretion from whole blood cultures was not different between those receiving 2.7 g of omega-3 polyunsaturated fatty acid supplementation daily and those receiving placebo.32 Although some studies in nonpregnant individuals, cell cultures, and animal models have reported a suppressive effect of omega-3 fatty acid supplementation on peripheral blood mononuclear leukocyte production of TNF-α and a stimulatory effect on production of IL-10,20–22 the majority of intervention studies in humans have found no effect of omega-3 supplementation or fish consumption on cytokines or other biomarkers of inflammation.33–39
Strengths of our study include a well-characterized, large cohort of women at high risk for preterm birth. We examined the response to an inflammatory stimulus with measurement of both an anti-inflammatory and a proinflammatory cytokine over time in the second trimester with paired samples.
The weaknesses of the study must be acknowledged. The study included women with a prior preterm delivery; therefore, the results may not be generalizable to other obstetric patients. All women received 17 alpha-hydroxyprogesterone caproate and it is unclear what effect this may have had on our study results. Progesterone is an immunomodulator at the maternal-fetal surface, affecting production of proinflammatory cytokines by macrophages and altering T-cell clone cytokine secretion in favor of IL-10.40–42
The role of activation of the maternal inflammatory response in the preterm parturition syndrome continues to be investigated. Our data suggest a role of premature downregulation of IL-10 production and may have implications for treatment; however, other studies across a wider gestational age range are needed to confirm these results. Study of subgroups such as women with a shortened cervical length in the midtrimester, women with chronic bleeding in pregnancy, and women with multifetal pregnancies might offer new insight about the preterm parturition syndrome in different clinical situations.
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