Innate Immunity and Defensins in Pediatric Disease.

摘要

The innate immune system is our first line of defense against invading microorganisms and includes barriers, signaling molecules, bioactive molecules and cellular components. Its actions are generally rapid and non-specific. In addition, innate immunity governs important interactions with the colonizing microbiota on the skin and mucosal surfaces. Given the limited exposure to antigens in utero and undeveloped adaptive immunity, newborns rely mainly on their innate immune system for protection and homeostasis. Perinatal exposure to environmental microbial products affects the neonatal immune maturation process. Preterm infants are particularly vulnerable to infection because of immature innate and adaptive immunity, prolonged hospitalization in the neonatal intensive care environment, and the many diseases associated with prematurity. Our study focused on a class of potent bioactive molecules, antimicrobial peptides, in neonatal disease. The studies included two major approaches, a human study and a neonatal rat model; both were focused on an important neonatal disease, necrotizing enterocolitis (NEC).;NEC is a devastating bowel disease that mostly affects premature infants. The pathogenesis of NEC is not clearly known, but the major contributing factors likely include: i) an immature intestinal tract and its defense mechanisms, and ii) a dysbiosis of the intestinal microbiota. The prematurely delivered rat is a common animal model of NEC, in which NEC-like symptoms are successfully induced in rat pups. Rats express many orthologs of human antimicrobial peptides, and are born with an immature intestine, similar to preterm infants who develop NEC.;In the human study reported here, we focused on the possible association of beta-defensins and their gene copy number (GCN) variation with the incidence and severity of NEC. Our data indicated that beta-defensins are expressed at low levels in neonatal small intestine, but are induced significantly in infants with moderate/severe NEC, compared to healthy controls. The beta-defensin GCN distribution was similar in both NEC and healthy control infants, with a slight decreased in mean GCN in the NEC group and a trend towards protection against NEC for those infants with GCN 6 or higher; however, neither of these analyses reached statistical significance. The beta-defensin GCN was correlated with expression levels in a primary keratinocyte model, where we observed an increase expression with higher GCN, under induction conditions. Overall, our data suggest that a low beta-defensin GCN may result in an inadequate production of antimicrobial peptides rendering infants more susceptible to NEC, compared to infants with normal or high GCN. However, a future study, with a much larger sample size, will be required to more definitively test this hypothesis.;A neonatal rat model was used to study the role of the probiotic bacterium, Bifidobacterium bifidum, in the prevention of NEC and the impact on antimicrobial expression and the composition of the intestinal microbiota. The data indicated that several antimicrobial products are increased in rat pups with NEC, compared to controls. Rat pups that received B. bifidum were protected from NEC and had correspondingly minimal increases in antimicrobial production. The microbiota composition in pups with NEC showed a reduction in Firmicutes and an increase in gamma-Proteobacteria, compared to pups that did not develop NEC. The B. bifidum was identified in the cecum, but did not affect the overall microbiota composition. Future investigations with this animal model of NEC would be of value, including more detailed assessments of changes in the microbiota with probiotic administration, with and without induction of NEC (e.g. next-generation sequencing of bacteria and assessment of fungi, archaea, and viruses).;In conclusion, the increased expression of beta-defensins in infants with NEC, the associations between expression and GCN for key-beta-defensins, and the increased expression of several antimicrobial peptides in animal NEC are entirely consistent with two proposed models: an innate immunity model (inadequate innate immunity increases NEC susceptibility directly) and a microbiota model (changes in innate immunity alter microbiota thereby increasing NEC susceptibility. The lack of association of defensin GCN variation with NEC risk suggests against the innate immunity model; however, the number of infants investigated is too small to completely rule out this possibility. The clear association of changes in the microbiota with protection against NEC in the rat model supports the microbiota model, but not necessary prove it. While our data are consistent with the notion that an inadequate expression of defensins by the intestinal mucosa may render infants more susceptible to disease, further study is needed to rigorously test these hypotheses.