2 Trainee Award: Evaluation of the neonatal hepatic innate immune response to <i>E. coli</i> -induced systemic infection

Abstract Abstract: Introduction Neonates are more susceptible to systemic infections than adults. There is a growing recognition that in adults, the hepatic innate immune response is a key first line of defense against systemic gram-negative bacterial (GNB) infection, such as E. coli. This robust innate immune response will initiate a pro-inflammatory response, activating local and systemic responses, thus mitigating the threat of E. coli and systemic infection. Whether an attenuated hepatic innate immune response contributes to the increased risk of infection in the neonate is unknown. Methods Postnatal day 3 (P3) and adult (6-8 weeks) C57BL/6J (B6) mice were exposed to E. coli (105 CFU/g, IP x 1 and 5 hrs). Hepatic tissue was assessed for neutrophil accumulation [myeloperoxidase (MPO) DAB staining] and E. coli [Colony Forming Units (CFU)]. Hepatic mRNA was assessed for induction of pro-inflammatory primary response genes and Acute Phase proteins (APP) using RT-qPCR. Results An evaluation of the MPO DAB staining helped determine hepatic neutrophil accumulation and revealed a decrease in baseline recruitment in P3 compared to adult mice. In adults, we expected a gradual increase in hepatic neutrophil recruitment throughout the passing time points. In contrast, hepatic neutrophilic count stayed stagnant in E. coli-exposed neonates. In adults, E. coli hepatic collections had no significant increases throughout the time points. In contrast, consistent with impaired neutrophil recruitment, E. coli burden in P3 mice significantly increases by the 5-hour timepoint. E. coli CFU quantification assessed that P3 mice have a significant growth of E. coli while the CFU for adult mice showed no significant growth. A contributing factor to the recruitment of hepatic neutrophils and E. coli growth in the liver is pro-inflammatory genes. We evaluated common pro-inflammatory genes Il6, Il1𝛽, and Cxcl1, known to express at significant levels in adult mice. In bacteremic adult mice, we observed an increase in the pro-inflammatory genes after 1 hour of E. coli exposure. In contrast, P3 mice do not express the same level of significant expression, and rather demonstrate a delayed initial response. We observed a decrease in hepatic neutrophilic recruitment and significant E. coli growth in the P3 mice coupled with an initial delay in the pro-inflammatory transcriptional response of P3 mice, showing a significant increase at the later 5-hour time point. The APR, activated by the pro-inflammatory transcriptional response, showed a parallel behavior, also exhibiting a delayed and stunted expression of the APR genes Crp, Lbp, and Saa1. Conclusions In response to systemic GNB infection, the neonatal hepatic innate immune response is attenuated when compared to adults. This includes decreased hepatic neutrophil accumulation, a delay in pro-inflammatory and APR transcription, and increased E. coli growth. We speculate that impaired neonatal hepatic innate immune response to E. coli exposure increases susceptibility to infection.