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Unexpected Macrophage Activity Alters the Inflammatory Process in Necrotizing Enterocolitis

Jessica L Raque, Paul J Matheson, Kevin Connolly, Jessica A Shepherd, Eric A Stamper, Kelly MMcMasters, Victoria Graham, R Neal Garrison, Jason W Smith, Cynthia D Downard

Objective: Macrophages play a role in clearing bacteria from the gut in the initial stages of necrotizing enterocolitis (NEC). Macrophage Inflammatory Protein-1α (MIP-1α), also known as CCL3, is a chemokine produced by macrophages that enhances the immune response. It acts as both a recruiter of immune cells and a coactivator of other macrophages. Interleukin-12 (IL-12) is a pro-inflammatory chemotactant produced by several types of phagocytic cells such as dendritic cells and neutrophils, but primarily by macrophages. We hypothesized that the pro-inflammatory state associated with NEC pathophysiology would induce increased expression of MIP-1α and IL-12 that would be detectable in serum and intestinal tissue.

Methods: Timed pregnant Sprague-Dawley rats were randomized by litter. Controls were delivered vaginally and dam-fed. NEC pup groups were delivered 12 h prematurely via Cesarean section, formula fed, given a single oral dose of lipopolysaccharide, and subjected to intermittent cold and hypoxia as part of a proven NEC rat model protocol. Animals were sacrificed at 0, 12, 24, 48, 72, and 96 h of life and serum and intestinal tissue samples were collected. Samples were analysed via western blot using antibodies with affinity to MIP-1α and IL-12 respectively.

Results: Serum and ileal levels of MIP-1 alpha were increased from 48 to 72 h in NEC animals when compared with controls. Serum and ileal IL-12 was downregulated in NEC groups at 12 and 24 h compared to controls.

Conclusion: Macrophage function plays an important role in the first 48 h of NEC pathophysiology. Downregulation of IL-12 in the setting of increased MIP-1α expression may represent deranged or inhibited macrophage function in the context of NEC pathogenesis. Further work to elucidate the significance of these findings is warranted.