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A7388 - Neonatal CCR2 Knockout Mice Are Protected from Chronic LPS-Induced Pulmonary Inflammation and Hypoalveolarization
Author Block: C. Fulton, T. X. Cui, A. P. Popova; Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor, MI, United States.
Rationale: Bronchopulmonary dysplasia (BPD) is the most common chronic pulmonary complication of preterm birth and is characterized by impaired alveolar growth with fewer and larger alveoli and interstitial fibrosis. In premature infants undergoing mechanical ventilation for respiratory distress syndrome, neonatal airway colonization with Gram-negative bacteria is associated with development of severe BPD. Tracheal aspirate chemokine (C-C motif) ligand 2 (CCL2) levels are higher in premature infants with respiratory distress syndrome who go on to develop BPD. Lipopolysaccharide (LPS) is a membrane component of Gram negative bacteria capable of inducing a strong proinflammatory immune response, including increased CCL2 expression and CCR2-dependent monocyte/macrophage accumulation. We hypothesized that chronic intranasal LPS exposure of neonatal mice increases lung inflammation and impairs lung alveolar growth in a CCR2-dependent manner.
Methods: Neonatal C57BL/6J and CCR2 knockout mice were inoculated with 3ug/10ul of LPS from E. coli O26:B6 or 10ul of PBS intranasally on day of life 3, 7, and 10. Lungs were harvested on day of life 14. Lung alveolar growth was assessed by lung morphometry. Whole lung gene expression was analyzed by qPCR. Lung myeloid cells were assessed by flow cytometry.
Results: Chronic intranasal LPS administration to neonatal mice disrupted alveolar growth leading to larger alveolar spaces and significantly increased alveolar chord length. In addition, chronic intranasal inoculation with LPS induced mRNA gene expression of: CC-chemokines Ccl2, Ccl3, and Ccl4, proinflammatory cytokines Tnfa, Ifng, and Il1b, and matricellular proteins Periostin and SPARC. Chronic intranasal LPS exposure also increased the number of exudative macrophages (F4/80+CD11c+CD11b+) and both lung resident CD11c+F4/80- DC subsets (CD103+ and CD11b+ cells). Neonatal CCR2 knockout mice were protected from LPS-induced pulmonary inflammation and impaired alveolar growth, showing no significant increase in mRNA expression of the above proinflammatory chemokines, cytokines and matricellular proteins or in alveolar chord length. Compared to wild-type mice, neonatal CCR2 knockout mice showed an attenuated increase in exudative macrophages, but continued to show increases in both CD103+ and CD11b+ DCs in response to chronic LPS exposure.
Conclusions: Our findings demonstrate that in neonatal mice, CCR2 signaling is required for chronic LPS-induced hypoalveolarization, a mechanism linking Gram-negative bacterial LPS-stimulated innate immune response and alveolar development. Future studies will examine the mechanisms by which CCR2+ signaling affects lung growth during chronic LPS exposure.