UC San Diego

Alveolar macrophages (AMs) play critical roles in metabolizing surfactant and protecting the lung against inhaled pathogens. AMs mature following the newborn period and their immaturity at birth may contribute to the variety of infectious and inflammatory lung diseases specifically affecting infants. However, the specific molecular features differentiating neonatal and adult AMs remain poorly understood. Here we identify the unique transcriptomes and enhancer landscapes of neonatal and adult AMs that establish the molecular phenotypes specific to each developmental age. Adult AMs expressed higher levels of genes involved in lipid transport and metabolism (i.e. Fabp1, Pnpla5), consistent with their role in surfactant recycling. Neonatal AMs expressed higher levels of proinflammatory genes (Il1b, Il6, Tnf, and S100a8). ATAC-seq data detected adult AM peaks enriched with motifs recognizing KLF, GR, PPAR-gamma, and STAT. Accessible chromatin regions in neonatal AMs were highly enriched with binding motifs for the innate immunity transcription factors AP1, NF-kappa B, and IRF. To test how these baseline differences might impact AM innate immune function, we exposed neonatal and adult mice in vivo with inhaled LPS. AMs from both neonatal and adult mice demonstrated robust induction of Toll-Like Receptor 4 (TLR4) signaling pathways and similar patterns of chromatin accessibility. While neonatal and adult AMs did exhibit divergent expression patterns for some genes, the overall core innate response was similar. Intriguingly, neonatal AMs expressed higher basal levels of many LPS-induced genes, suggesting constitutive innate immune priming or activation in the neonatal lung. The lung microenvironment was a major factor regulating the unique molecular features of neonatal and adult AMs. Culturing isolated AMs for only 20h minimized the differences in gene expression between neonatal and adult cells. Interesting, while neonatal AMs in culture more closely resembled adult AMs, a core inflammatory signature of gene expression was retained. These data suggest that the proinflammatory phenotype of neonatal AMs results from both inherent properties of the cell and the lung microenvironment. Collectively, these studies provide new insights into the molecular mechanisms of lung innate immune development.