Developmental and Environmental Regulation of Lung Macrophage Immune Response

Abstract

Abstract Neonates are uniquely susceptible to lung infection and injury. Alveolar (AM) and interstitial (IM) lung macrophages are the major cell populations protecting the lung against inhaled microbes and facilitating wound repair. AM and IM express unique cell surface markers and transcriptional profiles. We therefore hypothesized that neonatal and adult lung environments lead to differences in macrophage function. When lung macrophages were isolated by flow cytometry and stimulated with LPS in vitro for 2 h, neonatal AM expressed higher levels of Il1b and Il6 compared to adult AM. We next tested our hypothesis in vivo, challenging neonatal and adult mice with intranasal LPS. Contrary to our in vitro experiments, LPS induced higher cytokine expression in AM compared to IM. In addition, the adult AM response was higher than neonatal AM. To specifically test how the lung microenvironment regulated macrophage response, we stimulated neonatal and adult AM and IM with LPS either immediately after isolation or after 10–20 h of tissue culture. Consistent with our hypothesis, expression of key AM transcription factors fell quickly during cell culture. While neonatal AM and IM had similar LPS responses immediately after isolation, adult IM had higher cytokine expression compared to adult AM. Intriguingly, for both adults and neonates, longer culture time enhanced cytokine expression in activated AM while dampening the expression in IM. Our data suggest that the developmentally regulated lung microenvironment regulates the macrophage innate immune response.