IκB kinase activity drives fetal lung macrophage maturation along a non-M1/M2 paradigm.

In preterm infants, exposure to inflammation increases the risk of bronchopulmonary dysplasia, a chronic, developmental lung disease. Although macrophages are the key cells that initiate lung inflammation, less is known about lung macrophage phenotype and maturation. We hypothesized that fetal lung macrophages mature into distinct subpopulations during mouse development, and that activation could influence macrophage maturation. Expression of the fetal macrophage markers CD68, CD86, CD206, Ym1, fibrinogen-like protein 2, and indolamine-2, 3-dioxygenase was developmentally regulated, with each marker having different temporal patterns. Flow cytometry analysis showed macrophages within the fetal lung were less diverse than the distinctly separate subpopulations in newborn and adult lungs. Similar to adult alveolar macrophages, fetal lung macrophages responded to the TLR4 agonist LPS and the alternative activation cytokines IL-4 and IL-13. Using a macrophage-specific constitutively active IκB Kinase transgenic model (IKFM), we demonstrated that macrophage activation increased proinflammatory gene expression and reduced the response of fetal lung macrophages to IL-4 and IL-13. Activation also increased fetal lung macrophage proliferation. Fetal IKFM lungs contained increased percentages of more mature, CD11b(low)F4/80(high) cells that also expressed higher levels of the alternative activation markers CD204 and CD206. Development of fetal lung macrophages into mature alveolar macrophages may therefore include features of both proinflammatory and alternative activation paradigms.

Interactions between NF-κB and SP3 connect inflammatory signaling with reduced FGF-10 expression.

Inflammation inhibits normal lung morphogenesis in preterm infants. Soluble inflammatory mediators present in the lungs of patients developing bronchopulmonary dysplasia disrupt expression of multiple genes critical for development. However, the mechanisms linking innate immune signaling and developmental programs are not clear. NF-κB activation inhibits expression of the critical morphogen FGF-10. Here, we show that interactions between the RELA subunit of NF-κB and SP3 suppress SP1-mediated FGF-10 expression. SP3 co-expression reduced SP1-mediated Fgf-10 promoter activity, suggesting antagonistic interactions between SP1 and SP3. Chromatin immunoprecipitation of LPS-treated primary mouse fetal lung mesenchymal cells detected increased interactions between SP3, RELA, and the Fgf-10 promoter. Expression of a constitutively active IκB kinase ß mutant not only decreased Fgf-10 promoter activity but also increased RELA-SP3 nuclear interactions. Expression of a dominant-negative IκB, which blocks NF-κB nuclear translocation, prevented inhibition of FGF-10 by SP3. The inhibitory functions of SP3 required sequences located in the N-terminal region of the protein. These data suggested that inhibition of FGF-10 by inflammatory signaling involves the NF-κB-dependent interactions between RELA, SP3, and the Fgf-10 promoter. NF-κB activation may therefore lead to reduced gene expression by recruiting inhibitory factors to specific gene promoters following exposure to inflammatory stimuli.