Dysbiosis of the skin microbiota is increasingly implicated as a contributor to the pathogenesis of atopic dermatitis (AD). We previously reported first-in-human safety and clinical activity results from topical application of the commensal skin bacterium Roseomonas mucosa for the treatment of AD in 10 adults and 5 children older than 9 years of age. Here, we examined the potential mechanism of action of R. mucosa treatment and its impact on children with AD less than 7 years of age, the most common age group for children with AD. In 15 children with AD, R. mucosa treatment was associated with amelioration of disease severity, improvement in epithelial barrier function, reduced Staphylococcus aureus burden on the skin, and a reduction in topical steroid requirements without severe adverse events. Our observed response rates to R. mucosa treatment were greater than those seen in historical placebo control groups in prior AD studies. Skin improvements and colonization by R. mucosa persisted for up to 8 months after cessation of treatment. Analyses of cellular scratch assays and the MC903 mouse model of AD suggested that production of sphingolipids by R. mucosa, cholinergic signaling, and flagellin expression may have contributed to therapeutic impact through induction of a TNFR2-mediated epithelial-to-mesenchymal transition. These results suggest that a randomized, placebo-controlled trial of R. mucosa treatment in individuals with AD is warranted and implicate commensals in the maintenance of the skin epithelial barrier.

Lung disease causes significant morbidity and mortality, and is exacerbated by environmental injury, for example through lipopolysaccharide (LPS) or ozone (O3). Toll-like receptors (TLRs) orchestrate immune responses to injury by recognizing pathogen- or danger-associated molecular patterns. TLR4, the prototypic receptor for LPS, also mediates inflammation after O3, triggered by endogenous hyaluronan. Regulation of TLR4 signaling is incompletely understood. TLR5, the flagellin receptor, is expressed in alveolar macrophages, and regulates immune responses to environmental injury. Using in vivo animal models of TLR4-mediated inflammations (LPS, O3, hyaluronan), we show that TLR5 impacts the in vivo response to LPS, hyaluronan and O3. We demonstrate that immune cells of human carriers of a dominant negative TLR5 allele have decreased inflammatory response to O3 exposure ex vivo and LPS exposure in vitro. Using primary murine macrophages, we find that TLR5 physically associates with TLR4 and biases TLR4 signaling towards the MyD88 pathway. Our results suggest an updated paradigm for TLR4/TLR5 signaling.

Hematopoietic cell survival and death is critical for development of a functional immune system. Here, we report that a protein kinase, TAK1, is selectively required for resident macrophage integrity during embryogenesis. Hematopoietic lineage-specific deletion of Tak1 gene (Tak1HKO) caused accumulation of cellular debris in the thymus in perinatal mice. Although no overt alteration in thymocytes and blood myeloid populations was observed in Tak1HKO mice, we found that thymic and lung macrophages were diminished. In the in vitro setting, Tak1 deficiency caused profound disruption of lysosomes and killed bone marrow-derived macrophages (BMDMs) without any exogenous stressors. Inhibition of the lysosomal protease, cathepsin B, partially blocked Tak1-deficient BMDM death, suggesting that leakage of the lysosomal contents is in part the cause of cell death. To identify the trigger of this cell death, we examined involvement of TNF and Toll-like receptor pathways. Among them, we found that deletion of Tnfr1 partially rescued cell death. Finally, we show that Tnfr1 deletion partially restored thymic and lung macrophages in vivo. These results suggest that autocrine and potentially paracrine TNF kills Tak1-deficient macrophages during development. Our results reveal that TAK1 signaling maintains proper macrophage populations through protecting lysosomal integrity.