404 The collagen network maintained by macrophages provides a niche for Staphylococcus aureus skin infection

Abstract

Macrophages are an integral component of tissue homeostasis but their role(s) in skin is incompletely understood. Particularly, the immune component of hypodermis, the deepest skin layer and common site for Staphylococcus aureus infection in the form of cellulitis, remains unexplored. Through approaches including flow cytometry, single-cell RNAseq and bone marrow transfer we found that hypodermal macrophages were comprised of tissue-resident and monocyte-derived macrophages with distinct transcriptome profiles and chemokine receptor dependency. Both macrophage subsets relied on colony stimulating factor 1 (CSF1) for local maintenance. Single-cell RNAseq analysis of dermal and hypodermal stromal compartments identified fibroblasts and endothelial cells as major sources of CSF1 in both skin layers. We further found that Tek (a vascular marker) expression was limited to endothelial cells in the dermis but broadly expressed by stromal cells in hypodermis, enabling layer-specific ablation of CSF1 in the hypodermis in Tek-cre x Csf1-flox mice (Csf1DTek). Remarkably, this anatomical depletion of macrophages resulted in an impaired collagen network, indicating that hypodermal macrophages facilitated the homeostatic maintenance of the extracellular matrix. Interestingly, S. aureus injected into WT hypodermis, readily adhered to collagen bundles where they proliferated robustly prompting to abscess formation. In contrast, altered collagen network in Csf1DTek mice limited S. aureus adherance and proliferation leading to a decreased microbial load as determined by colony forming unit count. Thus, while macrophages were crucial for hypodermal homeostasis, the collagen network that they maintained served as a niche for S. aureus and rendered susceptibility to cellulitis. These findings may provide a foundation for the development of novel therapeutic strategies in S. aureus soft-tissue infections.