Identification of a Biofilm-Inducible Wound Macrophage Subset Characterized by Unique MARCO Expression: Significance in Chronic Wound Infection

Abstract

Staphylococcus aureus (SA) biofilm infection, a common occurrence in human chronic wounds, exhibits sustained inflammation resulting in wound chronicity. Current studies claim that SA biofilm transforms host macrophages (m) towards an alternatively activated phenotype that suppresses the inflammatory response. Given a key role of m in wound healing, we investigate the phenotype and function of wound m (wm) in SA biofilm infection. To address the biofilm component of SA pathogenicity, two isogenic mutant strains of wild-type SA (USA300) were used that possess varying degrees of biofilm-forming ability. The strains, USA300::sarA (ΔsarA) and USA300::rexB (ΔrexB), have hypo- and hyper biofilm-forming ability, respectively. Mechanistic studies were performed on peripheral blood monocytes subjected to conditioned media (CM) derived from ΔsarA and ΔrexB biofilm cultures. scRNAseq and bulk RNAseq studies revealed presence of a unique m subtype that displays a distinct gene expression signature when exposed to CM derived from ΔrexB mutant as compared to CM from ΔsarA mutant. We identified macrophage receptors with collagenous structure (MARCO) as one of the highly abundant uniquely expressed biofilm-responsive genes in wm. MARCO, a member of the scavenger receptor, belongs to a larger group of pattern recognition receptors primarily involved in immunosurveillance and plays important roles in host defense and apoptotic cell clearance activities. scRNAseq and immunohistochemistry from human chronic biofilm-infected wounds demonstrated abundance of MARCO expression on wm. Significant upregulation of MARCO was observed in ΔrexB CM treated m in a dose and time-dependent manner as compared to ΔsarA CM treated m. Knockdown of biofilm-induced MARCO in m resulted in dysregulated m functions and inflammatory responses, suggesting its critical significance in biofilm infection. Immunohistochemistry studies in SA biofilm-infected porcine wounds validated the in vitro findings. In summary, this study identifies a SA biofilm inducible m receptor that has a significant role in wound chronicity.