Revealing the Role of Lipid-Laden Macrophages in Staphylococcus aureus Osteomyelitis Abscess Formation

Inflammation of the bone or bone marrow, known as osteomyelitis, is most frequently caused by Staphylococcus aureus infection, with the prevalence of this pathogen making osteomyelitis a critical disease state to study. Bone possesses a unique cellular environment and serves a critical role in producing immune cells, including the differentiation of hematopoietic stem cells into monocyte precursors. S. aureus is capable of surviving via various mechanisms, such as the formation of abscesses in the bone marrow. These characteristic tissue lesions contain a central staphylococcal abscess community surrounded by both viable and necrotic neutrophils and are encapsulated by fibrin deposits. Previous work in our laboratory suggested the presence of a unique set of macrophages with a lipid-laden phenotype localized to the outer border of the abscess. Macrophages are innate immune cells recruited to the site of infection to phagocytose invading pathogens and recruit additional immune cells. Heterogeneous macrophage phenotypes have previously been described using techniques like flow cytometry, but spatial interrogation of heterogeneous macrophage populations and physiology has been difficult. Given the presence of lipid-laden macrophages around the abscess border and previous studies indicating the necessity of macrophage-derived factors for adequate abscess formation, I hypothesize that lipid-laden macrophages mediate abscess formation in the context of S. aureus-induced osteomyelitis, exhibiting anti-inflammatory molecular profiles and facilitating a protective niche for the SAC. Understanding the overall tissue architecture and localization of these cells in relation to the abscess is crucial, prompting the use of a multimodal imaging approach to assess this hypothesis. Matrix- assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is a label-free analytical tool capable of detecting hundreds of analytes within a tissue section with high specificity and sensitivity. My research group has pioneered new approaches for IMS of bone that will provide an unprecedented view of the lipid landscape in the context of osteomyelitis. Furthermore, spatial transcriptomics is a powerful, untargeted technique for identifying gene transcripts present within specific tissue regions. The novel integration of MALDI IMS and spatial transcriptomics in bone tissue, as proposed in Aim 1, will provide a comprehensive view of the impact of S. aureus on the bone and how macrophages impact the infection response. To further interrogate the role of macrophages in staphylococcal abscess formation, I will use multiple approaches to deplete these cells in our murine osteomyelitis model followed by the integrated multimodal imaging workflow. Additionally, I will establish a lipid-laden macrophage cell culture model, as described in Aim 2, to resemble the molecular profile observed in vivo and to characterize their interactions with S. aureus. These data will provide insight into the pathological progression of S. aureus infections and lipid-laden macrophage-induced inflammatory responses employed to fight off infection in the unique bone marrow microenvironment. Project Number: 1F31AI194801-01 | Fiscal Year: 2025 | NIH Institute/Center: National Institute of Allergy and Infectious Diseases (NIAID) | Principal Investigator: Lauren Emmerson | Institution: VANDERBILT UNIVERSITY, Nashville, TN | Award Amount: $34,880 | Activity Code: F31 | Study Section: Special Emphasis Panel[ZRG1 F07B-C (21)] View on NIH RePORTER: https://reporter.nih.gov/project-details/1F31AI19480101