Revealing the role of lipid-laden macrophages in staphylococcus aureus osteomyelitis abscess formation

The National Institute of Allergy and Infectious Diseases awarded Vanderbilt University $105,216 on December 23, 2025, under the Allergy and Infectious Diseases Research program (CFDA 93.855) to investigate the role of lipid-laden macrophages in Staphylococcus aureus osteomyelitis abscess formation.

The research employs multimodal imaging techniques, including matrix-assisted laser desorption/ionization imaging mass spectrometry, to examine the spatial localization and physiology of lipid-laden macrophages positioned at the outer border of bone marrow abscesses. The work tests the hypothesis that these macrophages mediate abscess formation by exhibiting anti-inflammatory molecular profiles and facilitating a protective niche for bacterial survival within the abscess microenvironment. The project addresses a critical gap in understanding heterogeneous macrophage phenotypes in infected bone tissue, building on prior laboratory observations of lipid-laden macrophages adjacent to staphylococcal abscess communities.

Performance takes place at Vanderbilt University in Nashville, Tennessee, with a period of performance extending through June 30, 2027. The award is classified as a Project Grant, a mechanism supporting investigator-initiated research in infectious disease and related immunologic and allergic disease areas.

Abstract

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.