Assistant Professor Stanford University Stanford, California, United States
Disclosure(s):
Trung Pham, MD PhD: No financial relationships to disclose
Introduction/Rationale: Macrophages maintain tissue homeostasis by phagocytosing spent cells, recycle nutrients, and mounting antimicrobial responses to eliminate pathogens. Yet, they can also act as a cellular niche for intracellular bacteria, such as Salmonella enterica, to persist in infected tissues within granulomas, which are immunological structures comprised of macrophages and other cell types. We seek to define mechanisms controlling bacterial persistence in macrophages and tissues.
Methods: Fully virulent Salmonella Typhimurium (STm) is used to infect 129x1/SvJ mice, which have intact NRAMP1 function required for efficient macrophage iron recycling and the host’s resistance to intracellular pathogens in humans and mice. ScRNA-seq, flow cytometry, confocal microscopy, and novel knockout mice are employed to define macrophage mechanisms influencing bacterial tissue persistence.
Results: Comparative scRNA-seq analysis of monocytes and macrophages in infected spleens identifies a functional correlation between bacterial persistence and SPIC, a transcription factor that controls macrophage capacity for recycling spent red blood cells (RBCs), heme, and iron in steady-state and inflamed tissues. STm infection robustly induces SPIC in macrophages and alters tissue iron distribution. SPIC-expressing macrophages have significantly higher CD86, cytokine production, and intracellular RBCs, linking macrophage activation with erythrophagocytosis. Intriguingly, these macrophages are more likely to harbor STm compared to other macrophages. Using novel Spic-/- mice generated with CRISPR editing, we find that Spic deletion reduces macrophage CD86 levels and erythrophagocytic macrophage abundance. Unexpectedly, Spic-/- mice have a defect in granuloma formation and lower tissue bacterial levels, indicating impaired bacterial persistence.
Conclusion: Our findings suggest that SPIC is essential for macrophage activation and recycling of RBCs, heme, and iron in infected tissues, yet this function promotes pathogen persistence.
