Structural integrity of the teleost granuloma: a macrophage barrier driven by coordinated activation of junctional, cytoskeletal and extracellular matrix gene networks

Granulomas are conserved immune structures that sequester persistent pathogens. However, the architectural mechanisms by which macrophages form a physical barrier remain incompletely understood. Here, by integrating histopathology, ultrastructural analysis, transcriptomics, and fluorescence in situ hybridization, we systematically dissected the cellular strategy and molecular basis of granuloma formation in the head kidney of largemouth bass (Micropterus salmoides) infected with Nocardia seriolae. Our results demonstrate that granuloma formation is accompanied by a distinct immune transcriptional signature, namely the upregulation of myeloid-lineage gene sets and the downregulation of lymphoid-lineage gene sets. Mechanistically, we show that the construction of the granuloma barrier is achieved through a profound epithelialization program in macrophages. Driven by a transcriptional signature that coordinately activates cell junction and cytoskeletal gene networks, macrophages establish a complete set of canonical intercellular junctions (desmosome, adherens, and tight junction) anchored to an epithelial-specific keratin cytoskeleton (e.g., krt8a, krt99), thereby integrating into a junction-cytoskeleton continuum that structurally mimics stratified squamous epithelium. Together, these findings reveal a novel picture of how teleosts control pathogens via a unique innate immune strategy focused on structural architecture, underscoring the universality of this mechanism across evolutionarily diverse teleosts and providing not only a new structural biology framework for understanding chronic infections but also potential new targets for controlling granulomatous diseases in aquaculture.