Trained Immunity in Neutrophils and Mononuclear Phagocytes: Mechanisms and Pathophysiological Functions

Trained immunity refers to the enduring functional reprogramming of innate immune cells after particular stimuli, driven by epigenetic and metabolic alterations that augment non-specific responses upon subsequent exposure. Neutrophils and monocytes/macrophages, as essential innate effectors, are crucial for the induction and control of trained immunity, which is the primary emphasis of this review. Neutrophils, the predominant circulating leukocytes, were historically considered incapable of memory owing to their brief lifespan. Emerging evidence indicates that trained immunity functions at the bone marrow progenitor level, influencing granulopoiesis to produce neutrophils with lasting functional modifications. This research offers new insights into neutrophil functions in infection, cancer, and inflammation. Monocytes and macrophages, characterized by phenotypic plasticity and tissue residence, function as conventional models of trained immunity. They experience direct peripheral reprogramming or emerge as primed descendants of trained bone marrow precursors, performing pro-inflammatory or reparative roles in malignancies, infections, and ischemia lesions. This study comprehensively outlines the regulatory mechanisms of trained immunity in these cells, clarifies their functions in various clinical situations, and examines therapeutic applications. Comprehending these pathways is crucial for elucidating the cellular foundation of innate immunological memory, uncovering its multiple functions in disease, and guiding innovative therapeutics aimed at granulopoiesis and monocyte-macrophage polarization.