A Revolutionary Concept in Innate Immunity and its Implications for Vaccine Development and Immune Therapies: A Comprehensive Review of Trained Immunity

Innate immune cells can acquire a form of memory through epigenetic and metabolic reprogramming following exposure to pathogen-associated molecular patterns (PAMPs), resulting in an enhanced, heterologous inflammatory response upon subsequent stimulation, a phenomenon termed trained immunity. This emerging concept challenges the traditional view that immunological memory is restricted to the adaptive immune system and is reshaping current understanding of host defense. Trained immunity is driven by long-lasting functional reprogramming of innate immune cells, particularly monocytes, macrophages, natural killer (NK) cells, dendritic cells, and their progenitors, leading to heightened responsiveness to secondary, often unrelated, stimuli. Comparable forms of innate immune memory have been documented across diverse biological systems, including systemic acquired resistance in plants, immune priming in insects, and trained immunity in mammals, highlighting its evolutionary conservation. The capacity of trained immunity to enhance immune responses provides a mechanistic basis for improved protection against reinfection and strengthened tumor surveillance. However, its dysregulated or excessive activation may also contribute to the development of autoinflammatory and autoimmune diseases, underscoring its dual and context-dependent nature. Consequently, trained immunity holds significant relevance for a wide range of clinical and translational applications, including infectious disease control, cancer immunotherapy, inflammatory disorders, and vaccine development. Harnessing trained immunity in vaccine design offers promising opportunities to achieve broader protective coverage, prolonged immune durability, and improved vaccine efficacy. Despite these advances, key challenges remain, including elucidating the precise molecular mechanisms underlying trained immunity, understanding its crosstalk with adaptive immune responses, and identifying optimal inducers and adjuvants capable of safely modulating trained immune pathways. Addressing these knowledge gaps will be essential for translating the concept of trained immunity into effective and safe therapeutic and vaccine strategies for human health.