Trained immunity: induction of an inflammatory memory in disease

The innate immune system adapts its behavior based on previous insults, mounting an enhanced response upon re-exposure. Hematopoietic progenitors in the bone marrow and peripheral innate immune cells can undergo epigenetic and metabolic reprogramming, establishing an innate immune memory known as trained immunity. The concept of trained immunity recently gained relevance in our understanding of how innate immunity is regulated in various diseases. This review explores the role of trained immunity in infections, autoimmune disease, cardiovascular disease, cancer, and neurodegenerative disease. We discuss how trained immunity can provide heterologous protection against infections, as it has been induced for decades by the Bacillus Calmette Guérin vaccine, how it can help counteract immunosuppression, and how it can be inappropriately induced leading to chronic inflammation. By understanding how trained immunity is involved in processes leading to health and disease, novel therapeutic strategies can be developed.