Trained immunity and epigenetic memory in long-term self-renewing hematopoietic cells

Immunologic memory is a feature typically ascribed to the adaptive arm of the immune system. However, recent studies have demonstrated that hematopoietic stem cells (HSCs) and innate immune cells such as monocytes and macrophages can gain epigenetic signatures to enhance their response in the context of reinfection. This suggests the presence of long-term memory, a phenomenon referred to as trained immunity. Trained immunity in HSCs can occur via changes in the epigenetic landscape and enhanced chromatin accessibility in lineage-specific genes, as well as through metabolic alterations. These changes can lead to a skewing in lineage bias, particularly enhanced myelopoiesis and the generation of epigenetically modified innate immune cells that provide better protection against pathogens on secondary infection. Here, we summarize recent advancements in trained immunity and epigenetic memory formation in HSCs and self-renewing alveolar macrophages, which was the focus of the Spring 2022 International Society for Experimental Hematology (ISEH) webinar.