The dogma that adaptive immunity is the only arm of the immune response with memory capacity has been recently challenged by several studies demonstrating evidence for memory-like innate immune training. However, the underlying mechanisms and location for generating such innate memory responses in vivo remain unknown. Here, we show that access of Bacillus Calmette-Guérin (BCG) to the bone marrow (BM) changes the transcriptional landscape of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs), leading to local cell expansion and enhanced myelopoiesis at the expense of lymphopoiesis. Importantly, BCG-educated HSCs generate epigenetically modified macrophages that provide significantly better protection against virulent M. tuberculosis infection than naïve macrophages. By using parabiotic and chimeric mice, as well as adoptive transfer approaches, we demonstrate that training of the monocyte/macrophage lineage via BCG-induced HSC reprogramming is sustainable in vivo. Our results indicate that targeting the HSC compartment provides a novel approach for vaccine development.

Macrophages promote both injury and repair following myocardial infarction, but discriminating functions within mixed populations remains challenging. Here we used fate mapping and single-cell transcriptomics to demonstrate that at steady state, TIMD4 + LYVE1 + MHC-II lo CCR2 − resident cardiac macrophages self-renew with negligible blood monocyte input. Monocytes partially replaced resident TIMD4 − LYVE1 − MHC-II hi CCR2 − macrophages and fully replaced TIMD4 − LYVE1 − MHC-II hi CCR2 + macrophages, revealing a hierarchy of monocyte contribution to functionally distinct macrophage subsets. Ischemic injury reduced TIMD4 + and TIMD4 − resident macrophage abundance within infarcted tissue while recruited, CCR2 + monocyte-derived macrophages adopted multiple cell fates, including those nearly indistinguishable from resident macrophages. Despite this similarity, inducible depletion of resident macrophages using a Cx3cr1 -based system led to impaired cardiac function and promoted adverse remodeling primarily within the peri-infarct zone, highlighting a non-redundant, cardioprotective role of resident cardiac macrophages. Lastly, we demonstrate the ability of TIMD4 to be used as a durable lineage marker of a subset of resident cardiac macrophages.

Most tissues of the body harbor resident macrophages. Yet, macrophages are phenotypically and functionally heterogeneous, a reflection of the diversity of tissue environments in which they reside. In addition to maintaining tissue homeostasis and responding to invading pathogens, macrophages contribute to numerous pathological processes, making them an attractive potential target for therapeutic intervention. To do so, however, will require a detailed understanding of macrophage origins, the mechanisms that maintain them, and their functional attributes in different tissues and disease contexts.Macrophage ontology has long engendered controversy 1,2 . Nevertheless, the concept that tissue macrophages develop exclusively from circulating bone marrow-derived monocytes has prevailed for nearly a half century 3 . Accumulated evidence, however, including recent studies using sophisticated fate-mapping approaches, have determined that some tissue macrophages and their precursors are established embryonically in the yolk sac (YS) and fetal liver before the onset of definitive hematopoiesis [4][5][6][7][8][9][10][11] . Regardless of their origin, tissue macrophages can maintain themselves in adulthood by self-renewal independent of blood monocytes 12,13 .Gene-expression profiling of macrophage populations from several tissues has established that only a small number of transcripts are expressed by all macrophages 14 , indicating the importance of the context provided by the tissue when studying macrophage function in homeostasis and disease. The normal arterial wall contains many tissue resident macrophages that contribute crucially to immunity, tissue homeostasis and wound healing following injury 15. However, the regulatory networks, ancestry and mechanisms that maintain arterial macrophages remain unknown.Using gene expression analysis, we show that arterial macrophages constitute a distinct population among tissue macrophages. Multiple fate mapping approaches demonstrated that arterial macrophages arise embryonically from CX 3 CR1 + precursors and postnatally from bone marrow-derived monocytes that colonize the tissue during a brief period immediately after birth.In adulthood, arterial macrophages were maintained by CX 3 CR1-CX 3 CL1 interactions and local proliferation without significant further contribution from blood monocytes. Self-renewal also sustained arterial macrophages after severe depletion during polymicrobial sepsis, rapidly restoring them to functional homeostasis. ResultsPhenotype and gene expression profiling of arterial macrophages. (Fig. 1a).Principal component analysis revealed a distinct transcriptome in arterial macrophages, which clustered near other macrophage populations including microglia, alveolar macrophages, and splenic red pulp macrophages, as characterized by the Immunological Genome Consortium (Fig. 1b, Supplementary Fig. 1a) 14. Stringent comparison of gene-expression profiles among arterial, brain, alveolar and splenic red pulp macrophages revealed 212 transcripts that were at ...