Hematopoietic Stem and Progenitor Cell Trained Immunity Is Heterogeneous and Cross-Protective in a Mouse Model of M. Avium Infection

Recent studies suggest that infection epigenetically reprograms hematopoietic stem and progenitor cells (HSPCs) to enhance innate immune responses upon secondary infectious challenge, a process called “trained immunity.” Indeed, transfer of hematopoietic progenitors from mice infected with Mycobacterium bovis has been shown to confer long-lasting immunity against tuberculosis in the transplant recipients via an interferon gamma (IFNg) dependent process (Kaufmann 2018). However, much remains to be discovered about the breadth of protection conferred by trained immunity and the specific progenitor cell types responsible. We aimed to build upon our prior work examining the effects of IFNg signaling on hematopoietic stem cells to begin to answer these questions. We established a model of trained immunity in mice in response to Mycobacterium avium infection. Transfer of lineage-negative cKit+ hematopoietic progenitors from M. avium infected animals to naïve hosts was sufficient to confer protection against rechallenge in the transplant recipients up to 1 year post transplant. Macrophages derived from trained HSPCs demonstrated enhanced bacterial killing and metabolism, indicative of reprogramming in HSPCs that is conferred to innate immune cells. scRNA-seq analysis revealed that HSPCs activate IFNg-response genes heterogeneously upon primary challenge, suggesting that HSPC responses are neither monolithic nor confined to a specific cell type. Single cell transcriptomic analysis also revealed expansion of rare cell populations in the bone marrow. However, transfer of purified long-term HSCs was not sufficient to transmit trained immunity, suggesting cross-talk is required. To test whether IFNg alone is sufficient to induce trained immunity, mice were treated with a single dose of recombinant IFNg. Strikingly, this exposure was sufficient to induce enhanced bacterial killing and metabolism in bone marrow derived macrophages cultured from treated mice. To test the specificity of trained immunity, mice transplanted with influenza -trained HSPCs were challenged with M. avium infection and vice versa. Mice transplanted with influenza-trained HSPCs displayed enhanced immunity against M. avium challenge and vice versa, demonstrating cross protection against antigenically distinct pathogens. Together, these results indicate that primary infections can cause heterogeneous and long-standing HSPC responses, leading to bone marrow derived macrophages with enhanced killing capacity and cross-protectivity against alternative pathogens. These data will inform future studies to understand long term impacts of inflammatory stress on hematopoietic progenitors and the immune cells they produce.