Decoding immunocompetence code that can reactivate aged hematopoietic stem cells

2021 Fiscal Year Research-status Report

• Project/Area Number: 21K16271
• Research Institution: Kumamoto University
• Principal Investigator: HO Pui・Yu 熊本大学, 国際先端医学研究機構, 特定事業研究員 (40888385)
• Project Period (FY): 2021-04-01 – 2023-03-31
• Keywords: Hematopoietic stem cells / Trained immunity

Outline of Annual Research Achievements

Recent works show that not only adaptive immunity but also innate immunity is capable of memorizing previously-encountered pathogens to develop trained immunity. Accumulating evidences show that HSCs can develop immune memory after microbial infection, although the underlying mechanism still remains unclear. We have previously found that upon gut inflammation, Bacteroides species, one of gram negative bacteria in the gut, infiltrate into body and activate BM haematopoiesis in which hematopoietic progenitor (HPCs) are directed to gut-associated lymph node and generate anti-inflammatory myeloid cells for tissue repair (Hayashi et al., bioRxiv 2021). Thus, we hypothesize that Bacteroides might be able to epigenetically imprint an innate immune memory on HSCs and their derived innate immune cells that exert stronger immunocompetence. To test our hypothesis, we primarily challenged mice with Bacteroides and performed in vivo sepsis model as secondary challenge. Microbial prestimulation can provide cross-protective effect to improve host survival. To elucidate the underlying mechanism, we establish a novel bioinformatic pipeline to integrate single-cell sequencing data, and identified epigenetic priming in anti-inflammatory, innate immunity- and metabolism-related genes at 1-month post-stimulation. The Bacteroides also induced substantial changes in active histone modification in the primed HSCs. Our data suggest that understanding of formation, maintenance and abrogation of innate immune memory in HSCs might help to enforce immune-competent of aged hemato-immune system.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

We have performed different single-cell sequencing to study the transcriptomic and epigenetic changes in HSCs after microbial infections. We have adjusted the screening criteria for identifying critical immune memory regulating genes for multiple times. More epigenetic study is performed to increase the data confidence and accurately predict the memory regulators.
On the other hand, although we have successfully proved that Bacteroides can improve survival from polymicrobial septic shock, the dynamics of immune cells change drastically within a short period of time. We have spent some time to find the optimal time window for investigating the critical immune cell population responsible for the Bacteroides-induced immuno-protection.

Strategy for Future Research Activity

To validate the potential immune memory regulating genes that we identify from the sequencing data, we will introduce lentiviral knockdown and overexpression system to demonstrate the function of the gene candidates on immuno-protection. In order to eliminate the non-specific effect from myeloid cells, like neutrophils and macrophages, we will also use Flt3-Cre mice to manipulate the gene expression specifically in HSCs.
On the other hand, to demonstrate the increased trained immunity during secondary infection, we will examine the bactericidal ability of the primed neutrophils and macrophages after sepsis. Preliminary data showed that microbial pre-stimulation with Bacteroides improves antigen presentation of macrophages derived from primed bone marrow cells.