Impact Of Sepsis On Epigenetic Regulation Of Hematopoietic Stem Cells (Hsc)

Sepsis is a systemic inflammatory over-response to microbial infection associated with high mortality rates. In addition, survivors of sepsis exhibit long-lasting effects of the disease, remaining immunosuppressed and at high risk for infections for the rest of their life. Hence, new therapies for sepsis are an unmet clinical need and novel ideas and approaches are needed. Using a murine model of severe sepsis induced by P. aeruginosa (P. aer.) bacteria or LPS, our group has previously reported that bone marrow (BM) responds to infections with a significant expansion of HSC associated with a blockage in their differentiation and inability to continuously generate mature neutrophils. As a result, the most critical defense against bacterial pathogens is suppressed, leading to profound neutropenia. Furthermore, when transplanted into healthy mice, HSC that have been exposed to a septic environment showed reduced ability to contribute to the myeloid lineage and to engraft at long-term, suggesting that they retain memory of the septic injury and maintain long-lasting changes. We hypothesize that the latters are due to sepsis-induced rapid and permanent epigenetic modifications in HSC, reshaping their transcriptional programs and affecting cell fate decisions and differentiation. To address our hypothesis, we challenged wild-type mice with P.aer. LPS and performed ATAC-seq on sorted HSC at 24 hrs or 108 hrs after challenge. Immunophenotypic analysis showed that HSC levels renormalized after 108 hr following their rapid expansion and differentiation arrest in response to LPS. However, alterations in the abundance and cell-cycle of myeloid progenitors and mature cells still persevered at long-term. Interestingly, ATAC-seq analysis revealed the persistence of euchromatic regions at 108 hrs from challenge in genomic regions involved in cell fate decisions and cell-cycle, suggesting that inflammation has long-lasting effects on epigenetic regulation, despite apparent HSC phenotypic normalization. Differently from previous works, our studies focus on epigenetic changes at HSC level, during the initial stages of the disease, when opportunity for effective therapeutic intervention may still be available. Our ultimate goal is to identify strategies to re-direct “septic” HSC to a normal differentiation program, thus allowing an efficient innate host response.