Oncostatin M (OSM) Regulates an Inflammatory Cytokine Network in Dnmt3a-Mutant Hematopoietic Stem Cells

During aging, somatic mutations are known to accumulate in the hematopoietic stem and progenitor cell (HSPC) compartment. While most of these somatic mutations are considered ‘neutral’, a subset can confer a selective growth advantage to HSPCs, leading to a condition termed age-associated clonal hematopoiesis (CH). While CH is not a disease per se, it is associated with increased risk of age-associated pathologies such as blood cancers and ischemic stroke. Understanding the cellular and molecular mechanisms underlying the selective advantage of HSPCs harboring somatic mutations, and how and in whom this is a risk factor for age-associated disease, will further inform our understanding of CH-associated conditions. Using an inducible Dnmt3a-mutant mouse model of CH ( Dnmt3a^R878H/+), we previously reported that transplant of Dnmt3a-mutant HSCs into 14-month-old recipient mice promotes their selective growth advantage compared to transplant into 2-month-old recipient mice (SanMiguel et al., Cancer Discovery 2022). This led us to hypothesize that differences in soluble factors in the middle-aged versus young bone marrow context contribute to Dnmt3a-mutant HSC expansion. We began with re-analysis of our HSC RNA-seq data from that study, identifying a hallmark inflammatory response signature in Dnmt3a-mutant versus control HSCs in middle-aged but not in young recipient mice. Examining this gene signature, we found that the top gene encoding a soluble factor and increased in expression in Dnmt3a-mutant HSCs was Oncostatin M ( Osm), anIL-6 family cytokine that activates JAK/STAT signaling and is involved in the immunopathogenesis of myeloma. Given that activation of OSM signaling is associated with expanded Dnmt3a-mutant hematopoiesis in the middle-aged BM microenvironment, we hypothesized that OSM as a single stimulus would be sufficient to promote the selective advantage of young Dnmt3a-mutant HSPCs. We found that recombinant OSM did not result in altered proliferation, apoptosis, hematopoietic engraftment, or myeloid differentiation of young Dnmt3a-mutant HSCs. Despite this, we found that young Dnmt3a-mutant HSCs do transcriptionally upregulate an inflammatory cytokine network in response to acute OSM stimulation as evidenced by significant upregulation of the genes encoding IL-6, IL-1β and TNFα. OSM-stimulated Dnmt3a-mutant HSCs also demonstrate upregulation of the anti-inflammatory genes Socs3, Atf3 and Nr4a1 which creates a negative feedback loop that limits sustained activation of the inflammatory network. In the context of an aged bone marrow context with chronically elevated levels of OSM, Dnmt3a-mutant HSCs upregulate pro-inflammatory genes but not the anti-inflammatory genes Socs3, Atf3 and Nr4a1. The results from our studies suggest that chronic inflammation with aging exhausts the regulatory mechanisms that HSCs in young mice use to resolve inflammatory states and that OSM is a master regulator of an inflammatory network that contributes to age-associated CH.