Flt3- and Tie2-Cre tracing identifies regeneration in sepsis from multipotent progenitors but not hematopoietic stem cells

Increased hematopoietic stem cell (HSC) proliferation in response to infections might reflect enhanced HSC contributions for rapid replenishment of blood and immune cell compartments. However, there is no direct evidence in vivo for enhanced HSC differentiation to natural challenges. Here we studied by non-invasive fate mapping HSC differentiation for a comprehensive set of hematopoietic challenges, including irradiation, polymicrobial sepsis, bleeding, antibody-mediated ablation of granulocytes or B lymphocytes (akin to Rituximab in humans), and genetic lymphocyte deficiency. HSC sensed systemic infection, upregulated transcription factors promoting stem cell self-renewal (Foxp1, Atf4), stem cell maintenance (cMyc and nMyc), transcription in stressed cells (Hsf1) and suppression of apoptosis (E2f4), and enhanced their proliferation. However, none of these acute or chronic hematopoietic stressors enhanced HSC output in situ, suggesting HSC protection and maintenance rather than heightened differentiation. This disconnect may prioritize the long-term integrity of HSC over short-term enhanced productivity in response to hematopoietic perturbations. Overall design: HSC, ST-HSC, MPP, and CMP were purified by cell sorting from male C57BL/6 (RosaYFP) mice one day (#29 and #31), or three days (#40) after infection, or uninfected mice (#24, #25 and #42). For the “day 1” experiment, two biological replicates for each group (infected and uninfected) were sorted. Single cells were captured using the Chromium System according to the Chromium Single Cell 3' Reagent Kit protocols (10x Genomics).