Burn Injury Results in Myeloid Priming during Emergency Hematopoiesis.

Introduction: Hematopoiesis proceeds in a tiered pattern of differentiation, beginning with hematopoietic stem cells (HSC) and culminating in erythroid, myeloid, and lymphoid lineages. Pathologically altered lineage commitment can result in inadequate leukocyte production or dysfunctional cell lines. Drivers of emergency hematopoiesis after burn injury are inadequately defined. Burn injury induces a myeloid predominance associated with infection that worsens outcomes. This study aims to further profile bone marrow HSCs following burn injury in a murine model.

Methods: C57BL/6 mice received burn or sham injury with ~12% total body surface area (TBSA) scald burn on the dorsal surface with subsequent sacrifice at 1, 2, 3, 7 and 10 days post-injury. Bone marrow (BM) from hindlimbs were analyzed for HSC populations via flow cytometry and analyzed using FlowJo Software (version 10.6). Event counts and frequencies were analyzed with multiple unpaired t-tests and linear mixed-effect regression. RT-PCR performed on isolated lineage negative BM cell RNA targeted PU.1, GATA-1, and GATA-3 with subsequent analysis conducted with QuantStudio 3 software. Statistical analysis and representation were performed on GraphPad software (Prism).

Results: Flow cytometry revealed significantly elevated proportions of Long-Term HSCs at 3 days post-injury (p < .05) and Short-Term HSCs at days 2, 3, and 10 (all p < .05) in burn-injured mice. There was a sustained, but not significant, increase in proportions in the multi-potent progenitor (MPP) 2 and 3 subpopulations in the burn cohort compared to sham controls. The common myeloid progenitor (CMP) proportion was significantly higher on days 3 and 10 (both p < .01), while the granulocyte-macrophage progenitor (GMP) proportion increased on days 1, 2, and 10 (p < .05, p < .01, p < .01). Although the megakaryocyte-erythrocyte progenitor (MEP) proportion appeared consistently lower in the burn cohort, this did not reach significance. mRNA analysis resulted in a downregulation of PU.1 on day 1 (p = 0.0002) with an upregulation by day 7 (p < 0.01). GATA-1 downregulation occurred by day 7 (p < 0.05), and GATA3 showed downregulation on days 3 and 7 (p < 0.05).

Discussion: Full-thickness burn results in an emergency hematopoiesis via proportional increase of Long Term-HSC and Short Term-HSC/MPP1 subpopulations beginning in the early post-injury period. Subsequent lineage commitment displays a myeloid predominance with a shift toward myeloid progenitors with mRNA analysis corroborating this finding with associated upregulation of PU.1 and downregulation of GATA-1 and GATA-3. Further studies are needed to understand how burn-induced emergency hematopoiesis may predispose to infection by pathologic lineage selection.