Posttraumatic pneumonia exacerbates bone marrow erythropoietic dysfunction

Introduction: Pneumonia is a common complication after severe trauma that is associated with worse outcomes with increased mortality. Critically ill trauma patients also have persistent inflammation and bone marrow dysfunction that manifests as persistent anemia. Terminal erythropoiesis, which occurs in bone marrow structures called erythroblastic islands (EBIs), has been shown to be impacted by trauma. Using a preclinical model of polytrauma (PT) and pneumonia, we sought to determine the effect of infection on bone marrow dysfunction and terminal erythropoiesis.

Methods: Male and female Sprague-Dawley rats aged 9 to 11 weeks were subjected to either PT (lung contusion, hemorrhagic shock, cecectomy, and bifemoral pseudofracture) or PT with postinjury day 1 Pseudomonas pneumonia (PT-PNA) and compared with a naive cohort. Erythroblastic islands were isolated from bone marrow samples and imaged via confocal microscopy. Hemoglobin, early bone marrow erythroid progenitors, erythroid cells/EBI, and % reticulocytes/EBI were measured on day 7. Significance was defined as p < 0.05.

Results: Day 7 hemoglobin was significantly lower in both PT and PT-PNA groups compared with naive (10.8 ± 0.6 and 10.9 ± 0.7 vs. 12.1 ± 0.7 g/dL [ p < 0.05]). Growth of bone marrow early erythroid progenitors (colony-forming units-granulocyte, erythrocyte, monocyte, megakaryocyte; erythroid burst-forming unit; and erythroid colony-forming unit) on day 7 was significantly reduced in PT-PNA compared with both PT and naive. Despite a peripheral reticulocytosis following PT and PT-PNA, the percentage of reticulocytes/EBI was not different between naive, PT, and PT-PNA. However, the number of erythroblasts/EBI was significantly lower in PT-PNA compared with naive (2.9 ± 1.5 [ p < 0.05] vs. 8.9 ± 1.1 cells/EBI macrophage). In addition to changes in EBI composition, EBIs were also found to have significant structural changes following PT and PT-PNA.

Conclusion: Multicompartmental PT altered late-stage erythropoiesis, and these changes were augmented with the addition of pneumonia. To improve outcomes following trauma and pneumonia, we need to better understand how alterations in EBI structure and function impact persistent bone marrow dysfunction and anemia.