The impact of clonal hematopoiesis on neuroinflammation after ischemic stroke in a chimeric bone marrow mouse model

Clonal hematopoiesis, which refers to somatic mutations in hematopoietic stem cells (HSC), is recognized as a risk factor for stroke. Tet2 mutations exerted the greatest influence on complex cardiovascular endpoints in a study of incident strokes. The aim of this preclinical study was to explore if mutant HSC exhibit an aggressive phenotype, releasing pro-inflammatory cytokines and causing more damage to the penumbra area, heart, endothelial cells, and the blood-brain barrier (BBB). One-leg irradiated mice grafted into with WT or Tet2-/- (induced knockout) chimeric bone marrow were subjected to 30-minute middle cerebral artery occlusion (MCAO) after 18 months. We assessed lesion sizes 1 and 7 days after MCAO using T2-MRI. Functional recovery was investigated by means of a behavioral test panel. Peripheral blood, bone marrow, and brain underwent flow cytometry-based immunophenotyping to assess expansion and invasion of Tet2-deficient cells. There was no difference in stroke lesion sizes between mice harboring WT and Tet2-deficient immune cells. Mice with mutant bone marrow also displayed similar functional deficit or recovery. We have not identified clonal expansion of Tet2-deficient cells in peripheral blood. Flow cytometry of the brain tissue revealed higher invasion of host immune cells and higher number of host microglia in lesion area of Tet2-deficient mice. We observed some evidence of a pro-inflammatory state in mice harboring Tet2-deficient cells after stroke that corresponded with clinical findings. However, the next round of study will require model refinement including a raise of the frequency of Tet2-deficient cells and the analysis of younger animals.