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Background and purpose: Accumulating epidemic evidence suggest that a considerable number of ischemic stroke patients had 1 or more cancer diagnosis before stroke attack. How the pre-existing cancer impact the cerebral ischemic injury is remarkably unknown. We tested the hypothesis that pre-existing cancer exacerbates cerebral ischemic stroke via regulatory T cell (Treg) redistribution.Methods: MC38 colon cancer cells or B16 melanoma cells were injected subcutaneously at the dose of 5*105 cells in 200μl PBS 3 weeks before distal middle cerebral artery occlusion (d-MCAO) surgery. Infarct volume was assessed at 3 days after surgery by staining the mice brain with 2,3,5-triphenyltetrazolium chloride. Sensorimotor assessments, such as body proprioception, climbing, forelimb walking, lateral turning, foot fault and adhesive removal were examined at 3, 5, 7, 14, 21 and 28 days after stroke. Neuro-inflammation was examined by measuring inflammatory cytokines with RT-PCR and immune cell infiltration using immunofluorescence and flow cytometry.Results: Pre-existing colon cancer and melanoma both exacerbated infarct volume growth in d-MCAO mice at 3 days after surgery. Mice with colon cancer exhibited prominently deterioration in their performance in sensorimotor functions after stroke compared with mice without cancer. Pre-existing colon cancer augmented peripheral immune cell infiltration into the ischemic brain but hindered Tregs’ recruitment into the brain. Cerebral ischemic stroke induced reduction in the number of Tregs in the peripheral blood were significantly aggravated in mice with pre-existing colon cancer. Depletion of Tregs with CD25 monoclonal antibody increased infarct volume in stroke mice but did not further exacerbate infarct growth in colon cancer stroke mice.Conclusion: Pre-existing cancer exacerbates ischemic brain injury and neuro-inflammation after stroke. Tregs redistribution plays an indispensable role in the cancer related deterioration of ischemic brain injury and may represent a promising target for treating stroke patients with pre-existing cancer.