Intracerebral hemorrhage leads to infiltration of several leukocyte populations with concomitant pathophysiological changes

Intracerebral hemorrhage (ICH) is a stroke subtype with high rates of mortality and morbidity. The immune system, particularly complement and cytokine signaling, has been implicated in brain injury after ICH. However, the cellular immunology associated with ICH has been understudied. In this report, we use flow cytometry to quantitatively profile immune cell populations that infiltrate the brain 1 and 4 days post-ICH. At 1 day CD45^hi GR-1⁺ cells were increased 2.0-fold compared with saline controls (P≤0.05); however, we did not observe changes in any other cell populations analyzed. At 4 days ICH mice presented with a 2.4-fold increase in CD45^hi cells, a 1.9-fold increase in CD45^hi GR-1^- cells, a 3.4-fold increase in CD45^hi GR-1⁺ cells, and most notably, a 1.7-fold increase in CD4⁺ cells (P≤0.05 for all groups), compared with control mice. We did not observe changes in the numbers of CD8⁺ cells or CD45^lo GR-1^- cells (P=0.43 and 0.49, respectively). Thus, we have shown the first use of flow cytometry to analyze leukocyte infiltration in response to ICH. Our finding of a CD4 T-cell infiltrate is novel and suggests a role for the adaptive immune system in the response to ICH.