Gr1+ Macrophages and Dendritic Cells Dominate the Inflammatory Infiltrate 12 h After Experimental Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a devastating disease lacking an effective treatment. While the initial injury occurs within minutes, an inflammatory response contributes to ongoing tissue damage over hours to days. Relatively little is known about leukocyte trafficking into the brain in the hours after ICH onset. Understanding these events may lead to identification of new therapeutic targets. Using the blood injection mouse model of ICH, the numbers of leukocytes in the ipsilateral and contralateral brain were quantified by flow cytometry 12 h after surgery. Perihematomal inflammation was confirmed by histology and chemokines and cytokines in the brain quantified by multiplex ELISA. Few neutrophils were detected in the brain 12 h after ICH. The majority of leukocytes consisted of inflammatory macrophages (CD45.1^hiCD3⁻Ly6G⁻CD11c⁻CD11b⁺Gr1⁺ cells) and inflammatory dendritic cells (CD45.1^hiCD3⁻Ly6G⁻CD11c^intCD11b⁺Gr1⁺ cells). Microglia numbers did not differ between the hemispheres. These results indicate that blood-derived monocyte populations traffic into brain early after ICH and outnumber neutrophils at 12 h.