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Original research
M1 macrophages are required for murine cerebral aneurysm formation
  1. Kamil W Nowicki1,
  2. Koji Hosaka2,
  3. Frank J Walch3,
  4. Edward W Scott4,
  5. Brian L Hoh2
  1. 1Department of Neurosurgery, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania, USA
  2. 2Department of Neurosurgery, University of Florida, Gainesville, Florida, USA
  3. 3Department of Neurosurgery, University of Colorado, Denver, Colorado, USA
  4. 4Department of Molecular Genetics and Microbiology, University of Florida, Gainesville, Florida, USA
  1. Correspondence to Dr B L Hoh, Department of Neurosurgery, University of Florida, Gainesville, PO Box 100265, Gainesville, FL 32610, USA; brian.hoh{at}


Introduction Macrophages and neutrophils have been separately implicated in cerebral aneurysm formation. The interactions between different myeloid subsets and the contributions of macrophage phenotypes in these lesions over time are not known. The purpose of the study was to examine macrophage phenotypic changes in cerebral aneurysms.

Methods We induced aneurysm formation in C57BL/6 mice and quantified contributions of M1 and M2 macrophages in aneurysm specimens with or without neutrophil blockade. In our aneurysm model, the left common carotid and right renal arteries were ligated, and mice were placed on a hypertensive high fat diet. One week later, stereotactic injection with elastase solution into the basal cisterns was performed. An angiotensin II secreting osmotic pump was implanted. The mice were then treated with anti-CXCL1 antibody or IgG control antibody. Animals were euthanized at 3 days, or 1 or 2 weeks. The circle of Willis was analyzed using immunohistochemistry for M1 and M2 macrophage phenotype contributions.

Results Proinflammatory M1/M2 ratio increased in cerebral aneurysm formation over time, from 0.56 at 3 days to 1.75 at 2 weeks (p<0.0001). In contrast, anti-CXCL1 antibody blockade led to polarization towards an anti-inflammatory phenotype with an M1/M2 ratio of 0.95 at 2 weeks compared with IgG treated mice (p=0.0007).

Conclusions CXCL1 dependent neutrophil inflammation appears to have an important role in macrophage polarization to M1 phenotype in cerebral aneurysm development.

  • Aneurysm
  • Inflammation
  • Inflammatory Response
  • Brain

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  • Contributors KWN conceived the study, carried out the animal experiments, performed immunohistochemistry, interpreted and analyzed the data, and drafted the manuscript. KH helped with animal studies and immunohistochemistry. FJW helped with immunohistochemistry studies. EWS participated in drafting the manuscript. BLH participated in designing the study, interpreting the data, and drafting the manuscript. All authors read and approved the final manuscript.

  • Funding This work was supported by a Shirley Dudek Demmer Chair of Research grant from the Brain Aneurysm Foundation to KWN and NIH grant K08 NS067058-01 to BLH.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data sharing statement Data available on request.