Annexin A1: checkpoint between blood brain barrier and leukocytes trafficking

E Maggioli, S McArthur, E Cristante, D Cooper, E Solito. William Harvey Research Institute-Queen Mary University London, London, EC1M 6BQ, UK

Annexin A1 (ANXA1) is expressed in the endothelial cells of the brain microvasculature1, where it plays a major role in promoting the integrity of the blood brain barrier (BBB)2. In particular, ANXA1-/- mice exhibit significantly increased BBB permeability due to disrupted inter-endothelial cell tight junctions, essentially related to changes in the actin cytoskeleton, which stabilizes tight and adherens junctions2. Furthermore, ANXA1 is selectively lost in the BBB endothelium in multiple sclerosis (MS), but not in other neuropathologies including Parkinson’s and Alzheimer’s diseases. We have previously shown ANXA1 to exert a powerful anti-inflammatory action by limiting leukocyte extravasation via binding to and blockade of the α4β1 integrin; hence its absence removes an endogenous braking mechanism in situations of an ongoing immune response3. Here we studied the involvement of ANXA1 in leukocyte-BBB interactions and migration, utilising the immortalised human brain microvascular cell line (hCEMC/D3) stably infected with shRNA for ANXA1, in a transwell system under shear stress conditions to ensure the correct cellular polarity and formation of tight junctions. Significantly greater numbers of peripheral blood mononuclear cells (PBMCs) (25±3.1 vs 13±1.6; p=0.0263; n=3) adhered to endothelial monolayers lacking ANXA1 under inflammatory conditions. Furthermore endothelial ANXA1 loss correlated with higher expression of ICAM1 measured by flow cytometry (MIF 903±17 vs 600±15; p=0.0002; n=3) and constitutively higher CD4 migration in non-inflammatory condition. We present here evidence for a dual role of ANXA1 in leukocyte migration into the central nervous system: 1) as a safeguard at the level of the endothelium in non-inflammatory conditions and 2) as a blocker of leukocytes extravasation in the presence of circulating cytokines and inflammatory conditions.

(We are grateful to the Wellcome Trust for financial support)

1. Solito E, McArthur S, Christian H, Gavins F, Buckingham JC and Gillies GE (2008) Annexin A1 in the brain-undiscovered roles? Trends Pharmacol Sci 29(3):135-142.

2. Cristante E, McArthur S, Mauro C, Maggioli E, Romero IA, Wylezinska-Arridge M, Lopez-Tremoleda J, Couraud PO, Christian E, Weksler BB, Malaspina A and Solito E. Identification of an essential endogenous regulator of blood brain barrier integrity: pathological and therapeutic implications.(PNAS in revision)

3. Solito E, Romero IA, Marullo S, Russo-Marie F & Weksler BB (2000) Annexin 1 binds to U937 monocytic cells and inhibits their adhesion to microvascular endothelium: involvement of the alpha 4 beta 1 integrin. J Immunol 165(3):1573-1581.