The role of the anti-inflammatory protein annexin A1 in the acute inflammatory response

Ellen Hughes, Julia Buckingham, Felicity Gavins. Imperial College, London, United Kingdom.

The inflammatory response is the body’s response to trauma, infection or other stimuli. Inflammation is critical to survival, however it can become exaggerated or persistent resulting in tissue damage, an effect found in the progression of many major diseases. The endogenous Ca2+-binding protein annexin A1 (AnxA1) is abundantly expressed on leukocytes. AnxA1 acts as an anti-inflammatory agent by causing detachment of leukocytes adherent to the vascular endothelium and inhibition of leukocyte migration in both in vitro and in vivo studies(1,2).

This study investigated the role of AnxA1 in the acute inflammatory response to lipopolysaccharide (LPS) in-vivo. Male wild-type (WT; C57BL/6) and AnxA1-null (AnxA1-/-) mice were challenged with LPS (10μg/mouse) given intra-peritoneally for 2h, 6h or 24h. The technique of intravital microscopy was employed to visualize and quantify venular and arteriolar interactions in the mesentery (1). Plasma samples were taken for inflammatory cytokines (IL-1B, IL-6, IL-10 and TNFα) assay by ELISA.

No significant cellular interactions were observed in arterioles of either WT or AnxA1-/- mice treated with saline or LPS. In venules of WT animals at 2h, LPS caused an inflammatory response: reduced rolling cell velocity (VWBC: 11.8±0.8 vs. 4.5±0.3); increased leukocyte rolling flux (109.7±14.1 vs. 254.0±38.8); increased adherent (1.1±0.4 vs. 5.0±0.6) and emigrated (1.1±0.19 vs. 6.5±1.9) cells (P<0.05, n=6mice/group). These effects increased over the 24h period that was assessed. In venules of AnxA1-/- mice, LPS treatment reduced VWBC and increased rolling cell flux, in line with its WT counterparts. However, AnxA1-/- mice showed significantly fewer adherent and emigrated cells (P<0.05, n=6mice/group) 2h after treatment with LPS, suggesting a delay in the inflammatory response of these mice in comparison to their WT counterparts. By 6h the adherent cells matched the WT values, and by 24h there was a further increase . LPS did not cause an increase in the number of emigrated cells in AnxA1-/- mice at any of the time-points tested (P>0.05, n=6mice/group).

Overall these data indicate that whilst there is an initial delay in the inflammatory response in AnxA1-/- mice, they demonstrate an exacerbated response in adherence at 6 and 24h. This phenomenon appears to be restricted to the venular system, and not arterioles.

(1) Gavins F et al. Blood. 2003;101: 4140-7.
(2) Lim LH et al., Proc Natl Acad Sci U S A. 1998;95:14535-9.