Ion in tendon explants from a 4 year old horse showing non-stimulated control (left) compared to stimulation with 5 Biotin NHS web ngml21 IL-1b (right). FPR2/ALX expression was not detectable in non-stimulated controls. Immunopositive staining is green, with Hoechst nuclear counter stain in blue. Scale bar = 25 mm. doi:10.1371/journal.pone.0048978.gtendon ECM via the induction of pro-resolving LXA4 and switching of lipid mediators from the prostaglandin to the lipoxin axis. Furthermore, in the setting of a pro-inflammatory environment, the presence of higher levels of PGE2 may exert an autoregulatory feedback effect on IL-1 activity in order to modulate the inflammatory reaction [50]. Although the cell types responsible for lipid mediator class switching have not been identified in inflamed tendons, we hypothesise that the interaction between resident tendon cells and infiltrating pro-inflammatory macrophagesFigure 8. Mean LXA4 levels 24 hours after stimulation with proinflammatory mediators. Explants were derived from macroscopically normal tendons from 3 horses aged between 9?4 years of age and stimulated with 5 ngml21 IL-1b or combined stimulation with low (0.01 mM) or high (1.0 mM) doses of PGE2 with 5 ngml-1 IL-1b compared to non-stimulated controls. LXA4 release was increased in all stimulated samples compared to respective controls (P = 0.005). Treatment with IL1b induced greater LXA4 production compared to controls (P = 0.011). Combined stimulation with high dose PGE2 enhanced LXA4 release compared to low dose PGE2 (P = 0.032). Error bars represent standard deviation. * P,0.05. doi:10.1371/journal.pone.0048978.gduring early stage injury initiates activation of pro-resolving processes. LXA4 levels were reduced during the chronic injury phase where the tendon does not return to normal structure and function. As LXA4 is a key determinant of pro-resolving processes [51] it is therefore plausible that incomplete resolution sustains a low level of inflammation, perpetuating chronic disease. Although the present study did not measure the multiple enzymes that synthesise the components of prostaglandin and lipoxin pathways, it is hypothesised that control of class switching involves the regulation of some of these enzymes. The lipoxin A4 receptor FPR2/ALX is reported to have a pivotal role in controlling the duration and magnitude of the inflammatory response, providing endogenous stop signals for inflammation [33,34]. Despite the anticipated importance of specialised pro-resolving mediators such as LXA4 in healing, these resolving pathways are not widely studied in injured tendons. We 1662274 recently identified significantly increased expression of FPR2/ ALX by tenocytes in early equine tendon injury [16] and studies in other inflamed connective tissues have emphasised the importance of resolution processes for regulating inflammation, BTZ043 biological activity including inhibition of leukocyte recruitment and modification of vascular permeability [33]. The current study provides novel data illustrating levels of FPR2/ALX are markedly diminished in the tendons of aged injured individuals. Because these mediators are essential for controlling the inflammatory cascade, this suggests an age-related deterioration of tendons to mount a counter-response to inflammation via FPR2/ALX. A component of immunosenescence is `inflamm-aging’ whereby aged individuals exhibit diminished ability to modulate inflammation [37,52]. Studies in humans and rodents report an age related decline in cutaneous.Ion in tendon explants from a 4 year old horse showing non-stimulated control (left) compared to stimulation with 5 ngml21 IL-1b (right). FPR2/ALX expression was not detectable in non-stimulated controls. Immunopositive staining is green, with Hoechst nuclear counter stain in blue. Scale bar = 25 mm. doi:10.1371/journal.pone.0048978.gtendon ECM via the induction of pro-resolving LXA4 and switching of lipid mediators from the prostaglandin to the lipoxin axis. Furthermore, in the setting of a pro-inflammatory environment, the presence of higher levels of PGE2 may exert an autoregulatory feedback effect on IL-1 activity in order to modulate the inflammatory reaction [50]. Although the cell types responsible for lipid mediator class switching have not been identified in inflamed tendons, we hypothesise that the interaction between resident tendon cells and infiltrating pro-inflammatory macrophagesFigure 8. Mean LXA4 levels 24 hours after stimulation with proinflammatory mediators. Explants were derived from macroscopically normal tendons from 3 horses aged between 9?4 years of age and stimulated with 5 ngml21 IL-1b or combined stimulation with low (0.01 mM) or high (1.0 mM) doses of PGE2 with 5 ngml-1 IL-1b compared to non-stimulated controls. LXA4 release was increased in all stimulated samples compared to respective controls (P = 0.005). Treatment with IL1b induced greater LXA4 production compared to controls (P = 0.011). Combined stimulation with high dose PGE2 enhanced LXA4 release compared to low dose PGE2 (P = 0.032). Error bars represent standard deviation. * P,0.05. doi:10.1371/journal.pone.0048978.gduring early stage injury initiates activation of pro-resolving processes. LXA4 levels were reduced during the chronic injury phase where the tendon does not return to normal structure and function. As LXA4 is a key determinant of pro-resolving processes [51] it is therefore plausible that incomplete resolution sustains a low level of inflammation, perpetuating chronic disease. Although the present study did not measure the multiple enzymes that synthesise the components of prostaglandin and lipoxin pathways, it is hypothesised that control of class switching involves the regulation of some of these enzymes. The lipoxin A4 receptor FPR2/ALX is reported to have a pivotal role in controlling the duration and magnitude of the inflammatory response, providing endogenous stop signals for inflammation [33,34]. Despite the anticipated importance of specialised pro-resolving mediators such as LXA4 in healing, these resolving pathways are not widely studied in injured tendons. We 1662274 recently identified significantly increased expression of FPR2/ ALX by tenocytes in early equine tendon injury [16] and studies in other inflamed connective tissues have emphasised the importance of resolution processes for regulating inflammation, including inhibition of leukocyte recruitment and modification of vascular permeability [33]. The current study provides novel data illustrating levels of FPR2/ALX are markedly diminished in the tendons of aged injured individuals. Because these mediators are essential for controlling the inflammatory cascade, this suggests an age-related deterioration of tendons to mount a counter-response to inflammation via FPR2/ALX. A component of immunosenescence is `inflamm-aging’ whereby aged individuals exhibit diminished ability to modulate inflammation [37,52]. Studies in humans and rodents report an age related decline in cutaneous.