Folate receptor positive macrophages in rat models for OA: phenotype switch in response to anti-inflammatory medication

Abstract

Purpose: Folate-based radiotracers have been used to visualize macrophages in a range of inflammatory diseases, including RA and OA, and have shown that folate receptor-expressing macrophages accumulate at the site of inflammation. Macrophages are thought to arise from monocytes that migrate from the bloodstream to the site of inflammation or injury and then mature to the appropriate pro-inflammatory or anti-inflammatory phenotype. Methods: From previous work using folate-SPECT imaging in both animal models and in patients with OA we know that folate receptor-expressing macrophages are present in OA affected joints. We have performed studies with a variety of rat models using folate-SPECT imaging in OA: running at various intensities, papain injections combined with running (Figure 1), high-fat diet, placement of grooves on femoral condyles and a combination of high fat diet + grooves (Figure 2). In all models cartilage damage, osteophytosis and synovial inflammation was assessed by histology. Results: We observed that folate uptake is increased with increasing OA severity, and seems to correlate with synovial membrane inflammation. This has led to the hypothesis that the folate-SPECT scan could be a useful tool to monitor disease severity in vivo. Interestingly, two subsequent studies with anti-inflammatory medication in the papain-groove model showed folate uptake increased up to three-fold after treatment with local delivery of either triamcinolone acetonide or celecoxib in the knee joint (Figure 1). This increase was not associated with increased disease severity, but rather with reduced osteophyte formation and increased cartilage thickness. Further in vitro evaluation confirmed that triamcinolone acetonide induces an anti-inflammatory folate-positive phenotype in macrophages during the early stages of maturation but also showed that the folate receptor is not specifically expressed in anti-inflammatory macrophages.Together these results suggest that as new monocytes arrive in the joint at the site of inflammation and mature into macrophages, they become active folate expressing pro-inflammatory macrophages. When anti-inflammatory medication is administered intra-articularly, the newly arrived monocytes mature instead to anti-inflammatory macrophages with possibly even higher folate expression (Figure 3). Conclusions: In conclusion, the folate receptor may be a useful tool to monitor macrophage activation during OA development and progression. However, folate-based imaging cannot distinguish between macrophage phenotypes and the use of anti-inflammatory medication could thus result in unexpectedly high uptake of folate by (anti-inflammatory) macrophages.Figure 2Relative activity on folate SPECT in the high-fat diet model in the grooved vs the sham operated knee.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 3Corticosteroids drive macrophages towards an anti-inflammatory phenotype, but only during the early stages of maturation.View Large Image Figure ViewerDownload Hi-res image Download (PPT)