Functional folate receptor beta-expressing macrophages in osteoarthritis synovium and their M1/M2 expression profiles

The distribution of folate receptor (FR)-β+ macrophages and their M1/M2 expression profiles were examined in osteoarthritis (OA) synovial tissues, and compared to those in rheumatoid arthritis (RA) synovial tissues and CD163+ macrophages in both OA and RA synovial tissues.

The phenotypes and fluorescein isothiocyanate (FITC)-folate uptake of FR-β+ synovial macrophages were analysed by flow cytometry. The distribution of FR-β+ macrophages in OA and RA synovial tissues was examined by immunofluorescent microscopy. Tumour necrosis factor (TNF)-α, inducible nitric oxide synthase (iNOS), interleukin (IL)-10, and transforming growth factor (TGF)-β expression in FR-β+ macrophages was detected by double-immunostaining in both OA and RA synovial tissues.

FR-β+ macrophages were predominantly present in the synovial lining layer in OA patients. The proportion of CD163-FR-β+ cells in synovial mononuclear cells (MNCs) was increased in OA compared to RA synovial tissues. FR-β(high) macrophages from OA synovial tissues represented the majority of folic acid-binding cells. Although FR-β+ or CD163+ macrophages in the synovial tissues of OA and RA patients expressed a mixed pattern of M1 and M2 macrophage markers, there were more M2 markers expressing synovial macrophages in OA than in RA patients.

The distribution and M1/M2 expression profiles of FR-β+ synovial macrophages were different between OA and RA synovial tissues. Thus, the findings underscore that the M1/M2 paradigm using surface markers FR-β and CD163 is an oversimplification of macrophage subsets. Functional FR-β present on OA synovial macrophages provides a potential tool for the diagnosis and treatment of OA.