Fig. 3: Axl and MerTK have distinct and characteristic expression patterns in RA synovium, and Axl ectodomain can be cleaved and released in the synovial fluid.

A, C Representative images of Axl (A) and MerTK (C) immunohistochemistry (IHC) staining of synovial tissue sections. Scale bar = 100 μm. Representative images of n = 20 samples stained. B Double immunostaining of Axl (red) with CD68 (green, upper panel) and CD55 (yellow, lower panel) in the synovium of RA patients. Nuclei were counterstained with DAPI (blue). White arrows indicate double-positive cells. Scale bar = 50 μm. Representative images of n = 13 samples stained. D Triple immunostaining of Axl (red), MerTK (yellow), and CD68 (green) in the synovium of RA patients showing the presence of both Axl⁺ and MerTK⁺ double-positive CD68⁺ macrophages (orange arrow) and Axl⁺ or MerTK⁺ single positive CD68⁺ macrophages (white arrow). Nuclei were counterstained with DAPI (blue). Scale bar = 50 μm. Representative images of n = 18 samples stained. E Double immunostaining of Axl (red) with ADAM10 (green) in the synovium of RA patients. Nuclei were counterstained with DAPI (blue). White arrows indicate double-positive cells. Scale bar = 50 μm. Representative images of n = 5 samples stained. F Levels of soluble Axl (sAxl), soluble MerTK (sMerTK) and soluble Gas6 (sGas6) in ng/mL assessed by ELISA in the synovial fluid of RA patients (n = 18). p values indicated were calculated using the Kruskall–Wallis test, with Dunn’s post hoc test. G Levels of soluble Axl (sAxl) in ng/mL assessed by ELISA in the synovial fluid of RA patients (n = 18) divided according to synovitis score (low [0–4], high [5-9]). p values indicated were calculated using the two-tailed Mann–Whitney test. F, G Data are represented as mean ±SEM. H Correlation between sAxl synovial fluid levels and the erythrocyte sedimentation rate (ESR) of RA patients (n = 18). p value and r coefficient were calculated according to the two-tailed Spearman correlation test.