TGFB1 and rheumatoid arthritis: TGF-β1 has been shown to have a direct role on class switching to IgA [19, 34, 35], and we also demonstrated a positive correlation between LPS-specific IgA response and TGF-β1 levels in SF from RA and SpA patients (r = 0.5; P < 0.05) (Fig. 3b).