MR1 and rheumatoid arthritis: After exposure to fixed E. coli, CD3+MR1-tet+ cells from controls and SpA patients strongly increased CD69 expression (median control MFI = 1307, SpA MFI = 1257), while CD3+MR1-tet+ cells from RA patients did not (median RA MFI = 466.6; p = 0.0004, controls vs RA p = 0.0021, SpA vs RA p = 0.0014).