Impaired proteolysis by SPPL2a causes CD74 fragment accumulation that can be recognized by anti-CD74 autoantibodies in human ankylosing spondylitis

Ankylosing spondylitis (AS) is associated with autoantibody production to class II MHC-associated invariant chain peptide, CD74/CLIP. In this study, we considered that anti-CD74/CLIP autoantibodies present in sera from AS might recognize CD74 degradation products that accumulate upon deficiency of the enzyme signal peptide peptidase-like 2A (SPPL2a). We analyzed monocytes from healthy controls (n = 42), psoriatic arthritis (n = 25), rheumatoid arthritis (n = 16), and AS patients (n = 15) for SPPL2a enzyme activity and complemented the experiments using SPPL2a-sufficient and -deficient THP-1 cells. We found defects in SPPL2a function and CD74 processing in a subset of AS patients, which culminated in CD74 and HLA class II display at the cell surface. These findings were verified in SPPL2a-deficient THP-1 cells, which showed expedited expression of MHC class II, total CD74 and CD74 N-terminal degradation products at the plasma membrane upon receipt of an inflammatory trigger. Furthermore, we observed that IgG anti-CD74/CLIP autoantibodies recognize CD74 N-terminal degradation products that accumulate upon SPPL2a defect. In conclusion, reduced activity of SPPL2a protease in monocytes from AS predisposes to endosomal accumulation of CD74 and CD74 N-terminal fragments, which, upon IFN-γ-exposure, is deposited at the plasma membrane and can be recognized by anti-CD74/CLIP autoantibodies.