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...