Antikeratin antibodies in serum and synovial fluid show specificity for rheumatoid arthritis in a study of connective tissue diseases.

Tests for antikeratin antibodies (AKA) were performed on 2152 disease-associated and control sera by indirect immunofluorescence (IF) on rat oesophagus substrate. The incidence of AKA was significantly raised in rheumatoid arthritis (37%) in comparison with systemic sclerosis (8%), psoriasis (7%), ankylosing spondylitis (6%), systemic lupus erythematosus (3%), and normal controls (2%). AKA were detected in synovial fluid obtained from patients with rheumatoid arthritis (RA) (48%) but not from patients with other conditions. Further experiments on AKA-positive sera showed reactivity with stratum corneum of rabbit prepuce and lips. A specific rabbit antihuman keratin antiserum was shown, by IF and inhibition studies, to have a different specificity from that of spontaneous human AKA. AKA were associated with the presence of subcutaneous nodules in RA (p = 0.05), but not with Raynaud's phenomenon, Sjögren's syndrome, or HLA-DR4 positivity. Rheumatoid factor (RF) was not associated with AKA either in RA or in RF-positive disease controls.