Unexpected presence of polyreactive catalytic antibodies in IgG from unimmunized donors and decreased levels in rheumatoid arthritis

Polyclonal IgG from healthy humans and unimmunized mice was screened for peptide-methylcoumarinamide (peptide-MCA) hydrolyzing activity. The activity was detected in every IgG sample examined. Contaminant enzymes were precluded as an explanation, as the activity tracked exactly with the 150-kDa IgG peak separated by gel filtration in denaturing solvent (6 M guanidine hydrochloride) and with the 50-kDa Fab fragment peak produced by papain- digestion of the IgG. Patients with rheumatoid arthritis displayed a 3.2- fold reduced peptide-MCA hydrolyzing activity (mean) compared to healthy subjects. Control osteoarthritis patients showed no diminution in activity. A progressive decrease in the activity by 7.4-fold of the pre-immune levels was observed in mice over the course of hyperimmunization with SRBC, indicating that exogenous Ag challenge, like rheumatoid arthritis, is associated with decreased catalytic activity. Apparent K(m) values of the IgG for Pro-Phe- Arg-MCA were 0.39 to 0.53 mM, values approximately 3-orders of magnitude greater than observed previously for Ag-specific catalysis by Abs. The only common structural feature in peptide-MCA conjugates utilized by the Abs as substrates was the presence of Arg-MCA and Lys-MCA bonds. The IgG hydrolyzed Pro-Phe-Arg-Phe at the Arg-Phe peptide bond, showing that the activity is relevant to cleavage of peptide bonds in natural Ags. The universal occurrence of this polyreactive catalytic activity in unimmunized donors and its diminution in an autoimmune disease and nonspecific Ag challenge suggest that it may possess an important, but as yet unidentified, biologic role.