The immunology of proteoglycans.

Abstract

This discussion will touch on some of the central features of proteoglycan (PG) immunology and suggest that despite the recent introduction of very sophisticated immunochemical techniques, this remains a problematic field. As I shall indicate, there are difficulties in understanding the immunological properties of PGs which are due to the nature of PG, to a basic uncertainty of the details of PG structure and organization and, finally, to our inability to isolate truly homogeneous PGs or PG fractions for use in immunological studies. The earliest work in PG immunology showed that although the glycosaminoglycan (GAG) portion of the PG comprises the bulk of its composition and determines its physicochemical properties, isolated chondroitin sulfate (CS) or keratan sulfate (KS) chains are immunologically silent. Immunization of experimental animals with the isolated GAGS does not lead to the production of antibodies detectable by immune precipitation, hemagglutination inhibition or cutaneous reactivity. Furthermore, GAG chains isolated by (3-elimination or papain digestion of PG do not influence the binding of multivalent anti-PG sera with PG radio-labeled either in the core protein or in the CS Thus, it appears that the immunogenicity of PG is due to its protein constituents. This has both good and bad consequences. The chemical and enzymatic methods for the analysis of the GAG chains are well established and our knowledge of their structure and synthesis is well developed, whereas our knowledge of the detailed structure of the PG core protein is limited. The specificity of anti-PG antibodies for PG protein offers a potentially valuable direct probe for the study of PG proteins in face of the relatively small amount present in PG. On the other hand, in trying to understand and interpret the immunological findings, we are sorely handicapped by our incomplete knowledge of what the PG proteins are and how they fit together. A second early discovery of PG immunology was that the PG molecule must be modified in order to allow it to be studied. Treatment of cartilage sections or of the extracted chondromucoprotein or proteinpolysaccharide, as PG was then known, with testicular hyaluronidase was found necessary in order to demonstrate full immunological reactivity, even though the antisera used were produced by immunization with PG fractions which had not been digested with hyal~ronidase.~* ~*~ The explanations offered for this were that PG molecules are just too big to diffuse through agarose, so they can’t be identified by Ouchterlony immunodiffusion, or that the CS chains are so big and bulky that the antibody molecules can not get near the antigen.3 I believe that the observation is correct, but I am not sure about the explanations. PG molecules are quite capable of diffusing through agarose and many of us make our livings by characterizing this process. In addition, if PG molecules are digested with trypsin, which does not affect the CS chains that are